Temperature functions of thermal death in yeasts and their relation to the maximum temperature for growth

Summary Parameters of thermal death were determined in 10 strains of yeast species whose maximum temperatures for growth (T _max) ranged from 22 to 49°C. Arrhenius plots of the specific thermal death rates (k _d) formed a positional sequence at the level of the experimental points that corrresponded in all but one case to the sequence of the respective T _max values. Extrapolated k _d values at higher or lower temperatures no longer formed this sequence.The correlation of the temperature functions with T _max could be characterized in terms of a new activation parameter, for which the name thermal death activation constant is introduced. It has the following form: T.D.A. – S where H and S are respectively the apparent heat and entropy of activation of thermal death and n is the number of degrees above T _max (expressed in °K) at which the T.D.A. constant exists.Seven mesophilic yeasts had a T.D.A. constant between 72 and 79 calxmol^-1 degree^-1 at n values between 1 and 4°. This suggested that the destructive process that limits k _d in these strains is of the same species as one that contributes to the establishment of T _max. Two psychrophilic yeasts apparently had a similar T.D.A. constant but at a high n value (about 12.5°C) which suggested that in these strains T _max is governed by a destructive process unrelated to the one that underlies thermal death. The strain of the nearly thermophilic Hansenula angusta (T _max 49°C) did not fit in either group.The significance of the T.D.A. constant is discussed and expressions for H and S in terms of bond activation parameters are proposed.