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Inactivation of E. coli in UCFMFigure 5.Arrhenius plot of ‘thermal’ and non-thermal (low water activity) inactivation of E. coli strains(Reproduced from Shadbolt et al., 1999).There is a distinct difference in slope above and below 50°C, reinforcing the proposition thatthe mechanisms involved in thermal and non-thermal inactivation are different. This will befurther supported by data presented in Section 5.2. Importantly, the relative effect of achange in temperature on the rate of inactivation is much greater at temperatures above theupper temperature limit for E. coli growth than temperature in the normal range for growth.4.3.4 ConclusionsFrom the studies and data reviewed above, several general rules about non-thermalinactivation in laboratory broth under conditions of relevance to UCFM processing emerge:i. increased temperature results in faster rates of inactivation over all phases of death;ii. the sensitivity of inactivation to temperature change is greater at temperatures > ~50°Ciii. the initial rapid decline in cell numbers is influenced by the sensitivity of the cell(exponential phase cells are known to be more susceptible to many stresses 7 );iv. the initial rapid decline in cell numbers (“Phase 1”) is influenced by the magnitude of the‘shock’, i.e. the rate and magnitude of environmental change;v. the relative effect of temperature on rate of inactivation appears to be more importantthan that of pH and water activity in the ranges of relevance to UCFM.4.4 Kinetics of Inactivation during UCFM ProductionFigures 6a – c indicate the changes that occur during the fermentation and maturation ofUCFM. Those data are reproduced from Grau (1996), but are representative of the results ofmany other studies (e.g. Glass et al., 1992; Hinkens et al., 1996; Riordan et al., 1998; Caseyand Condon, 2000).The results in Figures 6 require some interpretation in relation to the kinetics of inactivation.The first is that the initial rapid decline is not necessarily the same process as the Phase 1inactivation discussed in the Section 4. Fermentation temperatures are usually much higherthan maturation temperatures. As was suggested by the data presented earlier, and as willbe demonstrated clearly in Section 5.1, temperature has a relatively large effect on the rateof inactivation. Hence, the apparent rapid decline in survivors is most likely due to the higher7Recent results (J. Brown, pers. comm., October 2000) suggest that E. coli reaches an optimum level of resistance to acidshock in early stationary phase, but that this resistance then begins to decline again. See Figure 15, Section 6.Page 22 of 59