Department of microbiology microbial food technology group a diploma in quality assurance in microbiology diploma

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Eg: cheddar cheese

Eg: Grana, Parmesan, Asiago old.

  • Off flavor is produced by gas forming organisms. Eg: Clostridium, coliforms, yeasts.

  • Gassiness is produced by Clostridium, Bacillus polymyxa [produces gas and defects in ripening cheese].

  • Bitter flavor is produced by coliforms, Micrococci, Yeasts [acid proteolytic bacteria].

  • Leuconostoc produces Holes/openness in cheddar cheese.

  • Proteolysis, gas production is by undesirable microbes.

  • Sliminess/Off flavor is produced by Pseudomonas fragi, Alcaligenes metaalcaligenes.


    • Physical changes [hole formation, change nature of texture]

    • Chemical changes [undesirable end product, metal discoloration]

    • Gas holes/eyes/cracks/splitting [Clostridium-butyric acid+gas]

    • Undesirable acid [Propionibacterium sp.]

    • Bitterness [Streptococci]

    • Acid + proteolytic [coliforms, Micrococci]

    • Yeast flavor/sweet fruity flavor [yeast]

    • Putrefaction [Clostridium tyrobutyricum, Cl.lentoputrescens, Cl.sporogenes]

    • Discoloration ;

Eg: rusty spots  Lactobacillus plantarum, L.brewis

Yellow/pink/brown Propionibacterium

Reddish brown to grayish brown due to oxidation of tyrosine by bacteria.



  • Dairy mold  G.lactis

  • Red colour G.rubrum/G.crustacea

  • Red spot G.aurianticum

  • C
    4.BLACK SPOT/OFF FLAVOR  Monilia sp.,/M.nigra

    5. DISCOLOURATION  Aspergillus/Mucor/

    Alternaria/ Scopulariopsis

    6.Yellow /red growth Brevibacterium linens

    heese cancer  G.caseocorans


  • Dark green to black colour  C.herbarum


  • Green sporesP.puberulum

  • Yellowish brown spot  P.casei

  • Camembert discolouration  P.aurantiovirens

22.Write a brief note on Yoghurt Production.
Yoghurt [Bulgarian milk].

Yoghurt is the fermented milk product characterized by its viscous consistency, a strong acidulous taste due to high acidity [pH 4.6] and a distinct aroma caused mainly by acetaldehyde. Large-scale manufacture only started in the UK in the 1960s but since then yoghurt has become an increasingly important dairy product with many different varieties now available in supermarkets and other retail outlets.


1. Bloom cartons/frothy consistency and yeasty off flavor, odour yeast ferments sugar into CO2 and ethanol.

2. Mould growth is less but spoils the surface of yoghurt particularly in under filled cartons.


  1. Sterilization of filling equipment.

  2. Careful storage of packaging.

  3. Installation of filtered air laminar and airflow facilities in filling rooms.

  4. Use of UV in filling areas.

  5. Periodic fumigation of filling rooms.

  6. Control of spillages.

  7. Use of sulphate in fruit.

  8. Heat treatment of final product.

  9. Use of preservative in the final product.

  10. Proper use of fruit and fruit syrups to prevent contamination.

Whole milk, Skim milk + water, Whole milk + cream

Pasteurization [850c – 30 mins batch process,90 - 950 c – 10minutes continuous ] inhibits Salmonella, Listeria, Camphylobacter.

Homogenize [60 – 650c] – smooth texture

Emulsifier’s addition [agar, gums, alginate to increase the viscosity]

Sweeteners addition [5% sugar inhibit lactic acid production].

Heat [90 – 950c] & cool

Inoculate with starter [Strep. salivarius ssp thermophilus, Lactobacillus delbreukii ssp bulgaricus]

Incubate [4 – 16 hrs at 30 – 450c] & Cool [ 10 – 150c]

Add fruit and flavor

Package [Maintain at chill temperature at 4.50c – 2 wks].

Recently, a different type of yoghurt has been produced that uses a mixture of;

L.acidophilus+Bifidobacterium bifidum AB yoghurt

L.acidophilus+Bifidobacterium bifidum+S.salivarius thermophilus ABT yoghurt

These bio or therapeutic yoghurt are said to have health promoting properties. Manufacture of this type of yoghurt involves direct vat inoculation with the starter followed by incubation at 370 c for about 16 hrs giving a final product with a pH of 4.2 to 4.4 and a milder creamier flavor.

