Thermal death slope

In Figure 9.11a, we see a representation of a thermal death curve for Bacillus stearothermophilus spores steam-sterilized at 121°C for x minutes. Figure 9.11b shows the piecewise delimiters. Generally, the shoulder values (near time 0) are not used. So, the actual regression intercept is near 8 logio scale. The slope changes at x 10 min. 

The inactivation rate constants at reference temperature To and pressure po are given by kr and kj, , respectively. is the activation energy and is the activation volume, both derived from the corresponding slopes of the semilogarithmic plots [2j. Alternatively, for the first-order kinetics, the thermal death time and pressure death time approach can be applied (compare with Eq. (9.8)) with Dr and respectively being the decimal reduction times at reference temperature and pressure [2,46—48]. 

As predicted by the Arrhenius equation (Sec. 4), a plot of microbial death rate versus the reciprocal or the temperature is usually linear with a slope that is a measure of the susceptibility of microorganisms to heat. Correlations other than the Arrhenius equation are used, particularly in the food processing industry. A common temperature relationship of the thermal resistance is decimal reduction time (DRT), defined as the time required to reduce the microbial population by one-tenth. Over short temperature internals (e.g., 5.5°C) DRT is useful, but extrapolation over a wide temperature internal gives serious errors. 

The slope of this straight line is referred to as the isokinetic temperature, Tp (Barnes, Vogel, and Gordon, 1969 Leffler, 1966). This relationship with a linear thermodynamic compensation of rates is called enthalpy/entropy compensation it is important for the physiological stability of proteins (Lumry and Eyring, 1954). The value Tp is generally between 270°K and 320°K for the thermal destruction of cells Tp is between 320°K and 350°K. The importance of this fact is the possible implication of a uniform mechanism for cell death through protein denaturation. 

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