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Types
of
Microorganisms
in
Milk |
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Types
of
Microorganisms
in
Milk -
The types of micro organisms found in milk vary considerably, and are dependent upon the specific conditions associated with a batch of milk.
Bacteria, yeasts, moulds, and bacteriophages are commonly encountered. Other viruses and Protozoa are seldom observed in milk products, except as occasional contaminants.
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Bacteria
Bacteria
are
the
most
common,
and
probably
the
most
numerous
of
microorganisms
with
which
the
dairy
processing
industry
is
concerned.
They
belong
to
four
main
groups:
(1)
cocci,
usually
gram
positive,
(2)
gram
positive
non-sporeforming
rods,
(3)
gram
positive
sporeforming
rods,
and
(4)
gram
negative
non-spore
forming
rods.
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Yeasts
The yeasts most frequently encountered in milk and milk products act upon the lactose to produce acid and carbon dioxide.
Yeasts are more commonly found in raw, cream during hot weather, but are potential contaminants throughout the year.
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Moulds
Moulds often grow in large concentrations and are visible as a fuzzy or fluffy growth. They are sometimes observed on the surface, of butter, old cream, khoa, or cheese.
They are black, grey, green, blue or white. They discolour milk products and often produce undesirable, at times repulsive, odours and flavours. Moulds are essential in production of the certain kinds of cheese.
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Bacteriophages
Bacteriophages are particularly obnoxious in starter cultures used for making cultured milk, butter and cheese.
Phages will, kill the bacterial culture and the whole process of fermentation will be lost.
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Microorganisms found in milk can also be described on the basis of the following characteristics;
1. Biochemical activities.
2. Temperature response.
3.
Ability
to
cause
infection
and
disease.
Biochemical activities
If allowed to stand under condition that permit bacterial growth, raw milk of a good sanitary quality will rapidly undergo a series of chemical changes. The principal change is lactose fermentation to lactic acid.
This change is brought about by acid uric lactic organisms, especially Strepotococcus lactis and certain lactobacilli. These include two distinct biochemical types, homo-and heterofermentative. In homofermentation lactic acid is the major product of lactose fermentation.
Heterofermentative organisms, however, produce lactic, acetic, propionic, and some other acids, and some alcohols and gases such as CO2 and H2 Organisms continue to form lactic acid until the concentration of acid is itself too great for the organisms to remain live.
Microbacteria, micrococci, coliforn18, etc. also ferment lactose to lactic acid and other products. Many Clostriifiul1J species and, some yeasts such as Torula lactic, and Torula cremoris ferment lactose with acid and gas production.
As the acidity continues to increase and reaches a pH of 4.7, it eventually causes a precipitation of casein. Organisms capable of metabolizing lactic and other acids develop especially acid uric, yeasts and moulds.
The acidity of milk is diminished and the alkaline products of protein decomposition such as amines, ammonia and the like are produced.
This is accomplished by many species of the genera Bacillus, Clostridium, Pseudomonas, Proteus and numerous other forms.
The action of microorganisms does not involve fat as readily as it does lactose and protein. Lipolysis results from the action of lipase produced by bacteria such as Pseudomonas, Achromobacter and by some yeasts and moulds.
Fat is hydrolysed to glycerol and fatty acids. Some of the fatty acids, for example, butyric and caproic acid give milk products, distinctive and usually rancid, odours and flavours.
Several microorganisms also bring about certain objection able changes in the milk which may not be deleterious to health. Rapines in milk is sometimes encountered. The milk become ropy or slimy and may be pulled out into long threads.
It is produced by several organisms but the most important species is Alcaligenes viscolactis. A rapid fermentation of lactose in milk is sometimes observed and is known as stormy fermentation. This is brought about by Clostridium perfringens.
The curd become torn to shreds by the vigorous fermentation and gas production. Several organisms have been isolated from milk which impart brilliant colours. Pseudomonas syncyanea imparts blue colour, pseudomonas synxantha yellow colour and Serratia marcescens red colour to the milk.
Temperature response
Microorganisms found in milk can also be described according 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 activi1y'and by high temperatures to reduce microbial population and destroy pathogens. All the four types of microorganisms i.e. psychrophilic, mesophilic, thermophilic and thermoduric are found in milk.
Psychrophiles grow at temperatures just above freezing and at refrigeration tempratures. They produce a wide variety of spoilage defects. The defects may result in the production bf many "off" flavours
and
odours.
The
most
commonly
encountered
psychrophilic
bacteria
are
members
of
the
genera
pseudomonas,
Achromobacter,
Vibrio,
Flavobacterium
and
Alcaligenes,
They
arc
killed
in
the
pasteurization
process,
but
are
sometimes
found
in
pasteurized
milk.
The
contamination takes place after pasteurization from equipment, cans, bottles, and water.
The most important mesophilic bacteria are streptococci, lactobacilli and coliforms, which produce acid and gas and off flavours. They are killed in the pasteurization process
Thermophilic bacteria grow well at the temperature used in pasteurization, specially when the low temperature holding method is followed. Most thermophilic forms are found in two genera, Bacillus and Clostridium.
Thermoduric organisms are regarded as those which survive pasteurization but do not grow at pasteurization temperatures. The most common thermoduric bacteria are found in the genera Microbacterium, Corynebacterium, Micrococcus, Streptococcus and Bacillus.
Excessive numbers of thermoduric bacteria in milk make it difficult to meet the grading standard.
Ability to cause infection and disease
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.
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.
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