The document summarizes microbial population dynamics in milk. It discusses that the initial microflora of good quality milk consists of non-thermoduric micrococci and streptococci. Pathogens predominate in mastitis milk. Psychrotrophic bacteria originate from water and milking equipment and include Pseudomonas species. Lactic acid bacteria grow when milk is not refrigerated, producing acid that inhibits other microorganisms. Pasteurization reduces microorganisms and the surviving microflora consists mainly of spore-forming bacteria and lacto bacilli. Changes in milking systems have caused the microflora to change from gram-positive to gram-negative psychrotrophic microorganisms like P

1. MICROBIAL POPULATION
DYNAMICS
IN MILK
By
Dr.J.Roopavathy
Assistant Professor
Department of Zoology

2.  Microbial population in milk are continually changing and
interacting with the milk components and with each other
 The initial microbial population of raw milk is affected by
factors such as equipment and animal cleanliness, season,
feed and cow health
 The initial microflora of good quality milk consists mainly
of organisms found within the udder and of the normal
teat skin flora.
 These microorganisms include non thermoduric
micrococci and streptococci, which do not grow well at
low temperatures
 Pathogens will predominate in milk from mastitis cows

3.  Seasonal factors include feed source and housing,
both of which influence the level of bacterial spores
in milk
 Psychrotrophic bacteria are at low or even
undetectable levels in the best quality fresh raw milk
 They originate from the water and milking equipment
 After collection, milk is cooled, which initiates a
change in microbial population from Gram –Positive
rods and cocci, which do not grow, to gram negative
which can grow slowly at low temperatures
 These Gram negative rods are primarily
Pseudomonas species

4.  When raw milk is not kept cold, lactic acid bacteria
will grow
 The resulting increase in acidity will inhibit growth
of many other microorganisms including spore
forming Bacillus and Clostridium sp, and Gram –
negative Pseudomonas species and colifoms
 Growth of lactic acid bacteria produce bacteriaocins,
which inhibit other lactic acid bacteria as well as
some pathogens such as Listeria and E.coli
 Both the lactic acid bacteria and the psychrotrophic
bacteria that grow in raw milk are heat sensitive, so
when milk is pasteurized, microorganisms that have
previously grown in the product are greatly reduced
in number

5.  The surviving microflora consists mainly of
spore forming bacteria, micrococci and lacto
bacilli.
 Pasteurized milk that is not recontaminated with
the microorganisms will often keep at
refrigeration temperature for 3 weeks or longer,
because the surviving microorganisms grow
slowly or not at all in the cold milk
 Spoilage may occur more rapidly if significant
amounts psychrotrophic B. cereus are present in
the raw milk
 Most pasteurized milk is recontaminated with
low numbers of psychrotrophic bacteria resulting
in shorter shelf life

6.  Development of closed milking systems, use
of bulk tanks to store and transport raw milk
and improvement in refrigeration systems
have caused the micro flora naturally present
in raw milk to change from predominantly
gram – positive, acid producing bacteria to
gram – negative psychrotrophic
microorganisms chiefly species of genus
pseudomonas

7.  Microorganism found in milk can also be
described according to their optimum
temperature for growth and heat resistance
 This is a very practical consideration since milk
is preserved by employing low temperatures to
prevent changes due to microbial activity and by
high temperatures to reduce microbial
population and destroy pathogens. All the four
types of microorganisms i.e.,
Mesophilic
Thermoduric
Thermophilic
Psychrophilic

8.  Growing optimally at 30-32oc are called as
Mesophilic
 The most important mesophilic bacteria are
streptococci, lactobacilli and coliforms,
 which produce acid and gas and off flavours.
 Other microorganisms in this group are
Micrococci, Corynebacterium spp, Mycobacterium
spp, yeasts and moulds etc.
 They are killed in the pasteurization

9.  Thermoduric microorganisms are those which
survive pasteurization conditions but do not
grow at pasteurization temperature
 Most of them do not multiply well at ambient
temperature
 The most common thermoduric bacteria are
found in the genera microbacterium,
Corynebacteriaum, Micrococcus, streptococcus
and Bacillus
 Excessive number of the thermoduric bacteria in
milk make it difficult to meet the grading
standard

10.  Thermophilic bacteria grow well at the temperature
used in pasteurization, specially when the low
temperature holding method is followed
 These bacteria can grow at 55°c and most of them
have an upper limit of growth at about 70°c
 Although some facultative thermophiles may grow at
37°c or lower in standard plate count analysis, the
standard method of enumerating this group of
organisms is to incubate agar plates at 55°c
 Most themophilic forms are found in two genera,
Bacillus and Clostridium

11.  Psychrophilic bacteria are those which are able to grow at
refrigeration temperatures regardless of their optimum growth
temperature
 Usually the term psychrophilic is used for organisms that have an
optimum temperature below 20°c
 In the psychrophilic microflora of milk and milk products, very
few are classical psychrophiles and majority of them are
mesophilic in nature
 They produce a wide variety of spoilage defects
 The defects may result in the production of many off flavours
and odours
 The most commonly encountered psychrophilic bacteria are
members of the genera pseudomonas, Achromobacter, Vibrio,
Flavobacterium and Alcaligenes
 They are killed in the pasteurization process, but are sometimes
found in pasteurized milk
 The contamination takes place after pasteurization from
equipment, cans , bottles and water

