Food chemistry 2nd sem (full sylabus)

POWER RANGERNOTES FOOD CHEMISTRY
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Introduction
Foods are derived from plants, carcasses of animals, and single-cell organisms.
Main components include water, carbohydrates, proteins, lipids and minerals
A host of other compounds present in minor quantities - Non-protein nitrogenous
compounds, vitamins, colorants, flavor compounds and functional additives
These have significant impact on the quality of many products.
Sources of food Components
Water Carbo-
hydrates
Proteins Water Minerals Vitamins
Juices Sugars Soybean Oils Vegetables Vegetables
Fruits Honey Beans&
peas
Lard Fruits Fruits
Milk Cereals Meat Milk,
Butter,
Ghee,
Vanaspathy
Meat Fish liver
Vegetables Chocolate&
sweets
Fish Chocolate Fish
products
Meat
Jellies Potato Wheat Nuts Dairy
products
Cereals
Lean fish Cassava Cheese Egg yolk Cereals Milk
Lean meat Fruits &
Vegetables
Eggs Pork Nuts Yeast
Types of food components supplied by food
• The main food components supply the human body
• Necessary body building material,
• Energy yielding material
• Eelements and compounds indispensable for metabolism.

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1. Body building material
• Polysaccharides, proteins, and lipids serve as the building material of
different structures of the plant and animal tissues used for food.
• The structures made of these materials are responsible for the form and tensile
strength of the tissues, and created the necessary conditions for metabolic
processes to occur.
• Compartmentalization resulting from these structures plays a crucial biological
role in the organisms.
• Some of the main components, as well as other constituents, are bound to
different cell structures or are distributed in soluble form in the tissue fluids.
2. Energy yielding material
Carbohydrate
• The cheapest source of food.
• Can be readily digested, absorbs and utilized for producing energy.
• The most efficient source of energy.
• Can furnish 50-70% of the total calorie intake. Carbohydrates are almost
entirely derived from vegetable sources.
• Main sources: Starch in the granular form in cereals, pulses and tubers and
sugars that are present in milk, fruits and vegetables and sucrose.
Protein
• Oxidation of carbon skeletons of amino acids of proteins furnishes a minor but
significant fraction of the daily energy requirement.
Fat
• Fat (Triglyceride) from plant and animal sources rank close behind
carbohydrates as major source of energy. Fat has high fuel value. It has got
high capacity to be stored as energy in the body. There is little difference
between animal and vegetable fat as a source of energy.

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3. Protective
• Many of the minor components present food are nutritionally essential, such as
vitamins and minerals.
4. OTHER FUNCTIONS IN FOOD
3. Other materials
Many of the minor components originally present in the raw materials of food are
nutritionally essential, such as vitamins and minerals. Others such as most free
amino acids can be utilized by the body or impart desirable sensory properties to
food products. Numerous groups, including tocopherols, ubiquinone, carotenoids,
ascorbic acid, thiols, amines, and several other non-protein nitrogenous
compounds serve as endogenous muscle antioxidants, playing an essential role in
postmortem changes in meat. Some minor components are useless or even
harmful if present in
FUNCTIONS OF FOOD
a. The main food components supply the human body with the necessary building
material and source of energy, as well as elements and compounds indispensable for
metabolism.
b. Some plant polysaccharides are only partly utilized for energy.
c. As dietary fiber they affect, in different ways, various processes in the
gastrointestinal tract.
d. The distribution of lipids in food raw materials depends on their role in the living
animal and plant organisms.
e. In an animal body, lipids occur primarily as an energy rich store of neutral fat in
the subcutaneous adipose tissue; as kidney, leaf and crotch fat; as the intramuscular
fat known as marbling; and as intramuscular or seam fat.
• Many of the minor components originally present in the raw materials are
nutritionally essential, such as vitamins.

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• Some of them, although not indispensable, can be utilized by the body,
including most free amino acids, or impart desirable sensory properties to food
products.
• Numerous groups, including tocopherols, ubiquinone, carotenoids, ascorbic
acid, thiols, amines, and several other non-protein nitrogenous compounds
serve as endogenous muscle antioxidants, playing an essential role in
postmortem changes in meat.
• Some minor components are useless or even harmful if present in excessive
amounts.
• Most food raw materials are infected with different microorganisms,
putrefactive and often pathogenic, and some contain parasites and the
products of microbial metabolism.
• A number of compounds are added intentionally during processing, to be used
as preservatives, antioxidants, colorants, flavorings, sweeteners, and
emulsifying agents, or to fulfill other technological purposes.
WATER
• The content of water in various foods ranges from a few percent in dried
commodities (dried milk) to 90% in many fruits and vegetables
• about 15% in grains
• 16 to 18% in % in butter
• 20% in honey
• 35% in bread,
• 65% potato and cassava
• 75% in meat and fish
• 90% in many fruits and vegetables
Carbohydrates
• Carbohydrates are widely distributed in plant and animal tissues.
• The carbohydrates commonly occurring in foods are starch, glucose, fructose,
sucrose and lactose. About 50-70 % of energy value in the average diet is
provided by carbohydrates.

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• They are the cheapest source of energy. Glucose derived by digestion of
carbohydrates is stored as glycogen in liver and muscle tissues and used as
the main source of energy in the body.
• Hence food must always contain adequate amounts of carbohydrates.
Important sources of carbohydrates
• The important sources of carbohydrates in the diets of children and adults are
cereals, millets, roots, tubers, pulses, sugar and jaggary, while milk and sugar
are important sources in the diets of infants.
Carbohydrate content of some important food
Name of food Carbohydrate g/100g
Cereals and millets (rice, jowar, etc.) 63 – 79
Pulses (Bengal gram, red gram, etc.) 56 – 60
Nuts and oilseeds 10 – 25
Roots and tubers (Potato, tapioca,
sweet potato, etc.)
22 – 39
Arrow root flour 85 – 87
Cane sugar 99
Sago 87 – 89
Honey 79 – 80
Jaggery 94 – 95
Milk (fluid) 4 – 5
Dried fruits (Raisin, dates, etc.) 67 – 77
Fresh fruits 10 - 25
Protein
• The protein content in foods is present mainly as crude protein (i.e., as
Nx6.25).

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• The nitrogen-to-protein conversion factor (N:P) of 6.25 has been recommended
for most plant and animal food products under the assumption that the N
content in their proteins is 16% and they do not contain non-protein N.
• The N content in the proteins in various food, however, is different because it
depends o the amino acid component of protein compounds, such as free
peptides and amino acids, nucleic acids and their the non-protein N may
constitute up to 30% of total N.
• In many of these compounds the C:N ratio is similar to the average in amino
acids
Protein contents of different groups of foods
Food groups Protein content %
Cereals and millets 6.14
Pulses (legumes) dry 18-24
Oilseeds and nuts (except coconut) 18-40
Meat, fish and liver 18-20
Eggs 12-14
Milk (fresh) 3.5-4.0
Milk, dried whole 26-28
Milk, dried, skimmed 33-38
Vegetables, fresh Leafy 1-4
Roots and tubers 1-1.5
Other vegetables 1-7
Lipids
• A group of naturally occurring substances characterized by their insolubility in
water and solubility in organic solvents. They occur in plant and animal tissues.

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• They include simple lipids, compound lipids and derived lipids.
• Oils and fats serve as the main source of energy.
• They also provide the essential fatty acids. They are good source of fat soluble
vitamins.
• Fat serve as an insulating material in the subcutaneous tissues and around
vital organs.
• It provide materials for the synthesis of cholesterol and certain hormones.
• Lipoproteins and glycoproteins are essential for maintaining cellular integrity.
• Fat is essential for maintaining good health
Fat contents of different groups of foods
Food groups Fat content
%
Fruits, vegetables <1
Lean fish muscle <1
Oilseeds and nuts
(except coconut)
98
Beef ,Meat, 6
Egg yolk 32
Milk (fresh) 3.5-4.0
Butter 85
Minerals
• Minerals are naturally occurring inorganic element in the soil which is
transformed into an organic compound for use and assimilation by the human
body.

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• There are 16 minerals that the human body needs in order to function
properly.
• Most of the important minerals are easily supplied in common foods.
• Fruits, vegetables, and cereals are the chief sources of mineral elements in
diet.
• Milk products supply the majority of calcium and phosphorus in the diet.
Factors affecting food composition
a) Raw Materials
i. The content of different components in food raw materials depends on
- the species and variety of the animal or plant crop;
- on the conditions of life, and age of the farm harvesting of the plants;
- on the feeding, conditions of life, and age of the farm animals or
- the fishing season for fish and marine invertebrates; and
- on post harvest changes that take place in the crop during storage.
ii. The food industry, by establishing quality requirements for raw materials, can
encourage producers to control, within limits, the contents of the main components in
their crops;
e.g: starch in potatoes, fat in various meat cuts, pigments in fruits and vegetables
and in the flesh of fish from aquaculture, or protein in wheat and barley, as well as
the fatty acid composition of lipids in oilseeds and meats.
iii. The contents of desirable minor components can also be effectively controlled for
example, the amount of natural antioxidants to retard the oxidation of pigments and
lipids in beef.
iv. Contamination of the raw material with organic and inorganic pollutants can be
controlled by observing recommended agricultural procedures in using fertilizers,
herbicides, and insecticides, and by seasonally restricting certain fishing areas to
avoid marine toxins.
v. The size of predatory fish like swordfish, tuna or shark which are fished
commercially can be limited to reduce the risk of excessive mercury and arsenic in
the flesh.
ii) Processed foods

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• The composition of processed foods depends on the recipe applied and on
changes taking place due to processing and storage.
• These changes are mainly brought about by endogenous and microbial
enzymes, active forms of oxygen, heating, chemical treatment, and processing
at low or high pH.
Changes occurring in food due to processing
1. Leaching of soluble, desirable and undesirable components, such as vitamins,
minerals and toxins during washing, blanching and cooking.
2. Dripping after thawing or due to cooking
3. Loss of moisture and volatiles due to evaporation and sublimation
4. Absorption of desirable or harmful compounds during salting, pickling,
seasoning, frying or smoking.
5. Formation of desirable or harmful compounds due to enzyme activity, such as the
development of typical flavor in cheese or decarboxylation of amino acids in fish
marinades.
6. Generation of desirable or objectionable products due to interactions of reactive
groups induced by heating or chemical treatment, such as flavors or carcinogenic
compounds in roasted meats, or trans-fatty acids in hydrogenated fats.
7. Formation of different products of oxidation of food components, mainly of lipids,
pigments, and vitamins
8. Loss of nutrients and deterioration of dried fish due to attacks by flies, mites and
beetles.
2. WATER IN FOODS
Introduction
 Water is the most critical of all nutrients.
 It is an essential constituent of all cell structures and is the medium in which all
the chemical reactions of a cellular metabolism take place.
 Water is the major component of all living organisms.
 It constitutes 60% or more of the weight of most living things, and it pervades
all portions of every cell.
 Water is the universal solvent and dispersing agent, as well as a very reactive
chemical compound.

