Thermal Food Processing

pH-Assisted Thermal Processing 581
CFU / ml
10000
1000
100
10
1
0 5 10 15 20
pH
FIGURE 18.5 Survival curves of B. coagulans STCC 4522 heat treated at 108°C in pH 7
McIlvaine buffer and incubated for 24 h at 37°C in agar acidified to pHs of 6.22 (♦), 5.98
(◊), 5.85 (�), and 5.52 (�).
the physicochemical characteristics of the recovery medium. 95,121,122 In this regard,
the pH is a very important factor.
Microbial heat resistance is estimated from the number of survivors of the heat
treatment. Hence, any factor influencing cell damage and repair mechanisms might
also influence the estimated heat resistance values. Figure 18.5 shows survival curves
of B. coagulans STCC 4522 spores heat treated at 108°C in pH 7 McIlvaine buffer,
plated on nutrient agar acidified to different pH values, and incubated for 24 h at
37°C. The heat treatment was exactly the same in all cases, but spores cultured on
acid media appeared less heat resistant. D 108°C decreased from 3.2 min when spores
were incubated at pH 6.22 to 1.3 min when incubated at pH 5.52, and at this pH
there was growth only in the plates corresponding to the first times of treatment,
that is, when thermal injury was not important. At pH 5.05 no growth was observed
at any of the heating times. However, as shown in Figure 18.1, the same microorganism
was able to grow up to pH 4.5 before any heat treatment was applied,
depending also on the type of acid used. It was also observed that the time needed
for the colonies to develop was longer at acid pHs and at longer heating times.
Similar results have been shown by Leguérinel et al. 123 with B. cereus spores,
and by Cook and Brown, 124 Cook and Gilbert, 125 Mallidis and Scholefield, 126
López et al., 127 and Fernández et al. 128 with B. stearothermophilus spores. The
last authors 128 did not find differences when the recovery medium was acidified
with citric acid or with glucono-d-lactone.
Other parameters of the recovery medium that have an influence on the
apparent heat resistance of microorganisms are the sodium chloride concentration