Nutritive value of yoghurt:

During fermentation of milk the composition of minerals remain unchanged while proteins, carbohydrates, vitamins and fats to some extent are subjected to changes. The substances formed are lactic acid, alcohol, CO2 ,antibiotics and vitamins. The following processes make yoghurt


Proteolysis in milk takes place by exopeptidases and endopeptidases of lactic acid bacteria. So biological value increases 85.4 to 90%. This increases due to breakdown of proteins into peptides,

amino acids. The contents of essential amino acid such as leucine, isoleucine, methionine, phenyl alanine, tyrosine, tryptophan and valine increases which offers special advantage.

2.Hydrolysis of lactose:

Lactose in milk is hydrolysed by metabolic activity of bacteria. Lactic acid inhibits the growth of putrifactants. It is important for organolectic properties and calcium absorption.


The homogenization process reduces the size of globules which become digestible, as a result of lipolytic activity the free fatty acid increases, which have some physiological effect.

4.Changes in vitamins:

There is more than 2 fold increase in vitamins of B group especially thiamine, riboflavin and nicotinamide.

5. Antibacterial activities:

The antibiotic properties are associated with Lactobacilli in yoghurt and materials responsible are lactic acid, H2O2 and lactobacilline.
6.Therapeutic properties:

  1. Easy absorption and better assimilation. Eg; milk [32% in 1 hr], yoghurt [91% in 1 hr].

  2. Improves appetite due to its pleasant refreshing and pungent taste. It is highly nourishing invigorating.

  3. Gastric juice secreted by the action of yoghurt and desirable ratio of calcium and phosphorous induced by it leads to a high digestive capacity.

  4. Removes excessive fat from liver and enhances bile secretion. It has therapeutic importance in GI disturbances hepatitis, nephritis, diarrhea, colitis, anemia, and anorexia.

  5. It provides relief to chronic diarrhea in spruce and ulcerative colitis. Fat free yoghurt is importance to those who suffer from heart diseases.

  6. Yoghurt possesses potent anti-tumour activity. Pathogenic bacteria are not able to survive due to low pH.

23.Write a brief note on Saurkraut Production.

Fermented Vegetables


The sauerkraut fermentation is an example of a microbial succession. Microbial succession involves the growth of a group or species of micro organisms in an environment, the conditions of which then change as a result of their activities so that another group or species is favoured and becomes dominant. The microbial succession involved in the fermentation of sauerkraut can pass through 3 phases.

Phase 1:

  1. Leuconostoc mesenteroides initiates the fermentation. The organism is heterofermentative, converting sugars in the brine into lactic acid, acetic acid, ethanol and Co2.

Sugars L.mesenteroides L.A + A.A + Ethanol + Co2

  1. The role of this organism in the fermentation is complex and fundamental to the production of good quality sauerkraut.

    1. Rapid reduction of pH [below 4.0] within 2 days due to fermentation of lactic and acetic acids. This reduces and inhibits the bacteria other than lactic acid bacteria that may cause the cabbage to putrefy and enzymes that may cause the cabbage to soften.

    2. Co2 production helps to purge O2, from the brine. This creates an anaerobic condition which is important in restricting the growth of organisms other than lactic acid bacteria. Co2 will also inhibit the growth of some G-ve bacteria and stimulate the growth of other lactic acid bacteria that form part of the fermentation flora.

    3. The anaerobic conditions produced stabilize vitamin C in the cabbage so that a large percentage of the vitamin present in the raw material is retained.

    4. Reducing sugars produced from the breakdown of excess sucrose in the brine can cause the product to darken by combining with amino acids present [Maillard Browning] Leuconostoc prevents this process by converting fructose to mannitol and glucose to dextran. Both are available as a carbohydrate source to other lactic acid bacteria and although the dextran produces a slime, this is only temporary.

    5. Leuconostoc may produce growth factors that help to stimulate the growth of more fastidious lactic acid bacteria.

    6. Leuconostoc contributes in a major way to the final flavour and aroma of the finished product.

Early in this phase [the 1st 15 hrs] there is also some growth of gram-negative organisms. These organisms, mainly coliforms, help to remove oxygen from the brine and disappear within a day or two.

Phase II:

As lactic acid accumulates in the brine and the pH drops, the more acid-tolerant Lactobacillus brevis and Lactobacillus plantarum start to increase in numbers. Both organisms produce lactic acid [L.brevis is heterofermentative and L.plantarum is homofermentative] and after about 6-8 days become the dominant flora.

Phase III:

After about 16-18 days the numbers of L.brevis decline and the population becomes dominated by L.plantarum. The organism continues to ferment any residual sugars to produce lactic acid and a fully stable product in which all the sugars have been fermented.

The final sauerkraut has a stable pH of 3.8 and contains 1.7 to 2.3% acid [calculated as lactic acid] with a ratio of acetic : lactic acid of about 1 : 4. Diacetyl, acetaldehyde and a number of esters have been identified in the final product, which contribute to its characteristic odour and flavour.