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 Biologically active structures of macromolecules are spontaneously formed
only to aqueous media.
 Intracellular water is a medium in which structural arrangement and all
metabolic processes occur.
 It is an active partner of molecular interactions, participating directly in
many biochemical reactions as a substrate or a product.
 Its high heat capacity allows water to act as a heat buffer in all organisms.
 Regulation of water contents is important in the maintenance of
homeostasis in all living systems.
 Stability, wholesomeness, and shelf life are significant features of foods that
are, to a large degree, influenced by the water content.
 Dried foods were originally developed to overcome the constraints of time and
distance before consumption.
 Canned and frozen foods were developed next.
 The physical properties, quantity, and quality of water within food have a
strong impact on food effectiveness, quality attributes, shelf life, textural
properties and processing.
Water is the major component of many foods. Its quantity, location and orientation
profoundly influence the structure, appearance and taste of foods. Stability,
wholesomeness, and shelf life are significant features of foods that are, to a large
degree, influenced by the water content. Fresh foods contain large quantity of water
and hence effective forms of preservation is needed for long time storage.Water
content of different groups of foods is presented in table 2.1.1 Removal of water
by drying or converting it into ice crystals by freezing greatle alters the native
properties of foods. The physical properties, quantity, and quality of water within food
have a strong impact on food effectiveness, quality attributes, shelf life, textural
properties and processing.
Table 2.1.1 Water content of different groups of foods
Food Water content %
Fruits
Apple,grapes,oranges 90

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Pears 80-85
Tomato, strawberries 90-92
Vegetables
Banana, peas 73-80
Beet root, potatoes,carrots 86-94
Cabbage, cauliflower,
lettuce
90-95
Meat
Fish 65-84
Beef/Mutton 55-70
Chicken 65-75
Pork 55-60
Structure and Properties of water
• Water is a familiar material, but it has been described as the most anomalous
of chemical compounds.
• Although its chemical composition, HOH, or H2O, is universally known, the
simplicity of its formula belies the complexity of its behavior.
• Its physical and chemical properties are very different from compounds of
similar complexity, such as HF and H2S.
• Although a water molecule is electrically neutral as a whole, it has a dipolar
character.
• The high polarity of water is caused by the direction of the H-O-H bond angle,
which is 104.5o
, and by an asymmetrical distribution of electrons within the
molecule.
O
/ 104.5°
+
H H+
• In a single water molecule, each hydrogen atom shares an electron pair with
the oxygen atom in a stable covalent bond.
• However, the sharing of electrons between H and O is unequal because the
more electronegative oxygen atom tends to draw electrons away from the
hydrogen nuclei.
• The electrons are more often in the vicinity of the oxygen atom than in the
vicinity of the hydrogen atom.

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• The result of this unequal electron sharing is the existence of two electric
dipoles in the molecule, one along each of the H-O bonds.
• The oxygen atom bears a partial negative charge and each hydrogen atom a
partial positive charge. Because the molecule is not linear, H-O-H has a dipole
moment.
• Water molecules can interact through electrostatic attraction between the
oxygen atom of one water molecule and the hydrogen of another.
Type of hydrogen bond in water
• Such interactions, which arise because the electrons on one molecule c an be
partially shared with the hydrogen on another, are known as hydrogen bonds.
• The H2O molecule, which contains two hydrogen atoms and one oxygen atom
in a nonlinear arrangement, is ideally suited to engage in hydrogen bonding.
• It can act both as a donor and as an acceptor of hydrogen.
• The nearly tetrahedral arrangement of the H orbital about the oxygen atom
allows each water molecule to form hydrogen bonds with four of its neighbors.
• An individual, isolated hydrogen bond is very labile. It is longer and weaker
than a covalent O-H bond.
• The hydrogen bond’s energy, that is, the energy required to break the bond, is
about 20kJ/mol.
• These bonds are intermediate between those of weak Van der Waals
interactions (about 1.2 kJ/mol) and those of covalent bonds (460kJ/mol).
• Hydrogen bonds are highly directional; they are stronger when the hydrogen
atom and the two atoms that share it are in a straight line.
• Hydrogen bonds are not unique to water.
• They are formed between water and different chemical structures, as well as
between other molecules (Intermolecular) or even within a molecule
(Intramolecular
Water in foods
• Most natural foods contain water up to 70% of their weight.
• Water in foods is classified in to two types: (a) bound water and (b) free water

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• Water that can be extracted easily from foods by squeezing or cutting or
pressing is called as free water
a) Bound Water
Water that is held so tightly by another molecule (usually a large molecule such as a
protein) that it no longer has the properties of free water; water that is not easily
removed from the food is called bound water.
• This water is not free to act as solvent for salts and sugars.
• It can be frozen only at very low temperatures. It exhibits no vapour pressure.
Its density is greater than water.
• The water molecules are bound to polar groups or ions on molecules such as
starches, pectin, and proteins.
• This water is held firmly.
• The subsequent water layers are held less firmly.
The bound water is of three types
i. Constitutional
ii. Vicinal
iii. Multilayer
• i. Constitutional: They form an integral part of a non aqueous constituent
forming <0.03%. It is constituted by a monolayer of water molecules absorbed
on the polar absorption site of the molecule is almost immobilized and thus
behaves, in many respects, like part of the solid or like water in ice.
• ii. Vicinal: It is the bound water that strongly acts with specific hydrophilic sites
of non-aqueous constituents to form a monolayer coverage; water-ion and
water-dipole bonds forming 0.1 to 0.9%.
• iii. Multilayer: Bound water that forms several additional layers around
hydrophilic groups, water-water and water-solute hydrogen bonds. It forms 1-
5%.

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b) Free or entrapped water
• Water that can be extracted easily from foods by squeezing or cutting or
pressing is called as free water.
• Flow is unimpeded; properties close to dilute salt solutions.
• Free water is held within matrix or gel, which impedes flow forming 5-96%.
• Entrapped water is immobilized in capillaries or cells but if released during
cutting or damage, it flows freely.
Water activity
• Water activity or aw is a measurement of water content.
• It is defined as the vapour pressure of a liquid divided by that of pure water at
the same temperature; therefore, pure distilled water has a water activity of
exactly one.
aw=P/P0
• where p is the vapor pressure of water in the substance, and P0 is the vapor
pressure of pure water at the same temperature.
• As the temperature increases, aw typically increases, except in some product
with crystalline salt or sugar.
• Higher aw substances tend to support more microorganism.
• Bacteria usually require at least 0.91, and fungi at least 0.7.
• Many of the chemical and biological processes that cause deterioration of
foods, and ultimately spoilage, are water dependent.
• Water activity aw represents the water which is made available for the microbial
action.
• Microbial growth is directly linked to water activity.
• Essentially, water activity is the measure of the degree to which water is bound
within the food, and hence is unavailable for further chemical or microbial
activity
• Relative humidity of moist air is defined in the same way except that by
convention, relative humidity is reported as a percentage whereas water
activity is expressed as a fraction.
• Thus if a sample of meat sausage is sealed within an airtight container, the
humidity of the air in the head space will rise and eventually equilibrate to a
relative humidity of, say 83%, which means that the water activity (aw) of the
meat sausage is 0.83.

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Water activity and Shelf life of Foods
• It is an important consideration for food product design and food safety.
• Food designers use water activity to formulate products that are shelf stable.
• If a product is kept below a certain water activity, then mold growth is inhibited.
This results in a longer shelf-life.
• Water activity is used in many cases as a critical control point for Hazard
Analysis and Critical Control Points (HACCP) programs.
• Samples of the food product are periodically taken from the production area
and tested to ensure water activity values are within a specified range for food
quality and safety.
• Measurements can be made in as little as five minutes, and are made regularly
in most major food production facilities.
Water activity of some foods
Substance aw
Distilled Water 1
Tap water 0.99
Raw meats 0.99
[
Milk 0.97
Juice 0.97
Cooked bacon < 0.85
Saturated NaCl
solution
0.75
Point at which
cereal loses crunch
0.65
Dried fruits 0.60
Typical indoor air 0.5 - 0.7
Honey 0.5 - 0.7
Dried fruit 0.5 - 0.6
Microbial growth
• Many of the chemical and biological processes that cause deterioration of
foods, and ultimately spoilage, are water dependent.
• Microbial growth is directly linked to water activity.

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• No microbes can multiply at a water activity below 0.6.
Dehydration
• Dehydration is arguably the oldest form of food preservation;
• The sun drying of meat and fish has been traces to the beginning of recorded
history.
• Drying relies on removing water, thus making it unavailable for microbial
growth.
Salting or curing
• Salting or curing has the same effect.
• A saturated solution of common salt has a water activity of close to 0.75.
• Thus by adding sufficient salt to foods, the water activity can be lowered to a
level where most pathogenic bacteria are inactivated but the moisture content
remains high.
• Intermediate moisture content foods (IMF), such as pet food and continental
sausages; rely on fats and water-binding humectants such as glycerol to lower
water activity.
• Fat, being essentially hydrophobic, does not bind water, but acts as filler for
IMF to increase the volume of the product.
• The water activity of the salted food is 0.8.
Benefits of drying of food
• None of the dangerous pathogenic bacteria associated with food, such as
Clostridium or Vibrio spp. which cause botulism and cholera, can multiply at
water activity values below about 0.9.
• Drying or providing sufficient water-binding humectants is an effective method
of preventing the growth of food-poisoning bacteria.
• Only osmophilic yeast and some molds can grow at water activities in the
range 0.6 to 0.65.
• Thus, by reducing the water activity below these values, foods are microbial
stable.
• That is, unless the packaging is such that the food becomes locally rewet, in
which case local spoilage can occur, for example, when condensation occurs
within a hermetically sealed package subject to rapid cooling.

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Chemical reactions and water activity
• Various chemical reactions that proceed, and may be accelerated, at low
values of water activity.
• Maillard reactions leading to lysine loss and brown color development peaks at
aw around 0.5 to 0.8.
• Nonenzymatic lipid oxidation increases rapidly below aw = 0.4.
• Enzymatic hydrolysis decreases with water activity down to aw = 0.3 and is
then negligible.
• Water is facilitator of biochemical deterioration of foods.
• Dry foods are much more stable than wet foods, because any water remaining
to them has low activity, aw.
• Freezing removes water from the food matrix by forming ice crystals.
• Although the ice crystals remain in the food, the remaining water which is in
contact with the food matrix becomes concentrated with solutes and it’s aw
becomes low.
• Freezing is therefore akin to drying and this is the rationale for preserving food
by freezing.
• Most micro-organisms cease functioning below the water activity of about 0.7.
FOOD LIPIDS AND FISH LIPID
Fat/ lipids of Food
• Fat is a generic term for a class of lipids.
• Fats are produced by organic processes in animals and plants.
• These are extracted and used as an ingredient.
• All fats are insoluble in water and have a density significantly below that of
water (i.e. they float on water.)
• Fats that are liquid at room temperature are often referred to as oil.
• Most fats -composed primarily of triglycerides;
• some monoglycerides and diglycerides are mixed in products with a lot of
saturated fats tend to be solid at room temperature.
• Products containing unsaturated fats, which include monounsaturated fats and
polyunsaturated fats, tend to be liquid at room temperature.

