Drying of agriculture product (Cereals, Pulses and Oilseed ) and Different Dryers

Subject: Protected Cultivation and Secondary Agriculture
Topic: DRYING
by
Dr. Sanjay Singh Chouhan
Assistant Professor
College of Agriculture, JNKVV,
Powarkheda, Hoshangabad
1

DRYING
• Oldest method of Preservation.
• Important Unit Operation under post harvest phase, can occur is conjunction with other
processes.
• Drying is removal of water/moisture from a product to predetermined level.
• Dehydration means removal of moisture to very low levels usually to bone dry conditions.
• Drying makes the food grains and other product suitable for safe storage.
• Ex: Harvesting of paddy at 20-22% moisture content and then drying to 14% increase the saving
of 6-7% more grain. And also increase rice yield by 10% in comparison to harvesting paddy at
14%.
• For e.g. Liquid milk is highly perishable, whereas milk powders are more stable and easier for
preservation and handling.
JNKVV- College of Agriculture, Powarkheda
2

Utilities of Drying
1. Protection of grain and safe storage of agricultural product.
2. Attack of insect and other micro-organism activity during the storage can be kept negligible.
3. Drying permits early harvest of crops. This leads to reduction of losses by shattering.
4. Early harvest also permit more time of preparation for next crop.
5. Adequate drying facility help in proper planning of harvesting season as per the availability
of labours.
6. Storage of products after drying makes it available during off-season give additional income
as prices are higher in off-season.
7. Some agriculture by-product and waste product can also be converted into useful product by
proper drying.
3

MOISTURE CONTENT
• The moisture content of food grains and other agriculture product plays an
important role in maintaining the desirable quality of the product
• Expressed either in
• wet basis i.e. mass of water per unit mass of wet material
• dry basis i.e. mass of water per unit mass of dry solids
• Relation b/w dry basis and wet basis is
4

Moisture Content Determination Methods
1. Direct Method, also called primary methods
- More accurate
- time consuming
2. Indirect Methods, also called Secondary methods
- calibrated by direct method
- faster & employ the electrical properties of grain
- used at field, market, ware house etc.
5

Direct Methods of moisture determination
1. Air oven methods:
• 2-3 gm of representative sample are placed in oven at temperature of 130˚c for 1-
2 hr.
• 25-30 gm of representative sample are placed in oven at temperature of 100˚c for
72-96 hr.
2. Vacuum-oven methods:
• 2-3 gm of representative sample are placed in a vacuum-oven (25mm vacuum)
and dired at temperature of 100˚c for 72-96 hr.
6

3. Brown-Duvel Fractional Distillation Methods
• This method is recognised as an official method for determination of moisture content.
• 100gm of whole grains along with 150ml of mineral oil is taken in a flask.
• The sample is boiled, thus moisture from sample is evaporated.
• Evaporated moisture collected and condensed in a
graduated cylinder.
• The millilitre of moisture collected shows the %
moisture content.
• The determine moisture content is on wet basis.
• The temp. of mineral oil in flask should reach to
200˚c within 26 min.
• Total time required for MC determination is 30 min.
7

4. Infra-Red Method
• In this method, grain moisture content is directly measured by
evaporation of the water from a sample of grain with an infra-red
heating lamp.
• The instrument consists of a balance, a pan counter balance by a
fixed weight and a variable length of weighing chain.
• An infra-red lamp is mounted on an arm above the pan with a
provision to change its height.
• A scale calibrated in % mc is incorporated in the stem of the
instrument.
• At the end of the test, when the balance is zero, a direct reading of
mc is obtained.
8

INDIRECT METHODS
1. Electrical Resistance Method:
• The electrical conductivity or resistance of a product depends upon its moisture
content.
• This principle is employed in resistance measuring devices.
• The Universal moisture meter measure the electrical resistance of the grain at a
given compaction.
• It give a fairly accurate reading of moisture content of wet basis.
2. Dielectric Method
• Such devices measure the dielectric constant of grains.
• The grains are filled in chamber, the sides of chamber is formed by the plates of a
condenser.
• A high frequency current is passed in the chamber to measure the capacitance of
the sample.
• The capacitance varies with the MC present in the sample, the degree of
compaction and the grain temp.
9

