C in vacuum-packed minced beef samples heated in a laboratory water-bath. The experiment was repeated using vacutainers, which allowed heating of the beef to the desired temperature before inoculation. D-values of between 0Á15 and 36Á1 min were obtained for L. monocytogenes. Pre-heating the beef samples signi®cantly affected (P < 0Á05) the D 60 value only. D-values for Y. enterocolitica ranged from 0Á55 to 21Á2 min and all the D-values were signi®cantly different (P < 0Á05) after pre-heating. In general, the D-values obtained for core inoculated solid beef samples were signi®cantly higher (P < 0Á05) than those generated in minced beef when heated in a Barriquand Steri¯ow commercial retort.

The heat resistance of a wild type and nalidixic acid resistant strain of Yersinia enterocolitica, and Listeria monocytogenes, was measured in meat (minced beef and minced beef homogenate) and potato substrates over the temperature range 50–60C. Comparisons of heat resistance were determined using D‐values calculated using a linear survival model. The results showed that the wild‐type strain of Y. enterocolitica was more heat resistant than the mutant (p<0.05). Under most conditions, the use of a nonselective/overlay recovery medium resulted in higher D‐values compared to a selective recovery medium (p<0.05). Analysis of the data using a nonlinear survival model (D1 and D2 ‐values) suggested the presence of heat resistant subpopulations and was particularly evident for the mutant strain, and in potatoes compared to minced beef.

The effect of culture growth phase on induction of the heat shock response in Yersinia enterocolitica and Listeria monocytogenes, was examined. Exponential or stationary preconditioned cultures were heat shocked and survivor numbers estimated using selective and overlay/resuscitation recovery techniques. The results indicate that prior heat shock induced increased heat resistance in both micro-organisms to higher heat treatments. Heatshocked cells of each micro-organism were able to survive much longer than non-heatshocked cells when heated at 55 C. The size of the change in heat resistance between heatshocked and non-heat-shocked cells was greatest for exponential cultures (X:X). Results indicate that the overall relative thermal resistance of each pathogen was dependent on cell growth phase. Stationary cultures (S:S) were signi®cantly (P < 0Á01) more thermotolerant than exponential cultures (X:X) under identical processing conditions. Under most conditions, the use of an overlay/resuscitation recovery medium resulted in higher D-values (P < 0Á05) compared with a selective recovery medium.

1999. The effect of sodium lactate (NaL) (0, 2·4 or 4·8%), in heating and recovery media, on Yersinia enterocolitica and Listeria monocytogenes numbers recovered from minced beef heated at 55°C, was examined. Survivors were enumerated on selective media at pH 5·7/7·4 (Y. enterocolitica) or pH 5·7/7·2 (L. monocytogenes). Recovery of the organisms depended on the pH and NaL levels in the recovery medium. The heat resistance of Y. enterocolitica (P ³ 0·001) and L. monocytogenes (P ³ 0·01) decreased as the concentration of NaL in the minced beef increased from 0 to 2·4% or 4·8%. The thermal destruction of pathogens in foods processed using mild temperatures may be enhanced by the addition of 2·4% NaL.