Contamination spoilage and preservation of milk and milk products
1. Dr. Madhuri Kaushish Lily
Associate Professor, Department of
Biotechnology & Microbiology
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2. CONTAMINATION & SPOILAGE OF MILK &
MILK PRODUCTS
Contamination of milk and milk products:
Milk contains relatively few bacteria when it leaves
udder of a healthy cow, and generally those bacteria
do not grow in milk under the usual conditions of
handling.
However, Micrococci and Streptococci have been
recovered from aseptically drawn milk.
Probably the most significant sources of
contamination are dairy utensils and milk contact
surfaces including the milk pail or milking machines,
as the case may be, strainers, milk cans or pipelines
and the bulk milk cooler.
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3. Undesirable bacteria from those sources include
Lactic, Streptococci, Coliform bacteria, Psychotrophic
gram negative rods, and thermodurics, those which
survive pasteurization eg. Micrococci,Enterococcci,
Bacilli and Brevibacteria.
In general, these bacteria grow well in milk and hence
endanger its keeping quality.
The number and types of organism in milk or other
dairy product may be increased either by
contamination or by growth of organism already
present.
Methods of production, handling, storage and
manufacture are designed to prevent both.
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4. Spoilage of Raw Milk
Raw milk contains many types of microorganisms coming
from different sources.
The average composition of cow’s milk is protein 3.2%,
carbohydrates 4.8%, lipids 3.9%, and minerals 0.9%.
Besides casein and lactalbumin, it has free amino acids,
which can provide a good N source (and some C source, if
necessary).
As the main carbohydrate is lactose, those microorganisms
with lactose-hydrolyzing enzymes (phospho-b-galactosidase
or b-galactosidase) have an advantage over those unable to
metabolize lactose.
Milk fat can be hydrolyzed by microbial lipases, with the
release of small-molecular volatile fatty acids (butyric,
capric, and caproic acids).
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5. Microbial spoilage of raw milk can potentially occur
from the metabolism of lactose, proteinaceous
compounds, fatty acids (unsaturated), and the hydrolysis
of triglycerides.
If the milk is refrigerated immediately following milking
and stored for days, the spoilage will be predominantly
caused by Gram-negative psychrotrophic rods, such as
Pseudomonas, Alcaligenes, Flavobacterium spp., and
somecoliforms. Pseudomonas and related species, being
lactose negative, metabolize proteinaceous compounds
to change the normal flavor of milk to bitter, fruity, or
unclean.
They also produce heat-stable lipases (producing rancid
flavor) and heat-stable proteinases that have important
implications. 5
6. The growth of lactose-positive coliforms produces
lactic acid, acetic acid, formic acid, CO2, and H2 (by
mixed-acid fermentation) and causes curdling,
foaming, and souring of milk.
Some Alcaligenes spp. (Alcaligenes faecalis) and
coliforms can also cause ropiness (sliminess) by
producing viscous exopolysaccharides.
However, if the raw milk is not refrigerated soon,
growth of mesophiles, such as species of
Lactococcus, Lactobacillus, Enterococcus,
Micrococcus, Bacillus, Clostridium, and coliforms,
along with Pseudomonas, Proteus, and others,
predominates.
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7. But lactose-hydrolyzing species, such as
Lactococcus spp. and Lactobacillus spp.,
generally predominate, producing enough acid to
lower the pH considerably and prevent or reduce
growth of others.
In such situations, curdling of milk and sour
flavor is the predominant spoilage.
If other microorganisms also grow, gas formation,
proteolysis, and lipolysis become evident.
Yeast and mold growth, under normal conditions,
is generally not expected.
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8. Spoilage of Pasteurized Milk
Raw milk is pasteurized before it is sold for
consumption as liquid milk.
Thermoduric bacteria (Micrococcus, Enterococcus,
some Lactobacillus, Streptococcus,
Corynebacterium,and spores of Bacillus and
Clostridium) survive the process.
In addition, coliforms, Pseudomonas, Alcaligenes,
Flavobacterium, and similar types can enteras post-
pasteurization contaminants.
Pasteurized milk, under refrigerated storage, has a
limited shelf life; mainly due to growth of this
psychrotrophic contaminants.
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9. Their spoilage pattern is the same as described for
raw milk spoilage.
Flavor defects from their growth are detectable
when the population reaches-106 cells/ml.
Growth of psychrotrophic Bacillus spp., such as
Bacillus cereus, has been implicated in the spoilage
of pasteurized refrigerated milk, especially when
the levels of post-pasteurization contaminants are
low.
Spores of psychrotrophic Bacillus spp., surviving
pasteurization, germinate and multiply to cause a
defect known as bitty.
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10. They produce the enzyme lecithinase, which hydrolyzes
phospholipids of the fat globule membrane, causing
aggregation of fat globules that adhere to the container
surfaces.
Production of rennin-like enzymes by psychrotrophic
Bacillus spp. and others can cause sweet curdling of milk at
a pH higher than that required for acid curdling.
Ultrahigh temperature-treated (UHT) milk (150oC for a few
seconds) is an essentially commercially sterile product that
can only contain viable spores of some thermophilic
bacteria.
The UHT milk is not susceptible to spoilage at ambient
storage temperature but can be spoiled if exposed to high
temperature (40oC or above).