Microbiological Problems:

1. High Temperature:

At high temperatures of 32 C and above growth of Leuconostoc mensenteroides is prevented and the population becomes dominated by Lactobacillus plantarum and pediococcus pentosaceous. Both organisms are homofermentative, their growth resulting in product that darknes readily and has a poor flavour.

2. Aerobic conditions:

Aerobic conditions produced when the fermenting cabbage is not covered properly or air pockets are allowed to form when the cabbage is packed into vats, will allow the growth of yeasts and moulds. Discolourations E.g., the pink colour is due to growth of the yeast Rhodotorula, off flavours [yeasty or mouldy flavour] and softening due to pectinolytic activity of moulds are resulting defects.

3. Uneven or Low salt Concentrations:

Uneven or low salt concentrations may allow putrefactive bacteria to grow, resulting in a spoiled product.

Sauerkraut Defects and Spoilage:

Sauerkraut may be inferior quality because of abnormal fermentation and excessively high T inhibits the growth of Leuconostoc and consequently the flavour production. It may permit the growth of pediococcus cerevisiae and development of undesirable flavours. Low T may prevent lactic bacteria and encourages the growth of containments from soil. E.g., Enterobacter and flavobacterium. The long fermentation may favour the growth of L.brevis which yields a sharply acid flavour. Too much salt may encourage Pediococcus and Yeast. Abnormal fermentation of cabbage may result in cheese like odour caused by propionic butyric, caproic and valeric acid along with isobutyric and isovaleric acid.

1. Soft Kraut:

It may result from a faulty fermentation and from exposure to air or excessive pressing and / or tamping.

2. Dark Brown / Black Kraut:

It is due to oxidation during exposure to air and is caused by combined action of plant enzyme and microorganisms destruction of acid by film yeast and molds make cardition favourable for proteolytic and pectolylic microorganisms to rot the kraut. Darkening is encouraged by uneven salting and high T. Brown colour may result from iron in hoops and tanning from barrels.

3. Pink Kraut:

It is caused by red asporogenous yeast in the presence of air and high salt that has been distributed unevenly. The development of pink colour is favoured by high T, dirty vats, low acidity and iron salts.

4. Ropy Krauts / Slimy Krauts:

It is caused by encapsulated varieties of L.plantarum. The sliminess may disappear on longer holding and cooking of the kraut.

Sauerkraut is subjected to spoilage at its surface, where it is exposed to air. Film yeasts and molds destroy the acidity permitting other microorganisms to grow and cause softening.

5. Salt Burn:

If salt concentration increases the surface may be darken to black. [high salt and high T]


Cucumber pickles may be prepared without fermentation or partial or complete fermentation. They can be pasteurized to improve their keeping quality. Brined acidified cucumbers are heated so that the interior of the cucumber will be maintained at 73.9 C for atleast 15 min. Both heating and cooling should be rapid. These are 2 chief types of fermented pickles;

  1. Salt or salt stock pickles b. Dill pickles.

I. Preparation of Salt or Salt Stock Pickles:

Immature cucumber are washed, placed in barrels or tanks and brined. Sometimes about 1% of glucose is added if the cucumbers are low in sugar. The addition of sugar will favour the production of gassy pickles or bloaters.

Addition of Salt:

The rate of addition of salt and total amount added varies considerably 2 methods of salting, low salt method and high salt method.

High salt method  50 salometer [10.5% NaCl] and final 60 salometer [15% NaCl]

Low salt method  30 salometer [8% NaCl] and final 45 salometer

The cucumbers are keyed down under a surface layer of brine and fermentation begins. In both methods salt is added at weakly intervals to increase the salometer reading by about 3 salometer up to 60. In the low salt method the increase is about 2 per week up to 50. In warm climates the salt content of brine may be increased more rapidly and cool climates a weaker brine may be added initially.

1. The Traditional Fermentation:

The traditional process usually takes 6-9 weeks for completion, depending on he salting method and T employed. The number of salt tolerant sp. Of bacteria may grow initially in the newly brined fresh cucumbers. These may be marked difference in the kinds of bacteria growing in different lots, depending on the number of and kinds introduced by the cucumbers or dirt left on them and by the water of the brine, initial concentration of sodium chloride and rate of increase, and the T of the brined cucumbers. Low salt concentration will favour more kinds of bacteria, faster the acid production and greater the acidity. First to grow is Pseudomonas and Flavobacterium, types considered undesirable. Bacillus sp are likely to come on from the soils on the cucumber and their growth is undesirable. In brines of low salt content coliform bacteria, Leuconostoc mesenteroides, str. Faecatis, Pediococcus cerevisiae may grow and form acid and in 15% brines gas forming cocci may produce some acid. Later L.brevis may contribute to acidity if the salt concentration is not too high. L.plantarum developes acidity in both low and high salt brines. It becomes decreasingly active as the salt concentation increased. The total lactic acid content is 0.6 to 0.8%. Heterofermentative lactics yield pickles that are firm and have better density than homofermentative lactics.

