Comparison of the Thermal Inactivation Pattern of Zygosaccharomyces Fermentati Using Bigelow and Weibull Model

Introduction: The conventional approach for determination of D and z values, developed by Bigelow, has been used for decades to predict the effectiveness of thermal processing. However, the effective prediction of thermal inactivation cannot be evaluated from classical Bigelow model any longer because many microbial thermal inactivation patterns show not only linear survival curves but also various non-linear ones. Therefore, a number of mathematical models such as Weibull have been suggested to describe various microbial survival curves.

Purpose: The objective of this study was to compare Bigelow and Weibull models for prediction of the thermal resistance of Zygosaccharomyces fermentati which was isolated as a putrefactive microorganism from orange juice.

Methods: The BPS buffer (pH 7.4) and orange juice (pH 3.9, Brix 12%) were used as heating menstrum and inoculated by spoilage yeast which was grown at 35°C for 3 days to achieve roughly >10⁷ CFU/ml. The 0.07 ml of each inoculated menstrum was put into capillary tube followed by flame sealed. Then 3 tubes for each temperature were submerged in a water bath at 52, 55, 57 and 60°C, respectively, removed at various time intervals, cooled down immediately in cold water and enumerated those survivors from PDA plates at 35°C for 3 days.

Results: R² (0.95-0.99 in BPS, 0.93-1.00 in Juice) and RMSE (0.07-0.34 in PBS, 0.004-0.62 in juice) values of Weibull model were shown more reliable data than those of Bigelow model. Hazard plot for judging the applicability of Weibull model presented straight lines in both PBS (R² = 0.82-0.99) and orange juice (R² = 0.93-1.00). Through the 2nd fitting step, p of shape parameter and δ of scale parameter were recorded as 0.81, Log (δ) =0.24(T)-13.52 (R² = 0.99) in PBS respectively, on the other hand, 0.62 and Log (δ) = 0.26(T)-14.11 (R² = 0.98) were obtained from orange juice.

Significance: These comparative data suggest that the Weibull model is reliable to predict thermal inactivation and evaluate sterilization process against Zygosaccharomyces fermentati, spoilage yeast in orange juice, more effectively than the conventional Bigelow model.