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Inactivation of E. coli in UCFM 59.APPENDIX 5: Discussion of Grau's Broth Model Results with In-ProductResultsOn the basis of his studies of E. coli inactivation Grau (1996) concluded that “model [broth]systems can be used to indicate general effects …. but cannot be used to provide specificrates of destruction”. Grau’s conclusion seems to have been based on his data forinactivation in an acidified, water activity modified broth compared to an acidified, wateractivity modified minced beef model. These data were shown in Figures 9 as the Grau #3,Grau #4 and Grau #5 data sets. In each of those data sets the relative temperature responseis nearly identical, and while the absolute rate of inactivation at 25°C is approximately twofoldfaster in the broth system than in the minced beef, the rates are very similar at 15°C.Those results are based on the average inactivation of 5 strains, however, and there isconsiderable variation in the data for individual strains as shown in Table 8 below. It shouldalso be noted that the inactivation was reported to be non-linear in some cases in bothsystems. The rate data summarised below has ignored this to attempt to simplify thecomparison of the data.Table 8.Data of Grau (1996) comparing rates of inactivation of 6 strains of Escherichiacoli in meat- of broth-based model of UCFM.Inactivation Rate (logCFU/day)25°C 15°CStrain Meat Broth Ratio Meat Broth RatioL1A 0.150 0.180 0.83 0.067 0.005 13.4USS 0.250 0.550 0.45 0.100 0.240 0.42Adelaide 0.183 0.150 1.22 0.100 0.080 1.25Human H7 0.167 0.700 0.24 0.071 0.160 0.449001 0.250 0.660 0.38 0.124 0.210 0.59While there is some variation among the pattern of response of the individual strains, theacidified, water activity-modified broth system produces greater rates of inactivation at 25°Cthan the meat-based model system. However, the rate of inactivation in the meat system isrelatively slow when compared to the rate of inactivation in salamis from many other studiesshown in Figure 9b. This suggests that the meat based data at 25°C is anomalously slow,perhaps due to uncontrolled changes in the minced beef due to the growth of competingflora, as alluded to by Grau (1996).Page 59 of 59