To study the factors and mechanisms involved in microorganisms' death or resistance to temperature in low-water-activity environments, a previous work dealt with the viability of dried microorganisms immobilized in thin-layer on glass beads. This work is intended to check the efficiency of a rapid heating-cooling treatment to destroy microorganisms that were dried after mixing with wheat flour or skim milk. The thermoresistance of the yeast Saccharomyces cerevisiae and the bacterium Lactobacillus plantarum were studied. Heat stress was applied at two temperatures (150 or 200 degrees C) for treatments of one of four durations (5, 10, 20, or 30 s) and at seven levels of initial water activity (a(w)) in the range 0.10 to 0.70. This new treatment achieved a microbial destruction of eight log reductions. A specific initial water activity was defined for each strain at which it was most resistant to heat treatments. On wheat flour, this initial a(w) value was in the range 0.30-0.50, with maximal viability value at a(w)=0.35 for L. plantarum, whatever the temperature studied, and 0.40 for S. cerevisiae. For skim milk, a variation in microbial viability was observed, with optimal resistance in the range 0.30-0.50 for S. cerevisiae and 0.20-0.50 for L. plantarum, with minimal destruction at a(w)=0.30 whatever the heating temperature is.

A continuous pasteurization system was designed based on a domestic microwave oven. Broth was pumped through helical coils of glass tubing placed in the center of the oven cavity. Inactivation of two selected spoilage microorganisms, Bacillus cereus and Saccharomyces cerevisiae in broth were evaluated under continuous flow microwave heating conditions and compared with conventional batch heating in a well stirred hot water bath. Inoculated broth was heated in a microwave oven (700 W, 2450 MHz) under continuous flow conditions to selected exit temperatures of 90°C for B. cereus and 60°C for S. cerevisiae at five power levels (210, 280, 350, 420, 490 W) and five time intervals (1, 2, 3, 4, 5 minute). Broth treated in hot water bath at 90˚C for B. cereus and 60˚C for S. cerevisiae was taken as control. There was a decrease in B. cereus and S. cerevisiae count after microwave and hot water bath treatment (control) with increasing treatment time. Higher microbial inactivation was observed at lower power levels. For all the microwave power levels, higher inactivation of B. cereus and S. cerevisiae was observed in comparison to control, this may be due to some non thermal effects associated with microwave. Heating rate and flow rate also increased with the increasing power level with decrease residence time to kill the contaminants. In future, this system may be useful for effective pasteurization of liquid foods e.g. sugarcane juice and soymilk without affecting the taste of processed juices.

The particular effect of 4 kinds of amino acid and peptide-rich food material (APRM) containing different charged amino acid contents on the gelatinization and retrogradation behavior of potato starch granules and on the water-vaporization behavior was analyzed by differential scanning calorimetry, rapid viscoanalysis, x-ray diffractometry, thermal gravimetry-differential thermal analysis, and pulsed NMR. APRM with a high-charged amino acid content produced unique gelatinization and retrogradation behavior in terms of an elevated gelatinization temperature, reduced viscosity, higher setback, and lower retrograded starch melting enthalpy. The recovered x-ray diffraction intensity decreased with increasing charged amino acid content. APRM with high-charged amino acid content could provide an improved paste having easy vaporization of external water in the swollen starch granules due to the reduced swelling.