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Types of fat
• Predominantly saturated fats (solid at room temperature)
• All animal fats (e.g. milk fat, lard, tallow), as well as palm oil, coconut oil, cocoa
fat and hydrogenated vegetable oil (shortening).
• Predominantly unsaturated and remain liquid at room temperature
• Vegetable fats- from olive, peanut, maize (corn oil), cottonseed, sunflower,
safflower, and soybean, are.
• However, both vegetable and animal fats contain saturated and unsaturated
fats.
• Some oils (such as olive oil) contain in majority monounsaturated fats, while
others present quite a high percentage of polyunsaturated fats (sunflower,
rape).
Saturated fats
If the fatty acid has all the hydrogen atoms it can hold it is said to be saturated (see
below)
H H H H H
│ │ │ │ │
C C C C C
│ │ │ │ │
H H H H H
This type of fat is typically found in large amounts in foods from animals, e.g. meat,
butter, cheese and cream.
Many baked goods such as cakes, biscuits and pastries are also high in saturated
fat.
Excessive intake of saturated fat can increase blood cholesterol levels.
Unsaturated fats

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• In unsaturated fats, some of the carbon atoms are joined to others by a double
bond and, therefore, could accept more hydrogen atoms.
• They are not completely saturated with hydrogen, so are called unsaturated
fats.
H H H H H
│ │ │ │ │
C C ═C C C
│ │ │
H H H
There are two main types of unsaturated fats–
1. Monounsaturated (containing one double bond) and
2. Polyunsaturated (containing more than one double bond).
Most monounsaturated and polyunsaturated fats have good qualities, with one
exception - trans-fatty acids.
Trans fatty acids are, an unsaturated fat but offer no health benefits.
Monounsaturated fatty acids
There is one double bond is present
Found in significant amounts in most types of fats of plant origin, such as nuts,
avocado pears, rapeseed oil and olive oil.
Monounsaturated do not raise blood cholesterol and evidence shows that they
reduce blood cholesterol levels if they replace saturated fat in the diet
• Oleic acid is the main monounsaturated fat in our diets and this is sometimes
called omega-9 (because the double bond is in position 9 of the fatty acid
chain).
• Found in significant amounts in most types of nuts, avocado pears, rapeseed
oil and olive oil and spreads made from these.

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Polyunsaturated fatty acids
• There is more than one double bond,
• These come mostly from vegetable sources, such as sunflower oil or seeds,
but are also found in, nuts, green leafy vegetables and oily fish such as
mackerel and sardines.
• Polyunsaturated fatty acid can actively reduce blood cholesterol levels.
• The polyunsaturated found in oily fish specifically appear to have no effect on
blood cholesterol levels, but they do alter the consistency of blood.
• There are two 'series' of polyunsaturated fats in food
• They are also known as essential fatty acids. They are omega 3 and omega 6.
• Essential fatty acids are so called because the body cannot make them but
they are essential to the body's normal functioning, therefore, must be supplied
through diet.
• Humans are unable to make these essential fatty acids, because they do not
have the particular destaurase enzymes that insert double bonds in position 3
and 6 of the fatty acid chain
• Fats also carry the fat soluble vitamins A, D, E and K.
• Fats and lipids are energy storage materials in plants and animal tissues.
Functions of lipid
1. Important sources of metabolic energy (ATP)
• Lipids are the most energy rich of all classes of nutrients:
• Gross energy value of lipid 9.5 Kcal/g, protein 5.6 Kcal/g, carbohydrate 4.1
Kcal/g and the net values were 9 K.cal /g, 4.0K.cal/g and 4.k.cal/g for fat,
protein and carbohydrate respectively.
• Dietary lipids may be used to spare the more valuable fo protein r growth.
• The free fatty acids derived from triglycerides (fats and oils) are the major
aerobic fuel sources for energy metabolism.
2. Forms part of membrane
Lipids are essential components of all cellular and subcellular membranes
(polyunsaturated fatty acids containing phospholipids, and sterol esters).
3. Serve as biological carriers:

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• For the absorption of the fat soluble vitamins A, D, E and K.
4. Source of essential fatty acids:
• Linoleic, Linolenic and Arachidonic acids- which in turn are essential for the
maintenance and integrity of cellular membranes, are required for optimal lipid
transport (bound to phospholipids as emulsifying agents)
• Precursors of the prostaglandin hormones.
5. Mechanical cushion/support:
Play a role as mechanical cushion/support for the vital body organs, and aid in
the maintenance of neutral buoyancy.
6. Source of essential steroids:
• Needed to perform a side range of important biological functions.
• Sterol cholesterol is involved in the maintenance of membrane systems,
for lipid transport,
• A precursor of vitamin D3, the bile acids, and the steroid hormones –
androgens, estrogens, adrenal hormones, and corticosteroids.
7. Food flavor / mouth feel:
• They play a role in food flavor / mouth feel, palatability, texture and aroma.
Fish lipids
• Lipid in fish generally carries natural flavour components and provides and
preserves other generated during cooking, pickling or other processing.
• A certain amount of fat and fatty acid assists in providing smoothness of texture
during mastication of lean fish. In fatty fish the influence of fat on texture is
even more important.
• The lipid content of fish varies widely from species to species and even within
the same species from one individual to another depending on age, sex,
environment and season.
Lipid content of seafood
Type of fish Fat %

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Fatty fish 10.0
Lean fish 0.5
Crustaceans 2.1
Mollusks 1.5
Distribution of Fat in Fish
• The term lipid will be used for total fat component in fish.
• However, term fat is used for selected anatomical deposits, which are mostly
triglyceride.
• In lean fish the dark (red or lateral line) muscle has about twice the lipid of
white muscle.
• The percentage of cellular lipid in the white muscle is normally altered by
season.
• The lean muscle fish generally have more fat in livers (e.g. cod) which show
seasonal variation.
• In the fatty fish species, the muscle shows fluctuating levels of seasonal
variation in neutral fat
a. Neutral fat (Triglycerids)
• Fforms the major constituent of fish lipid.
• There are variations in the amount of neutral fat in muscle.
• The belly flap is a high fat section of many fishes. (E.g. In mackerel -29% lipid
in belly flaps, 18.3% lipid in dark muscle and 7.6% in while muscle).
• In male mackerel the skin fat forms 40% of the total fat in the whole fish.
• Triglyceride distributed through fish muscle tends to have a homogeneous fatty
acid composition Most species of marine organisms try to obtain an optimal fat
and fatty acid composition and their behavior and food preferences lead
towards this objective.
b. Basic cellular lipids (Phospholipids)
• Lean white fish muscle contain a minimum of about 0.7% of basic cellular lipid,-
85-95% is ‘polar’ lipids, mostly phosphatidyl ethanolamine and phosphatidyl
choline.
• The balance of this type of basic lipid includes sterol ester and free sterol, free
fatty acidsand triglyceride.

23
• This basic mixture represented the structural lipid of cell walls, and that any
excess of triglyceride and/or certain other non-polar lipids such as wax esters
or glyceryl ethers, provided the ‘fat’ of fatty fish.
c. Fatty acids of fish lipid
Classification of fish lipid fatty acids
1. Saturated acids
2. Monoenoic acids
3. Polyenoic acids.
The fatty acid s present in fish lipid is classified into three groups, saturated acids,
monoenoic acids and polyenoic acids.
1. Saturated fatty acids
Myristic or tetradecanoic (14:0) - 5-10% of the total
Palmitic or hexadecanoic(16:0) - 10-30% of the total
Stearic or octadecanoic(18:0). - 1-3% of of the total
Longer fatty acids (20:0, 22:0 and 24:0) - 0.01-0.1%.
These fatty acids can all be biosynthesized by the organisms, but they are also
freely absorbed from dietary fats.
Odd number straight-chain fatty acids: If Propionate molecule primes the two-
c arbon fatty acids chain extension process instead of an acetate molecule- odd-
chain fatty acids are also formed.
Bacteria also contributethese fatty acids
The fatty alcohols of copepod esters also have C15 and C17 methyl-branched
and odd carbon structures which are probably oxidized to corresponding acids and
then deposited in the fats of capelin, mackerel and herring.
2. Monoenoic Fatty acids
Monoenoic acids offish lipids
Oleic C18:1 or 9-octadecenoic (1ω7),
Gadoleic or 11-eicosenoic (20:1 ω9) and
Cetoleic or 11-docosenoic (22:11 ω1).
Palmitoleic acid (16:1) and oleic (18:1 ω9) of fish oils can be synthesized by fish
and other marine organisms from acetate units.

24
Palmitoleic acid can be carbon extended to cis-vanccenic acid (18:1 ω7), which
forms 10-30% of the total 18:1 isomers.
20:1 ω9 is by chain elongation of 18:1 ω9.
20:1 ω11 and 20:1 ω7 ,20:1 ω9 –occur in several fish oils.
In pacific herring oil 20:1 ω11 –Higer proportions.
Dominant 22:1 isomer in marine fish oil is 22:1 ω11.
In herring only the 22:1 ω9 isomer is biosynthesized.
3. PolyenoicFatty acids
• Important polyenoic acid present in fish lipids
• Eicosapentaenoic (EPA) (C20:5 ω3)
• Dohosahexaenoic (DHA) (C22:6 ω3).
• These fatty acids give marine oils their most specific characteristics.
• They originate in unicellular phytoplankton or in some seaweed.
• The average fatty acids composition of phytoplankton includes all the principal
fo fatty acids und in the oils and lipids of the higher organisms.
• The two common ‘plant’ C18 fatty acids 18:2 ω6 (linoleic acid) and 18:3 ω3
(linolenic acid) –Not more than 1% or 2% of fatty acids.
• Many invertebrate lipids-20:5 ω3 is the dominant polyunsaturated fatty acids -
possibly due to dietary algae.
d. Wax Easters and Glyceryl Ethers
Wax esters of marine organisms
Two main classes: (1) those rich in 16:0 acids
(2) those rich in 22:1 acids.
The chain length of fatty acids: In the range C32 to C36.
The copepod wax ester rich in 22:1 alcohol, has a high content of 14:0 acids.
They also have a high proportion of 16:0 fatty alcohols in their wax esters as part of
their body lipid.

25
The ‘castor oil’ fish Ruvettus pretiosus contains wax esters, which are
responsible for the purgative effect.
Properties of fat
(a) The fats are insoluble in water, but readily soluble in ether, chloroform, benzene,
carbon tetra chloride
(b) They are readily soluble in hot alcohol but slightly soluble in cold.
(c) They are themselves good solvents for other fats, fatty acids etc.
(d) They are colourless, odourless, tasteless and neutral in reaction.
(e) Several neutral fats are readily crystallised, e.g. mutton, beef
(f) Their melting points are low.
(g)The specific gravity is about 0.86. Hence the fatreadily float in water.
They spread uniformly over the surface of wates and this spreading effect is to
lower surface tension.
Functions of lipoprotein
The various types of lipoproteins have different functions.
Chylomicrons and VLDLs: Chylomicrons and VLDLs are the principal carriers of
triglycerides and Choleterols in blood.The concentrations are increased in
atheroscorosis and coronary thrombosis.
LDLs:In LDLs the predominant lipid is cholesterol and phospholipids. Increased in
atherosclerosis and coronary thrombosis, etc.
HDLs: HDLs are predominant lipid is phospholipid and proteins.
LDLs and HDLs are involved in the cholesterol transport. LDLs carry about 80% of
cholesterol while the remaining is carried by HDLs. LDLs carry cholesterols to cells
for their use where as HDLs carry excess cholesterol away from the cells to the liver
for processing and excretion from the body. The levels of LDL correlate directly with
heart disease, where as HDL levels correlates inversely with heart disease risk. Thus
HDL is some times referred to as “good” cholesterol and LDL as “bad” cholesterol.

