THERMAL DEATH KINETICS OF B. STEAROTHERMOPHILUS SPORES IN SUGARCANE MUST

Thermal death kinetic parameters for Bacillus stearothermophilusspores were evaluated in sugarcane must (21.5°Brix, pH = 6.14) at temperatures ranging from 98 to 130C, using the thermal-death-time tube method and survivors count. Resulting survival curves showed strong nonlinearity and different shapes according to heating temperature. The 98 and 110C curves showed an initial shoulder or thermal lag and were adjusted to the logistic model. At the temperature range of 120–125C, the two-term exponential model for population with heterogeneous heat resistance was fitted, and at 130C, the classic linear model was suitable. It was shown that rate constants are influenced by temperature according to two irreconcilable methods: the Arrhenius and the Bigelow methods. Activation energy (Ea) obtained was 249.52 kJ/mol while thermal resistance parameter (z-value) calculated from Ea and the Bigelow method were 11.48 and 9.19C, respectively. Thermal death kinetic constant, k values, varied from 0.019 to 13.63/min. PRACTICAL APPLICATIONS The thermal degradation kinetics from this work, in conjunction with the appropriate sugar degradation kinetic data (Nolasco Junior 2005, Nolasco Junior and De Massaguer 2006), can be used to properly design an optimal thermal process to inactivate sugarcane must contaminants with optimal sugar content retention. Contaminant control in ethanolic fermentation processes could namely be based on temperature, a physical agent, instead of the current control practice based on antibiotics. Besides, the data from this research meet the more frequently found nonlinear microbial thermal death kinetic model application.