Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish (including goldfish and carp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.
2. ETHANOL
• Ethanol (ethyl alcohol, C2H5OH)
• Melting point = -114°C
• Boiling point = 78.4°C
• Bioethanol is derived from alcoholic fermentation of
sucrose(C12H22O11) or simple sugars, which are
produced from biomass
• Absolute and 95% ethanol are good solvents and are
used in many industrial products such as paints,
perfumes and tinctures
• Solutions of ethanol (70-85%) are used as
disinfectants in Medicine
3. GENERATION OF BIOFUELS
• 1st Generation of biofuels: ethanol from sugar, corn,
molasses, starchy biomass, etc
• 2nd Generation of biofuels : biodiesel from vegetable
oils and bioethanol from lignocellulosic biomass
• 3rd Generation of biofuels : algal biofuels (Biodiesel,
biobutanol, gasoline, methane, ethanol, vegetable oil, jet
fuel)
• 4th Generation of biofuels : biohydrogen
6. CARBOHYDRATES
• Carbohydrates are polyhydroxy aldehydes, polyhydroxy
ketones or compounds that can be hydrolyzed from them
• The smallest carbohydrates (glucose and fructose) that
cannot be hydrolyzed to smaller carbohydrate units are
called monosaccharides
• Those consisting of same two monosaccharides (lactose
and maltose) or different compounds are called
disaccharides
• Carbohydrates consisting of a more than two
monosaccharides (raffinose) are called oligosaccharides
• Polysaccharides contain thousands of covalently linked
monosaccharides. Among the most important
polysaccharides in nature are starch (amylose and
amylopectin), cellulose and hemicellulose
7. PRIMARY PLANT MONOSACCHARIDES
• The two dominant simple sugars (monosaccharides) are
the five-carbon sugar, D-xylose and the six-carbon sugar
D-glucose
• D-glucose serves as readily available chemical energy
and as a supply of carbon for producing more-complex
materials (disaccharides, oligosaccharides,
polysaccharides)
8.
9. • These two sugars in combination with several other
minor sugars, serve as building blocks for the
production of more-complex carbohydrates (sucrose,
a disaccharide containing one molecule of D-glucose
and one molecule of D-fructose, starch, cellulose and
hemicellulose).
• These more-complex carbohydrates function as
structural components and as long-term energy stores
10. PRIMARY PLANT POLYSACCHARIDES
• The plant´s primary method of storing energy for
extended periods of time is starch production
• There are two major types of starch namely amylose
and amylopectin, which differ in bond structure,
reactivity and associated physical properties
• Most starch is 10-30% amylose and 70-90%
amylopectin
11. AMYLOSE
Amylose is a linear dextrose polymer of α-1,4
bonds with a molecular weight ranging from 4,000 to
340,000. It can be hydrolysed with acid or with
enzymes
H O
OH
H
OH
H
OH
CH2OH
H
O H
H
OH
H
OH
CH2OH
H
O
H
H H O
O
H
OH
H
OH
CH2OH
H
H H O
H
OH
H
OH
CH2OH
H
OH
H
H O
O
H
OH
H
OH
CH2OH
H
O
H
1
6
5
4
3
1
2
amylose
12. AMYLOPECTIN
• Amylopectin is a nonlinear carbohydrate polymer,
which contains millions of D-glucose units linked by
α-1,4 and α-1,6 bonds resulting in a branched
configuration
H O
OH
H
OH
H
OH
CH2OH
H
O H
H
OH
H
OH
CH2OH
H
O
H
H H O
O
H
OH
H
OH
CH2
H
H H O
H
OH
H
OH
CH2OH
H
OH
H
H O
O
H
OH
H
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OH
H
OH
CH2OH
H
H H O
H
OH
H
OH
CH2OH
H
H
O
1
OH
3
4
5
2
amylopectin
13. BASIC CHEMISTRY
LIGNOCELLULOSIC BIOMASS
Lignocellulosic biomass chemically consists of three
basic polymers
Cellulose (C6H10O5)x
Hemicelluloses (xylan (C5H8O4)m
Lignin [C9H10O3- (OCH3)0.9−1.7]n (in trunk, foliage and
bark)
16. COMPOSITION OF LIGNOCELLULOSIC
BIOMASS
• Lignocellulosic material - world’s largest bio-ethanol
renewable resource.