26
Role of Fish Lipids in Human Nutrition
To achieve a balanced diet there is a need to reduce total fat intake and it is also
important to make sure that the type of fat we eat is right.
The fat that is not beneficial to human is the hard "saturated" fat which comes mainly
from the fat of land animals such as cows and sheep.
Fish fat has several beneficial effects.
1. Providefood with low fat
• Fish is a good food for a low fat diet. It is low in calories and many types of fish
do not contain any unsaturated fat.
• The nutritional value of fish will vary slightly according to the location it is
harvested, the cut of fish, and the age of the fish.
• The method used for cooking will have an affect on it also.
2. Reduce the cholesterollevelin the blood
• Cholesterol is the type of fat which is naturally produced by our bodies and is
also found in the diet.
• It is usually deposited in the lining of the blood stream vessels, causing them to
narrow.
• The heart then has to work harder to pump blood around the body.
• Blood clotting can result and the cholesterol deposits can be very hard.
• Tissues can become deprived of oxygen when the blood vessels become
blocked.
• Unsaturated fats can help to reduce the cholesterol level in the blood, thus
lowering the risk of heart disease.
• Oil-rich fish such as mackerel, sardines, herring and sprats are rich in
unsaturated fats containing Omega-3 so valuable for health.
3. Source Omega-3 fatty acids
• Two fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic (DHA),
collectively known as Omega-3, are essential fatty acids.
• Schizophrenia symptoms can be eliminated or at least vastly diminished by oral
supplementation with EPA.

27
• DHA is the building block of human brain tissue and is particularly abundant in
the grey matter of the brain and the retina.
• Low levels of DHA have been associated with depression, memory loss,
dementia and visual problems.
• DHA is particularly important for fetuses and infants; the DHA contents of the
infant’s brain triples during the first three months of life.
• Optimal levels of DHA are therefore crucial for pregnant and lactating mothers.
• Oil-rich fish, such as salmon, trout, mackerel, herring and sardines, are an
excellent source of Omega-3 fatty acids, which are essential to our diet.
• Eating oil-rich fish provides the Omega-3 fatty acids needed for the body.
• Omega-3 oils from fish have a lowering effect on blood fats.
• This decreases the chance of the blood vessels clogging with cholesterol.
• Omega-3 can also make blood less "sticky", and it therefore flows more easily
around the body. This can reduce the risk of a heart attack.
• They also help to reduce blood pressure a little and keep the heart beat steady.
Omega-3 oil in fish can reduce the risk of dying from heart attacks.
4. Fish oils preventcancer
• Fish oils can help to prevent cancer cells progressing to the tumor stage.
• They may also reduce inflammation and provide relief for people suffering from
rheumatoid arthritis and even some skin disorders such as psoriasis.
5. Needed for the developmentof brain
• Omega-3 oils can play an important part in aiding the development of brain.
• Expectant mothers are advised to eat a lot of oil-rich fish in the last three
months of pregnancy to assist the baby's brain growth.
• A good supply of Omega-3 oils assists the development of nerves and
eyesight.
OXIDATION OF OIL/FAT
Lipid oxidation is one of the major causes of food spoilage.
In edible oils and fat-containing foods, it leads to the development of various off
flavors and off odors, generally known as oxidative rancidity, which renders the foods
less acceptable.
It may also be able to decrease the nutritional value of food and in some cases may
produce potentially toxic products.
It may be sometimes desirable as in the case of cheese.

28
Oxidation is caused by a biochemical reaction between fats and oxygen called as
autoxidation and it is the main reaction involved.
The lipids of foods can be oxidized by
(i) non enzymic and
(ii) enzymic mechanisms.
Lipid oxidation generally occurs after a long induction period.
Once started it is generally a very rapid reaction.
(i) Non enzymic oxidation
• Non enzymatic lipid oxidation (Autoxidation) proceeds by a free radical
mechanism.
• It is catalysed by light and free radical-producing substances and yields
hydroperoxide (ROOH).
• This primary product is relatively unstable.
• It enters into numerous reactions involving substrate degradation and
interaction, resulting in numerous compounds of various molecular weigh,
flavour generating and biological significance.
Free radical mechanism
A free radical is a compound with an odd number of unpaired electrons.
H H H
— C —C — C —
H H H
↓
H H H
— C —C•
— C — + H•
H H
When initiated two free radicals are formed. These radicals are very reactive and
generally do not have long life time.
Lipid oxidation
There are three main steps

29
i. Initiation
ii. Propagation
iii. Termination
i. Initiation
 The initiation occurs by direct attack of oxygen in its most stable form on
double bonds of fatty acids (RH).
 The presence of a double bond in the fatty acid (RH) weakens the C-H bonds
on the carbon atom adjacent to the double bond and so makes H removal
easier.
 Oxygen attack at the end carbon of the double bond and forms hydrogen
peroxide.
Hydroperoxides breakdown in several steps to form free radicals.
Initiator
RH +O2 → ROOH → Free radicals (R•
, ROO •
)
• The carbon radical tends to be stabilized by a molecular rearrangement to form
a conjugated diene.
•
Oxygen attack the end carbon of the double bond and forms hydrogen
peroxide.
• Hydroperoxide breakdown, in several steps, to form free radicals.
ii. Propagation
• Once the initial radicals have formed, the formation of other radicals proceeds
rapidly.
• The new radicals will not be at the double bond. To remove a hydrogen from a
double bond requires 80 Kcal/mole.
• To remove a hydrogen alpha to a double bond only requires 15 Kcal/mole.
• As a peroxyl radical is able to abstract H from another lipid molecule (adjacent
fatty acid), especially in the presence of metals such as copper or iron, thus
causing an autocatalytic chain reaction.
• The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide).
• This reaction characterizes the propagation stage.

30
• The peroxyl radical combines with H to give a lipid hydroperoxide (or
peroxide).
• This reaction characterizes the propagation stage.
• R •
. + O2 → ROO •
.
•
• ROO•
. + RH → ROOH + R •
.
iii) Termination
Any kind of alkyl radicals (lipid free radicals) R•
can react with a lipid peroxide
ROO•
to give non-initiating and
Non-propagating species such as the relatively stable dimers ROOR or two
peroxide molecules combining to form hydroxylated derivatives (ROH).
R•
+ R•
→ R-R
R •
+ ROO•
→ ROOR
2ROO•
→ ROO-OOR
Effects of Lipid Oxidation in Foods
• When lipids in food are oxidised, some of the product formed impart odour and
flavours, usually undesirable, to the food.
• The free radicals generated during the oxidation reaction and some of the
molecules formed when oxidized compound decombos (aldehydes, acids,
alcohols, ketones etc.) can interact with and alter other constituents including
pigments, vitamins, proteins and amino acids.
• These interactions can result in colour, texyure and nutritive value.
• If foods containing oxidized lipids are consumed, the oxidation products could
be involved in reactions leading to pathological changes.
• For e.g. malonaldehyde and oxidation product of certain polyunsaturated fatty
acids found in many foods, is a potential carcinogen.
(ii) Enzyme catalysed lipid oxidation
• Enzyme reaction starts with the action of lipolysis.
• Released polyunsaturated fatty acids are then oxidized by either lipoxygenase
or cyclooxygenase to form hydroperoxides or endoperoxides, respectively.

31
• Then these compounds are hydrolysed to yield a variety of breakdown
products, which are responsible for the characteristic flavours of natural
products.
Pro-Oxidants
• Transition metals, particularly those possessing two or more valency states and
a suitable oxidation reduction potential between them are effective pro-
oxidants.
• e.g.copper, iron, manganese, cobalt and nickel
• If present even at very low concentrations(0.1ppm) can decrease the induction
period and increase the rate of oxidation.
• Trace metals are naturally occurring in all food tissues and all fluids of
biological origin (eggs, milk and fruit juices) and they are present in both free
and bound forms.
• Heme compounds are also important pro-oxidants.
Antioxidant
• An antioxidant is a molecule that can delay onset, or slow the rate of oxidation
of oxidisable material. Oxidation reactions can produce free radical. In turn,
these radicals can start chain reactions.
• Antioxidants terminate these chain reactions by removing free radical
intermediates, and inhibit other oxidation reactions by acting as hydrogen
donors or free radical acceptors.
• ROO. + AH → ROOH + A.
• Antioxidants are found in varying amounts in foods such as vegetables, fruits,
grain cereals, eggs, meat, legumes and nuts.
• Antioxidants are widely used as preservatives in food and as ingredients in
dietary supplements.
• . Natural antioxidants: Ascorbic acid and tocopherols,
• Synthetic antioxidants: Propyl gallate, isoamyl gallate, tertiary
butylhydroquinone (TBHQ),butylated hydroxy anisole (BHA) and butylated
hydroxy toluene (BHT).
• Used as help guard against food deterioration
• Antioxidants can be directly added to vegetable oils or to melted animal fats
after they are rendered.

32
• Food products can also be dipped in or sprayed with solutions of
antioxidants
Digestion and absorption of lipids
Introduction
Foods are enzymatically digested to prepare them for absorption. During digestion in
the gastrointestinal tract of mammals, the three major nutrients (carbohydrates, lipids,
and proteins) undergo enzymatic hydrolysis into their building block components.
This is necessary for their absorption, since the cells lining the intestine are able to
absorb them into the bloodstream only as relatively small molecules. Lipids must be
hydrolyzed into fatty acids and glycerol.
Digestion
The digestion of triglycerides begins in the small intestine. In this region the
zymogen, prolipase is secreted by the pancreas (Fig.1). There it is converted into
lipase, which in the presence of bile salts and a special protein called colipase, binds
to droplets of triglycerides and catalyzes the hydrolytic removal of one or both of the
outer fatty acid residues. Monoglycerides remain unhydrolyzed. The fatty acids and
the uncleaved gycerides are emulsified into fine droplet by peristalsis, the churning
action of the intestine, aided by the detergent effect of the bile salts and the
monoglycerides, which are amphipathic molecules. Phospholipids are split by
phospholipases to the acyl chains, glycerol and choline. Cholesterol esters are
converted to cholesterol and free fatty acids.
Absorption
The fatty acids, glycerol and monogycerides, in these droplets are absorbed by
intestinal cells, where they are largely reassembled into triglycerides. The free fatty
acids are activated by thiokinase in the presence of coenzyme A and ATP for the
resynthesis of triglyceride. Some free glycerol passes directly to the lymp vessel. The
others will be activated by glycerokinase in the presence of ATP to form glycerol 3
phosphate and combine with acyl CoA to form triglycerides. All the long chain fatty
acids present are reincorporated into the triglycerides. The triglycerides do not pass
into the blood capillaries but into the small lymph vessels in the villi. The choline from
phospholipids may be absorbed and send to liver via lymph vessels. Cholesterol is
absorbed into the lymphatic vessels and converted into cholesterol esters and
transported.
Chylomicrons: The lymph draining the small intestine, called chyle, has a milky
appearance after a fat-rich meal, due to the suspended chylomicrons, droplets of
highly emulsified triglycerides, about 1µm in diameter. Chylomicrons contain