3. Chemical Methods
Karl Fischer titration method
• The Karl Fischer titration method is a standard laboratory technique used to
measure moisture content in liquids and solids.
• Moisture content is determined with a calibration curve correlated to the volume
of reagent used to titrate the water of a sample.
• The sample is completely dissolved in a solution consisting of a primary alcohol
(methanol) as the solvent, and a base (pyridine) as a buffering agent.
• Next, a brown titrant solution of sulfur dioxide (SO2) and iodine (I2) is added drop-
wise to the sample solution.
• The iodine reacts with water and the titrant solution loses its color.
10

• Further titrant solution is added until the reaction between the water and the
iodine is completed, as indicated by the titrant solution recovering
• its color again because of iodine availability.
• The amount of water in the original sample solution is determined based on the
amount of titrant solution added before re-coloration occurs.
11

Equilibrium Moisture content
• Most of the agricultural products, like food grains absorb moisture from the
surrounded environment of loose moisture.
• This properties of gaining or loosing due to vapor pressure deference between
grain and environment is known as hygroscopicity.
• The equilibrium moisture content (EMC) of grain surrounded at least partially by
air is the moisture content at which the material is neither gaining nor losing
moisture.
• The value of the EMC depends on the material and the relative
humidity and temperature of the air with which it is in contact.
12

DRYING THEORY
• The moisture migration into or from a product is dependent on the difference of
vapour pressure between atmosphere and product.
• If the vapour pressure of grain is greater than atmospheric vapour pressure,
transfer of moisture from grain to atmosphere take place.
• But when the atmospheric vapour pressure is greater than grain vapour pressure,
grain absorbs moisture from atmosphere.
• So it is necessary to understand the nature of air and composition of product to
know the drying process.
13

Methods of Drying
• There are many methods to develop water-vapour pressure difference between product
and atmosphere.
• For increasing of water vapour pressure of grain, the methods are:
1. Solar or sun drying
2. Conduction drying
3. Infra-red drying
• The methods usually employed to lower the water vapour pressure of air are:
1. Heated air drying
2. Desiccated air drying
3. Refrigerated air drying
14

Natural Drying
• Natural drying is the method of drying, in which we are using the natural source (viz.
Sun) for drying of food products.
• It is also known as sun/solar drying.
• Sun drying is cheaper method due to natural source of drying. While it is slow process,
very prone to contaminants as well as weather dependent. That is why it is not most
common in commercial scale.
Types of sun drying:
1. Drying of standing crops
2. Drying of grains on stalks
3. Drying of threshed grains
15

Advantages:
• Free
• No added cost for equipment
• No added cost for electricity or gas
Disadvantages:
• Contaminations from the environment.
• Product losses and contaminations by insects and birds.
• Floor space requirements.
• Difficulty in controlling the process.
• Inconsistent sensory quality.
16

Mechanical Drying
• Mechanical drying is the method of drying of foods by means of mechanical
systems.
• Hot air is produced by the system which is used for the drying of food material.
• Several mechanical dryers are developed by the researcher in the field of food
technology.
• Various types of mechanical drying system available in the market which are hot
air convective drying, freeze drying, vacuum drying, fluidized bed drying, spray
drying, microwave drying, vacuum assisted microwave drying, microwave assisted
fluidized bed drying.
17

Advantages
• Quick method
• Perfect drying is possible even under unfavorable weather condition.
• Seed loss is minimized
• Small no of unskilled labors
• Required little space
Disadvantages
• Requires specialized equipment and machine, which is costly.
• Care should be taken while drying the seed using hot air, as it causes damage to the seed.
• Tempering is to be followed while drying the seed in this method.
18

Mechanical Drying Methods
1. Contact drying
• In this methods, heat is supplied to wet products by conduction.
• Heat is transferred through a heated surface like plate, dryer wall etc.
• Thermal efficiency is higher, but product quality may be affected.
2. Hot air convective drying
• The principle of hot air convective drying is based on conventional heat transfer from heated air to the
material being dried.
• Hot air is forced through the material and does the moisture diffusion process that result in the
drying.
• This method has been widely used in industries.
• Different types of dryers have been developed and employed in commercial production.
• Heated air is blown through the material by cross flow or by fan generated flow.
19

