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FT 5105 FOOD MICROBIOLOGY POSTGRADUATE INTITUTE OF AGRICULTURE
BY
Terrence Madhujith
Professor in Food Science and Technology
University of Peradeniya
At the of the session, the learner should be able to ◦ Define fermentation
◦ Describe the importance of fermentation as a food preservation method
◦ Describe the factors affecting fermentation
FT 5105 Food Microbiology 2
First explained by Louis Pasteur – fermentation by yeast
Pasteur’s “life without air”
Originates from the Latin word fervere [boiling ]
He explained that fermentation occurs under anaerobic conditions and can bring about CO2, ethanol and other products
Fermentation is considered the oldest food preservation method
FT 5105 Food Microbiology 3
Even today fermentation plays a vital role in food preservation
Fermentation occurs naturally
However, uncontrolled fermentation leads to spoilage
Controlled growth may bring about desirable changes.
Fermentation brings about food and beverages
Alcoholic beverages
Industrial products
Pharmaceuticals
Hormones
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Fermentation is the metabolic process in which CHO and related compounds are oxidized with the release of energy in the absence of any external electron acceptors
The final electron acceptors are organic compounds therefore, only partial oxidation happens and only little energy is released
FT 5105 Food Microbiology 8
• Fermentation is all about CHO and related materials
• However, almost all foods contain fat and protein as well
• These compounds too undergo simultaneous changes
• Thus, a distinction is necessary – Changes of CHO fermentation
– Changes of proteins proteolytic/putrefaction
– Changes in lipids lipolytic
FT 5105 Food Microbiology 9
When a food undergoes fermentation all three processes are possible
However, one process usually dominates the others
Which process dominates is decided by ◦ Type of food
◦ Type of MO
◦ Environmental conditions
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Alcoholic fermentation of glucose
C6H12O6 → 2 C2H5OH + 2 CO2
glucose ethanol carbon dioxide
Glucose Glycolysis Pyruvic Acid
Acetaldehyde Ethyl Alcohol
CO2
ATP
ATP
NADH
NADH NAD+
NAD+
Pyruvic acid from glycolysis is reduced to lactic acid by NADH, which is oxidized to NAD+.
This commonly occurs in muscle cells. Lactic acid fermentation allows glycolysis to continue by ensuring that NADH is returned to its oxidized state (NAD+).
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Result of a positive role of microorganisms
Fermentation has been used for over 5000 years
Fermented foods show more shelf life and nutritional qualities compared to their non-fermented counterparts
Lactic acid bacteria and yeasts play a major role
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Dairy Products
Acidophilus milk
Bulgarian buttermilk
cheese
Kefir
Yoghurt
Kumiss
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Starter cultures are used
Lactic starter culture MOS convert lactose to lactic acid L. cremoris L. lactis L. diacetilactis Leuconostoc citrovorum
Starter culture can be single or mixed
Usually the cultures are preserved by freeze drying or freezing
pH drops down to 4.3 – 4.5
Titratable acidity increases up to 0.8-1.0%
FT 5105 Food Microbiology 19
1. Yoghurt S. thermophilus & L. bulgaricus
are used at 1:1 ratio
Cocci grow faster than rod.
