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.

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.

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.

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.

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

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

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

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

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

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.

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.

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.

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.

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 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

Bacteria

Yeasts

Moulds

Bacteriophages

Biochemical activities

Temperature response

Ability to cause infection and disease

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.

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

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

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.

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

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.

REVATHI.S

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