Thermal Food Processing

Thermal Processing of Ready Meals 367
12.2.1.1 Freezing
Freezing of food does not render the food sterile. Although the freezing process
can reduce the levels of some susceptible microorganisms, this is not significant
in the context of the overall microbial quality of the food. Thus, a frozen ready
meal will have been subjected to a thermal process at some stage or stages in its
manufacture. At commercial freezing temperatures (–18 to –24°C), all microbial
activity is suspended and the length of time for which the food can be kept is
dependent on quality factors. It is important to note, however, that once a frozen
food is defrosted, those viable microorganisms present will grow and multiply.
12.2.1.2 Chilling
Chilling is the process that lowers the food temperature to a safe storage temperature
of 0 to 5°C. Chilled foods can potentially present a greater risk to public
safety than frozen foods. Keeping products at a low temperature reduces the rate
of microbiological and chemical deterioration of the food. In most processed
chilled foods, it is microbial growth that limits the shelf life; even the slow growth
rates that occur under chilled conditions will eventually result in microbial levels
that can affect the food or present a potential hazard (Table 12.2). This microbial
growth can result in spoilage of the food (it may go putrid or cloudy or show the
effects of fermentation), but pathogens, if present, may have the potential to grow
and may show no noticeable signs of change in the food.
To reduce microbial effects to a minimum, chilled ready meals are usually
given a thermal process, sufficient to eliminate a variety of pathogens, such as
Salmonella, Listeria, and E. coli O157. A process equivalent to 70°C for 2 min is
TABLE 12.2
Minimum Growth Temperatures for Selected Pathogens
Pathogenic Microorganism Minimum Growth Temperature (°C)
Bacillus cereus 4.0
C. botulinum (psychrotrophic) 3.3
E. coli O157 (and other VTEC) 7.0
Listeria monocytogenes –0.4
Salmonella species 4.0
Staphylococcus aureus 6.7
Vibrio parahaemolyticus 5.0
Yersinia enterocolitica –1.0
Source: Betts, G., Ed., A Code of Practice for the Manufacture of Vacuum
and Modified Atmosphere Packaged Chilled Foods with Particular Regard
to the Risks of Botulism, CCFRA Guideline 11, Campden & Chorleywood
Food Research Association, Gloucestershire, U.K., 1996. With permission.