3. Freeze drying
• Freeze drying is the technique of removing the moisture content from the liquid
food, by freezing and removing the water portion of the food materials.
• Some pharmaceuticals are heat sensitive.
• Some fruits and vegetables lose their aroma and flavor if they remain in high
temperature for significant figure of time.
• For such cases freeze-drying is an alternative.
• Freeze drying requires several successive steps, as pre-freezing, primary drying,
secondary drying, conditioning and dehydration.
• It is expensive and requires sophistication.
• Hence, it is difficult to apply to all commercial drying needs.
20

4. Vacuum drying
• The basic principle of vacuum drying is to remove the water by means of vacuum.
• There are four essential elements in a vacuum drying system: a vacuum chamber,
vacuum generating device, system for collecting water vapor and means for
supplying heat required for vaporization of water.
• For reasons similar to freeze-drying vacuum drying is also an expensive drying
method.
• It is used only for costly products.
21

5. Fluidized bed drying
• Fluidized bed drying is the drying technique in which fluidization is take place.
• Fluidization provide better surface area of heat and mass transfer.
• Fluidized bed dryer are found throughout all industries, from heavy mining through food,
fine chemicals and pharmaceuticals.
• They provide an effective method of drying relatively free flowing particles with a
reasonable narrow particles size distribution.
• Fluidized bed dryers can process a wide variation of feed rates from pounds to several
hundred tons per hour.
• Two principle types of fluidized bed dryers exist- 1 static fluid bed, 2. vibrating fluidized
bed.
22

DRYERS
23

Types of Dryers
Different types of dryers can be categorised as follow:
1. Dryers in which product are kept in a bin and the heated or drying air is ventilated through
them.
Ex: Deep bed dryer, Tray dryer, Tunnel dryer
2. Dryers in which products are kept moving. The movement of products is by gravity or by
some mechanical means and the drying air is ventilated through them.
Ex: Continuous Flow Dryer, Recirculating Dryer, LSU Dryer, Rotary Dryer
3. Dryers with free falling bed due to gravity and the drying air is ventilated through the falling
film of product.
Ex: Spray dryer, Pneumatic Dryer
4. Dryers in which the product are dried in a state of fluidisation by strong bowing heated air.
Ex: Fluidised bed dryer
24

SELECTION OF DRYER
1. Production capacity
2. Initial moisture content of product
3. Particle size distribution or shape of the product
4. Drying characteristic of product
5. Maximum allowable product temperature
6. Explosion characteristics (spray or fluid bed dryer)
7. Moisture isotherm
8. Physical data of material
25

26
Deep bed drying characteristics at different depths
DEEP BED DRYER

DEEP BED DRYER
JNKVV- College of Agriculture, Powarkheda 27

Deep Bed Dryer
• These batch-in-bin dryer are of large capacity.
• All grains are not fully exposed to the same condition of drying air.
• Condition of drying air changes with time and depth of grain bed.
• An air flow rate of 2.94 to 3.92 m3/min/tonne is recommended.
• Rate of airflow per unit mass of grain is small.
• Drying of grain in deep bin can be taken as sum of several thin layers.
• Paddy with 2.5 m deep layer may take 20 days to dry.
• The net perforated area of the floor should be 15% of the total floor area.
• Air velocity of 300 m/min through the opening is preferable.
28

FLAT BED DRAYR
29

FLAT BED DRYERS
• It is similar to deep bed dryers except that the surface area of the dryer is more
and the depth of drying layer is less.
• Low capacity usually 1-2 tonnes.
• Designed for farm level operation.
• Depth of spread grains is 0.6 to 1.2 m.
• Low air pressure is required to force the drying air.
• The whole batch is dried quickly and these are less chance of spoilage due to
moulding.
32

