Thermal Food Processing

Thermal Processing of Dairy Products 279
or inadequately pasteurized milk was implicated in several outbreaks of gastroenteritis
in Great Britain from 1978 to 1984 61 ; some cases were traced to pasteurized
milk in glass bottles whose lids had been pecked by magpies and
jackdaws, which are probable carriers of this bacterium. 65 Yersinia enterocolitica,
which causes yersiniosis in children younger than 7 years of age, 62 is heat labile
and completely inactivated by pasteurization 66 ; however, several outbreaks of
yersiniosis have been reported in the U.S. and Europe. 65 This bacterium can grow
in milk during refrigeration, and thus poses a potential heath hazard in underpasteurized
milk and milk contaminated with the organism after heat treatment.
Although L. monocytogenes is more heat tolerant than most other nonspore-forming
pathogens, pasteurization should result in its total destruction 67 ;
however, some evidence exists of it being linked to a serious food poisoning
incident involving pasteurized milk in the U.S. during 1983. 23
As indicated in Table 9.3, most of the food poisoning outbreaks due to
consumption of milk are believed to have been due to either inadequate heating
or postheating contamination. One means by which pathogenic organisms can
contaminate pasteurized milk is by leakage of raw milk into the pasteurized milk
in the regeneration section of pasteurizers. 68 This should not occur in properly
installed pasteurizers, where a booster pump prevents flow of milk from the raw
into the pasteurized streams.
Maintaining proper hygiene in raw milk handling and ensuring adequate
pasteurization can thus prevent contamination of pasteurized milk with pathogenic
microorganisms. However, control of spoilage bacteria in pasteurized milk
is more difficult. The main causative agents, thermoduric and psychrotrophic
bacteria, are discussed below.
9.7.4 EFFECTS OF PASTEURIZATION ON MILK SHELF LIFE
Thermoduric organisms are those that will survive pasteurization. The main types
of these organisms in milk and cream are shown in Table 9.4.
Common examples that can be isolated from pasteurized milk and cream
include coryneforms, micrococci, and some streptococci, which all grow very
slowly at refrigeration temperatures, 69 and also spore-forming organisms, such
as Bacillus spp. whose spores can survive much more intense heating conditions,
such as 80°C for 10 min. The spores can be activated, germinate, and grow in
heat-treated products under certain conditions, subsequently causing spoilage.
As mentioned above, some may even compromise the safety of the heat-treated
product.
Postprocessing contamination (PPC) with Gram-negative, heat-sensitive psychrotrophic
bacteria is the most important cause of spoilage in pasteurized milk
and cream. These bacteria, which produce off-flavors due to enzymatic action
when the bacterial count exceeds about 10 7 cfu/ml, 31,70 enter the product from
nonsterile surfaces of processing and packaging equipment, the air, and packaging
material. Constant monitoring of the PPC level with sensitive test methods such
as the Psychrofast test 71 and strict attention to the cleanliness of surfaces with