cellulose (30-50%),
hemicellulose (15-35%)
lignin (10-20%)
• Cellulose and hemicelluloses make up approximately
70% of the entire biomass and are tightly linked to the
lignin component through covalent (bonding between
non-metal characteristics) and hydrogenic bonds (here
hydro means hydrogen, H+ bonds with highly
electronegative elements like fluorine and bromine) that
make the structure highly robust and resistant to any
treatment
18. CELLULOSE
• Cellulose is a homopolysaccharide, which is
composed of β-d-glucopyranose units linked together
by (1→4)-glycosidic bonds which consists of
approximately 40 – 50 weight percentage of dry
wood and provides wood´s strength.
• After removal of water from each glucose-molecule
(glucose anhydride), long cellulose chains containing
5,000 – 10,000 glucose units are formed (namely
cellobiose units)
20. • In the cellulose chain, the glucose units are in 6-
membered rings, called pyranoses while 5
membered rings are called furanoses
• They are joined by single oxygen atoms (acetal
linkages) between the C-1 of one pyranose ring
and the C-4 of the next ring
• Since a molecule of water is lost when an alcohol
and a hemiacetal react to form an acetal, the
glucose units in the cellulose polymer are referred
to as anhydroglucose units
STRUCTURE OF CELLULOSE
21. • The pyranose rings of the cellulose molecule have all
of the groups larger than hydrogen sticking out from
the periphery of the rings (equitorial positions)
• The stereochemistry at carbons 2, 3, 4 and 5 of the
glucose molecule are fixed; but when glucose forms a
pyranose ring, the hydroxyl at C-4 can approach the
carbonyl at C-1 from either side, resulting in two
different stereochemistries at C-1
• When the hydroxyl group at C-1 is on the same side
of the ring as the C-6 carbon, it is said to be in the a
configuration (not to be confused with a -cellulose,
which is not related)
22. • In cellulose, the C-1 oxygen is in the opposite, or b
configuration (i.e., cellulose is poly[b -1,4-D-
anhydroglucopyranose])
• This b configuration, with all functional groups in
equatorial positions, causes the molecular chain of
cellulose to extend in a more-or-less straight line,
making it a good fibre-forming polymer
• A covalent bond which bonded a carbohydrate
molecule to another molecule or C-O bond is called
as glycosidic bond
23. HEMICELLULOSE
• Hemicelluloseis an amorphous and variable structure
formed of heteropolymers including hexoses (D-glucose,
D-galactose and D-mannose) as well as pentose (D-
xylose and L-arabinose) and may contain sugar acids
(uronic acids) namely, D-glucuronic, D-galacturonic and
methylgalacturonic acids
24. LIGNIN
• Lignin - a phenylpropane-based polymer, is the
largest non-carbohydrate fraction of lignocellulose
• The main function of lignin is the support through
strengthening of wood (xylem cells), filling the
spaces in the cell wall between cellulose,
hemicellulose and pectin components
• Lignin is indigestible by animal enzymes. Only
some fungi and bacteria secrete ligninases, which
can biodegrade the polymer
• Some lignolytic enzymes are manganese peroxidise,
lignin peroxidase and cellobiose dehydrogenase
29. LIQUEFACTION
α-amylase - Sources: Grain – malt
Bacteria – Bacillus subtilis
Fungi – Aspergillus spp.
Optimum conditions
Fungi Bacteria
Liquefaction : The conversion of a solid or a gas into
a liquid
32. DISACCHARIDES TO ETHANOL
PROCESS
First, invertase (an enzyme present in the yeast) catalyzes the hydrolysis
of sucrose to convert it into glucose and fructose
Then, another enzyme (zymase), also present in the yeast, converts
the glucose and the fructose into ethanol and CO2
34. HYDROLYSIS OF CELLULOSE
Why is the hydrolysis of cellulose difficult?
• Celluloses have crystalline structures due to the dense
packing of cellulose chains
• They are very stable under many chemical conditions
• They are not soluble in water, many organic solvents,
weak acids or bases
• The crystalline structure can be destroyed and turned
into amorphous form under high temperature (>300˚C)
and pressure (25 MPa)
• There are normally two ways to hydrolyze cellulose:
chemically and enzymatically
35. HYDROLYSIS OF CELLULOSE
• The chemical method is to use concentrated strong
acids to hydrolyze cellulose under high temperature
and pressure.