33
triglycerides, free and esterified cholesterol; have a hydrophilic coat of phospholipids
and a special protein, which function to keep the chylomicrons suspended. The
chylomicrons pass from the thoracic duct into the subclavian vein and then to liver.
Emulsification: The emulsification and digestion of lipids in the small intestine is
facilitated by the bile salts. The major human bile salts are sodium glycocholate and
sodium taurocholate, derivative of cholic acid, the most abundant of four major
human bile acids. The bile salts are powerful emulsifying agents secreted by the liver
into the bile, which empties into the upper portion of the small intestine. After the fatty
acids and monoglycerides of the emulsified fat droplets have been absorbed in the
lower small intestine, the bile salts aiding this process are also reabsorbed. They
return to the liver, to be used over again.
Metabolism
Metabolism of Triglycerides
Triglycerides are first converted to fatty acids and glycerol mostly in adipose
tissue. The fatty acids are released into the plasma where they combine with serum
albumin. Long chain fatty acids are oxidized in liver, heart, kidney, muscle, lung, brain
and adipose tissue. Glycerol is utilized by liver, kidney, intestine and lactating
mammary gland where the activating enzyme glycerokinase is present.
b) Metabolism of fatty acids
The fatty acids components of the lipids entering the liver also have several
different pathways
1. Oxidation to CO2 with ATP production: Free fatty acids may be activated and
oxidized to yield acetyl-CoA and ATP. The acetyl-CoA is oxidized via the citric acid
cycle to yield ATP by oxidative phosphorylation. Fatty acids are the major oxidative
fuel in the liver.
2. Synthesis of fatty acids: There are three types of fatty acid synthesis. (1)
Elongation of existing short chain fatty acid in the mitochondria (2) Microsomal
system of chain elongation and (3) The cytoplasmic synthesis of fatty acid from acetyl
CoA.
3. Biosynthesis of cholesterol: Some of the acetyl-CoA derived from fatty acids
(and from glucose) will be used as the major precursor for the biosynthesis of
cholesterol, which in turn is the precursor of the bile acids and bile salts, which are
essential for the digestion and absorption of lipids.
4. Biosynthesis of lipids of plasma lipoproteins( Triglyceride and
phospholipids): Fatty acids are also used as precursors for the synthesis of the lipid
portion (triglycerides and phospholipids) of the plasma lipoproteins, which carry lipids
to adipose or fat tissue for storage as triglycerides.
5. Fomation of ketone bodies: Excess acetyl-CoA released on oxidation of fatty
acids and not required by the liver is converted into the ketone bodies, acetoactate
and D-β-hydroxy butyrate, which are circulated via the blood to peripheral tissues, to
be used as fuel for the citric acid cycle. The ketone bodies may be regarded as a
transport form of acetyl groups. They can supply significant fraction of the energy to
some peripheral tissues, up to one-third in the case of the heart

34
Metabolism of fat
• The lipids of metabolic significance include synthesis and degradation of
• triglycerides
• phospholipids
• steroids together with long chain fatty acids
• glycerol and
• ketone bodies
• Oxidation of triglycerides takes place in the adipose tissue.
• The complete degradation of fatty acid in the body leads to the oxidation to
CO2 and water
Metabolism of fatty acids
The fatty acids components of the lipids entering the liver also have several
different pathways
1. Oxidation to CO2 with ATP production
2. Biosynthesis of cholesterol
3. Biosynthesis of lipids of plasma lipoproteins
(Triglyceride and phospholipids)
4. Formation of free fatty acids
5. Formation of ketone bodies
Metabolism of Triglycerides
• Triglycerides are first converted to fatty acids and glycerol mostly in adipose
tissue.

35
• The fatty acids are released into the plasma where they combine with serum
albumin.
• Long chain fatty acids are oxidized in liver, heart, kidney, muscle, lung, brain
and adipose tissue.
• Glycerol is utilized by liver, kidney, intestine and lactating mammary gland
where the activating enzyme glycerokinase is present.
1. Oxidation of fatty acids to CO2 with ATP production
• Fatty acids are oxidized by β, α, and ω oxidation. β- Oxidation is the most
important pathway for the production of energy.
• The term β -oxidation means the oxidation takes place in the β -carbon in the
fatty acid with the removal of 2 carbon atoms at a time from the carboxyl end of
the molecule.
• The saturated fatty acids containing even number and odd number of carbon
atoms and the unsaturated fatty acids are oxidized by β -oxidation.
(a) β-Oxidation of saturated fatty acids
• Saturated fatty acids are oxidized to acetyl-CoA by β oxidation.
• It takes place in mitochondria.
• Five steps are involved and each step involves acy1-CoA derivatives catalyzed
by separate enzymes, utilizes NAD+ and FAD as coenzymes, and generates
ATP.
• Fatty acid oxidation is an aerobic process, requiring the presence of oxygen.
Step 1 Activation of Fatty Acids
• Long chain fatty acids are first converted to an ‘active fatty acid’ or acyl CoA in
the cytosol
• But activation of lower fatty acids occurs within the mitochondria.
• Thiokinase is found both inside and outside the mitochondria.
• Thiokinase
Fatty acid+ATP+coenzyme A +Mg2+→Acyl CoA +AMP
• The presence of inorganic pyrophosphatase ensures that activation goes to
completion by facilitating the loss of the additional high-energy phosphate
associated with pyrophosphate.

36
• Two high energy phosphates are expended during the activation of each fatty
acid molecule.
• (i)Transport of smallerfatty acids
• Small fatty acids are able to penetrate the inner membrane off mitochondria
and become oxidized within the mitochondria.
• (ii) Transport of long-chainfatty acids
• Long-chain fatty acid penetrate the inner mitochondrial membrane only as
carnitine derivatives.
• Carnitine acyl transferase I, -in outer mitochondrial membrane, converts long-
chain acyl CoA to acyl carnitine, -penetrate the inner membrane of
mitochondria
• Carnitine-acyl carnitine translocase -in mitochondria, catalyses the transfer
the acylcarnitine into inner membrane.
• Carnitine acyl transferase II- in the inner mitochondrial membrane, converts
acyl carnitine to long-chain acyl CoA and carnitine.
• Acyl CoA then undergoes further reactions of β-oxidation
Step 2 Dehydrogenation of Aceyl CoA
Aceyl CoA dehydrogenase
Acyl CoA + NAD+
↔ α-β unsaturated acyl
CoA + NADH + H+
NADH + H+
is reoxidised via electron transport chain.
Step 3 Conversion of α-β unsaturated acyl CoA to β hydroxyl acyl CoA
Enoyl-CoA hydratase.
α-β unsaturated acyl CoA + H2O↔ β hydroxyl acyl CoA
Step4 Dehydrogenation at the β-carbon of β-hydroxyacyl CoA
β-hydroxyacyl-CoA dehydrogenase
β hydroxyl acyl CoA +NAD+
↔β-ketoacyl-CoA+ NADH+H +
The NADH+H+
formed is reoxidised via electron transport chain.

37
Step5. Cleavage by thiolase
Thiolase
β-ketoacyl-CoA ↔ Acetyl-CoA + acyl-CoA
The products of this reaction are acetyl-CoA and an acyl-CoA derivative
containing two carbons less than the original acyl-CoA molecule that underwent
this oxidation.
The acyl-CoA formed in the cleavage reaction renters the oxidative pathway at
reaction 1.
A long chain fatty acid may be degraded completely to acetyl-CoA (C2 units).
In the case of palmitic acid the reactions are repeated 7 times and 8 molecules of
acetyl CoA are formed.
Since acetyl-CoA can be oxidized to CO2 and water via the citric acid cycle, the
complete oxidation of fatty acids is achieved
Production of ATP
Synthesis of high energy phosphates (ATP) for Electron transport chain
reoxidation of FADH2 and NADH of formation of 7 acetyl-CoA molecules for β-
oxidation of palmitate : five.
No.of ATP derived from β-Oxidation Since 8 molecules of acetyl CoA are formed
: 7x5 = 35
No. of ATP formed on oxidation of
8 acetyl-CoA molecules (via citric acid cycle) : 8x12 = 96
--------
Total : 131
ATP utilized for initial activation of the fatty acid: -2
-------
Net total yield : 129
Calorific value per mole of palmitic acid:
• The calorific value is 129x7.6=980 K.cal/mole.

38
• The calorific value per mole of combustion of palmitic acid is 2340 K.cal/mole.
• The process captures as high-energy phosphate in the order of 41% of the total
energy of combustion of the fatty acid.
(β)Oxidation of a fatty acid with an odd number of carbon atoms
Fatty acids with an odd number of carbon atoms are oxidized by the
pathway of β - oxidation, producing acetyl-CoA until a three- carbon (propionyl-CoA)
residue remains.
This compound is converted to succinyl-CoA, a constituent of the citric acid cycle
and metabolized.
Propionyl-CoA carboxylase
Propionyl CoA + CO2 + H2O → D-methylmalonyl-CoA
D-Methylmalonyl-CoA is converted to its steroisomer, L- methylmalonyl-CoA, by
methylmalonyl-coA racemase before its final isomerization to succinyl-CoA by the
enzyme methylmalonyl-CoA isomerase.
Methylmalonyl-CoA racemase
D-Methylmalonyl-CoA ↔ L- methylmalonyl-CoA
Methylmalonyl-CoA isomerase
L- methylmalonyl-CoA ↔ Succinyl-CoA
Thus the propiony fatty acid l residue from an odd-chain fatty acid is the only part of
a
b) Oxidation of unsaturted fatty acids
Oxidation of unsaturated fatty acids occurs by a modified beta- oxidation pathway.
1. Initial reaction
The CoA ester of these acids are degraded by the enzymes normally responsible
for β - oxidation until either a Δ3
-cis- acyl-CoA compound or Δ4
-Cis-acyl-CoA
compound is formed, depending upon the position of the double bonds.
2. Reaction of Isomerase

39
The former compound is isomerized (Δ3
cis-Δ2 Acyl CoA isomerase) to the
corresponding Δ2 -trans-CoA stage of β - oxidation for subsequent hydration and
oxidation.
(One cycle of Beta Oxidation)
3. Conversion of α-β unsaturated acyl CoA to β hydroxyl acyl CoA
Enoyl-CoA hydratase.
α-β unsaturated acyl CoA + H2O↔ β hydroxyl acyl CoA
4.Dehydrogenation at the β-carbon of β-hydroxyacyl CoA
β-hydroxyacyl-CoA dehydrogenase
β hydroxyl acyl CoA +NAD+
↔β-ketoacyl-CoA+ NADH+H +
5. Action of thiolase
6. Conversion of Δ4 -cis-acy1-CoA to Δ2 -trans enoy1-CoA
Any Δ4 -cis-acy1-CoA remaining, as in the case of linoleic acid, is converted to
Δ2 -trans enoy1-CoA by an NADP dependent enzyme, Δ2 - trans - Δ3-cis
dienoy1-CoA reductase.
7. Action of Acyl-CoA dehydrogenase
Δ-cis (or trans) Δ2 enoy1-CoA isomerase will attack the trans double bond to
produce Δ2 - trans enoy1-CoA,
This compound is further metobolised via β - oxidation
DIGESTION AND ABSORPTION OF LIPIDS
INTRODUCTION
• Foods are enzymatically digested to prepare them for absorption.
• During digestion in the gastrointestinal tract of mammals, the three major
nutrients (carbohydrates, lipids, and proteins) undergo enzymatic hydrolysis
into their building block components.
• This is necessary for their absorption, since the cells lining the intestine are
able to absorb them into the bloodstream only as relatively small molecules.
• Lipids must be hydrolyzed into to fatty acids and glycerol.

