Study of Milk and Microorganisms ( Saugat Bhattacharjee )


 Milk is a white liquid produced by the mammary glands of mammals.

 Primary source of nutrition for young mammals before they are able to digest other types of food.

 Throughout the world, there are more than 6 billion consumers of milk and milk products.

Physical & Chemical properties

 Milk is a white emulsion or colloid of butter fat globules

 Presence of fat globules and the smaller casein micelles provide the opacity to the milk.

 Yellow-orange carotene imparts the creamy yellow colour to a glass of milk.

 It contains many other nutrients.

 Milk contains 30–35 grams of protein per liter of which about 80% is arranged in casein micelles.

 Different carbohydrate including lactose, glucose, galactose, and other oligosaccharides.

 Calcium, phosphate, magnesium, sodium, potassium, citrate, and chlorine are all included.

 Vitamins A, B6, B12, C, D, K, E, thiamine, niacin, biotin, riboflavin, folates, and pantothenic acid are all present in milk.

Microorganisms in milk

 Milk is sterile at secretion in the udder but is contaminated by bacteria even before it leaves the udder.

 Further infection of the milk by microorganisms can take place during milking, handling, storage, and other pre-processing activities.

Milk supports the growth of a variety of bacteria including pathogenic one.

 Acid-forming bacteria, such as Streptococcus lactis, Str. faecalis Lactobacilli. These ferment lactose, forming lactic acid, and lead to the formation of curd.

 Alkali-forming bacteria, such as Alkaligenes sp. Achromobacter. Aerobic spore-forming bacilli These render the milk alkaline.

 Gas-forming bacteria, such as Coliform bacteria Cl. peifringens Cl. Butyricum. These produce acid and gas.

 Proteolytic bacteria, such as Bacillus subtilis, B. cereus, Proteus vulgaris, Staphylococci Micrococci. These bacteria are responsible for proteolytic activity.

 Inert bacteria, such as Achromobacter do not produce any visible change.

Lactic Acid Bacteria

 lactococci

 L. delbrueckii subsp. lactis (Streptococcus lactis )

 Lactococcus lactis subsp. cremoris (Streptococcus cremoris )

 lactobacilli

 Lactobacillus casei

 L.delbrueckii subsp. lactis (L. lactis )

 L. delbrueckii subsp. bulgaricus

 (Lactobacillus bulgaricus )


 Coliforms: coliforms are facultative anaerobes with an optimum growth at 37° C.

 Coliforms are indicator organisms; they are closely associated with the presence of pathogens but not necessarily pathogenic themselves.

 They also can cause rapid spoilage of milk because they are able to ferment lactose with the production of acid and gas, and are able to degrade milk proteins.

 They are killed by HTST treatment, therefore, their presence after treatment is indicative of contamination.

 Escherichia coli is an example belonging to this group.

Pathogenic Microorganisms in Milk

 The following bacterial pathogens are still of concern today in raw milk and other dairy products:

 Bacillus cereus

 Listeria monocytogenes

 Yersinia enterocolitica

 Salmonella spp.

 Escherichia coli O157:H7

 Campylobacter jejuni

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