• However, this method is not preferred by biofuel
industry, because toxic by products remaining in the
glucose products will be introduced into the
fermentation step, affecting the fermenting
bacteria/yeast.
• Hence, the milder enzymatic method seems to be a
much more potential candidate to hydrolyze cellulose.
36. COST OF ENZYMATIC HYDROLYSIS
• About half of the total cost of producing biofuel from
cellulose is allocated on enzymatic cellulose hydrolysis, in
which cellulase is the most expensive part, consuming 15-
25% of the total cost
• Enzymatic hydrolysis is a very slow step. As a result, a lot
of cellulases are needed to achieve a reasonable
hydrolyzing rate
• Typically, the ratio of cellulase to cellulose used in
hydrolysis step is 25 g/1 kg
• Despite that the fermentation can produce a great amount
of cellulase, about 100 g from 1liter broth, the cost of
cellulase still remains a large portion of the total cost
39. CELLULOLYTIC ENZYMES
Cellulase (Endoglucanase)
• Randomly attack the β-(1, 4) glycosidic bonds of
cellulose
• Normally act on only amorphous cellulose not
crystalline
• Cellulase can be produced from fungi and bacteria
• Optimum reaction conditions depend on the source
organism
42. CELLOBIOHYDROLASE (EXOGLUCANASE)
• Release cellobiose from the non-reducing ends of a
cellulosic substrate
• Hydrolyze both amorphous and crystalline cellulose
• Mainly from fungi
45. OTHER ENZYMES
Xylanases
1. Attack β-(1,4) bonds between D-xylose residues of
heteroxylans and xylo-oligosaccharides
2. Do not degrade xylobiose
3. Endo-acting enzyme
β-Xylosidase
1. Hydrolyze xylo-oligosaccharides to xylose
2. Not active on xylan
47. YEAST CELL COMPOSITION
Water 80%
Dry matter 20%
C - 50%
O - 30-35%
N - 5%
H - 5%
P - 1%
Mineral 5-10%
Or Proteins - 40-45%
Carbohydrates - 30-35%
Nucleic acids - 6-8%
Lipids - 4-5%
48. YEAST PROPAGATION
Carbon source
•Glucose, maltose, etc
Nitrogen source
•Need ammonium or organic N
•(NH4)2SO4, (NH4)3PO4, urea
Phosphorus source
•Need P mainly at early fermentation
•Need small amount, usually enough from raw starch
materials such as corn or other grains
•Addition of P is needed when sugar beet is used
50. YEAST PHYSIOLOGY IN ETHANOL
PRODUCTION
• The primary industrial yeast used in bioethanol
production is Saccharomyces cerevisiae
• S. cerevisiae is an ideal candidate as it is able to
tolerate and produce high concentrations of alcohol
• S. cerevisiae is an unicellular eukaryotic fungus that
reproduces by budding
• S. cerevisiae cells are generally ellipsoidal in shape
ranging from 5 to 10 μm at the large diameter and 1
to 7 μm at the small diameter
• The yeast cell contains numerous organelles, all of
them important for yeast functions
51. YEAST AS CANDIDATE FOR ETHANOL
PRODUCTION
• Dextrin (starch) is converted to fermentable sugars (mostly
glucose) by the enzyme α-amylase
• Yeast grows and ferments glucose through pyruvic acid to
ethanol with liberation of carbon dioxide
• The metabolic state of the yeast is such that only anaerobic
metabolisms of glucose takes place
• But a small amount of oxygen is an absolute requirement of the
yeast, because the small amounts of oxygen present in the
medium are sufficient to ensure that the yeast cell can synthesise
both the unsaturated fatty acids(a fatty acid whose carbon chain
can absorb additional hydrogen atoms) and the sterols(any of a
group of natural steroid alcohols derived from plants or animals;
they are waxy insoluble substances), that it needs for cell
membrane synthesis during growth
52. THEORY OF METABOLISM
Embden-Meyerhoff Pathway
• This path utilizes 1 mol of glucose to yield 2 mol of
pyruvate which are then decarboxylated to acetaldehyde
and reduced to ethanol.
