Survivor curves

The resistance of an organism to a sterilizing agent can be described by means of the D-value. For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors Fig. 20.2k). The calculation of the D-value assumes a linear type A survivor curve (Fig. 20.1), and must be corrected to allow for any deviation from linearity with type B or C curves. Some typical D-values for resistant bacterial spores are given in Table 23.2 (Chapter 23). 

Data obtained by the survivor-curve method are plotted semilogarithmi-cally. Data points are connected by least-squares analysis. In most cases the equation used is the first-order death rate equation,... 

Many micro-organisms produce nonlinear survivor curves, such as 1-B in Figure 1. The cause of nonlinear survivor curves has been explained by several theories, such as the multiple critical sites theory [9], experimental artifacts [10], and the heterogeneity of spore heat resistance [11]. Mathematical models for concave survivor curves have been developed by Han et al. [12], They are quite... 

Figure 4 Survivor curves showing the effect of decreasing the microbial load (A) from 106 to 102 on the time required to achieve a probability of nonsterility (B) of 10 6.

Han, Y. W. Death rates of bacterial spores Nonlinear survivor curves. Can J Microbio 21 1464-1467 (1975). 

Fig. 20.1 Typical survivor curves for bacterial spores exposed to moist heat or gamma-radiation.

The term sterile , in a microbiological context, means no surviving organisms whatsoever. Thus, there are no degrees of sterility an item is either sterile or it is not, and so there are no levels of contamination which may be considered negligible or insignificant and therefore acceptable. From the survivor curves presented, it can be seen that the... 

Typical microbial survivors found can be classified into four general types as shown in Figure 4.2 (Moats, 1971). Curve A represents a typical survivor curve with a logarithmic death rate. Curve B is a type commonly found and shows an initial lag in death rate followed by a logarithmic death rate. Curve C is similar to curve A,... 

FIGURE4.3 Log arithmic survivor curve. (Modified from Stumbo, C.R., Thermobacteriology in Food Processing, Academic Press, New York, 1965, 57-96.)... 

The rapture correlation suggests random hits or death by chance , somewhat similar to survivor curves for the thermal destruction of bacterial spores. Effects of agitation conditions, viscosity of liquid, pellet diameter and culture age on rapture rate coefficient, k were studied, and the following experimental equation was obtained. 

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