Modeling non-linear survival curves to calculate thermal inactivation of Salmonella in poultry of different fat levels

Survival curves of a cocktail of eight serotypes of Salmonella in ground po ultry of different fat levels (1-12%), when heated rapidly to specified tem peratures (58-65 degreesC), were examined. Because many of the survival cur ves were concave, values for two parameters: the asymptotic D-value and the "lag" times were estimated and used to develop secondary models for estima ting the time needed to obtain a 7 log(10) relative reduction as a function of fat level and temperature. To compute the necessary time, at a given te mperature and fat level, the estimated lag time should be added to the prod uct of 7 and the estimated asymptotic D-value. A model was also developed f or estimating the standard error of the estimated times, so that upper conf idence bounds for the necessary times can be computed. It was found that la g times increase with higher fat levels. The effect of fat on D-values depe nded on the species; it is estimated that, for a given increase of fat leve l, the increase of the D-value would be greater for ground chicken than tha t for ground turkey. In addition, there was a statistically significant spe cies effect on D-values, with higher D-values for ground turkey than for gr ound chicken at the higher temperatures studied. The thermal death curves d isplayed a non-linear tendency, however, for estimation purposes, a linear curve was assumed. There was not a statistically significant interaction ef fect of fat levels and temperatures on D-values, thus, for modeling, it was assumed that z-values were not dependent on the fat levels. The z-values f or ground chicken and turkey were estimated to be 5.5 degreesC and 6.1 degr eesC, respectively, and are statistically significantly different. These fi ndings should have substantial practical importance to food processors of c ooked poultry, allowing them to vary their thermal treatment of ready-to-ea t poultry products in a safe manner. Published by Elsevier Science B.V.