Dairy microbiology


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Presentation Transcript

Slide 1:

Dairy Microbiology

Microbiology in Dairy Products:

Microbiology in Dairy Products Milk and dairy products constitute an important item of our food. These products are very suitable for microbial growth. It thus becomes necessary to know the chemistry of milk, its spoilage, method of preservation, and different dairy products where microbes play a positive rather than negative role.


cont … Milk is considered as a complete food and it contains proteins, fat , carbohydrates, minerals, vitamins and water. It is also a good medium for the growth of microorganisms. It is therefore, important to know the types of microorganisms present in milk, their control and use for beneficial purposes.

Slide 4:

Milk contains relatively few bacteria when it is secreted from the udder of an healthy animal. However, during milking operations it gets contaminated from the exterior of the upper and the adjacent  areas, dairy untensils, milking machines, the himds of the milkers from the soil and dust. In this way bacteria, yeasts and molds got into the milk and constitute the normal flora of milk. The number of contaminants added from various sources depends on the care taken to avoid contamination.

Slide 5:

The presence of these nonpathogenic organisms in milk is not serious but if these organisms multiply quickly, They can cause spoilage of milk, such as souring or putrefaction and develop undesirable odours . Control of their multiplication in milk is therefore, very essential.

Slide 6:

Milk may also contain pathogenic organisms, derived directly from the animal or from the surroundings. Microorganisms that are harmful and found in milk are Streptococcus cremoris, Pseudomonas sp., Mycobacterium spp. Serratia marcescens, enteric bacteria etc. Normally, milk is pasteurized before use. However, pasteurization does not kill all the bacteria; the survivors (thermodurics), depending on their initial number.

Slide 7:

If the initial number is high they cause rapid spoilage. It is imporant, therefore, that the milk be refrigerated at. around O°C soon after pasteurization to prevent the growth of these undersirable microorganisms.


Cont., Pasteurization, either at 145°F for 30 minutes or 161°P for 15"'30 seconds eliminates most of the pathogenic bacteria particularly Mycobacterium tuberculosis. Boiling of milk destroys all microorganisms except spore formers. Sometimes, on cooling or under improper refrigeration, spores germinate and cause spoilage of boiled milk.

Composition of Milk:

Composition of Milk Milk is a complete food, with about pH 7.0,that is an aqueous solution of proteins, fats and carbohydrates with many minerals and vitamins. The following Table 4 shows an average composition of cow milk. Component Percentage Water 87.0 Casein 2.5 Lactalbumin and other proteins 0.5 Lactose 5.0 Lipid 4.0 Sterols, Vitamins A, D, E 0.05 Miscellaneous 0.95


The process was developed by Louis Pasteur in the 1860s to eliminate bacteria in wines. The process for milk was adopted in 1895. Primary object of this process is to eliminate disease-causing bacteria from milk, though the total number of bacteria is also very much reduced during this process. It reduces the chances of milk-spoilage. Pasterurization

Holding Method Pasteurization:

Holding Method Pasteurization This is an old process, in which milk is heated in large tank at 62.9°C for 30 minutes. This method is also known as the LTLT method (low temperature, long time). To ensure uniform heating the milk is constantly stirred during the process.

Flash Method Pasteurization :

Flash Method Pasteurization This is modem method and also known as HTST (high temperature, short time) method. Raw milk is first warmed using the heat of the previously pasteurised milk. It then passes through a hot cylinder at 71.6°C for a period of 15 to 17 seconds. The milk is then cooled rapidly in part by transferring its heat to the incoming milk.

Fermented Dairy Products :

Fermented Dairy Products Many products are made through microbial fermentation of milk, including buttermilk, yogurt and many cheeses. Fermentation is primarily carried out by lactic acid bacteria. The lactic acid pathway and the accumu­lation of lactic acid from the metabolism of milk sugar, lactose are common to the production of fermented dairy products. The differences in the flavour and aroma of the various dairy products are due to additional fermentation products, that may be present in very low concentrations.

1. Buttermilk, Sour cream, Kefir and Koumis:

1. Buttermilk, Sour cream, Kefir and Koumis Different products are produced by using different strains of lactic acid bacteria as starter cultures and different fractions of whole milk as the starting substrate. Sour cream uses Streptococcus cremoris or S. lactis for producing lactic acid and Leuconostoc cremoris for characteristic flavour. Cream is starting substrate. Butter is normally made by churning cream that has been soured by lactic acid bacteria. Streptococcus cremoris or S. lactis is used to produce lactic acid rapidly and Leuconostoc citrovorum produces necessary flavors. Kefir and Koumis, popular in Europe are fermentation products of S. lactis, S. cremoris, other Lactobacillus spp and yeasts.

2. Yoghurt.:

2. Yoghurt. It is made from milk,skimmed milk or flavoured milk. For the preparation of yoghurt,the milk should be from contamination. The product can be improved by adding small amount of modified gums which bind water and impart thickening to the product.

