Infrared (IR) radiation is easily absorbed by water and organic materials. IR heating is used widely for the drying, dehydration, blanching, thawing and pasteurization [1] -[5] , as well as for medical treatment [6] [7] . The importance of microbial decontamination is increasing owing to outbreaks of food poisoning and the emergence of drug-resistant bacteria. As well as providing clean working areas, IR heating can save space because no heat- transfer medium is needed. Although the IR radiation cannot penetrate deep and heats up only a few millimeters below the surface of the sample, the main effect of IR is due to heating of a thin layer of food material on the surface [5] . The IR heating process improves the shelf life of foodstuffs [8] [9] , and is a promising method for the efficient inactivation of microbes [10] - [12] . During IR heating operations such as drying, blanching, roasting frying and cooking of food products, the pasteurization has progressed at the same time.

Previous studies have reported the use of IR, especially far-infrared (FIR), for pasteurization [13] - [20] . Compared with thermal conductive heating, FIR irradiation was found to be more effective in pasteurizing vegetative bacterial cells [13] [16] [20] . Furthermore, FIR irradiation caused heat activation and death of Bacillus subtilis spores in a temperature range where spore viability was not affected by thermal conductive heating [17] . Hamanaka et al. [21] also reported heat activation and the inactivation of bacterial spores by FIR irradiation. The pasteurization effects of FIR may be attributed to the absorption of radiative energy by the bacterial suspension in a very thin layer near the surface and an increase in the bulk temperature of the suspension [14] [18] . Although the effectiveness and validity of IR heating were confirmed in these studies, a quantitative evaluation of the FIR pasteurization effect against fungi was not performed, and pasteurization kinetics of fungi by FIR heating were not elucidated. In the present study, we evaluated the disinfection and pasteurization of liquid media and investigated the pasteurization kinetics of fungi suspended in saline by FIR heating under isothermal conditions.

Figure 2 shows the effect of the bulk temperature on the viability of C. albicans heated by FIR irradiation. The ordinate is the ratio of the C. albicans CFU post-irradiation (N) divided by the non-treated CFU (N₀). The effect tiveness of pasteurization by FIR heating increased with the bulk temperature of the irradiated suspension. As shown in Figure 2, with the logarithmic survival ratio and irradiation time (t) as the ordinate and abscissa, respectively, the population of the C. albicans decreased almost linearly at a constant temperature. Thus, assuming that the pasteurization of C. albicans by FIR irradiation follows first-order reaction kinetics as given by Equation (1) [22] , the apparent first-order death rate constant, k, of C. albicans by FIR heating can be determined.

Figure 3 summarizes the k values obtained under different conditions in an Arrhenius plot. Here, T represents the bulk temperature of the microbial suspension heated by FIR irradiation or thermal conduction. At lower temperatures, k values for FIR heating were larger than those for thermal conductive heating. A similar trend was observed for S. cerevisiae and A. niger.

Assuming that the k values followed the Arrhenius equation as shown by Equation (2) [22]

the activation energy (E_a) for the death of fungi used in this study by FIR and thermal conductive heating was determined (Table 1). Compared with the thermal conductive heating, the values of E_a for the IR irradiation decreased, indicating that FIR heating is superior to thermal conductive heating in energy expenditure. Ward et al. [23] investigated the pasteurization effectiveness of a high-power Nd:YAG laser light (1.064 μm), which emits energy in the IR wavelength range, on E. coli in saline and reported that the action of the Nd:YAG laser light at 50˚C was due partly to thermal heating and partly to an additional, undefined mechanism. Hamanaka et al. [24] hypothesized that inactivation of microorganisms by IR heating may involve an inactivation mechanism similar to that of UV light (DNA damage) and microwave heating in addition to thermal effects. Previous reports have evaluated stress-induced injury in bacteria on the basis of the sensitivity changes to antibiotics. The validity of this method was examined using chemical and physical stresses [16] . Although similar sensitivity changes were obtained from thermal conductive heating and IR heating, the damage caused by IR heating was more serious than that caused by thermal conductive heating [16] . Krishnamurthy [25] verified the cell wall damage, cytoplasmic membrane shrinkage, cellular content leakage and mesosome disintegration of IR-treated S. aureus.

Different letters indicate significant difference (P < 0.05).

In a previous study [14] [15] , temperature distribution of the bacterial suspension irradiated by IR was estimated; it was proposed that the temperature in a thin surface region, which cannot be measured by thermocoupling, was higher than the temperature of the bulk suspension. For example, when the irradiated power was 3.22 kW/m², the estimated surface temperature of the suspension rose to approximately 57˚C for 0.3 s, even when the bulk temperature of the suspension was maintained at 40˚C [14] . Thus, bacteria circulating in the suspension will be subjected to repeated episodes of rapid temperature fluctuations as they pass into and out of the surface region. The accumulated effect of this repeated stress might cause E. coli damage and death, even when the bulk temperature of the suspension irradiated by IR remains below the lethal temperature [18] . Therefore, variation in the thickness of the liquid layer affects the frequency and duration of bacterial cell contact with the surface region. In fact, the effectiveness of pasteurization by IR increased with a decrease in the thickness of the liquid layer, even at the same bulk temperature [20] . This difference, shown in Table 1, suggests that some mechanisms of action might be different, possibly because of the increase in the surface temperature and the direct absorption of IR radiative energy in the surface region of the liquid layer.

This study evaluated the pasteurization kinetics of fungi suspended in saline by FIR heating under isothermal conditions. For the same bulk temperatures, pasteurization by FIR heating was more effective than thermal conductive heating. The activation energy for the death of fungi by FIR irradiation was slightly lower than thermal conductive heating. This indicates differences in the mechanism of action. Although the understanding of the action mechanism is still unclear, IR heating can offer many advantages over conductive heating in heat transfer rate and heat flux, which results in time-savings as well as increased production line speed.

TubasaNakata,ArisaOkamoto,JunSawai,MikioKikuchi, (2015) Effects of Far-Infrared Irradiative Heating Pasteurization on Fungi. Journal of Biosciences and Medicines,03,60-65. doi: 10.4236/jbm.2015.37007