Thermal Food Processing

548 Thermal Food Processing: New Technologies and Quality Issues
TABLE 17.10
Parametric Values Obtained from Survival Curves of the Spores Used in
Present Study
Bacterium D Values* (min)/Temperature (°C) Z Values** (°C)
B. subtilis 26.1/102 13.9/105 6.0/108 2.3/111 8.3
B. coagulans 105.0/104 38.2/107 13.9/110 4.6/113 6.7
B. stearothermophilus 28.6/113 6.6/117 2.3/121 0.6/125 7.4
C. sporogenes 10.8/110 4.1/115 0.6/120 7.9
* D value is the time required to reduce the number of spores by 1/10 at the temperature shown.
** Z value is the temperature to reduce D value by 1/10.
17.6.1 BACTERIAL SPORES
The bacteria used were B. subtilis, B. coagulans, B. stearothermophilus, and
C. sporogenes. Each spore was prepared according to the procedure outlined
above. 5 The D values and Z values obtained from each survival curve of the
four bacteria are shown in Table 17.10. These results were obtained under
atmospheric pressure. Each survival curve became almost linear (r = 0.999 to
0.978). The D values were large enough compared to those of previous
reports, 29,43,44 so that it is difficult to inactivate the spores at temperatures lower
than 100°C, and the Z values were almost the same as those of the previous
reports. 29,43,44 Each kind of spore was suspended in a 1/15 M phosphate buffer
solution (pH 7.0) and used for the following experiments. The way of pressurizing
the spore suspension was mentioned beforehand in Section 17.2.1.
17.6.2 DEATH BEHAVIORS OF B. SUBTILIS SPORES
The survival curves of B. subtilis at 400 MPa from 25 to 65°C are indicated in
Figure 17.13a. The results show that the spores were inactivated even from a
relatively low temperature such as 35°C. The death rates became higher in
accordance with the increase in temperature. Thus, the temperature would affect
the sterilization of spores even from around room temperature. On the contrary,
the population of spores significantly declined in the first 5 min of treatment,
during which the temperature rose. In the case of 65°C, the degree of the decrease
reaches a 3 log cycle of the initial spores in the first 5 min.
After the first 5 min, the temperatures of the spore suspensions are thought to
reach the given ones and then become constant, shown in Figure 17.2. The shape
of each survival curve proved to be a downward convex. It is reported that survival
curves of spores under HHP do not obey a first-order rate equation, as was often
previously reported. 5,23,39,40
The survival curves replotted in the relationship between log (survivors) and log
(time) show a clear linear correlation (Figure 17.13b), and therefore obey an exponential
equation. The same behavior is also observed in the previous studies, though