3. Cheese. :

3. Cheese. Cheese consists of milk curds that have been separated from the liquid portion of the milk (whey). The curdling of milk is done by enzyme rennin (casein coagulase or chymosin) and lactic acid bacterial starter cultures. Cheeses are classified as soft (high, 50-80% water content), semi hard (about 45% water) and hard (a low water content, less than 40%). They are also classified as unriped if produced by single-step fermentation or ripened if additional growth is required during maturation of the cheese to achieve the desired taste, texture and aroma. Cottage and cream are soft, unripened cheese; Brie, Camembert and Limburger are soft, 1-5 months ripened cheeses; Blue, Brick, Gorgonzola, Monterey, Muenster and Roquefort are semi soft, 1-12 months ripened cheeses, whereas Cheddar and Colby are hard, 3-12 months ripened cheeses.

Slide 17:

Natural production of cheese involves lactic acid fermentation, with various mixtures of Streptococcus and Lactobacillus spp. used as starter cultures. The flavour results from use of different microbial starter cultures, varying incubation times and conditions and the inclusion or omission of secondary microbial species late in the process. Ripening involves additional enzymatic transformations after the formation of cheese curd.

Slide 18:

Swiss cheese formation involves a late propionic acid fermentation with ripening done by Propionibacteria shermanii. Various fungi are also used in the ripening of different cheeses. The unripened cheese is inoculated with fungal spores. Blue cheeses are produced by Penicillium spp. Roquefort cheese is produced by using P. roqueforti and Camembert and Brie by using P. camemberti and P. candidum.

Spoilage of Milk and Milk Products:

Spoilage of Milk and Milk Products Spoilage type Oraganisms involved Signs of spoilage Souring Lactobacillus sp. Streptococcus sp. Sour milk,Curd formation Sweet curdling Bacillus sp . Proteus sp . Micrococcus sp . Alkaline pH Curd formation Gas production Clastoridium sp. coliform bacteria Explosion of curds Ropiness Alcaligenes sp.,Klebsiella sp.,Enterobacter sp. Stringy or slimy milk Red rot Serratia marcescens Red colaration Gray rot clotridium sp. Gray colaration, Foul smell Dairy mould Penicilium sp.,Geotrichum sp. Mouldy appearance

Types of Microorganisms in Milk:

Types of Microorganisms in Milk Bacteria Yeasts Moulds Bacteriophages Biochemical activities Temperature response Ability to cause infection and disease

Milk Spoilage :

Milk Spoilage Spoilage occurs when microorganisms degrade the carbohydrates, proteins, fats of milk and produce noxious, end products. It may be seen that Lactobacillus or Streptococcus species ferment the lactose to lactic acid and acetic acids turning the mi1k sour. They may produce enough acid to curdle the protein and form sour curd.

Slide 22:

Attack of milk protein by Micrococcus, Bacillus or Proteus results into sweet curdling. There is little acid formation. If milk becomes contaminated with Gram-negative rods-of coliform group of bacteria, such as E. coli or Enterobacter aerogenes, or Clostridium sp., there is .acid and gas formation from the lactose. This stormy fermentation causes the explosion of curds. Ropiness , like bread develops from Alcaligenes, Klebsiella and Enterobacter. Serratia marcescens causes the development of a red pigment.

Milk Borne Diseases :

Milk Borne Diseases The important diseases are, tuberculosis, brucellosis and Q fever. Tuberculosis bacterium. Mycobacterium bovis is consumed in the milk and passes from human intestine to the blood, from which it spreads to most organs. Brucellosis, a blood-disease is caused by Gram-negative rod, Brucella abortus. When transmitted to man through cow milk, the bacterium infect the blood -rich organs. Q fever, caused by rickettsia, Cxiella burnetii is also a milk borne disease. Other important disorders associated with milk are primary atypical pneumonia, toxoplasmosis, anthrax, streptococcal infections etc.

Milk Microorganisms Ability to Cause Infection and Diseases :

Milk Microorganisms Ability to Cause Infection and Diseases Pathogenic organisms of both bovine and human origin have­ been isolated from milk. Milk, therefore, can serve as a carrier of diseases. Many serious epidemics were caused by the consumption of such products before this fact was clearly recognized. However, this became less common as milk sanitation has improved and pasteurization is being more widely practised.

Slide 25:

The disease organisms present in milk may be derived from., (1) diseased animals or (2) persons collecting and handling milk: Thus the danger is due to the inoculum and not to the growth of organisms in the milk. The health of animal is an important factor. Several diseases of cattle including staphylococcal and streptococcal infections, tuberculosis, brucellosis, salmonellosis, Q fever and Foot and mouth disease may be transmitted to man. The organisms causing these diseases may get into the milk either directly from the udder, or indirectly from infected body discharges, which may drop, splash, or be blown into the milk.

diseases transmitted by milk :

diseases transmitted by milk Some of the important diseases of human origin that have been transmitted by milk are (1) typhoid fever (2) diphtheria, (3) scarlet fever, (4) dysentery (5) septic sore throat and (6) poliomyelitis. It is also possible for humans to infect animals. For example, mastitis may be caused by a variety of organisms, including Staphylococcus aureus. The infecting organism, in s9me cases, has been traced to humans.

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