Continuous Flow Dryer
Columnar type dryer in which wet grains flow from top to the bottom of the dryer
Two types:-
1. Mixing
2. Non-mixing
Continuous flow Dryer (Mixing Type)
• Grains are diverted in the dryer by providing baffles
• Use low air flow rates of 50-95 m³/min/tonne
• Zig-zag columns enclosed by screens are used to achieve mixing
• High drying air temperature of 65°C is used
33

36
Recirculatory Batch dryer
Continuous Flow Dryer (Non-mixing)

Continuous flow dryer (Non-mixing)
Recirculatory Batch dryer
• Continuous flow non mixing type
• Consists of 2 concentric circular cylinders, set 15-20 cm apart
• Bucket elevator is used to feed & recirculated the grain
• Centrifugal blower blows the hot air into the inner cylinder, acts as a plenum
• The exhaust air comes out through perforations of the outer cylinder
• Grain is recirculated till it is dried to desired moisture content
• Drying is not uniform as compared to mixing type
37

Roller or Drum Dryers
Single drum type

39
Double Drum Type

Roller or Drum Dryers
• In these the food is spread over the surface of a heated drum.
• The drum rotates, with the food being applied to the drum at one part of the cycle.
• The food remains on the drum surface for the greater part of the rotation, during which
time the drying takes place, and is then scraped off.
• Drum drying may be regarded as conduction drying.
• Can be used for drying liquid and slurry forms of food, and foods which can withstand
relatively high temperature for short time.
• Used for drying milk, soup mixes, mashed potatoes.
• Double drum can be used for drying tomatoes, pumpkin purees.
40

CABINET, TRAY OR COMAPARTMENT DRYER

42

• Insulated cabinet with air circulating fan which moves the air through heater and
then through baffles which direct air between the trays of food.
• In this type of dryers heated air also removes the moist vapours.
• They vary in the number of trays, and have varied application in the industry,
which includes drying fruits and vegetables at 1-20 tons/day.
• The installation is cheap and flexible.
43

44
Tunnel Dryers

45
Tunnel Dryers

Tunnel Dryers
• These may be regarded as developments of the tray dryer, in which the trays on
trolleys move through a tunnel where the heat is applied, and the vapors removed.
• In most cases, air is used in tunnel drying and the material can move through the
dryer either parallel or counter current to the air flow.
• Sometimes the dryers are compartmented, and cross-flow may also be used.
• Vegetables and fruits cut into pieces can be dried in semi continuous basis.
46

Fluidized Bed Dryers
• In a fluidized bed dryer, the food material is maintained suspended against gravity in an
upward-flowing air stream.
• There may also be a horizontal air flow helping to convey the food through the dryer.
• Heat is transferred from the air to the food material, mostly by convection.
• Heated air is forced up through a bed of solids under fluidized conditions so that solids
are suspended in air.
• Heated air acts both as fluidizing and drying medium.
• It is used for drying peas, beans, carrots, coffee, salt, sugar, meat cubes, flours, etc.
49

LSU DRYER

LSU dryer
• Developed at Louisiana state university (LSU) . It is a continuous mixing type dryer
• Developed specifically for rice to ensure gentle treatment, good mixing & good air to
grain contact.
• Consists of rectangular chamber, holding bin, blower with duct, grain discharging
mechanism and air heating system .
• Layers of inverted V shaped channels are installed in the drying chamber; heated air is
introduced through these channels at many points.
• Alternate layers are air inlet & outlet channels; arranged one below the other in an
offset pattern.
53

• Ribbed rollers are provided at the bottom of drying chamber for the discharge of grain.
• Capacity varies from 2-12 tonnes.
• Recommended air flow rate is 60-70 m³/min/tonne
• Air temp. are 60-85°C for raw & parboiled paddy
• Uniformly dried product can be obtained.
• Can be used for different types of grain
• High capital investment
54

Spray Dryers

Spray Dryers
• In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet
dispersion into a current of heated air.
• Air and solids may move in parallel or counterflow.
• Drying occurs very rapidly, so that this process is very useful for materials that are damaged by
exposure to heat for any appreciable length of time.
• The dryer body is large so that the particles can settle, as they dry, without touching the walls
on which they might otherwise stick.
• Commercial dryers can be very large of the order of 10 m diameter and 20 m high
• Commonly used for processing of milk powders, coffee, tea, ice cream mix, milk-based baby
foods, yeast extract, edible proteins.
57

