Effect of the wavelength of infrared heaters on the inactivation of bacterial spores at various water activities

Bacterial spores (Bacillus subtilis subsp. subtilis NBRC 16183) inoculated onto a stainless steel Petri dish and treated at nine levels of water activity (a_w) for 2 days were inactivated by infrared radiation heating (IRH) using three kinds of infrared heaters with different radiation spectra. The peak wavelengths used were 950, 1100 and 1150 nm. In general, the inactivating efficacy of IRH treatment against bacterial spores with shorter wavelength heater (950 nm) was greater than that with other heaters. The decimal reduction times (D value) calculated using the linear portion of survival curves were affected by both the initial a_w values and the spectra of the infrared rays. Spores at approximately 0.9, 0.7 and 0.6 a_w were most resistant to IRH at wavelengths of 950, 1100 and 1150 nm, respectively. The a_w values that led to maximum D values for bacterial spores increased as the wavelength was shortened. Optimum a_w values were identified for the inactivation of bacterial spores by IRH. Spore resistance to IRH could also be affected by the spectral characteristics of the infrared absorption, which varied with the a_w of bacterial spores.