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Inactivation of E. coli in UCFMFigure 8 (facing page). Data of Wang et al. (1996) showing the population dynamics of E. coli O157:H7 inbovine faeces in relation to temperature, and pH and water activity changes in the product.Figures A and B (reproduced from Wang et al., 1996) show the population changes at 5°C(open circles), 22°C (open squares) and 37°C (closed circles) in the faeces for initial E. colipopulations of 10 3 or 10 5 cful/ml. Figure C and inset (first 10 days) show the time at whichthe conditions in the faeces first prevent growth of E. coli due to water activity depression(i.e. below ~0.95), and its co-incidence with the sudden decline in the population of E. coli inthe faeces. pH and a w values are shown by closed and open symbols respectively (5°C,squares; 22°C, circles; 37°C, diamonds). The arrows indicate data that were obtained by anenrichment method and that are not fully quantitative, but rather represent a detection limitof 1 CFU/g, i.e. 0 LogCFU/gOrganic acidsThe principle organic acid produced during fermentation is lactic acid. The potential forgrowth of E. coli will be limited also by undissociated lactic acid in the batter, initially presentin the meat at levels in the millimolar range, but this is insufficient on its own to preventE. coli growth (see Section 4.2).4.4.2 Ecology of E. coli during maturationAs stated earlier, there is little knowledge or understanding of the processes of non-thermalinactivation. Most of the available data (presented earlier and reviewed in detail in Section 5)suggest that once conditions in the batter prevent growth few factors, other thantemperature, have a strong effect on the rate of inactivation. A common feature of the datadescribing E. coli inactivation during maturation is that rate of inactivation is generally quitesteady at constant temperature despite the changes in water activity, pH and, by inference,organic acid concentration.4.4.3 In–product studies of the potential for a 3-log kill during UCFM productionIn response to the West Coast USA salami outbreak, and subsequently introducedlegislation, a series of studies of the kinetics of inactivation of E. coli in UCFM products wasundertaken. Many of these originate in USA, and were undertaken by the Food ResearchInstitute (FRI) at the University of Wisconsin at Madison. That work was initiated in anindustry funded collaboration, through the National Cattlemen’s Beef Association andcomprised 30 trial runs, collected over 70,000 pieces of data, and was intended to test theefficacy of the six most common UCFM processes of the US UCFM industry. Variablesincluded in those studies were:• fermentation temperature (21, 32, 43°C)• low and high acid targets (final pH in the ranges 4.6 - 4.8, or 5.0 – 5.3)• the effect of heating (38, 43, 49, 52°C) or holding at the fermentation temperature for aperiod of 7 days after the pH target was reached• final water activityThe variables combinations are shown in Appendix 3 and the results summarised in Table 7below. Unfortunately, the interim update report (Nickelson, 1996) did not specify thematuration times and temperatures, but drying temperatures of ~13°C are common in USprocesses.In addition to the specific results from that study, the main conclusions were that unlesseither a cook step, or a holding step at temperature higher than 30°C was applied, none ofthe processes yielded a 3-log reduction. Holding was more effective at higher temperatures.Inactivation during FermentationIn USA, large quantities of pepperoni are consumed in the form of pizza topping (Hinkens etal., 1996) and, subsequent to the Blue Ribbon Task Force studies, many studies of E. coliinactivation in UCFM have employed a pepperoni-style process. Many of those studies havePage 28 of 59
Inactivation of E. coli in UCFMconfirmed that standard fermentation and drying steps usually achieve a 2-log reduction butcannot deliver a 3-logTable 7. Summary of results of Blue Ribbon Task Force (Nickelson, 1996)Temperature (°C) pH Process Average ± SD21 ≤ 4.6 dry 2.08 0.23≤ 4.6 hold 2.26 0.5≥ 5.0 hold* 4.11 2.1≥ 5.0 hold 2.93 0.96≥ 5.0 heat 5.83 0.26≥ 5.0 heat 5.61 0.532 ≤ 4.6 hold 6.43 0.5≤ 4.6 hold 4.72 0.77≤ 4.6 heat 6.75 0.26≤ 4.6 heat 6.65 0.23≥ 5.0 hold 2.87 0.37≥ 5.0 heat 6.43 0.5543 ≤ 4.6 dry 2.45 0.59≤ 4.6 dry 2.12 0.2≤ 4.6 hold 6.34 0.36≤ 4.6 hold 6.42 0.32≥ 5.0 hold 5.88 0.65≥ 5.0 hold 6.03 0.42≥ 5.0 heat* 4.99 1.66≥ 5.0 heat 4.52 1.09* In both of these trials it was suspected that inadequate heat was applied in one of the triplicates.reduction in E. coli numbers (Glass et al., 1992; Grau, 1996; Nickelson et al., 1996; Sauer etal., 1997; Vanderlinde, 1999). Faith et al. (1997) reported that fermentation at 36°C for up to16 hours resulted in a 1 log decrease of E. coli O157:H7. This decline is similar to othersreported (Hinkens et al., 1996; Calicioglu et al., 1997; Faith et al., 1998a; Faith et al., 1998b;Riordan et al., 1998; Vanderlinde, 1999; Casey and Condon, 2000). Exceptions where ahigher kill was achieved involved the use of higher fermentation temperatures or increasedamounts of preservatives and lower pH levels.Grau (1996) reported a 2.1-log reduction with 161ppm nitrite, 3.05% NaCl, and fermentationto pH 4.6 at 26°C for 72 hours. Petchsing and Woodburn (1990) reported a 2.5-log reductionin E. coli after fermentation at 30°C for 72-96 hours to a pH of around 4.6, with 200ppm nitriteand 3% NaCl 9 . Riordan et al. (1998) observed a 3.36-log decrease in E. coli O157:H7 afterthe fermentation step of pepperoni manufacture.However, this was achieved in the presence of increased levels of preservatives (300ppmnitrite), NaCl (3.3%) and low pH (4.4) at a high temperature of 38°C. Those authors alsoobserved, however, that increasing the amount of preservatives in fermented products waslikely to result in unacceptable organoleptic changes.Inactivation during MaturationStudies report pathogen reductions from ~ 0.5 - 0.90-log during drying (Grau, 1996; Faith etal., 1997; Faith et al., 1998a; Faith et al., 1998b). Combined with typical reductions after9The safety of this product (Nham = Thai-style fermented sausage) is questionable, since no maturation step is employedand the bacteria used in this study were not of an acid-tolerant variety. See Appendix 4.Page 29 of 59
Page 1 and 2: Final reportFOOD SAFETYPrepared by:
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