Thermal inactivation of two yeast strains heated in a strawberry product: Experimental data and kinetic model

The thermal resistances of Saccharomyces cerevisiae and Debaryomyces hansenii were studied in a high-sugar strawberry product. The effect of the composition of the growth medium on the heat resistance of the two yeast strains was evaluated. The results showed both strains to have different sensitivities to heat, and the cultivation conditions were observed to affect one strain. The decrease in yeast number was not always exponential and survivor curves often presented a preliminary lag phase followed by either an exponential or a parabolic decrease of the surviving population. The results were modelled with a three-parameter formula: ln(N-0/mN(1))(a) = kt where k is the kinetic constant, N,and N, the viable cells at r = 0 and after heating time t, and m and a are two parameters of the model. m represents the number of cells occurring in clumps and a characterizes the deviation from the exponential decrease. The kinetic constants vary with temperature according to Arrhenius's law. The values obtained in the medium used were higher than those observed in classically studied media like YMPG, particularly when cells were cultivated on malt agar. The results emphasize the need to take account of the non-exponential parts of the survival curves in order to establish thermal process schedules. (C) 1997 Elsevier Science S.A.