Bulk milk is the mixture of all the milked udders in a given herd. Nowadays, about 15–40% of
the
udders
in
most
herds
are intramammary infected by different bacteria species, mainly
coagulase
negative staphylococci. The presences of bacteria in the lumen of the mammary
gland
induce
impairment of milk quality and increase the number of somatic cells. A positive
relationship
between % casein (casein/total protein) and curd firmness (CF) and negative
relationship
between
lactose,
or somatic cell count (SCC) and CF are associated with bacterial
infection and with late lactation milk, and therefore with reduction in cheese yield
and
quality.
On
the other hand, in milk of goats with intramammary infection, the correlation
between the levels of fat, protein, casein and curd yield is minor compared to
milk
of
uninfected animals. Thus, gross milk composition is an insufficient predictor of
milk
quality for cheese production, since a high percent of the bulk milk originates from
subclinically
infected
glands.
Research carried out in the past few years highlighted the
effectiveness
of lactose as a predictor of milk quality. The correlation between lactose and
CF
was
higher than that for % casein and SCC. Lactose concentration of ≤4% is associated
with
non-coagulating milk and therefore, such milk is unsuitable for making cheese, but
still
meets the criterion for consumption as pasteurized milk. A model that describes the
simultaneous
and close association between reduction in lactose concentration and milk
yield
on the one hand and reductions in lactose concentration and milk quality on the other
hand
is presented. The physiological and biochemical basis for deterioration of milk quality
in subclinically infected and in late lactation animals is reviewed and suggestions to
improve
the quality of milk produced by farmers and acquired by dairies are presented.