Thermal Food Processing

pH-Assisted Thermal Processing 577
more than 50% when the pH was reduced to 4. 84 Similar decreases were
observed for L. monocytogenes 105 and Yersinia enterocolitica. 83
The growth temperature of vegetative bacteria seems to influence the effect
of pH of the heating medium on heat resistance. Hence, the magnitude of the
influence of pH on heat resistance increases as the growth temperature
increases. 83,84 Moreover, the pH of maximum heat resistance for Y. enterocolitica
changed from 7 for cells grown at 37°C to 5 for cells grown at 4°C. 83 Pagán et al. 83
found that the outer membrane of the cells grown at higher temperatures was
more easily disrupted than for cells grown at lower temperatures.
The pH of the heating medium also influences the z values of bacteria, and it is
precisely this effect around which most controversy has arisen. Some authors have
found an increase in z value at acid pHs, 57,62,71,72,106 which means that at high temperatures
pH may even have no effect on heat resistance. 61,78 However, other authors
found a decrease, 65,76,77,107 and still others were unable to find any effect. 64,66,73–75,93,108
Figure 18.3 shows z values of different bacterial spore-forming species in
function of the pH of the buffer used as a heating medium. As it can be seen, the
z value remained approximately constant for B. subtilis spores (z = 9°C) and for
A. acidocaldarius (z = 7°C). It increased from 7.8 to 9.6°C for B. stearothermophilus
spores, from 8.9 to 10.5°C for B. coagulans spores, and from 7 to 11°C
z value
14
12
10
8
6
4 5
FIGURE 18.3 z values of different species of spore-forming bacteria in function of the pH
of the heating phosphate buffer. B. licheniformis (�), B. coagulans (♦), C. sporogenes (�),
B. stearothermophilus (▲), B. subtilis (◊). (Data taken from Palop, A. et al., Int. J. Food
Microbiol., 29, 1–10, 1996; Palop, A. et al., Int. J. Food Microbiol., 46, 243–249, 1999;
Cameron, M.S., Appl. Environ. Microbiol., 39, 943–949, 1980; López, M., Int. J. Food
Microbiol., 28, 405–410, 1996; Palop, A., Estudio de la influencia de diversos factores
ambientales sobre la resistencia de Bacillus subtilis, B. licheniformis y B. coagulans, Ph.D.
dissertation, Universidad de Zaragoza, Zaragoza, Spain, 1995.)
pH
6
7