Thus, cocci are mainly responsible for LA production
Rod adds flavor and aroma
Acetaldehyde is mainly responsible for yoghurt flavor
Combination produces more LA and acetaldehyde that they produce when grown alone
FT 5105 Food Microbiology 20
Milk is heated to 85-95C
cooled to 45C
Starter culture is added (2%)
Incubate at 42-45C for 3-5h
Place in cups and chill to 4C
Titratable acidity goes up to 0.9%
To achieve this, fermentation is to be stopped at 0.65%
Streptococcus can tolerate pH up to 3.6
Lactobacillus can tolerate only up to 4.2
During fermentation, pathogenic MOs are destroyed. E.g. Coliform
FT 5105 Food Microbiology 23
Freshly produced yoghurt contains up to 109 organisms/g
During storage the count drops to 106/g
Acetaldehyde is present at 30-40 ppm level
Diacetyl is present in minute quantities
Both contribute to the flavor
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treptococcus salivarius
Lactobacillus delbrueckii
Lactobacillus acidophilus
Lactobacillus casei
Bifidobacterium adolescentis
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Hundreds of different varieties produced around the world
Cheese can be defines as consolidated curd of milk solids in which milk fat is entrapped by coagulated casein
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• Ripened cheese types are produced through further microbial activity
• All hard cheese are ripened by bacteria for 2-16 months – e.g. Cheddar
• Semi hard cheese – by bacteria over 1-8 months – e.g. Gouda
• Semi hard – by mold 2-12 months – e.g. Roquefort and Blue
Four basic steps are involved
1. Curdling of milk
Performed by either adding rennet or through starter culture
Pure of mixed starter cultures can be used
Streptococcus cremoris S. lactis S. thermophilus Lactobacillus lactis L. helveticus L. bulgaricus
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Casein gets curdled during this step
LA produced by the starter culture brings pH down
Casein coagulates at pH between 4.64 - 4.78
Casein is present as calcium caseinate
LA forms its salt (Calcium lactate) by reacting with calcium caseinate
Casein gets precipitated when Ca level lowers to a certain extent
Rennin makes Ca paracaseinate
CP formed reacts with free Ca thus casein gets precipitated
pH does not go down
Sometimes a combination of the two are used
2. Draining the curd to remove excess moisture
Pressure may or may not be used depending on the type
3. Salting
4. Ripening
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Many varieties except few are ripened. E.g. cottage and cream cheese are not ripened
Roquefort Cheese: The curd is inoculated with Penicillium roqueforti
Inoculated curd is placed in ripening room of which T and RH are carefully controlled
When the fungal growth is adequate the temperature is brought down to control the fungal growth
FT 5105 Food Microbiology 32
P. roqeuforti is aerobic, however, can grow even at low oxygen concentrations such as 4.25%
To provide oxygen, holes are made in the curd
Camemberti Cheese
Ripening goes on for 2-4 weeks
Inoculated with Penicillium camemberti
They grow on the rind (outer layer)
The proteolytic enzymes secreted by the fungi soften the cheese
FT 5105 Food Microbiology 33
• Thus the texture of cheese changes
• Swiss Cheese:
• Ripening is carried out by – Lactobacillus
– Streptococcus and
– Propionibacterium
• They produce acetic acid, propionic acid and water
• CO2 makes holes in cheese
FT 5105 Food Microbiology 34
Swiss cheese – L. bulgaricus + S. thermophilus + Propionibacterium shermanii
Glu Pyruvate lactate propionate acetate CO2
Cheddar Cheese:
Various Streptococci and Lactobacilli are used to ripen cheddar cheese
Limburger Cheese:
Streptococcus, Lactobacillus and Brevibacterium linens are used
The latter is proteolytic and mainly responsible for producing flavor
High salt and low pH conditions are used
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Group Variety Primary ripening organism/s
Hard Cheddar Bacteria
Swiss Eye forming Bacteria and other bacteria
Semi hard Brick Bacteria
Roquefort Molds
Gorgonzola Molds
Soft Limburger Bacteria
Cammemberti Mold
Cottage Unripened
cream Unripened
FT 5105 Food Microbiology 37
Lactic acid
Imparts a fresh acid flavor to cheese
Assists rennet in coagulating casein
Improve texture
Suppress the growth of pathogens/spoilage Mos
Traces of aroma compounds produced by LABs contribute to the flavor of cheese
FT 5105 Food Microbiology 38
Produced by churning pasteurized cream (71C for 30 min)
Butter culture (starter culture) is introduced ◦ Increases yield
◦ Helps develop flavor
S. lactis and S. cremoris – form LA
Leuconostoc dextranicum and L. citrovorum - develop flavor
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Acetoin (3-hydroxybutanone) produced by latter is spontaneously oxidized to diacetyl
