66 Chapter 3
lethal primary stress (e.g., acid) which can
induce increased resistance to an unrelated
secondary stress (e.g., heat) (Rowe and Kirk,
1999 ). This has implications for the safety
margins of pasteurization, which must not
only account for variation of heat resistance
amongst strains, but also the prior incubation
conditions of the organisms.
The effectiveness of pasteurization is
usually monitored by measuring the activity
of alkaline phosphatase, which is an intrinsic
enzyme present in milk and is inactivated by
pasteurization (Allen et al., 2004 ). The resid-
ual phosphatase activity of properly pasteur-
ized milk should be less than 100 mU/l.Pasteurized Milk Safety
Pathogenic bacteria can be present in pas-
teurized milk through survival of the pas-
teurization process or as post - pasteurization
contaminants. In the case of the thermoduric
organisms, two species are of interest:
Bacillus cereus , which survives pasteuriza-
tion as endospores, and Mycobacterium
avium subsp. paratuberculosis (Map), which
is reputed to survive pasteurization and has
been implicated as the cause, or a contribu-
tory factor, in the human condition of Crohn ’ s
disease.
Bacillus cereus : The organism Bacillus
cereus can elicit two distinct syndromes: a
diarrheal type and an emetic type, both of
which are mediated by toxins. The diarrheal
type presents as watery diarrhea and abdomi-
nal cramps occurring 6 to 15 hours after con-
sumption of contaminated food. The emetic
type is characterized by nausea and vomiting
one - half to six hours after ingestion of
infected food and is usually associated with
rice or foods high in starch; therefore, dairy
products are minimally implicated unless
recombined into other products with the
requisite nutritional requirements to induce
toxin production. The rate of germination
is infl uenced by growth conditions upon
sporulation as well as genetic predetermina-the requirement for rennet during cheese
manufacture and result in an increase in
grading score compared to untreated controls
(McCarney et al., 1995 ).
Pasteurized Milk
Pasteurization has been defi ned by the
International Dairy Federation as a process
applied to a product with the object of mini-
mizing possible health hazards arising from
pathogenic microorganisms associated with
milk, by a heat treatment which is consistent
with minimal chemical, physical, and organ-
oleptic changes to the product. The lethality
of pasteurization was originally directed at
M. bovis, one of the causative agents of
human tuberculosis, but was later increased
to take account of C. burnetii , the causative
agent of the human disease Q - fever (Boor
and Murphy, 2002 ). Pasteurization can either
be performed using the holder process (i.e.,
62.8 ° C and not more than 65.6 ° C for at least
30 minutes before immediate refrigeration
[less than 10 ° C]), or more frequently, by the
high temperature short time (HTST) process
(i.e., not less than 71.7 ° C for at least 15
seconds before immediate refrigeration). If
the fat content of the milk product is 10% or
more, if it contains added sweeteners, or if it
is concentrated, the temperature is increased
by 3 ° C. Milk also can be subjected to ultra
pasteurization, which involves heating the
milk to not less than 138 ° C for 2 seconds and
results in a product with a longer shelf - life
but still requiring refrigeration. In central
Europe pasteurization often involves a heat
treatment of 74 ° C for 30 to 40 seconds,
whereas in other European countries, a brief
heat treatment at 85 ° C is used to give an
equivalent kill (Muir, 1990 ).
Bacteria generally exhibit greater heat
resistance when in the stationary growth
phase compared to the lag or log growth
phases, and can also exhibit the phenomenon
of cross - protection. This phenomenon can
occur when an organism receives a non -