The lactoperoxidase system is a naturally occurring antimicrobial
system
found in milk, with lactoperoxidase, thiocyanate and hydrogen peroxide
as its
components. The keeping quality of milk pasteurized at 72°C for 15
s was found to
be better than that of milk heated at 80°C for 15 s. This agrees with
previous findings
and is usually attributed to heat shocking of spores. However, complete
deactivation
of lactoperoxidase occurred at 80°C–15 s, whereas at 72°C–15
s residual lactoperoxidase
activity was ∼70%, which may provide an alternative explanation.
Higher levels of hypothiocyanite (the major antimicrobial agent produced
by the
lactoperoxidase system) were also detected in milk processed at 72 than
at 80°C,
which supports the theory that the lactoperoxidase system has a role in
the keeping
quality of pasteurized milk. Of all the methods evaluated, titratable acidity
and
alcohol stability gave the most consistent estimates of keeping quality,
while
dissolved oxygen was a good indication of the onset of spoilage. Lactoperoxidase
activity decreased with temperature more rapidly between 70 and 80°C
than is usual
for an enzyme over a 10 deg C range.