## Info

Log D

Log D

Temperature (T)

Temperature (T)

Figure 12. Time and temperature dependence of the thermal inactivation kinetics of bacterial spores in the thermal processing of canned foods. Source: Ref. 3, reprinted with permission, copyright 1992 by Marcel Dekker, Inc., New York.

Figure 12. Time and temperature dependence of the thermal inactivation kinetics of bacterial spores in the thermal processing of canned foods. Source: Ref. 3, reprinted with permission, copyright 1992 by Marcel Dekker, Inc., New York.

are known as survivor curves. The decimal reduction time, D, is expressed as the time in minutes to achieve 1 log cycle of reduction in concentration, C. As suggested by the family of curves shown, D is temperature dependent and varies logarithmically with temperature, as shown in the second graph. This is known as a thermal death time (TDT) curve and is essentially a straight line over the range of temperatures used in food sterilization. The slope of the line that describes this relationship is expressed as the temperature difference, Z, required for the line to traverse 1 log cycle (10-fold change in D). The temperature in the food product, in turn, is a function of the retort temperature (Tr), initial product temperature (Tt), location within the container (x), thermal diffusivity of the product (a), and time (t), as shown by the heat penetration curves at the bottom of Figure 12. Thus, the concentration of viable bacterial spores during thermal processing decreases as a function of the inactivation kinetics, which are a function of temperature. The temperature, in turn, is a function of the heat transfer considerations, involving time, space, thermal properties of the product, and initial and boundary conditions of the process.

### Microbiological Considerations

Heat Resistance. The heat resistance of microorganisms varies considerably. At any given temperature, this is generally expressed as a decimal reduction time (D-value), which is the heating time required to reduce the number of microorganisms by 90% (or to one-tenth of the initial). The temperature sensitivity of these D-values is expressed in terms of a Z-value, which represents the temperature range that results in a 10-fold change in the D-values. These two values can be realized as negative reciprocal slopes of logarithm of surviving microbial numbers vs time (D-value or survivor curve) and logarithm of D-values vs temperature (Z-value curve) as described by the functional expressions in Figure 12. Some typical D- and Z-values for selected microorganisms are given in Table 3.

As mentioned earlier, the Z-value curve, which describes the temperature dependency of the D-value, is often referred to as the TDT curve. It forms the basis upon which thermal process times and temperatures are determined and is shown in more detail in Figure 13. Once the TDT curve has been established for a given microorganism in a given food product, it can be used to calculate the time

Microorganism |
D25o-value, min |
z-value, °C |

B. stearothermophilus |
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