Thermal Food Processing

Thermal Processing of Meat Products 181
process temperatures. The selection of the Z value can have a significant impact
on the calculated process lethality and should be conservatively chosen. Tables
of lethal rates can be prepared for a range of product temperatures for a specific
temperature and Z value.
To obtain the kill time at reference temperature, the sum of the lethal rates
at each of the product temperatures (from product temperature history) is multiplied
by the effective time (time between the measurements):
F L xt
T ref T T =∑
(6.2)
Thus, the process lethality (decimal reductions of pathogen) can be obtained by
dividing F T ref by the D value of the particular pathogen at T ref . While this is a
simplistic form of evaluating process lethality, caution should be observed when
using this method in thermal process evaluations.
The general method for calculating process lethality has wide application in
the canning industry and can be applied to thermal processes in enclosed systems,
where the loss of moisture from the product (mass transfer) is minimal. An
example for such a cooking system is moist cooking in smokehouses, where the
humidity is high (wet and dry bulb temperatures are similar), such as cooking
the product in bags in water or in high-humidity environments (steam cooking).
Under these conditions, the predicted lethality will correlate well with the
observed lethalities for vegetative food-borne pathogens.
Typical thermal destruction parameters for common food-borne pathogens
are shown in Table 6.6. When dry heat is employed for thermal processing, such
as in cooking hamburgers or cooking roast beef products, chicken thighs, or
chicken breasts in a dry environment or environment with low humidity, the
predicted lethalities will be greater than the observed lethalities. An example for
this behavior is the work conducted by Goodfellow and Brown 52 on destruction
of Salmonella on roast beef, which indicates that survivors could be observed on
the surface-inoculated product using dry heat. Injection of steam either early or
late in the cooking process resulted in complete destruction of surface-inoculated
Salmonella. The authors 52 attributed the survival of the Salmonella during dry
heating to dehydration of the microorganisms, resulting in increased resistance.
In some cases, predicted process lethalities for vegetative microorganisms do
not agree with lethalities observed through microbial challenge studies. These
anomalies can be explained by several factors that affect the microbial heat
resistance. These factors can be categorized as those that pertain to the product
and those that are related to the microbial cells. They have been recognized as
early as the 1960s and include:
1. Product characteristics that affect microbial heat resistance
a. Water activity (a w)
b. Fat content
c. pH of the product