Purpose: The purpose of this study was to (i) compare spore formation of Sporolactobacillus nakayamae in four different growth media, (ii) compare first-order and Weibull models in describing the data, and (iii) calculate z-values for each model.
Methods: Sporolactobacillus nakayamae was isolated from a food model containing cooked mashed potatoes with fresh cleaned scallions added post-cooling Spores were prepared using MRS medium, reinforced clostridial medium (RCM), Glucose Yeast Peptone with 1.5 mmol/L papaverine (GYP), and All Purpose Tween medium with sucrose and bromocresol purple (APT) and diluted in PBS to 105-106 CFU/ml. Thermal inactivation was performed for different treatment times (0 -25 min) at three temperatures (70, 75, and 80°C) using 2 ml vials. Survivors were determined by plating on MRS, RCM, GYP, or APT agar. Weibull and first-order models used to describe survivor curve kinetics and thermal death times. A comparison test (ANOVA, Post Hoc test) was used to analyze the treatments on survival ratio.
Results: Thermal inactivation parameters for S. nakayamae spores varied widely depending upon media formulation. GYP media consistently yielded the highest D-value over the three temperatures tested. For GYP media, the D-values for S. nakayamae were in the range of 25.24±1.57 to 3.45±0.27 min for the first-order model and 24.18±0.62 to 3.50±0.24 min for Weibull model at 70 and 80°C, respectively. The z-values determined for S. nakayamae spores were 11.91±0.29°C using the Weibull model and 11.58±0.43°C using the first-order model. The Weibull model consistently produced the best fit for all the survival curves.
Significance: This study provides novel and precise information on thermal inactivation kinetics of S. nakayamae which will enable reliable thermal process calculations for eliminating this spoilage bacterium.