• Two moles of ATP are generated from one mole of
glucose in this process
53. ENTNER-DOUDOROFF PATHWAY
• The Entner - Doudoroff pathway is an additional
means of glucose consumption in many bacteria.
Glucose is phosphorylated and then oxidized to 6-
phosphogluconate
• At this point, dehydration occurs to form 2 keto - 3 -
deoxy - 6 - phospogluconate (KDPG) which is then
cleaved by KDPG - aldolase
• The net yield is 2 mol of pyruvate formed from 1 mol
of glucose and the generation of 1 mol of ATP
54. MIXED ACID 2, 3-BUTANEDIOL
FERMENTATION
• Multiple end products may be produced by organisms
which conduct mixed acid type fermentations such as
the 'enteric' group of facultative anaerobic bacteria
• It is a complex pathway Phosphoenol pyruvate
produced in Embden-Meyerhoff pathway may be
further broken down to such diverse products as
ethanol formate, acetate, succinate, lactate, and 2,3 -
butanediol
• The basis of fermentation ethanol production is the
specific chemical change under gone by the substrate,
which is induced by an enzyme or microorganism
56. • Nicotinamide adenine dinucleotide (NAD) is a coenzyme
found in all living cells. The compound is a dinucleotide,
because it consists of two nucleotides joined through their
phosphate groups
• One nucleotide contains an adenine base and the other
nicotinamide. Nicotinamide adenine dinucleotide exists in two
forms, an oxidized and reduced form abbreviated as NAD+ and
NADH (NAD+H, for hydrogen)respectively
• A cofactor is a non-protein chemical compound that is required
for the protein's biological activity. These proteins are
commonly enzymes, and cofactors can be considered "helper
molecules" that assist in biochemical transformations
• Cofactors can be subdivided into either one or more inorganic
ions, or a complex organic or metalloorganic molecule called a
coenzyme; most of which are derived from vitamins and from
required organic nutrients in small amounts
57. • Sugar enters the cell and most is immediately reacted upon by
the enzymes, which convert glucose via the glycolytic pathway
to pyruvate, which is then converted to carbon dioxide and
acetaldehyde and then to ethanol by alcohol dehydrogenase.
• In the process, one molecule of glucose is broken down into
pyruvate:
C6H12O6 → 2 CH3COCOO− + 2 H+
• During this reaction a size difference of two molecules of NAD+
to NADH and two ADP molecules converted to two ATP plus
two water molecules happens.
• Pyruvate is then converted to acetaldehyde and carbon dioxide
(by pyruvate decarboxylase). Subsequently, the acetaldehyde is
reduced to ethanol by the produced NADH (from previous
glycolysis), which is returned to NAD+.
CH3COCOO− + H+ → CH3CHO + CO2
CH3CHO + NADH → C2H5OH + NAD+
58. • Ethanol leaves the cell by diffusion
• Yeast cells produce a substrate level of ATP (the
energy storage chemical of cells), which is the major
source of energy for growth and metabolic processing
in the cell
• Yeast cell growth cannot happen, unless metabolic
production of ATP occurs through glycolysis as
ethanol is produced
• During glycolytic pathway, one glucose molecule is
converted into two ethanol molecules and two carbon
dioxide molecules:
• C6H12O6 → 2 C2H5OH + 2 CO2
59. List of yeast strains, which are actually of primary interest to
industrial operations in the fermentation of glucose into
ethanol
syn. Baker’s Yeast = synthetic Baker’s Yeast (here the
yeast is cultured in synthetic media)
60. HEAT PRODUCTION
• Overall net heat production for all stages: 157 kJ/mole
• Energy storage in ATP: 2 x 31 = 62 kJ
• Overall heat can be produced: 157 + 62 = 219 kJ/mole
61. NEW TECHNIQUES ON YEAST
• High temperature yeast: 40 – 50º C
Possible to combine saccharification and fermentation
• Ethanol-tolerant yeast: 18-20% (v) EtOH
Normal yeast: 10-12% (v) EtOH
• Genetically engineered yeast: directly convert starch to
EtOH
62. NEW TECHNIQUES ON YEAST
1.Active Dry Yeast
• A form of dry yeast in which the yeasts are not killed but
made dormant through dehydration, and return to becoming
active again when mixed with a warm liquid (about 105 to
115°F) or 40 to 46 c
• Normally, yeast contains ~ 80% water
• Under rapid vacuum drying at 50-60ºC, water content can be
reduced to 5%
• Active dry yeast has to be vacuum packed to keep it activity
• Active dry yeast: 30 – 40 billion cells/g
2.Immobilized yeast fermentation
• Yeast cells are immobilized during the fermentation process.
