Microbiology of Milk

1. College of Health Sciences
Dep. of Medical Laboratories
Food Microbiology Theory
3rd stage
Lecture 5
Dr: Shameeran S. Ismael
BVM & S, M.Sc Medical Microbiology(Parasitology),
PhD Molecular Parasitology

2. Microorganisms in Milk:
• Milk is an excellent culture medium for the
growth and multiplication of microorganisms
• 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

3. • Nearly all the changes that take place in the
flavor and appearance of the milk after it is
drawn form the animal, are the result of the
activities of microorganisms ,therefore, it is
very essential to control these microorganism

4. Origin of microorganism in milk
• Commensal micro flora- teat skin, epithelial
lining of the teat canal, duct that conveys the milk
from the mammary gland to the teat orifice:
Staphylococcus, Streptococcus, Bacillus,
Micrococcus, Corynebacterium, coliforms
• Environmental contamination- soil, water
equipment, dairy farm area are reservoir for many
food borne pathogens (Salmonella spp. and
Listeria monocytogenes)

5. • Diseased animal
Mastitis- Staphylococcus (S.agalactiae) and
Streptococcus species

6. • There are many microorganism found in milk
including: Bacteria, yeasts, moulds and
bacteriophages are commonly encountered.
• Viruses and protozoa are seldom observed in milk,
except as occasional contaminants.
1.Bacteria:Most common and most numerous of
microorganisms found in milk and milk products .

7. • They belong to four main groups:
1. Gram +ive cocci
2. Gram +ive non-spore forming rods
3. Gram +ive spore-forming rods
4. Gram -ive non-spore forming rods.
Normally present in milk and they are also used
as starter culture for the production of cultured
dairy products. They ferment lactose and yield
lactic acid.

8. 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.

9. • Gas-forming bacteria, such as Coliform
bacteria Clostridum peifringens, Clostridum
butyricum. These produce acid and gas.
• Proteolytic bacteria, such as Bacillus subtilis,
Bacillus cereus, Proteus vulgaris,
Staphylococci micrococci. These bacteria are
responsible for proteolytic activity.
• Inert bacteria, such as Achromobacter do not
produce any visible change

10. 2.Yeast:
• Most frequently encountered in raw cream during
hot weather produce acid and Co2. They are
potential contaminants throughout the year.

11. 3.Moulds:
Their growth is visible as a fuzzy or fluffy
growth on the surface of milk and milk
products.
• They may be black, green, grey, blue or white.
• They discolor the product and often produce
repulsive undesirable off odors
• Essential in production of certain varieties of
cheese.

12. 4.Bacteriophages :
• Particularly obnoxious in starter cultures used for
the production of cultured dairy products.
• Phages kill bacterial culture and entire
fermentation process fails (slow or dead vat).

13. Factors affecting Microbial growth
in milk:
There are two types of factors:
1. Intrinsic Parameters (inside the milk)
Factors inherent to the food. They are chemical and
physical characteristics of food such as:
• pH
• Moisture
• Oxidation-Reduction Potential Inside Food
• Nutrient Content
• Natural Antimicrobial Constituents
• Biological Structures & Natural Microflora

14. 2. Extrinsic Parameters (environment around
the milk)
Storage conditions of the food i.e. properties of
the environment in which the food is stored
including:
• Temperature
• Relative Humidity
• Presence of Gases or Oxygen
• Antimicrobials or Added Microorganisms

15. Significance of
Microorganisms in Milk
• Microbial content serves as an indicator of
production conditions and sanitary quality of
milk.
• Prevention of spoilage.
• Prevention of milk borne illnesses.
• Production of dairy products with desired
characteristics imparted via m.o. introduction

16. Products of Microbial Growth in Milk:
• Enzymes
• Decomposition products of protein, fat & CHO
etc.
• Pigments , Toxins etc (mycotoxins & bacterial
toxin )

17. Result of microbial growth in milk (Spoilage)
o Principal cause are Psychrotrophs
o Most of these are destroyed by pasteurization
o Some may survive e.g. Pseudomonas fluorescens ,
Pseudomonas fragi
o Other species and strains that survive pasteurization and grow at
Refrigeration temp. ,Produce heat stable proteolytic & lipolytic
enzymes and cause spoilage : ~Bacillus ~ Clostridium ~
Cornebacterium ~Arthrobacter ~ Lactobacillus ~
Microbacterium ~ Micococcus
~ Streptococcus

