Thermal Food Processing

Infrared Heating 515
Å Insulating material
Ç Aluminum foil
É Stainless steel Petri dish
Ñ Bacterial suspension
Ö Rotary shaker
Ü Holder
á Thermostat
à
Cooling plate
FIGURE 16.20 Sample for pasteurization by FIR. (From Hashimoto, A. et al., J. Chem.
Eng. Jpn., 25, 275–281, 1992.)
by conventional thermal mechanisms (e.g., thermal denaturation of proteins and
nucleic acids), whereas a nonthermal effect of FIR on enzyme activity has been
reported. 41,42 This thermal effect of FIR is supported by experiments 43 comparing
the characteristics of damaged cells exposed to either FIR or conductive heating.
However, Hashimoto et al. 44 observed that the bacteria were impaired and killed
by FIR even when the bulk of the suspension was kept below the lethal temperature.
They have investigated the effects of FIR radiation on Escherichia coli and
Staphylococcus aureus in phosphate-buffered saline under the condition that the
bulk of the suspension was kept below 40°C by cooling it (Figure 16.20).
Figure 16.21 shows the effect of irradiation power, q ir, on the ratio of the colonyforming
unit of the test bacteria irradiated by FIR for 20 min to that of the control
(N 1/N 10). The explanation given for the FIR effect was that the bacteria were
impaired and killed in the very thin region near the surface, since the FIR energy
was almost entirely absorbed at the surface, raising the surface temperature. It
should be noted that FIR heating is more effective for pasteurization than NIR. 45
The superiority of FIR pasteurization over NIR was explained by the very high
absorption coefficient of the bacterial suspension in the FIR region. In addition,
it was found experimentally that FIR was more effective than hot-air heating for
the pasteurization of bacteria on a solid model food (agar). 46
Bacteria such as the Bacillus or Clostridium group form spores under some
conditions. When the sterilization of the food is considered, this spore is important,
since it is heat resistant. Sawai et al. 47 have investigated the effect of FIR on spores.
They used spores of Bacillus subtilis, which were suspended in phosphatebuffered
saline, and irradiated FIR to the spore suspension, which was dispensed