Acetoin and diacetyl give the characteristic butter flavor
0.25 – 1% starter is used
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Milk based drink containing kefir grains
Lactobacillus bulgaricus
L. lactis naturally present in kefir are used
Lactose fermenting yeasts are
also added
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Lactobacillus produce LA while yeasts produce alcohol
The final alcohol content is 1-2%
Cow, goat or sheep milk is used
When mare milk is used the product is known as Kumiss
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Produced using lactic starter culture
Pasteurized cream
Starter culture
holding until acidity develops
churning
Butter Buttermilk
Commercial cultured buttermilk is prepared by inoculating skim milk with lactic culture
Curd is broken through agitation – this is called cultured buttermilk
Sour cream is prepared by homogenized and pasteurized light cream with lactic culture
Aroma is due to diacetyl and sourness is due to LA
Culture Action Diary Product
L. delbrueckii bulgaricus
Acid and flavor Bulgarian butter milk
L. Acidophilus Acid Acidophilus milk
Leuconostoc cremoris
Flavor Cultured buttermilk, sour cream, cottage cheese
S. lactis diacetylactis
Acid and flavor Sour cream, buttermilk
L. lactis and bulgaricus and yeast
Acid and alcohol Kefir & Kumiss
S. faecalis Acid and flavor Swiss and Cheddar cheese
• Two functions – Leavening
– Flavor development – yeasts do not produce flavor compounds in significant amounts but bacteria do.
• Yeasts are used for leavening
• Bakers add extra amount of yeast to encourage fermentation and thereby to reduce time spent
• Sugar naturally present in flour is enough, but to boost the activity of yeast sugar is added
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Yeasts ferment sugar to CO2
Released CO2 helps in softening the
dough
Their activity starts immediately after mixing them with dough and continues until heat in the oven kills enzymes
During fermentation dough conditioning also happens
San Francisco sourdough bread and similar bread types – both bacteria and yeast used
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• Historically, barm [mother sponge] was used as the source of starter culture
• L. sanfrancisco – souring
• S. exiguus – leavening
Idli
• Produced by fermenting pre-soaked rice and black gram overnight and steaming
• L. mesenteroides predominates in Idli
• Strains of S. cerevisiae are used
• Good strains – Produce a large no. of cells
– Stable and viable
– Produce CO2 rapidly
• Original mother culture is increased in number in few steps to final seed culture
• Finally in culture tanks, the seed culture cells are concentrated into a ‘cream’ through centrifugation
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This heavy suspension is added into the mash
E.g. sugar cane or beet molasses
Mineral salts and other additives are added to enhance the growth of yeasts
E.g. ammonium slats, urea, malt sprouts, phosphates, vitamins, vegetable extracts etc.
pH is adjusted to 4.3- 4.5
Mixtures are maintained at 30C
Mixture is aerated rapidly
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• Molasses are added gradually to maintain sugar content
• After 4-5 budding cycles the mixture is centrifuged to produce a thick ‘cream’
• ‘cream’ is filter pressed to remove excess liquid
• The mass of yeast is made into pellets, cakes after adding little amount of vegetable oil
• Active dry yeast is made by drying to reduce MC to 8%
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Fermented Sausages Dry Semi dry Intermediate
Dry sausages – ground meat is mixed with curing agents and seasonings
Mixture is stuffed in casings Incubate for varying periods Starters or natural flora can be used Fermentation time is reduced when starters
are used
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Curing mixture contains Glucose
Nitrites/nitrates
During incubation sausages are stored at low RH conditions – 60%
Time varies from 2wk to 6 months
pH goes down to 4.8
L plantarum
L brevis
L buchneri
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Semi-dry sausages follow the steps but subjected to less drying
These are usually heated or smoked and cooked before eating
E.g. Bologna, Summer sausages
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Uneviscerated fish is mixed with salt at 1:3 ratio
Placed in fermentation tanks and allowed to liquefy for 6 m
Liquid is collected, filtered and ripened for further 3m
Halophilic microbes grow
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Fish sauce is a dark colored, clear liquid with a distinct aroma
Streptococcus, Micrococcus and Bacillus predominate
Fish proteases too contribute to liquefication.