40
Digestion and absorption of fat
• The digestion of triglycerides beings in the small intestine, In this region the
zymogen, prolipase is secreted by the pancreas (Fig.4.1).
• There it is converted into active lipase, which in the presence of bile salts and a
special protein called colipase, binds to droplets of triglycerides and catalyzes
the hydrolytic removal of one or both of the outer fatty acid residues.
• Monoglycerides remain unhydrolyzed.
• The fatty acids and the uncleaved gycerides are emulsified into fine droplet by
peristalsis, the churning action of the intestine, acided by the detergent effect of
the bile salts and the monoglycerides, which are amphipathic molecules.
• Phospholipids are split by phospholipases to the acyl chains, glycerol and
choline.
• Cholesterol esters are converted to cholesterol and free fatty acids
Absorption
• The fatty acids, glycerol and monogycerides, in these droplets are absorbed by
intestinal cells, where they are largely
reassembled into triglycerides.
• The free fatty acids are activated by
thiokinase in the presence of
coenzyme A and ATP for the
resynthesis of triglyceride.
• Some free glycerol passes directly to
the lymp vessel.
• The others will be activated by
glycerokinase in the presence of ATP
to form glycerol 3 phosphate and
combine with acyl CoA to form
triglycerides.
• All the long chain fatty acids present
are reincorporated into the triglycerides.
• The triglycerides do not pass into the blood capillaries but into the small lymph
vessels in the villi.

41
• The cholin from phospholipids may be absorbed and send to liver via lymph
vessels.
• Cholesterol is absorbed into the lymphatic vessels and converted into
cholesterol esters and transported.
Chylomicrons
The lymph draining the small intestine, called chyle.
It has a milky appearance after a fat-rich meal, due to the suspended
chylomicrons, droplets of highly emulsified triglycerides, about 1µm in diameter.
Chylomicrons contain triglycerides, free and esterified cholesterol; have a
hydrophilic coat of phospholipids and a special protein, which function to keep the
chylomicrons suspended.
The chylomicrons pass from the thoracic duct into the subkavian vein and then to
liver.
Emulsification
The emulsification and digestion of lipids in the small intestine is facilitated by the
bile salts.
The major human bile salts are sodium glycocholate and sodium taurocholate,
derivative of cholic acid, the most abundant of four major human bile acids.
The bile salts are powerful emulsifying agents secreted by the liver into the bile,
which empties into the upper portion of the small intestine.
After the fatty acids and monoglycerides of the emulsified fat droplets have been
absorbed in the lower small intestine, the bile salts aiding this process are also
reabsorbed.
They return to the liver, to be used over again.
METABOLISM OF LIPIDS
a) Metabolism of Triglycerides

42
a) Metabolism of Triglycerides
• Triglycerides are first converted to fatty acids and glycerol mostly in adipose
tissue.
• The fatty acids are released into the plasma where they combine with serum
albumin.
• Long chain fatty acids are oxidized in liver, heart, kidney, muscle, lung, brain
and adipose tissue.
• Glycerol is utilized by liver, kidney, intestine and lactating mammary gland
where the activating enzyme glycero kinase is present.
The fatty acids components of the lipids entering the liver also have several different
pathways.
1. Oxidation to CO2 with ATP production
Free fatty acids may be activated and oxidized to yield acetyl-CoA and ATP via
glycolysis.
The acetyl-CoA is oxidized via the citric acid cycle to yield ATP by oxidative
phosphorylation.
Fatty acids are the major oxidative fuel in the liver.
2. Biosynthesis of cholesterol and bile
salts
• Some of the acetyl-CoA derived from fatty acids (and from glucose) will be
used as the major precursor for the biosynthesis of cholesterol,
• Cholesterol is the precursor of the bile acids and bile salts, which are
essential for the digestion and absorption of lipids.
3.Biosynthesis of plasma lipoproteins
Fatty acids are also used an precursors for the synthesis of the lipid portion of the
plasma lipoproteins.
Lipoproteins carry lipids to adipose or fat tissue for storage as trigycerides.
.
4.Formation of plasma free fatty acids

43
Free fatty acids become bound to serum albumin and are carried via the blood to
the heart and skeletal muscles, which absorb and oxidize free fatty acids as major
fuel.
5. Formation of ketone bodies
Excess acetyl-CoA released on oxidation of fatty acids and not required by the liver
is converted into the ketone bodies, acetoactate and D-β-hydroxy butyrate, which are
circulated via the blood to peripheral tissues, to be used as fuel for the citric acid
cycle.
The ketone bodies may be regarded as a transport form of acetyl groups.
They can supply significant fraction of the energy of some peripheral tissues, up to
one-third in the case of the heart.
Metabolism of Fatty acids
CARBOHYDRATES
Introduction
Nature commonly utilizes carbohydrates as source of energy, structure-forming
material, water-maintaining hydrocolloids and even sex attractants.
Amino acids synthesize in the concentrated space and polymerize into proteins on
already-available polysaccharide matrices.

44
Carbohydrates are organic compounds containing carbon, hydrogen and oxygen with
the general formula Cn(H2O)n.
They may be simple or complex molecules. Important food carbohydrates include
simple sugars, dextrins, starches, celluloses, hemicelluloses, pectin, and gums.
They are an important source of energy or fiber in the diet and they are important
constituents of food because of their functional properties.
They are used as sweeteners, thickeners, stabilizers, gelling agents, and fat
replacers.
The simplest carbohydrates are called monosaccharides or sugars and they have the
general formulae CnH2nOn.
The most common ones contain six carbon atoms.
Disaccharide contains two sugar units, trisaccharides contain three, oligosaccharides
contain several units, and polysaccharides are complex polymers containing as many
as several thousand units of monosaccharides linked by means of glycosidic bonds.
Monosaccharides
• Monosaccharides are simple carbohydrates containing between three and
eight carbon atoms, but only those with five and six carbon atoms are common.
• Most important ones are glucose and fructose with general formula C6H12O6.
• Glucose is an aldose as it contains an aldehyde group.
• Fructose is a ketose as it contains a keto group.
Monosaccharide and their natural derivatives
Pentoses
L-Arabinose
D-xylose
Plant gums,
hemicellulose,
saponins,
protopectin
Accompanies L-
arabinose
Alcoholic
fermentation,
furan-2, aldehyde
production
Reduction to
xylitol

45
sucrose substitute;
alcoholic
fermentation;
production of
furan-2 aldehyde
Hexoses
D-Glucose
D-Fructose
D-Galactose
L-Fucose
D-Mannose
L-Rhamnose
Plants and animals,
honey, inverted
sugar, saponins
Fruit, traces in plant
honey,
Constituent of milk,
dairy products
(Milk sugar)
algae, plant mucus
and gums
Oligo-and
polysaccharides,
plant mucus and
gums, saponins,
glycosides
Algae, plant mucus.
Plant mucus and
gums, pectins
saponins, glycosides
Alcoholic
fermentation;
sweetener ; energy
pharmacopeial
material;
nutrient; food
preservative
Preparation of
dairy products
Preparation of
mannitol, which is
used as an
alternative
sweetener in food
products
Hexuloses
D-Fructose
D-
Glucosylamine
L-Sorbose
Fruits, honey,
inverted sugar
maple syrup
Chitin, Chitosan
Rowan berries
Noncavity-causing
sweetener;
sweetener for
diabetics; food
humidifier and
preservative
Pharmaceutical
aid; antiarthritic
drugs; ion
exchanger
Synthesis of
ascorbic acid
Disaccharides
• Disaccharide contains two monosaccharides linked together by glycosidic
bond. Sucrose or the table sugar is the most common disaccharide.

46
• It contains glucose and fructose linked by α-1, 2-glycosidic bond.
• Maltose contains two glucose units linked by α1-4 glycosidic bond.
• Lactose known as milk sugar contains one glucose and one galactose
molecules.
• Maltose is the building block of starch, which contains
Disaccharides
Lactose
Maltose
Sucrose
Mammalian 'milk
Starch, sugar beet,
honey
Sprouted grain,
hydrolysis of starch
Sugar beet, sugar cane ,
maple syrup
Dairy product taste
improver; fermenting
component of milk
Food
fermentation;
Common sweetener;
caramel production;
food preservation
Oligosaccharides
• Oligosaccharides contain 3-10 monosaccharide units linked together by
glycosidic bonds.
• Common ones include raffinose and stachyose.
• Raffinose is a trisaccharide with galactose, glucose and fructose. Stachyose
contains glucose, fructose and two galactose units.
• Both occur in legumes and dry beans and peas.
• They are not hydrolyzed or digested by human digestive system and become
food for bacteria in the large intestine.
Polysaccharides
• Polysaccharides contain more than 10 monosaccharide units linked together by
glycosidic bond.
• The most important polysaccharides are starches, pectins and gums.
• All are complex polymers with different properties, which depend on the mono
saccharides that make up the structure the linkage by which they are linked
and the degree of branching of the molecules.
Occurrence

47
• All organism cells, including those of animals, contain components of
carbohydrates in their membranes.
• Frequently, carbohydrates exist in naturally derivatives forms, including
aminated forms, as in chitin and chitosan; esterified; alkylated as in glycosides;
oxidized; reduces; or linked to proteins, lipids, and other structures such as
glycoproteins.
• Lower monosaccharides, such as aldotrioses and aldo-and ketotetroses, do not
exist naturally in a free state.
• Glyceraldehyde in phosphorylated form is the product of alcoholic fermentation
and glycolic sequence.
• Erythrose, an aldotetrose, and erythrulose, a ketotetrose, also appear in
phosphorylated from in the pentose cycle of glucose, while ketopentose-
ribulose can be found as its phosphate ester.
• Several common and uncommon carbohydrates (erlose, turanose, trehalose,
isomaltose, melecitose) have been found in honey. In nature, various
carbohydrates derivatives are also found.
• Among them are so-called sugar alcohols (auditors).
• They are the natural products of the reduction of monosaccharide.
• Algal gums and mucilage’s constitute an abundant group of polysaccharides in
plants.
FIBER IN FOOD AND ITS ROLE
1. Introduction
• Non-starch polysaccharide is the main components of dietary fiber.
• Pectin, gum, mucilage, cellulose, hemicelluloses and lignin.
• Dietary fiber comes from the portion of plants that is not digested by enzymes
in the intestinal tract. Part of it, however, may be metabolized by bacteria in the
lower gut.
• Different types of plants- have varying amounts and kinds of fiber.
.
Soluble Fiber
• Pectin and gum are water-soluble fibers found inside plant cells.
• They slow the passage of food through the intestines but do nothing to
increase fecal bulk.

