Thermal Food Processing

182 Thermal Food Processing: New Technologies and Quality Issues
d. Solutes or salts and other antimicrobial agents (NaCl, nitrates and
nitrites, phosphates, lactates and other salts of organic acids, etc.)
e. Packaging system employed (vacuum packaged vs. moderately aerobic
conditions, such as meats’ or sausages’ moisture-permeable
casing)
2. Microbial cell characteristics that affect heat resistance
a. Species and strain of the organism
b. Growth conditions of the organism (growth medium, temperature, etc.)
c. Age of the population
d. Concentration of the cells
e. Method of preparation of the cells (centrifugation, washing, etc.)
The degree of variations in the heat resistance of microorganisms can be
observed (Figure 6.11) for L. monocytogenes, summarized from various reports. 53
The destruction of microbial cells as described by the D value (first-order kinetics)
assumes a log-linear relationship between the survivors and the heating time.
However, deviations from the log-linear relationship have been observed and
often are rationalized on the basis of some special property of the microorganism
or its physiological state. 54 These deviations, termed shoulder or tailing regions
on the survival curve, may be due to the clumps of microorganisms or the existence
of a more heat resistant subpopulation (Figure 6.12).
Log D (s)
5
4
3
2
1
0
47.5 50.0 52.5 55.0 57.5 60.0 62.5 65.0 67.5
Temperature (°C)
FIGURE 6.11 Heat resistance of Listeria monocytogenes in different substrates. (Adapted
from Mackey, B.M. and Bratchel, N., Lett. Appl. Microbiol., 9, 89–94, 1989.)