• Cell system helped reduce fermentation times in a significant
manner
63. ADVANTAGES AND DRAWBACKS OF POTENTIAL ORGANISMS IN
LIGNOCELLULOSIC-BASED BIOETHANOL FERMENTATION
67. FORMATION OF INHIBITORS
The above picture indicating main routes of formation of
inhibitors. Furan aldehydes and aliphatic acids are carbohydrate
degradation products, while lignin is the main source of phenolic
compounds, as indicated by guaiacyl (4-hydroxy-3-
methoxyphenyl) and syringyl (4-hydroxy-3,5-dimethoxyphenyl)
moieties found in many phenolics
While the contents of furan aldehydes and aliphatic acids are
relatively easy to determine, the quantification and identification
of phenolic compounds remain challenging
The insert shows the variety of peaks representing phenolic
compounds found in a hydrolysate of Norwegian spruce, as
indicated by analysis using liquid chromatography-mass
spectrometry (LC-MS)
69. Temperature
• Temperature control is necessary in order to ensure
that yeasts are not killed in the process
• Saccharomyces yeasts are rather tolerant to
temperatures near 35°C in the early stages of growth
• At high ethanol levels, every increased °C occurring in
the fermentor above 27°C is a risk factor, because of
reducing yeast activity, as well as because such
temperatures increasingly favours the growth of heat-
resistant Lactobacillus species
YEAST STRESS IN FERMENTATION PROCESS
70. Alarm levels of inhibitory chemicals that affect metabolism in
yeast-catalysed fuel alcohol fermentations
71. Organic acids
• The two major organic acids that are detrimental to yeast
metabolism are lactic and acetic acids
• Both of them are end products of fermentation by bacteria
(Lactobacillus spp.) and/or wild yeast
• Losses in ethanol yield are directly correlated to
contamination with lactic or acetic acid concentration
Ions
• Sodium was identified as a problem ion due to its use (as
NaOH) as virtually the only cleaner/sanitiser employed
in fuel alcohol plants
• In combination with other stressful agents (e.g.
temperature, organic acids, pH, etc), ions such as sodium
can exert such stress, that ethanol production rates can be
72. Mycotoxins
• Any toxic substance produced by a fungus
• The term 'mycotoxin' is usually reserved for the toxic
chemical products produced by fungi that readily
colonize crops
• One mold species may produce many different
mycotoxins, and the same mycotoxin may be produced
by several species
• Especially mycotoxins, like deoxynivalenol, a
trichothecene mycotoxin is said to be inhibitory to yeast
cells
73. Phytic acid
• Phytic acid (found in plants) contains bound
phosphorus, which is not nutritionally available to
yeast unless it is degraded to release inorganic
phosphorus
• The phytate molecule is known to be a chelator of
positively charged ions, e.g. magnesium, calcium,
zinc, iron and copper
• But these ions are important for enzyme function and
structure and therefore for yeast growth
74. • Saccharomyces cerevisiae converts only hexose sugars such
as glucose and is not able to co-ferment glucose and xylose.
(Ho NWY et al., 1989)
Natural ethanologenic yeast species such as
• Pichia stipilis,
• Pachysolen tannophilius,
• Kluyveromyces marxianus (K. marxianus )
• Candida shehatate
appeared to have promise in replacing S. cerevisiae in
lignocellulosic-based ethanol fermentation. (Chen YCB.,2009).
75. • Thermophilic anaerobic bacteria and yeasts such as
Thermoanaerobacterium saccharolyticum,
Thermoanaerobacter ethanolicus, Clostridium
thermocellum and K. marxianus IMB3 for their potential to
utilize a wide range of feedstocks at high temperatures
above 65˚C
• These thermophilic bacteria are able to ferment both
hexose and pentose sugars in addition to their ability to
produce cellulase enzymes and avoid the addition of
commercial enzymes
76. • Emerging technologies including SSCombF and CBP
represent potential improvements as they reduce operation
steps as well as chemical inhibitors and can be enhanced
by lignin, energy-self-sustaining co-products.