Pneumatic Dryer

59
Pneumatic Dryer

Pneumatic Dryer
• In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the
velocity and turbulence of the stream maintaining the particles in suspension.
• Heated air accomplishes the drying.
• Classifying device is included in the equipment in which, the dried material is separated.
• The dry material passes out as product and the moist remainder is recirculated for further
drying.
• Generally used for the finishing of dried powder or granulated materials.
• The incoming material is usually pre-dried (Spray or fluidized) to moisture level below
40%.
• Dried product are separated from exhaust air by cyclone or filters.
60

Rotary Dryer

Rotary Dryer
• The foodstuff is contained in a horizontal inclined cylinder through which it travels, being
heated either by air flow through the cylinder, or by conduction of heat from the
cylinder walls.
• In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle
or screw rotates within the cylinder conveying the material through.
• Rotatory dryers are used to dry particulate solid foods.
• Drying medium is either hot air or combustion gases co-currently or counter-currently.
• Size of dryers in usually 0.3m diameter-2m in length to 5m diameter-90m length.
• Application includes in drying of meat pellets, granulated sugars, cocoa beans.
64

Conveyor or Belt Dryers

66
Single Belt Dryers

67
Multiple Belt Dryers

• The food is spread as a thin layer on a horizontal mesh or solid belt and air passes
through or over the material.
• In most cases the belt is moving, though in some designs the belt is stationary and the
material is transported by scrapers the hot air.
• The heated air is either directed up or down through the conveyor, the layer of product
either maybe perpendicular or parallel to the surface spread in thin layer.
• Dryers consists of two or more conveyors, sometimes maybe up to five, they called
Multiple conveyors.
• Used for drying vegetables and fruits mainly in diced form.
68

Vacuum Dryers

70
Vacuum Dryers

Vacuum Dryers
• Batch vacuum dryers are substantially the same as tray dryers, except that they operate
under a vacuum, and heat transfer is largely by conduction or by radiation.
• The trays are enclosed in a large cabinet, which is evacuated.
• The water vapour produced is generally condensed, so that the vacuum pumps have
only to deal with non-condensable gases.
• Heat sensitive foods in solid or liquid state are dried by heat conduction.
• e.g. fruit juices concentrates, tomato concentrates and coffee extracts.
• ‘Puff Drying’ is a method to obtain rapid drying by adjusting the vacuum to make the
material puff by the evolution of gases
71

Freeze Dryer

73
Freeze Dryers

Freeze Dryers
• Freeze drying is a process used to dry extremely heat –sensitive materials.
• It allows the drying , without excessive damage, of proteins, blood products and even
microorganisms, which retain a small but significant viability.
• In this process the initial liquid solution or suspension is frozen, the pressure above the
frozen state is reduced and the water removed by sublimation.
• Thus a liquid to vapour transition takes place, but here three states of matter involved:
liquid to solid, then solid to vapour.
• The material is held on shelves or belts in a chamber that is under high vacuum.

Solar Dryer
75

Solar Dryer
Solar dryer can most conveniently be classified as direct or indirect.
Direct Solar Dryer
• The item to be dried is exposed directly to solar radiation through a transparent
material that covers the structure.
• The heat generated from the solar energy is used to dry the crops or food items and also
heats up the surroundings.
• The main disadvantage of using the direct mode is that the heat that will be absorbed
by the item cannot be controlled.
• Available in many sizes, ranging from kilograms to metric tons.
• Simple and Affordable
76

Direct Solar Dryer

Indirect Solar
Dryer

80
Indirect Solar Dryer

Indirect Solar Dryer
• As the name suggests, this method does not expose the crop directly to the sunlight.
• The solar radiation is absorbed and converted into heat by another surface (like a black
top) usually called the collector.
• Air that will be used for drying is passed over this surface and gets heated, which is then
used to dry the food item inside the dryer.
• The main advantage of indirect mode of drying is that the temperatures can be controlled.
• The sizes can vary from kilograms to metric tons.
• It is expensive and more complex to construct when compared to direct solar dryers.
81

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