48
• Beans, oat bran, fruit and vegetables contain soluble fiber
Water insoluble fibers
• Present in cell walls.
• Cellulose, hemicelluloses and lignin.
• Such fibers increase fecal bulk and speed up the passage of food through the
digestive tract.
• Wheat bran and whole grains - the most insoluble fiber- vegetables and beans
- are good sources
2. Benefits of Fiber
i) Prevent constipation
Insoluble fiber binds water, making stools softer and bulkier.
Therefore, fiber especially that found in whole grain products is helpful in the
treatment and prevention of constipation, hemorrhoids and diverticulosis.
Diverticula are pouches of the intestinal wall that can become inflamed and painful.
In the past, a low-fiber diet was prescribed for this condition.
A high-fiber diet gives better results once the inflammation has subsided.
ii) Lower cholesterol levels
• Low blood cholesterol levels (below 200 mg/dl.) have been associated with a
reduced risk of coronary heart disease.
• The body eliminates cholesterol through the excretion of bile acids. Water-
soluble fiber binds bile acids, and hence a high-fiber diet may result in an
increased excretion of cholesterol.
• Some types of fiber appear to have a greater effect than others.
• The fiber found in rolled oats is more effective in lowering blood cholesterol
levels than the fiber found in wheat.
• Pectin has a similar effect in that it, too, can lower the amount of cholesterol in
the blood.
iii) Reduce the risk of some cancers
Dietary fiber may help reduce the risk of some cancers, especially colon cancer.
This idea is based on information that insoluble fiber increases the rate at which
wastes are removed from the body.

49
This means the body may have less exposure to toxic substances produced during
digestion.
A diet high in animal fat and protein also may play a role in the development of
colon cancer.
iv) Useful for losing weight
High-fiber diets may be useful for people who wish to lose weight.
Fiber itself has no calories, yet provides a "full" feeling because of its water-
absorbing ability.
For example, an apple is more filling than a half cup of apple juice that contains
about the same calories.
Foods high in fiber often require more chewing, so a person is unable to eat a
large number of calories in a short amount of time.
3. Sources of Fiber
• Dietary fiber is found only in plant foods: fruits, vegetables, nuts and grains.
Meat, milk and eggs do not contain fiber.
• The form of food may or may not affect its fiber content.
• Canned and frozen fruits and vegetables contain just as much fiber as raw
ones.
• Other types of processing, though, may reduce fiber content.
• Drying and crushing, for example, destroy the water-holding qualities of fiber.
• The removal of seeds, peels or hulls also reduces fiber content.
• Whole tomatoes have more fiber than peeled tomatoes, which have more than
tomato juice.
• Likewise, whole wheat bread contains more fiber than white bread.
4. RDA
• Women
25 grams per day, for women younger than 50
21 grams per day, for women older than 50

50
• Men
38 grams per day, for men younger than 50
30 grams per day, for men older than 50
5. Adverse effect
• Although fiber is important, it is just one part of a properly balanced diet.
• Too much fiber may reduce the amount of calcium, iron, zinc, copper and
magnesium that is absorbed from foods.
• Deficiencies of these nutrients could result if the amount of fiber in the diet is
excessive, especially in young children.
• Fiber supplements are sold in a variety of forms from bran tablets to purified
cellulose.
• Many laxatives sold as stool softeners actually are fiber supplements.
• Fiber's role in the diet is still being investigated.
• Various types of fiber have different roles in the body.
• For these reasons fiber supplements should be avoided.
• Eating a variety of fiber-rich foods is the best way to receive the maximum
benefits from each type of fiber present in foods, and obtain necessary
nutrients.
BROWNING REACTIONS
1. Introduction
Browning is a common colour change seen in food during pre-preparation,
processing or storage of food.
It occurs in varying degrees in some foods.
The colour produced range from cream or pale yellow to dark brown or black.
Browning reactions observed in food may be classified as enzymatic browning or
nonenzymatic browning.
a) Enzymatic Browning

51
• Fruits such as apples, pears, peaches, apricots, and bananas, and vegetables
such as potatoes quickly turn brown when their tissue is exposed to oxygen.
• Such oxygen exposure occurs when the food is sliced or bitten into or when it
has sustained bruises, cuts or other injury to the peel.
• This “browning reaction” is related to the work of an enzyme called phenolase
(or polyphenoloxidase), a conjugated enzyme in which copper is present.
Phenolase
• Phenolase is classified as an oxidoreductase.
• The substrates for phenolase are phenolic compounds present in the tissues of
the fruits and vegetables. Phenolase hydroxylates monophenols to 0-diphenol
and oxidizes 0-diphenols to 0-quinones.
• The 0-quinones then enter into a number of other reactions, which produce the
“undesirable” brown discolorations.
• Quinone formation is enzyme and oxygen-dependent.
• Once the quinones have formed, the subsequent reactions occur
spontaneously and no longer depend on the presence of phenolase or oxygen.
Prevention
 Enzymatic browning can be prevented or slowed in several ways.
 Immersing the “injured” food (for example, apple slices) in cold water slows the
browning process.
 The optimum temperature for enzymes to act is 43ºC(109ºF).The lower
temperature decreases enzyme activity, and the water limits the enzyme’s
access to oxygen.
 Refrigeration slows enzyme activity even more, and boiling temperatures
destroy (denature) the enzyme.
 A long-used method for preventing browning involves lowering of pH to 2.5-2.7
by the addition of acids such as ascorbic acid, malic or citric acid
 Phenolase works very slowly in the acidic environment created by the added
acids.
 In addition, the vitamin C (ascorbic acid) present in lemon juice functions as an
antioxidant.

52
 It is more easily oxidized than the phenolic-derived compounds, and its
oxidation products are colorless.
b) Non Enxymatic Browning
1) Maillard reaction:
• The non enzymatic browning or Maillard reaction is a chemical reaction
between an amino acid and a reducing sugat, usually requiring heat.
• When aldoses and ketoses are heated with amines, a variety of reactions
ensue, producing numerous compounds some of which are flavours, aromas
and dark coloured polymeric material.
• They may be produced slowly during storage and much more rapidly at the
high temperature encountered during frying roasting or backing.
• The reducing sugar reacts with the amine to form a Schiff base (an imines)
which may cyclate to form glucosamine.
• In the case of glucose the Schiff base undergo a reaction called Amadori
rearrangement to give 1-amino-1-deoxy-D-fructose or Amadori compound.
• The Amadori compounds are early intermediates in the browning reaction
sequence.
• Amadori compounds undergo transformation via different pathways starting
with four different intermediates formed from them.
• The result is a complex mixture of intermediates and products.
The Maillard reaction occurs in three main steps:
• 1. Initial step- formation N glycoside: The carbonyl group of the sugar reacts
with the amino group of the amino acid, producing N-substituted glycosylamine
and water
• 2. After formation of N glycoside the immonium ion is formed and then
isomerizes, this reaction is called Amadori rearrangement and forms a
compound called ketosamine:
• 3. The ketosamine products then either dehydrates into reductones and
dehydro reductones, which are caramel, or products -short chain hydrolytic
fission products such as diacetyl, acetol or pyruvaldehyde which then undergo
the Strecker degradation and produce short-chain hydrolytic fission products
and brown nitrogenous polymers and melanoidins

53
• Important intermediates are formed by rearrangements and eliminations are 1-,
3- and 4-deoxydicarbonyl compounds called 1-, 3-, and 4-deoxyosones. They
finally form 5- hydroxy methyl furfural
• In the process, hundreds of different flavor compounds are created. These
compounds in turn break down to form yet more new flavor compounds, and so
on. Each type of food has a very distinctive set of flavor compounds that are
formed during the Maillard reaction. It is these same compounds have been
used over the years to create artificial flavors.
Food products with Maillard reactions
• The Maillard reaction is responsible for many colors and flavors in foods such
as bread, biscuit, malted barley as in malt whiskey or beer, roasted meat, dried
or condensed milk, roasted coffee etc 6-Acetyl-2,3,4,5-tetrahydropyridine is
responsible for the biscuit or cracker-like flavor present in baked goods like
bread and popcorn.
• The structurally related compound 2-acetyl-1 pyrrpoline has a similar smell and
occurs also naturally without heating and gives varieties of cooked rice their
typical smell.
• Maillard reaction may result in a reduction in nutritional properties and the
formation of potentially toxic and mutagenic compounds. In a food system, the
reactants are mostly amino acids (free forms or peptide-bound) and reducing
sugars.
• Since up to 50% of the food groups have been processed before consumption,
some of the amino acids and reducing sugars is lost during processing.
• Maillard reactions affect protein bioavailability by derivatizing protein-bound,
dietary limiting amino acids such as lysine, arginine, and histidine.
• Maillard reaction products also exhibit antinutritive effects by mechanism
involving complex formation with micronutrients, destruction of vitamins, and by
acting as inhibitors of digestive enzymes
• High temperature, low moisture levels and alkaline conditions promote the
Maillard reaction.
• The rate of Maillard reactions increases as the water activity increases,
reaching a maximum at water activities in the range of 0.6 to 0.7.
• However, as the Maillard reaction produces water, further increases in water
activity may inhibit Maillard reactions.
• Pentose sugars react more than hexoses, which react more than disaccharide.
• Different aminoacids produce different amounts of browning

54
2) Browning reactions which occur in meat
• The browning reactions which occur when meat is roasted or seared have
often been referred to as Maillard reaction browning.
• However, lean meat contains very few, if any, reducing sugars.
• Furthermore, red meat undergoes more extensive browning than does white
meat.
• The browning reactions in lean meat are most likely due to the breakdown of
the tetrapyrrole rings of the muscle protein, myoglobin.
• Thus, the browning of meat is technically not a Maillard browning since it does
not involve the reaction with a reducing sugar
3) Caramelization
• Caramelization is a browning reaction formed by heating carbohydrates like
sucrose or reducing sugars.
• Reactions are facilitated by small quantity of acids, base and certain salts.
• Caramelization is an entirely different process from Maillard browning, though
the results of the two processes are sometimes similar to the naked eye (and
taste buds).
• The final product caramel contains a complex mixture of polymeric compound,
formed from unsaturated cyclic compounds.
• Flavour and aroma compounds are also formed.
• Heating causes the dehydration of sugar molecule with introduction of double
bonds or formation of anhydro rings. Intermediates such as 3-deoxy osones
and furans are formed.
• The unsaturated rings may condense to form useful, conjugated double-bond
containing, brown coloured polymers.
• Catalysts increase the reaction rate and are used to direct the reaction to
specify types of caramel colour, solubility and acidities.
• To make caramel a carbohydrate is heated alone or in the presence of acid, a
base or salt.
• The carbohydrates most often used are sucrose, but fructose, glucose, invert
sugar, malt syrups and molasses may also be used.
• Acid used are food grade sulfuric, sulfurous, phosphoric, acetic and citric acids.

