Modelling thermal inactivation of Listeria monocytogenes in sucrose solutions of various water activities

The heat resistance of Listeria monocytogenes was determined in sucrose solutions with water activity (aw) ranging from 0.99 to 0.90. At all temperatures investigated shape of the survival curves depended on the aw of the treatment medium. The survival curves for aw=0.99 appeared to be linear, for aw=0.96 were slightly upwardly concaved and for aw=0.93 and 0.90 were markedly concave upward. A mathematical model based on the Weibull distribution provided a good fit for all the survival curves obtained in this investigation. The effect of the temperature and aw on the Weibull model parameters was also studied. The shape parameter (p) depended on the aw of the treatment medium but in each medium of different aw the temperature did not have a significant effect on this parameter. The p parameter followed a linear relationship with aw. The scale parameter (δ) decreased with the temperature following an exponential relationship and increased by decreasing the aw in the range from 0.99 to 0.93. However the δ parameter of survival curves obtained at aw=0.90 were lower than those obtained at aw=0.93. A mathematical model based on the Weibull parameters was built to describe the joint effect of temperature and aw on thermal inactivation of L. monocytogenes. This model provides a more complete information on the influence of the aw on the L. monocytogenes than the data initially generated. The model developed indicated that the effect of the aw on the thermal resistance of L. monocytogenes varied depending upon the temperature of treatment.