2. 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.
3. 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.
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.
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.
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.
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.
8. 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.
9. 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
10. Pasterurization
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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
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.
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.
19. 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
20. TYPES OF MICROORGANISMS IN MILK
Bacteria
Yeasts
Moulds
Bacteriophages
Biochemical activities
Temperature response
Ability to cause infection and disease
21. 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.
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.
23. 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.
24. 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.
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.
26. 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.