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• A canned or bottled made of shredded cabbage leaves
• Preserved mainly by lactic acid generated during fermentation
• Normal cabbage flora is used for fermentation
• 2.25% salt is added and mixed well with cabbage leaves
• Anaerobic conditions are maintained inside the container
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• Contents are well packed to release as much air as possible
• This helps in achieving anaerobic conditions inside
• The container is filled up to the top and weight is applied at the top
• This helps in bringing juice out and submerging the leaves in juice
• Otherwise molds and yeasts grow on the surface leading to spoilage
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Fermentation commences in a day or two and continues for 3-6 weeks
When the fermentation process is complete ◦ All fermentable CHOs are used up
◦ Typical aroma is developed
◦ Acidity rises to 1.8-2.2%
◦ pH is between 3.2 -3.5
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• The final product is stable when stored at low temperature and free of oxygen
• If temperature rises the product darkens and flavor is lost
• Three steps are involved in fermentation
• 1. fermentation by Leuconostoc mesenteroides – Consumes sugars producing lactic acid, acetic acid, ETOH, mannitol and CO2
• Pleasant odor is mainly developed by this
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CO2 generated helps bringing in anaerobic conditions quickly
When acidity reaches 1% they start to die off
2. Then Lactobacillus plantarum takes over
They mainly produce lactic acid
When acidity goes up they too die off
3. Then Lactobacillus brevis takes over
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• L. brevis can tolerate any acidity that can develop naturally
• A homofermentator thus no CO2 is produced
• During fermentation lactic acid and acetic acid is formed at 4:1 ratio
• L mesenteroides needs low temperature thus it is important to provide low temperature at the onset to develop good flavor
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Cucumbers are fermented in large wooden or plastic vats
Cucumber is placed in 5% brine
Strength is increased over 6-9 wks up to 15% every week
Salt extracts water from cucumber thus salinity goes down gradually
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Acidity will reach 0.5-1%
pH is 3.8
Aerobacter aerogenes predominate
Produces CO2 and H2 vigorously
They live for about 1 week and die off
Then LABS predominate The main bacterium is L. plantarum
LABs are involved Leuconostoc mesenteroides P. cerevisiae Lactobacillus brevis and Strptococcus faecalis
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Yeasts are found at all stages of fermentation
Their peak population is found in the third week
When brine fermentation is complete and salt concentration reaches 15% the mixture is microbiologically stable
FT 5105 Food Microbiology 65
Green and black olives
Fully mature fruits are harvested and placed in shallow vats
Olives are first treated with 2% lye solution to remove bitter taste
Alkali is washed away with water
Fruits are placed in barrels and brine (12%) is added
The fruits are graded, sorted and packed in bottles filled with 6-9% brine and 0.5% LA
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Olives are mostly pitted to remove the pits and stuffed with pimento
Spoilage of olives: A. Scum formation – due to growth of yeasts.
The whitish - cream colored scum develops on the brine surface
They utilize lactic acid as the energy source B. zapatera spoilage due to Clostridium C. Butyric acid fermentation: Sugars are
converted to butyric acid through fermentation
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Yeast spots: White spots develop just beneath the surface of fruits
Softening: Believed to be done by proteolytic molds
Discoloration: prevented by adding lactic acid. Promoted by iron thus iron vessels should never be used for olive processing
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Fermentation starts spontaneously
Natural flora is used
Brine from previous well fermented olives is used
First to appear is Aerobacter aerogenes
They produce hydrogen and CO2
Then LABS predominate ◦ L. mesenteroides
◦ P. cerevisiae
◦ L. plantarum
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• They produce lactic acid
• More LA can be obtained by adding small quantity of sugar
• This should not be done at the onset as sugar addition promotes excessive gassing
• Fermentation continues until all soluble CHOs are used up
• At this stage the product is fully cured
• It is stable as long as kept out of air
• Final pH reaches 3.8-4
• LA content should be above 0.5%
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Soybean and wheat flour are inoculated with A. oryzae or A. soyae
Leave for 3days
This produces a large amount of fermentable matter
Fungal covered material is mixed with 18% NaCl and water and incubate at RT for 6-12m
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Moromi is pressed to release soy sauce
L. delbrueckii Zygosaccharomyces rouxii carry out anaerobic
fermentation Some other fermented Products Tempeh Miso Ogi Gari
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• Fermented soybean
• Soybean is soaked and then cooked
• Inoculated with tempeh from a previous batch
• Wrap in banana leaves and leave for 1-2 days at ambient temperature
• Mold grow on tempeh and as a result it becomes a ‘cake’
• Rhizopus oligosporus R. arrhizus, R. oryzae
• Wheat too can be used
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Fermented soybean product common in Japan
Steamed soy is mixed with koji and salt
White miso – results in one week
Brown miso – fermented for a long time – up to 2 years
Aspergillus oryzae is used
Miso is a thick paste used for soups, sauces, spreads etc.