55
• Bases that may be used are ammonium, sodium, potassium and calcium
hydroxides.
• Salts that may be used are ammonium, sodium, potassium carbonates,
bicarbonates, phosphates, sulphates or bisulphates.
Classes of caramel
Class I caramel :Prepared by heating a plain carbohydrate
Class II caramel :Prepared by heating a carbohydrate in the presence of a
sulphite
Class III caramel :Prepared by heating a carbohydrate in the presence of a
source of ammonium ion.
Class IV caramel :Prepared by heating a carbohydrate in the presence of a both
sulphite and ammonium ions
• Caramelization may sometimes cause browning in the same foods in which the
Maillard reaction occurs, but the two processes are distinct.
• They both are promoted by heating, but the Maillard reaction involves amino
acids, as discussed above, while caramelization is simply the pyrolysis of
certain sugars.
DIGESTION AND ABSORPTION OF CARBOHYDRATES
INTRODUCTION
The most abundant carbohydrates ingested by human beings are they
polysaccharides, starch and cellulose, furnished by plant foods and glycogen,
provided by foods of animal origin.
2. Digestion- Starch
• Starch and glycogen are completely hydrolyzed by enzyme action in the
gastrointestinal tract to yield free D-glucose.
• This process begins in the mouth during chewing, through the action of
amylase secreted by the salivary glands.
• Salivary amylase hydrolyzes many of the α (14) glycosidic linkages of
starch and glycogen
Salivary amylase
Starch and glycogen → a mixture of maltose, glucose and

56
oligosaccharides.
The digestion of digestible polysaccharides to yield D-glucose is continued and
completed in the small intestine
1.Pancreatic amylase -made by the pancreas and secreted via the pancreatic
duct into the upper protein of the small intestine duodenum
2.Intestinal amylase secreted by small intestine continue and complete the
digestion of starch
Pancreatic amylase & intestinal amylase
Starch → a mixture of maltose, glucose and
oligosaccharides
Disaccharides
Disaccharides are hydrolyzed by enzymes located in the outer border of the
epithelial cells lining the small intestine.
sucrase or invertase
Surcose → D-glucose and D-fructose ,
lactase
Lactose → D-glucose and D-galactose
maltase
Maltose → two molecules of D-glucose.
The liver stores the glucose as glycogen and releases glucose as and when
needed to maintain blood glucose level.
Cellulose
• Cellulose cannot be enzymatically digested and used by most mammals for
lack of enzymes capable of hydrolyzing the β(14) linkages between
successive D-glucose residues of cellulose.

57
• Nevertheless undigested cellulose residues of plant foods provide bulk or fiber
(also called “roughage”) in the diet and are desirable for proper motility of
materials in the intestine.
• Cellulose can be digested by ruminant animals, but only indirectly. The rumen
bacteria hydrolyze cellulose to yield D-glucose, which they ferment to yield
lactate, acetate, and propionate, absorbed into the blood.
• Lactate and propionate are converted by the liver into glucose sugar in
ruminants.
Absorption
• In the epithelial cells lining the small intestine, D-fructose, D-galactose and D-
mannose are converted into D-glucose.
• The resulting mixture of simple hexoses is absorbed into the epithelial cells
lining the small intestine and brought via the blood to the liver.
Metabolisms involving glucose
• The absorbed free glucose is phosphorylated to glucose 6 phosphate by
hexokinase using ATP.
• The fructose, galactose and mannose absorbed are also converted into
glucose 6 phosphate.
• This compound is metabolized by five major metabolic path ways.
i. Conversion into blood glucose
• The absorbed free glucose is phosphorylated to glucose 6 phosphate by
hexokinase using ATP.
• The fructose, galactose and mannose absorbed are also converted into
glucose 6 phosphate.
• This compound is then metabolized by major metabolic path ways.
• Glucose 6-phosphate is dephosphorylated by glucose 6-phosphate to yield
free-D-glucose, which passes into the systemic blood to be transported to other
tissues.
Glucose 6 phosphatase
• Glucose 6-phosphate → Glucose + Pi
ii. Conversion into glycogen

58
Glucose 6-phosphate not immediately needed to form blood glucose
↓
Converted into liver glycogen by the sequential action of
phosphoglucomutase and glycogen synthase.
iii. Conversion into fatty acids and cholesterol
Excess glucose 6-phosphate not used to make blood glucose or liver glycogen
↓
Degraded via glycolysis and pyruvate dehydrogenase into acetyl-CoA
Acetyl-CoA → malonyl CoA -----→Fatty acids.
↓
triglycerides and phospholipids
↓
Transported to other tissues by plasma lipoproteins.
Acetyl-CoA → to cholesterol by liver
iv. Oxidative degradation to CO2
1. Glycolysis
Glucose 6-phosphate to pyruvate
2. Decarboxylation
Pyruvate to Acetyl-CoA, oxidized via the.
3. Citric acid cycle
Acetyl-CoA oxidized to Co2 and H2O
4.Electron transport and oxidative phosphorylation yield energy in the form of
ATP.
Fatty acids are the major oxidative fuel for the citric acid cycle in the liver.

59
5. Degradation via the Pentose
Phosphate Pathway
Glucose 6- phosphate →the
pentose phosphate
pathway
↓
(1) Reducing power in the form
of NADPH, needed in the
reducing steps in the
biosynthesis of fatty acids and
cholesterol and
(2) D-ribose 5- phosphate, a
precursor in nucleotide
biosynthesis.
Through the action of various regulatory enzymes and through hormonal
regulation, the liver directs the flow of glucose residues into these different pathways
according to the prevailing supply and demand economy of the organism.
Digestion of carbohydrates

60
GLUCONEOGENESIS
• Gluconeogenesis is the synthesis of glucose from noncarbohydrate and then
conversion to glycogen
• The major substrates for gluconeogenesis are the glucogenic amino acids,
lactate, glycerol, and propionate.
• Liver and kidney are the major tissues involved, since they contain a full
complement of the necessary enzymes.
Importance
• Gluconeogenesis meets the needs of the body for glucose when carbohydrate
is not available in sufficient amounts from the diet.
• A continual supply of glucose is necessary as a source of energy, especially for
the nervous system and the erythrocytes. Failure of gluconeogensis is usually
fatal. Below a critical blood glucose concentration, there is brain dysfunction,
which can lead to coma and death.
• Glucose is also required in adipose tissue as a source of glyceride-glycerol,
and it probably plays a role in maintaining the level of intermediates of the citric
acid cycle in many tissues.
• Even under conditions where fat may be supplying most of the caloric
requirement of the organism, there is always a certain basal requirement for
glucose.

61
• Glucose is the only fuel that will supply energy to skeletal muscle under
anaerobic conditions.
• In addition, gluconeogenic mechanisms are used to clear the products of the
metabolism of other tissues from the blood, e.g. lactate, produced by muscle
and erythrocytes, and glycerols, which is continuously produced by adipose
tissue.
• Gluconeogenesis involves glycolysis, the citric acid cycle, plus some special
reactions

62
The energy barriers obstruct a simple reversal of glycolysis
between pyruvate and phosphoenolpyruvate,
between furctose 1,6 bisphosphate and furctose 6-phosphate,
between glucose 6-phosphate and glucose, and
between glucose 1-phosphate and glycogen.
These reactions are all nonequilibrium, releasing much free energy as heat and
therefore physiologically irreversible.
These reactions are circumvented by the following special reactions.
A. Conversion of Pyruvate into Phosphoenolpyruvate:
Pyruvate carboxylase, present in mitochondria, converts pyruvate to oxaloacetate
in the presence of ATP, the B vitamin biotin, and CO2.
The function of the biotin is to bind CO2 from bicarbonate onto the enzyme prior to
the addition of the CO2 to pyruvate.
A second enzyme, phosphoenolpyruate carboxykinase, catalyzes the conversion
of oxaloacetate to phosphoenolpyruvate.
High energy phosphate in the form of GTP or ITP is requried in this reaction, and
CO2 is liberated.
Thus, with the help of these two enzymes catalyzing endergonic transformations
and lactate dehydrogenase, lactate can be converted to phosphoenolpyruvate,
overcoming the energy barrier between pyruvate and phosphoenolpyruvate.
B. Conversion of Fructose 1,6-Bisphosphate into Fructose 6- phosphate:
The conversion of fructose1,6 bisphosphate to fructose 6-phosphate, necessary to
achieve a reversal of glycolysis, is catalyzed by a specific enyzme, fructose-1,6
bisphosphatase.
This enzyme is present in liver and kidney and in striated muscle. It is absent in
heart muscle and smooth muscle.
C. Conversion of glucose 6-phosphate into Glucose:

63
The conversion of glucose 6-phosphate to glucose is catalyzed by another specific
phosphatase, glucose-6-phosphate.It’s presence allows a tissue to add glucose to
the blood.
D. Conversion of glucose 6-phosphate into Glycogen:
The break down of glycogen to glucose 1-phosphate is carried out by
phosphorylase.
The synthesis of glycogen involves an entirely different pathway through the
formation of uridine diphosphate glucose and the activity of glycogen synthase.
These key enzymes allow reversal of glycolysis to play a major role in
gluconeogenesis, the relationships between gluconeogenesis and the glycolytic
pathway.
After transamination or deamination, glucogenic amino acids form either pyruvate
or members of the citric acid cycle.
The reactions described above can account for the conversion of both glucogenic
amino acids and lactate to glucose or glycogen.
Thus, lactate forms pyruvate and enter the mitochondria before conversion to
oxaloacetate and ultimate conversion to glucose.
The source of pyruvate and oxaloacetate for gluconeogenesis is mainly amino acid
catabolism.
• Some amino acids are catabolized to pyruvate, oxaloacetate, or precursors of
these.
• Muscle proteins may break down to supply amino acids.
• These are transported to liver where they are deaminated and converted to
gluconeogenesis inputs.
E. Conversion of propionate into succiny1coA:
• Propionate enters the main gluconeogenic pathway via the citric acid cycle
after conversion to succiny1-coA.
• Propionate is first activated with ATP and CoA by an appropriate acy1-CoA
synthetase.

64
• Propiony1 CoA formed undergoes a CO2 fixation reaction to form D-
methylmalony1-CoA, catalyzed by propiony1-CoA carboxylase.
• This reaction forms a malony1 derivative and requires the vitamin biotin as a
coenzyme.
• D-Methylmalony1-CoA must be converted to its steroisomer, L-methylmalony1-
CoA, by methylmalony1-coA racemase before its final isomerization to
succiny1-CoA by the enzyme methlmalony1-CoA isomerase.
• It is converted into malate which is then converted into phosphoenol pyruvate
and finally to glucose.
• Fates of Pyruvate
• Three common fates for pyruvate are of prime importance: conversion into
acteyl CoA, lactate, and ethanol.
ATP production from glucose
• Glycolysis
• One mole. of glucose → 2 moles of pyruvate
• ATP produced = 8
• Pyruvate → Acetyl CoA
• ATP produced = 2x3=6
• Citric acid cycle
• Acetyl CoA →CO2 + H2O
• ATP produced = 12 x2=24

65
• Net production =30+8=38
Food proteins
Chapter 1: Native proteins
and denatured
proteins
7.1.1.Introduction
Proteins are important in foods, both nutritionally and as functional ingredients. They
play an important role in determining the texture of a food. Proteins are made up of
sequence of amino acids. There are 20 amino acids present in food proteins.
1. Essential amino acids and Nonessential amino acids
The body is able to synthesis about 10 amino acids and these amino acids are
called non-essential or dispensable amino acids as they can be synthesized in
animals from other compounds. They are glycine, cysteine, alanine, serine,
proline, tyrosine, aspartic acid, asparagine, glutamic acid and glutamine. The
remaining amino acids are called essential or non-dispensable as they cannot be
synthesized in animals. They are methionine, tryptophan, threonine, valine,
isoleucine, leucine, phenylalanine, lysine, arginine and histidine. Arginine and
histidine are essential for infants and children.
2. Limiting amino acid
Proteins of different foods have different proportions of essential amino acids.
Some of them may contain required mounts of essential amino acid and few of them
may not have adequate amounts of one or more of essential amino acids. An
essential amino acid of a protein which is present much below requirement is called
as limiting amino acid. Most of the plant proteins contain limiting amino acid.

Food chemistry 2nd sem (full sylabus)

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