Thermal Food Processing

580 Thermal Food Processing: New Technologies and Quality Issues
D 99
10
1
0.1
0.01 0.1 1
Normality (N)
FIGURE 18.4 Heat resistance (D 99 values) of B. coagulans STCC 4522 spores heat
treated in 1% peptone water acidified to pH 4 with increasing concentrations of different
acids: acetic (◊), ascorbic (�), citric (�), lactic (♦), malic (�), hydrochloric (�), and
glucono-δ-lactone (▲).
not only to determine bacterial growth (see Section 18.3), but also for heat
resistance. The surprisingly great increase in heat resistance caused by the
increase in concentration of hydrochloric acid could be related to the protective
effect that would cause the addition of salt (sodium chloride) to the heating
medium, 55 because NaOH was used in all cases to adjust the pH to 4.
However, Leguérinel and Mafart 113 showed that at neutral pH the heat resistance
of B. cereus spores was the lowest at the lowest acetic acid concentration
(0.01 M), while there were no differences in heat resistance at lower pH values
(up to pH 4.35) caused by the acid concentration. The authors attributed this
protective effect of the acid concentration at neutral pH to the dissociated form
of the acid, which accounts for 99.4% of the total concentration of the acid at
pH 7, but only 29.1% at pH 4.35. Perhaps at pH 4 the lower proportion of
dissociated acid present (that would leave spores unprotected) could account for
the opposite effect found by Palop 117 using acetic acid at this pH.
18.6 EFFECT OF PH ON THE RECOVERY
OF SURVIVORS TO HEAT TREATMENT
After a heat treatment, microorganisms are more sensitive to the environmental
stresses. Heat causes injuries of different severity, before the microbial cell is
irreversibly inactivated. Some of the damages can be repaired by the cells, depending
mainly on the severity of heat injury and on recovery conditions. 118 These
conditions include the temperature of incubation after heat treatment 119,120 and