Thermal Food Processing

Ohmic Heating for Food Processing 447
TABLE 14.2
Kinetic Constants and Thermal Inactivation Parameters for
Zygosaccharomyces bailii under Conventional and Ohmic
Heating
Temperature (°C) D conv (min –1 ) D oh (min –1 ) k 0conv (sec –1 ) k 0oh (sec –1 )
49.8 294.6 274.0 0.008 0.009
52.3 149.7 113.0 0.016 0.021
55.8 47.21 43.11 0.049 0.054
58.8 16.88 17.84 0.137 0.130
z (°C) 7.19 7.68 — —
E a (kJ·mol –1 ) — — 123.97 116.19
Source: Adapted from Palaniappan, S. et al., Biotechnol. Bioeng., 39, 225–232, 1992.
heat resistance of microorganisms. Microbial death during OH was mainly attributed
to thermal effects, while the nonthermal effects were insignificant (Table 14.2).
The effect of an electric field on the thermal inactivation kinetics of a highly
heat resistant microorganism, Byssochlamys fulva, has been studied. This is a
thermotolerant, ascospore-producing, filamentous fungus and was investigated
by Castro et al. (personal communication, 2003). It can also produce the
important mycotoxin patulin. B. fulva death kinetics were determined in an
industrial strawberry pulp (14.5°Brix, pH = 4.0). The experimental D values
for B. fulva obtained under OH (D oh) conditions were half the ones obtained
for conventional heating (D conv) (T = 85°C, D conv = 7.23, D oh = 3.27). Unsurprisingly,
these results are not consistent with those obtained for B. subtilis and
Zygosacharomyces bailii (a bacterium and yeast, respectively), and the nonthermal
inactivation mechanism needs to be studied in more detail, in terms of
the effect of the electric field on the membrane/wall integrity of ascospores and
enzymes participating in ascospores’ activation. Finally, the effect of OH on
patulin production in food by B. fulva and degradation of patulin requires
investigation.
Data on nonthermal effects are scarce and more studies are needed to determine,
for example, the effect of electricity on the physiological characteristics of microbes,
changes in glycosylation degree of proteins and lipids, and other elements that can
affect the heat resistance of microorganisms. The differences between microorganisms
such as bacteria, filamentous fungi, and yeast need to be fully recognized.
The effects of OH on fermentations using immobilized cells or high-cell-density
systems should also be further investigated, namely, in terms of substrate/metabolite
diffusional limitations, lag phase duration, and efficiency of metabolite production.
The effect on toxic metabolite production by microorganisms and degradation
in food requires investigation in general.