Kinetics of Listeria monocytogenes cell reduction in processed cheese during storage

The present work utilized data from a series of challenge tests conducted on a processed cheese product (pH = 5.1, a(w) = 0.93) aiming at elucidating whether the processed cheese can support or not the growth of Listeria monocytogenes when stored at 4,12 or 22 degrees C (Angelidis, Boutsiouki, & Papageorgiou, 2010). In these challenge tests three L. monocytogenes strains (Scott A, CA and IS951) were used for inoculating samples of processed cheese at three different inoculation levels. Although the results of this previous work have shown that the product did not support the growth of L. monocytogenes at either storage temperature, there were noticeable differences in the inactivation rates of the different L. monocytogenes strains at the different storage temperatures. Therefore, in the present study, the data from the early experiments were used to describe the kinetics of L. monocytogenes cell reduction in processed cheese during storage. A Weibull-type model was initially employed to describe the reduction of L. monocytogenes viable cells as a function of storage time, while the Arrhenius relationship was used to express the effect of temperature on the rate of cell reduction. For each strain, cell reduction data at all temperatures and all inoculum levels were treated as a single data set for the simultaneous determination, through non-linear fitting, of the cell reduction kinetic parameters and their temperature dependence. The proposed approach gave satisfactory results, in terms of agreement between measured and predicted cell reduction, as far as the effect of initial inoculum level and storage time and temperature is concerned. Strain variability was noticeable at the lower temperatures tested, with Scott A being the most resistant among the L. monocytogenes strains examined. (C) 2012 Elsevier Ltd. All rights reserved.