18. Spoilage of Milk and dairy products:
1.Changes in Milk Fat
2.Alkali Production
3. Color Changes:
•Yellow milk (Ps. Synxantha, Flavobacterium)
•Red milk (Serratia marcesans, Torula glutinis)
•Brown milk ( Ps. Putrfaciens)
•Blue milk (Ps. syncyannea)

19. 4. Flavor Changes:
• Sour or acid flavor: Clean acid flavor, Aromatic
acid flavor, Sharp acid flavor
• Bitter flavor- Proteolysis of casein (Bacillus sp.,
Clostridium)
•Burnt/Caramel flavor (burnt milk flavor-S. lactis
var. maltigenes)

20. 5.Gas production: accompanied by acid
formation- mainly by coliform bacteria,
Clostridium and gas-forming Bacillus sp. yield H2
and CO2.
6. Acid formers killed at pasteurization temps.-
however spores of clostridium and bacillus may
and cause spoilage of pasteurized
milk.

21. 7. Proteolysis: hydrolysis of milk proteins by m/o
accompanied by production of bitter flavor (due to
peptide released).
Proteolysis is favored by –storage at low temps,
destruction of lactics and other acid formers by heat,
destruction of formed acid in milk by molds and
yeasts
ex: Micrococcus, Akaligenes, Pseudomonas,
Proteus, Flavobacterium (non sporeformers);
Bacillus and Clostridium sp (spore formers).

23. Spoilage of Raw Milk:
•The temperature of freshly drawn milk is about
38°C.
•Milk sours rapidly if held at these temperatures.
•Some inhibitory substances (lactoperoxidase and
agglutinins) are present in freshly drawn milk but
soon become comparatively ineffective.

24. •Microbial spoilage of raw milk can potentially
occur from the metabolism of lactose,
proteinaceous compound, fatty acids and the
hydrolysis of triglycerides.
•Outbreaks of disease are occur due to
consumption of raw and pasteurized milk
contaminated with a variety of organisms,
including E. coli O157:H7, Salmonella spp.,
Campylobacter jejuni, Yersinia enterocolitica, and
Listeria monocytogenes.

25. •Raw milk may be a vehicle for the transmission
of Borrelia burgdorferi, the agent responsible for
Lyme disease, and it has been shown that the
organism can survive for at least 46 days in milk
stored at 5◦C.
•E. coli O157:H7 has also been shown to be able
to survive in yogurt

26. Spoilage of Pasteurized Milk:
Spoilage may result from either the growth of
psychrotrophic thermoduric organisms that
survive pasteurisation, or post-pasteurisation
contamination by psychrotrophs.
Thermoduric spoilage: Gram-positive spore
forming, mainly Bacillus spp., Clostridium and
organisms with heat-resistant vegetative cells,
such as Micrococcus, Lactobacillus,
Enterococcus,Streptococcus, Corynebacterium
and Alcaligenes.

27. •However, at slightly higher temperatures (7 - 8
°C), B. cereus in particular may grow quite
rapidly, producing a type of spoilage known as
'bitty cream' or 'sweet curdling’, caused by the
action of lecithinase on the phospholipids in fat
globule

28. Pathogenic Microorganisms in Milk:
• Milk borne illnesses occur as a result of :
~Ingestion of raw milk
~Improper pasteurization
~Poor handling / storage leading to milk contamination
• Measures to decrease the threat :
~Hygienic production practices
~Proper handling and storage
~Pasteurization:
• LTLT : Low temp. Long time 63°C 30 minutes
• HTST: High temp. Short time72°C 15 seconds
• UHT : Ultra high temp.,130-135 °C 1 second

29. Control and Practical Application:
1. Heat - pasteurization , sterilization etc.
2. Ionizing Radiations- UV., gamma rays
3. High frequency sound waves- super & ultrasonic
4. Electricity - by the heat generated.
5. Pressure - 600 x > atmospheric pressure
6. Chemicals Acids ,alkalis, halogens H2 02 etc.