• These processes are typically associated with thermophilic
and cellulolytic microorganisms including organisms such
as Trichoderma reesei along with Phanerochaete
chrysosporium, K. marxianus and Clostridium
cellulolyticum with some of them possessing fermentative
abilities in addition to their hydrolytic properties.
• Companies such as DDCE (DuPont Danisco Cellulosic
Ethanol) and Butalco prefer using genetically engineered
conventional strains, S. cerevisiae and ethanologenic
Zymomonas mobilis for their higher alcohol tolerance and
yield
77. • Theoretically, 1 ton of hexose (glucose or fructose)
yields 511 kg of ethanol. However, practical
efficiency of fermentation is about 92 percent of this
yield
• Lignocellulosic ethanol can reduce greenhouse gas
emissions by around 90% when compared with fossil
petroleum
78. ETHANOL PRODUCTION IN INDIA
11 units in Uttar Pradesh -75 million liters
7 units in Tamil Nadu -62.5 million liters
8 units in Karnataka- 66.5 million liters
4 units in Andhra Pradesh-over 40 million liters
• Similar steps have also be taken up by the cooperative
sector units in Maharashtra, Punjab and UP. By the
end of the year (2014) it is estimated that about 300
million liters capacity would have been created for
the production of anhydrous ethanol
The strong tendency of intra- and intermolecular hydrogen bonding in cellulose results in molecular aggregation to form microfibrils or a small fibril in the cytoplasm or wall of a cell, visible only under an electron microscope, and typically aggregated into coarser fibrils or structures.
A chemical bond that involves sharing a pair of electrons between atoms in a molecule – covalent, robust – strong/rich
Dry basis is an expression of the calculation in chemistry, chemical engineering and related subjects, in which the presence of water is ignored for the purposes of the calculation.
Wet basis : Wet basis moisture content (designated MW in the text) is described by the percentage equivalent of the ratio of the weight of water (WW) to the total weight of the material (Wt).
An aqueous solution containing 2 g of glucose and 2 g of fructose per 100 g of solution contains 2/100=2% glucose on a wet basis, but 2/4=50% glucose on a dry basis. If the solution had contained 2 g of glucose and 3 g of fructose, it would still have contained 2% glucose on a wet basis, but only 2/5=40% glucose on a dry basis.
A covalent bond which bonded a carbohydrate molecule to another molecule is called as glycosidic bond
Dextrins: Any of various polysaccharides obtained by hydrolysis of starch; a tasteless and odourless gummy substance that is used as a thickening agent and in adhesives and in dietary supplements
Liquefaction : The conversion of a solid or a gas into a liquid
Basic building block of linear polymer is cellobiose, a compound of two glucose molecules.
An essential structural component of living cells and source of energy for animals; includes simple sugars with small molecules as well as macromolecular substances; are classified according to the number of monosaccharide groups they contain or CARBOHYDRATES or SUGAR - Saccharides
Eukaryotic : Having cells with 'good' or membrane-bound nuclei
unsaturated fatty acid : A fatty acid whose carbon chain can absorb additional hydrogen atoms
Sterols : Any of a group of natural steroid alcohols derived from plants or animals; they are waxy insoluble substances
Yeast are commonly grown in the laboratory using two general types of media, rich media and synthetic media. Rich media is formulated to supplies yeast with ample metabolites, including a nitrogen source and a carbon source, which can be varied. Synthetic media can be formulated to be complete or selective and is more of a 'bare bones' type media, containing known types and amounts of carbon and nitrogen sources, essential minerals and vitamins as well as all or some of the necessary amino acids and nitrogenous bases, depending on the particular genetic background of the strain being used.
A form of dry yeast in which the yeasts are not killed but made dormant through dehydration, and return to becoming active again when mixed with a warm liquid (about 105° to 115°F). Or 40 to 46 degree celsius
Yeast stress in fermentation process
There are several stress agents, which can produce problems in the fermentation process.