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Inactivation of E. coli in UCFM 42.Figure 15.Results of Brown (in preparation) showing the decrease in resistance of E. coli as a theculture continues to age. Cells were removed from a stationary phase culture and subjectedto a pH 3.5 stress. Other data (not shown) indicates that resistance rises as the culturemoves into stationary phase, consistent with the observations of many other. The datashown, however, indicates that after 24h in stationary phase the rate of inactivation at pH 3.5becomes more rapid, and results in truncation of ‘Phase 2”. The times indicated are hours ofincubation at 25°C after commencement of stationary phase.Faith et al. (1998a) investigated the effects on inactivation of conditioning of UCFM batter bytemperature treatments, involving combinations of storage at 4°C (‘refrigerated’/’thawed’), at13°C (‘tempered’), and/or -20°C (‘frozen’) prior to initiation of the fermentation. Batter whichwas tempered, frozen and thawed experienced the greatest amount of inactivation at the endof maturation (-2.1 logCFU), followed by frozen and thawed (-1.6 logCFU), with batter thathad only been refrigerated achieving a 1.1 logCFU decrease.Grau (1996) observed that freezing of challenge organisms in the meat, prior to preparationof the batter did not have a marked effect on subsequent survival during UCFM processing,but that destruction was greater when cells had been grown on the meat. On the basis ofthese observations Grau proposed that: “a system of testing survival of E. coli duringproduction of fermented meats, which adds cells at the time of chopping and mixing mayunderestimate the real reduction in viable cells. A system closer to real life should grow thecells on the meat beforehand”.One interpretation of the above observations is that cells which have been grown on themeat, or which have been tempered at growth permissive temperatures, are more likely to bein exponential phase at the beginning of the fermentation. Our preliminary data (unpublished)for the effect of transferring growing E. coli cells across the temperature boundary for growthsuggest that this transition does not cause the induction of a lag phase 14 , i.e. anexponentially growing cell that is growth arrested by a reduction in temperature does notnecessarily adopt the more resistant state characteristic of stressed cells. Thus, whiletempering as a part of UCFM production would be an advantage, it will need to bedemonstrated that all producers use a tempering step. If not, this assumption could lead toover-prediction of the extent of inactivation, i.e. fail-dangerous observations. Furthermore, we14Note added in proof: Gill et al. (2001) presented results indicating that lag phases can be induced in E.coli by temperaturetransitions at temperatures below the lower limit for E.coli growth.Page 42 of 59