12.  The principal milk processing operation include
separation and clarification, homogenization,
pasteurization, sterilization, chilling and
packaging
 Contamination with microorganisms is minimal
to many thousands, depending upon whether
the equipment is properly cleaned and sanitized
 Vigorous agitation, such as with the use of
pumps, may increase the number of
microorganisms in milk by disrupting bacterial
clumps into individual cells, each of which can
give rise to a colony in plate count procedure

13.  Generally, water supplies do not contain sufficient chlorine
to destroy the gram – negative bacteria.
 Therefore, equipment washed with such water must be
sanitized before use to control these bacteria
 During separation, many microorganisms are physically
removed from milk and deposited along with somatic
cells and other particulate matter in the slime that
contains large number of microorganisms
 The heat treatment of milk is the key unit operation to
control the microbial quality of milk
 The scientific basis of the specific conditions used for heat
treating milk and milk products is derived from the
concepts of microbial thermal death times in which the
destruction of the organisms is assumed to be dependent
on a linear, semilogarithmic relationship of temperature to
time

14.  The overall objectives of the heat treatments applied to
market milks are to a) eliminate or reduce the risk of
disease associated with pathogens common to raw milk
and b) Extend the shelf life of milk by inactivation of
potential spoilage organisms and degradative enzymes
The specific objectives are to ensure that all particles of
milk are heated to the desired temperature for the desired
time and to avoid significantly damaging the flavour or other
quality characteristics of milk
Rapid cooling after heat process is also essential to minimize
product heat damage and to control the growth of
microorganisms that may survive the heat process
In general, the higher the heat treatment, the greater the
efficiency of microbial destruction, but also the greater the
chance that the quality of milk will be affected

15. It does not restrict ourselves to the bacteriological but also physical and
chemical quality of raw milk before it reaches even the first chilling station
As per the definition of adulteration the presence of residues of pesticides,
heavy metals, antibiotics, aflatoxins, drugs, hormones, etc in excess
quantities indicates adulteration
As these residues are not inherent to milk so directed measures should be
employed so that these residues can be eliminated and even if they are
present, their level does not exceed the established standards
Insecticide
Pesticide
Antibiotics
Industrial Chemicals
Aflatoxins
Physical Contaminant

16.  Insecticide/Pesticide:
Insecticide/Pesticides are the chemicals which are used to kill
insects / pest to save the crop.
When these chemicals are used irrationally could prove injurious to
the health of the consumers
Dairies should have infrastructure like gas liquid chromatographs
having electron capture detector to detect organo – chlorine
pesticides like DDT, aldrin, dialdrin, HCN, BHC, etc
The standard laid down by the PFA for pesticide residues in milk
and milk products should be adhered to
This is only possible if we have the commitment and infrastructure
to check the residue levels.
Dairies are not adding or removing pesticides, the message has to
flow to the farmers about the uses and their harmful effects if used
in excess

17.  The use of certain pesticides have been
prohibited because the residues of the chemicals
may pass into the milk
 Likewise, certain sprays for crops which will later
be eaten by milch animals have been prohibited
or their use permitted only at certain periods of
development because they may be recovered in
the milk supply
 During 1960s it was reported that milk
frequently contained pesticides, however, their
levels were well below the safety limits
 However, some reports in India have indicated
that the residues of the pesticides do sometimes
exceed the permissible limit prescribed by PFA

18.  In recent year, the dairy industry has come to use
considerable quantities of antibiotics in the treatment of
animal diseases, notably mastitis
 When such treatments are made, the milk from the treated
animal is not supposed to be used for human
consumption for at least 72 hours following the treatment
 If such milk is added to milk from untreated animals
before the 72 hours period is completed, residual
antibiotics may be detected in the entire supply
 These antibiotics may enter the milk or other dairy
products has serious health implication and hence they
must be used only as specified
 Penicillin apparently has greater influence on public
health then other antibiotics in common use and thus has
held the main interest in developing test procedures for
residual antibiotics

19.  Toxic industrial chemicals are a source of environmental contamination
and also source of serious exposure to certain works places
 Heavy metals, for example, lead, mercury, cadmium, arsenic, etc have
been reported in milk and milk products
 These environmental contaminants are harmful/toxic if they are ingested
in higher concentration than what is prescribed by PFA act
 Lead and mercury are most frequently studied because they have been
the cause of infant poisoning through milk.
 Lead is the single most common heavy metal pollutant of environment
 In general, urban dwellers are at greater risk than rural residents
 Higher values of lead in milk of urban population compared to rural
ones, and higher values in colostrum compared to mature milk, have
been reported
 At world level, concerns are being expressed for their harmful effects
and their prescribed limits are being lowered frequently in order to
ensure that the food available to the consumers is safe

20.  Aflatoxins are fungal metabolities found as
toxic contaminants of foods
 Aflatoxins are produced by the common
moulds, Aspergillus flavus and A. parasiticus
by virtue of their ability to produce aflatoxin
 These can be secreted in milk by the milch
animal which has been fed with aflatoxin
containing feeds

21.  Physical contaminant/hazards found in milk and
milk products could be metal fragments, glass,
wood splinters, stones, dirt, plastics, insects, etc
 The presence of these can cut the mouth, break
teeth, cause choking or perforation in the
alimentary tract
 If proper care is taken during production,
transportation, processing and handling these
physical contamination can be avoided

22. Thank you
Reference
Vol.1MilkandMilkProcessing(ShivashrayaSingh)