Thermal Food Processing

278 Thermal Food Processing: New Technologies and Quality Issues
particular strain, E. coli O157:H7, a food-borne pathogen of international importance,
is unable to grow in pasteurized milk at temperatures below 10°C. 57 In
addition, it is readily destroyed by pasteurization, and hence consumption of pasteurized
milk does not pose a significant risk for this pathogen. 58
In terms of spore-forming bacteria that can be present in raw milk, Clostridium
perfringens, a food poisoning bacterium, produces very heat resistant spores
that survive pasteurization, but C. perfringens does not pose a health hazard in
pasteurized milk due to the inability of the spores to germinate and grow at
refrigeration temperature. In contrast, B. cereus in pasteurized milk has caused
food poisoning outbreaks, as its spores can survive heat treatment, and some
strains are capable of growing at low temperatures. 59 However, as was discussed
earlier, this bacterium is rarely detected in milk stored below 5°C that has been
processed in a properly maintained dairy plant. 60
Thus, while hygienic milk production and handling, pasteurization, and refrigeration
are effective measures to ensure the safety of milk, significant problems can
arise if pathogenic organisms that can proliferate at low temperatures contaminate
pasteurized milk (Table 9.3).
In 1985, an unusually large outbreak of salmonellosis in Chicago, involving
more than 16,000 cases, was associated with the consumption of low-fat (2%)
pasteurized milk 64 ; a subsequent investigation concluded that the pathogen entered
the milk as a postpasteurization contaminant. Similarly, Campylobacter jejuni in raw
TABLE 9.3
Major Disease Outbreaks Involving Pasteurized Milk
Causative
Organism Location Year
Number of
Cases Cause Reference
S. aureus U.S. 1914–1942 29 PPC a 23
Campylobacter
jejuni
U.K. 1978–1984 27
3
Improper
pasteurization
PPC
Salmonella spp. U.S. 1985 1600 PPC 23
E. coli U.S. 1994 18 PPC 23
L. monocytogenes U.S. 1983 49 Unknown 23
Yersinia
enterocolitica
U.S.
U.S.
U.S.
Europe
1975
1982
1995
1985
217
172
10
36
Poor raw
material
handling
PPC
PPC
PPC
B. cereus Europe 1980 280 Unknown 59
a Postpasteurization contamination.
62
62
23
23
23
63