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• Popular in Nigeria
• Produced from corn
• Corn is soaked for 2- 3days and wet milled
• Corynebacterium predominated during steeping and it is responsible for diastatic activity
• Lactobacilllus plantarum and S. cerevisiae are also responsible for fermentation
• Used as the first weaning food
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Fermented cassava
Used in Wet Africa
Cyanogenic glucosides are released as hydrocyanic acid
Grated cassava (peeled) is left for 3-4 days for fermentation
Fermentation occurs in two stages
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in stage 1 – Corynebacterium manihot ferments starch and produces acids thus the pH goes down. CG is released during this stage
In stage 2 – Geotrichum grows and develop flavor
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Ale – top fermented product by S. cerevisiae Lager – bottom fermented by S. uvarum
Malted beverages produced by brewing
Yeasts convert fermentable sugars to ethanol and
CO2
Yeasts do not produce enough amylases
Thus, it is essential to convert starch into
fermentable sugars
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First step is malting
Barley is soaked and germinated for 5 days and kiln dried
Malt is a good source of amylases Alpha amylase – increase sugar content Beta amylase – liquefy
Kiln-dried product is ground to form a powder [malt milling]
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In the next step mashing
Objective is to make more fermentable substances
Hydrolysis occurs
Cereal adjuncts are added
Temperature is raised to 60C – at this temp saccharification occurs
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Insoluble matter settles down and the liquid part is separated ‘ Wort’
Hops are added to wort and boiled for 2.5h
Hops add bitter taste due to bitter acids
Also adds resins such as humulone, cohumulone and adhumulone
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• Resins are effective against g(+)ves
• Next step is fermentation
• S. cerevisiae [top fermenting]or
uvarum [bottom fermenting] is added into wort
• Fermentation is carried out at 3-14C for 8-14 days
• FT 5105 Food Microbiology 84
Yeasts produce EtOH and CO2
In addition, little amounts of glycerol, acetic acid, aromatic esters too are produced
Aging/ maturation
Adds flavor
Improves clarity
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Defects Turbidity Off flavors Poor physical characters
Beer infections
Sarcina sickness:
Caused by
P cerevisiae Lactobacillus pastorianus L. diastaticus and Zymomonas anaerobium
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Obesumbacterium proteus produce parsnips odor
Acetobactor and Gluconobactor also produce sourness
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• Traditionally made out of grapes
• Can also be made from fruit juices,
berries etc.
• Grapes are harvested when they have the correct sugar content
• Fruits are crushed and SO2 treated - must
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• Wine yeast is added – S. cerevisiae ellipsoideus
• Mixture is fermented at 24-27C for about a week.
• High temp deactivate yeast while low temp permit LABs to grow
• Must is to be cooled as fermentation generates heat
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• When the first fermentation is over clear liquid is siphoned off to another tank
• Secondary fermentation continues in this tank for about 10 days at about 25C
• At the end again clear portion is siphoned off
• Wine can be flash-pasteurized to precipitate proteins
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• Cooled for few days and filtered
• Placed in oak or redwood tanks and stored for months
• Periodically wine is racked
• In the initial fermentation stage, aeration is preferred to allow yeast growth
• Later on anaerobic conditions are favored
FT 5105 Food Microbiology 91
Starch from steamed rice is hydrolyzed by A. oryzae
Fermentation is carried out by Saccharomyces sake over 30 – 40 days
Product contains 12-15% alcohol and 0.3% LA
LA is produced by LAB
FT 5105 Food Microbiology 92
Seeds are fermented in tanks for 10-12 days
During fermentation liquids generated and temperature rises to 50C
Fermentation occurs in two stages
In the first stage sugars from acidic pulp is converted to alcohol
Then alcohol is oxidized to acetic acid
Yeasts play an important role
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• Acidity – Low pH is better
• Sugar content – dry wines are spoilt by bacteria
• Alcohol concentration
• Tannins
• Amount of SO2 – 100-750 ppm
• storage temperature
• Availability of air – oxygen favors unnecessary yeast films.
