5. The Decimal Reduction Time.  (TOC)

The time required to reduce the population of a particular organism by a factor of ten is a commonly mentioned quantity. It is very simple to calculate from Equation (9). Equation (9) can be recast as

from which the decimal reduction time is

Experimentally, suppose a time series of samples is taken from a suspension of vegetative cells held at a constant temperature greater than the minimum lethal temperature. If the data are then plotted with ln(N(t)/N(T₀)) as the ordinate and time t as the abscissa, a straight line of negative slope will usually be observed for at least a portion of the data. The absolute value of the slope of this portion of the line, if it exists, is the thermal death constant K, with dimensions of reciprocal time. The time between any two points with a ten-fold difference of viable organism numbers on the linear portion of the line is the Decimal Reduction Time D. From Equation (10) it follows that organisms with a large thermal death constant are less resistant to killing than are organisms with a smaller thermal death constant. Thermodynamically, the former have a lower activation energy.