D-value

Abstract: Initially, the effect of water activity (a w ) on heat resistance of Bacillus cereus spores (decimal reduction time) was investigated. A linear relationship was found between log D and 1-a w . The combined effects of temperature (85-105°C), pH (4.56.5) and water activity (0.80-1) were then studied. A four parameter model was fitted to the data. This model appeared to be parsimonious with each parameter having a biological significance. Interactions between factors were observed but they accounted for <2.4% of the total variation and they were not taken into account by the model.

Abstract: Aqueous spore suspensions of Bacillus stearothermophilus ATCC 12980 were heated at different temperatures for various time intervals in a resistometer, spread plated on antibiotic assay medium supplemented with 0.1% soluble starch without (AAMS) or with (AAMS-S) 0.9% NaCl, and incubated at 55 degrees C unless otherwise indicated. Uninjured spores formed colonies on AAMS and AAMS-S; injured spores formed colonies only on AAMS. Values of D, the decimal reduction time (time required at a given temperature for destruction of 90% of the cells), when survivors were recovered on AAMS were 62.04, 18.00, 8.00, 3.33, and 1.05 min at 112.8, 115.6, 118.3, 121.1, and 123.9 degrees C, respectively. Recovery on AAMS-S resulted in reduced decimal reduction time. The computed z value (the temperature change which will alter the D value by a factor of 10) for spores recovered on AAMS was 8.3 degrees C; for spores recovered on AAMS-S, it was 7.6 degrees C. The rates of inactivation and injury were similar. Injury (judged by salt sensitivity) was a linear function of the heating temperature. At a heating temperature of less than or equal to 118.3 degrees C, spore injury was indicated by the curvilinear portion of the survival curve (judged by salt sensitivity), showing that injury occurred early in the thermal treatment as well as during logarithmic inactivation (reduced decimal reduction time). Heat-injured spores showed an increased sensitivity not only to 0.9% NaCl but also to other postprocessing environmental factors such as incubation temperatures, a pH of 6.6 for the medium, and anaerobiosis during incubation.

Abstract: Prevention of browning in pineapple puree by thermal inactivation of enzyme, Polyphenoloxisase (PPO), was examined between 40 and 90 °C and in relation to exposure time. The amount of inactivation was measured as a function of time and temperature under isothermal conditions. Reaction rate constant and activation energy (Ea) as well as Decimal reduction time (D) and z-value of thermal inactivation, were determined. The rate of inactivation varied with temperatures and follows a logarithmic law. Kinetic studies showed that the thermal inactivation (40–90 °C) of the PPO followed first-order kinetics.