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• If there is no inherent acidity, then acidity has to be developed quickly through fermentation
• If exposed to oxygen/air, microbial growth on the surface can oxidizes acids. Thus, acidity goes down. So does the preservative action.
• This can potentially happen in ripening cheddar cheese
FT 5105 Food Microbiology 95
Certain yeasts produce alkaline by products such as NH3 as a result of their regular metabolism
This is purposely encouraged in the production of Limburger cheese
FT 5105 Food Microbiology 96
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A
B
C
D E
F
A-B - Germicidal action
B-C - S. lactis
C-D - Lactobacillus
D-E - molds (oxidize
acids) + yeasts
(produce NH3)
E-F - Proteolytic spore
former bac.
Acid
ity %
Time
Sequence of changes in raw milk in relation to acid concentration
Proteolytic action brings pH of milk higher than that of the original milk
During E-F region, the curdle made during B-D is digested and the curdle becomes gassy
FT 5105 Food Microbiology 98
• Alcohol too imparts a preservative action
• Alcohol content depends on initial sugar content, type of yeast, temp of fermentation and oxygen conc.
• Most yeasts cannot tolerate >12-15% (v/v)
• Natural wines contain 9-13%
• Fortified wines contain up to 20%
FT 5105 Food Microbiology 99
• Microbes in a mixed fermentation dominate depending on the fermentation temperature
• Thus, temperature may have a direct effect on the microbial products which in turn control the quality of the final product
• E.g. In sauerkraut production – Leuconostoc mesenteroides – acetic acid, LA, alcohol
and CO2 – Lactobacillus cucumeris – LA when L. mesenteroides
leaves
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– Lactobacillus pentoaceticus & plantarum – produce more LA when both leave
• L. mesenterodes require cool temp <21C
• If temp is >21C – Lactobacilli outgrow
• Acidity creates by Lactobacilli further prevents mesenteroides growth
• Thus, in sauerkraut making, low temp is given first.
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Acid
ity %
Time (days)
Sequence of acid fermentation in sauerkraut production
L. mesenteroides
L.curcumeris
L. Pentoaceticus
& L. plantarum
MOs may have different oxygen requirements for their growth and fermentation
E.g. S. cerevisiae (Bakers’ yeast)
and S. ellipsoideus (Wine yeast) grow best under aerated conditions while they ferment sugars rapidly under anaerobic conditions
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• Thus, in commercial production of yeast, air is bubbled through molasses
• In baking – large dough masses provide a relatively anaerobic condition
• Vinegar production is done in two steps
• 1. Sugars alcohol
Anaerobic conditions
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Alcohol Acid
Aerobic conditions
Aerobic conditions are provided by bubbling air through the ferment
This occurs in a vinegar generator until acetic acid conc. Reaches 4%
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• Mos can be separated on the basis of salt tolerance
• LA producers in pickles, olives, sauerkraut etc. tolerate up to 10-18% salt
• Many proteolytic and spoilage Mos can not tolerate above 2.5%
• Adding salts to above ferment gives advantage for LA producers
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• Generating acids make the envt. More unfavorable for others
• Slat also draws sugar and water from vegetables
• Water drawn from vegetables dilutes the brine solution thus, salt level is to be added gradually
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Sauerkraut - 2-2.5%
Olives – 7-10%
Gherkin – 15-18%
Cheese curd is salted to prevent the growth of proteolytic bac. During long ripening periods
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