Yeast may grow after some acid has been formed by the bacteria. Two types,

1. Film / Oxidative yeast:

Which grows on the surface of the brine and destroy lactic acid by oxidation. E.g., Debaryomyces, Endomycopsis, Candida.

The control includes daily agitation of the surface or the addition of the mineral oil, sorbic acid or other substances. Pickle vats are located out in the sunlight which inhibits surface growth on the brine.

2. Fermentative Yeast:

Which grows down in the brine and ferment sugar to alcohol and Co2. E.g., Torulopsis, Zygosaccharomyces, Hansenula.

Gas produced by these yeast, bubbles from the brine and may be responsible for bloated pickles.

When the cucumber are first brined they are chalky white and opaque but during the fermentation and cure the colour changes from bright green to yellowish green and the flesh becomes increasingly translucent. The salt pickles are prepared for use in making special products such as sour, sweet sour, mixed pickles, relishes or other products.

2. The controlled fermentation:

This process is designed to eliminate or minimize the defects of the traditional fermentation. First the cucumbers are washed, brined and sanitized [cl – 80 ppm] in the vat. The chlorimated brine is then acidified with glacial acetic acid. These 2 process suppers the growth of undesired bacteria. Following a purge with N2, sodium acetate is added [0.5%] to buffer the brine. This ensures effective utilization of all the fermentable carbohydrate present. After 10-24 hrs they are inoculated with special cultures of pediococcus cerevisiae and L.plantarum. During the active fermentation [10-14 days] N2 purges are repeated and additional salt is added to maintain 25 salometer.

II Preparation of Dill pickles:

  1. They are named because of the addition of dill herb and spices.

  2. They may be unfermented or fermented or made from salt stock.

  3. The fermentation to produce dill pickles have a lower concentration of salt and brine is acidified with vinegar at the start. The low salt content favours and increased rate of acid production, but adds to the risk of undesirable microbial changes. The flavouring materials dill, spices, garlic etc., also act as a source of undesirable micro organisms treated spices containing low micro organisms are available 2 types of fermented dill pickles

    1. Overnight b. Genuine









Slow acid fermentation [20 sal]
Weak acidified brine

Cured weed of dill added

Lactic acid  0.3 to 0.6%

Salt concn  5.3%

Kept in cold

Org  L.mesenteroides Str.faecalis, P.cerevisiae, L.plantarum

Vinegar added

T 15-30 C

Acid  1-1.5%

Salt concn initial  7.5 to 8.5%

Final concn  3.4 to 4.5%

Defects and Spoilage:

Fermented pickles are subjected to a number of defects most of which are caused by bacteria.

1. Hollow Pickles:

If cucumbers are allowed to stand for a while after harvesting and before fermenting or it may be due to loose packing in vat, insufficient weighting, too rapid a fermentation and too strong or too weak a brine cause hollow pickles.

2. Floaters or Bloaters:

It may result from gas being formed by yeast, L.plantarum or coliform which can produce Co2. This can be controlled by purging the brine with N2 to remove dissolved Co2. Floaters are favoured by thick skin that doesnot allow gas to diffuse out, by rapid gas production during fermentation, high initial salt by added sugar or / and acid.

3. Slippery pickles:

It occurs when cucumbers are exposed to air permitting the growth of encapsulated bacteria. Slipperiness also may be due to broken scums of film yeast that have grown on the surface of the brine and dropped on to the cucumber.

4. Soft Pickles:

They are made so by pectolytic enzymes mostly from molds and from cucumber flowers. These molds are mostly of the general Penicillium, Fusarium, Cladosporium, Alternaria.

Bacteria  Bacillus, Aeromonas, Coliforms.

These can degrade;

Pectinase  Pectin

Pectin Esterase  Pectinic acid

Poly methyl galacturnose  Galactournoic acid

Softening is favoured by;

  1. An insufficient amount of salt. b. Too high a T

c. Low acidity

  1. Presence of air favouring the growth of film yeast or mold.

  2. Infusions of may blossoms.

5. Black Pickles:

May owe their colour due to the formation of H2S by bacteria and combination with iron in the water to yield black ferrous sulfide. It is also due to the growth of black pigmented Bacillus nigrificans and B.subtilis.

i.e., Iron  Water

Sulphide  From Vat / H2O / CaCo3 [Gypsum]

6. Ropy Pickle Brine:

    • Favoured by unidentified motile, Gram –ve encapsulated rods.

    • Favoured by

      1. Low salt

      2. Low acid

      3. High


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