Microorganisms in Milk
Milk is sterile at
secretion in the udder but is contaminated by bacteria even before it leaves
the udder. Except in the case of mastitis, the bacteria at this point are
harmless and few in number. Further infection of the milk by microorganisms can
take place during milking, handling, storage, and other pre-processing
activities.
Lactic acid bacteria:
This group of bacteria is
able to ferment lactose to lactic acid. They are normally present in the milk
and are also used as starter cultures in the production of cultured dairy
products such as yogurt. Note: many lactic
acid bacteria have recently been reclassified; the older names will appear in
brackets as you will still find the older names used for convenience sake in a
lot of literature. Some examples in milk are:
·
Lactococci
o
L.
delbrueckii subsp. lactis (Streptococcus lactis )
o
Lactococcus
lactis subsp. cremoris (Streptococcus cremoris )
·
Lactobacilli
o
Lactobacillus
casei
o
L.delbrueckii subsp. lactis (L.
lactis )
o
L. delbrueckii subsp. Bulgaricus
(Lactobacillus bulgaricus)
·
Leuconostoc
Coliforms:
Coliforms are facultative anaerobes with an optimum growth at
37° C. Coliforms are indicator organisms; they are closely associated with the
presence of pathogens but not necessarily pathogenic themselves. They also can
cause rapid spoilage of milk because they are able to ferment lactose with the
production of acid and gas, and are able to degrade milk proteins. They are
killed by HTST (high temperature short time) treatment; therefore, their
presence after treatment is indicative of contamination. Escherichia
coli is an example belonging to this group.
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