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11. Spoilage of Concentrated Liquid Products
Evaporated milk, condensed milk, and
sweetened condensed milk are the principal
types of concentrated dairy products susceptible
to limited microbial spoilage during storage.
All these products are subjected to sufficient
heat treatments to kill vegetative
microorganisms as well as spores of molds and
some bacteria.
Evaporated milk is condensed whole milk with
7.5% milk fat and 25% total solids.
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12. It is packaged in hermetically sealed cans and
heated to obtain commercial sterility.
Under proper processing conditions, only
thermophilic spores of spoilage bacteria can
survive, and exposure to high storage
temperature (43oC or higher) can trigger their
germination and subsequent growth.
Under such conditions, Bacillus species, such
as Bacillus coagulans, can cause coagulation
of milk (flakes, clots, or a solid curd).
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13. Condensed milk is generally condensed whole
milk and has 10 to 12% fat and 36% total solids.
The milk is initially given a low-heat treatment,
close to pasteurization temperature, and then
subjected to evaporation under partial vacuum (at
50oC).
Thus, it can have thermoduric microorganisms
that subsequently can grow and cause spoilage.
Other microorganisms can also get into the
product during the condensing process.
Even at refrigerated temperature, this product has
a limited shelf life, as does pasteurized milk.
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14. Sweetened condensed milk contains 8.5% fat,
28% total solids, and 42% sucrose.
The whole milk is initially heated to a high
temperature (80 to 100oC) and then condensed at
60oC under vacuum and put into containers.
Because of a low Aw, it is susceptible to spoilage
from the growth of osmophilic yeasts (such as
Torula spp.), causing gas formation.
If the containers have enough headspace and
oxygen, molds (e.g., Penicillium and Aspergillus)
can grow on the surface.
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15. Spoilage of Butter
Butter contains 80% milk fat and can be salted or
unsalted.
The microbiological quality of butter depends on the
quality of cream and the sanitary conditions used in the
processing.
Growth of bacteria (Pseudomonas spp.), yeasts (Candida
spp.),and molds (Geotrichum candidum) on the surface
causes flavor defects (putrid, rancid, or fishy) and
surface discoloration.
In unsalted butter, coliforms, Enterococcus,and
Pseudomonas can grow favorably in the water phase
(which has nutrients from milk) and produce flavor
defects.
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16. Preservation of Milk & Milk Products
Asepsis: Keeping quality is usually improved when
smaller number of microorganism are present,
especially those which grow readily in milk.
A low microbial load, particularly the number of
spores present, is an important consideration in the
milk to the processed by ultra-high temperature or
commercial sterilization.
Packaging serves to keep microorganisms from
bottled milk, fermented milks, packaged butter,
canned milk, dry milk and packaged cheese, and so
do coating by plastic, wax or other protective
substances or finished cheese.
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17. Removal of Microorganisms: After microorganism have
entered milk, it is difficult to remove them efficiently.
The process of centrifugation, as in clarifying or separating
will remove some microorganism from milk.
Molds can be removed physically from the surface of some
kinds of cheese during the curing process by scraping or
periodic washing.
Use of Heat: (Pasteurization and Ultra-pasteurization):
Because milk and cream are so readily changed by heat, the
mild heat treatment called pasteurization is used for their
preservation.
Heat treatment should be accomplished without deleteriously
affecting the flavor, appearance, nutritional property or
creaming of the milk.
The cheese maker is also concerned that the heat treatment
does not harm the curdling properties of the milk.
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18. Vat Pasteurization: It’s a pasteurization process for
milk involving heating the milk in large tanks or vats at
62.8oC for 30 min to eliminate Coxiella burnetii, a
rickettsia responsible for Q fever which can be
transmitted in milk.
High Temperature-Short Time (HTST)
Pasteurization: It is a pasteurization process at a
temperature of at least 72oC for at least 15 seconds. The
HTST system is the most widely used commercial
pasteurization process today.
Ultra High Temperature (UHT) Pasteurization: It is a
pasteurization process of heating the milk at 137.8oC or
above for at least 2 seconds. Listeria monocytogenes and
other heat resistant microorganism can be killed by this
process. 18
19. Use of Low Temperature: With the exception of
canned milk and dry milk, most dairy products
requires the use of low temperature as one factor
in their preservation, often this is the most
important factor.
Refrigerated Storage: Newly pasteurized milk is
to be cooled to 7.2oC or less and maintained
there.
Freezing: Ice cream and other frozen dairy
desserts are frozen as part of the manufacturing
process and are stored at low temperature in the
frozen state, where microbial multiplication is
impossible.
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20. Drying: Various milk products are made by removing
different % of water from whole or skim milk to prevent the
growth of microorganism.
Use of Preservatives: The addition of preservatives to dairy
product is permitted only to a limited extent.
The use of sorbic or propionic acid or one of their salts is
permitted in cottage cheese, yogurt and some of the hard
cheeses or processed cheeses.
Added sugar acts as a preservative of sweetened condensed
milk; it reduces Aw, thereby making moisture unavailable to
microorganisms.
Sodium chloride or common salt is added in the manufacture
of various kinds of cheese but usually it is more for flavor or
for controlling growth of microorganism during
manufacturing and curing than for preservation of finished
product. 20
