Modeling of Salmonella growth in chopped onions stored at various temperatures

In this study, the growth and inactivation behavior of Salmonella spp. on diced onions under different temperature conditions was evaluated, and the data were analyzed using primary and secondary mathematical models. Experimental results showed that Salmonella spp. grew significantly within the temperature range of 15–45°C, whereas no growth was observed at 4°C, 7°C and 10°C, where inactivation occurred over time. The temperature range of 30–40°C was identified as optimal for Salmonella growth in terms of both growth rate and maximum population density. However, at 45°C, the growth rate declined, and stress effects were observed on the cells. For primary modeling, the Baranyi model was applied to data where growth was observed, while the log-linear model was used to describe inactivation at lower temperatures. The Baranyi model demonstrated a good fit to the growth data, with determination coefficients (R²) ranging between 0.95 and 0.99 The specific growth rate (µmax) increased with temperature, reaching its peak value at 40°C (3.77 h⁻¹). The lag phase duration (tlag) decreased with increasing temperature; while it was approximately 35 hours at 15°C, it shortened to about 2.5 hours at 45°C. The highest bacterial concentration (ymax) was recorded at 30°C (8.94 log CFU/g), with a subsequent decrease at higher temperatures. Analysis with the log-linear model showed that inactivation occurred at lower temperatures, with the fastest inactivation observed at 4°C, where the D-value was calculated as 53.81 hours. This suggests that Salmonella can be more effectively controlled at lower temperatures, with 4°C being particularly critical for microbial safety. For secondary modeling, the full Ratkowsky square root model was employed to describe the temperature dependence of the growth rate, achieving a high correlation (R² = 0.89). The model estimated the minimum and maximum growth temperatures for Salmonella spp. as (Tmin) 7.09°C and (Tmax) 45.25°C, respectively. In addition, the relationships between temperature and other parameters such as lag phase and maximum population density were modeled using polynomial equations. Notably, the inverse second-order polynomial used for lag time versus temperature showed perfect fit (R² = 1.00). The maximum population level exhibited a parabolic response to temperature, increasing up to an optimum point and then declining. Overall, the study highlights that fresh-cut vegetables like diced onions, due to their high-water activity, can serve as a favorable environment for rapid Salmonella proliferation under improper temperature conditions. The derived model parameters allow for more accurate predictions of Salmonella growth limits and kinetic behavior. These findings are highly relevant for improving microbial risk assessments and informing food safety strategies in production, storage, and distribution of diced onions.