198: Effects of Ascorbic Acid on Thermal Death Kinetics of Salmonella spp. and Enterococcus Faecium in High-Temperature and Extremely Low Humidity Conditions

Introduction

Modeling microbial inactivation under drying conditions is essential for effective pathogen control while preserving quality. Although studies suggest high acidity reduces bacterial thermal resistance in fruits, its impact under high-temperature, low-humidity environments relevant to drying processes remain unclear. This work sought to characterize the impact of high acidity on the thermal death kinetics of Salmonella spp. and Enterococcus faecium NRRL B-2354 (surrogate) under high-temperature and extreme low-humidity conditions that are relevant to drying process.

Methods

Sessile bacterial cells of three-strain Salmonella cocktail (S. Enteritidis PT30 STCC BAA-1045, S. Mbandaka 69858, and S. Tennessee K4643) and E. faecium were inoculated on sand (50 g), with or without adding 1 mL of 3.4% ascorbic acid (pH 2.30), reflecting high-acidity environments in certain fruits. The inoculated sand was dried in a chamber at room temperature and RH <10%. Thermal resistance of Salmonella and E. faecium, with and without the addition of ascorbic acid was evaluated under isothermal (80°C, 100°C, and 120°C) and constant-humidity conditions (RH <0.5%) in thermal water activity cells (TAC) with silica gels as humidity suppressor.

Results

Upon inoculation and drying on sand, initial populations of Salmonella and E. faecium were 9.1±0.7 Log CFU/g and 8.6±0.4 log CFU/g (mean±sd) respectively. The addition of ascorbic acid reduced E. faecium populations by 1.4 log while had no significant impact on Salmonella. Under all circumstances, the thermal inactivation (log reduction) increased linearly with treatment time, D-values decreased exponentially with increase of temperature, following the Arrhenius equation. The Z-values (16–21°C) showed temperature impacts were consistent with or without ascorbic acid on both bacteria. However, D-values were significantly lower with addition of ascorbic acid at certain temperature for Salmonella and E. faecium, with log D-values reduced by 1.1–1.6, corresponding to an 11–38-fold decrease in D-values. Overall, ascorbic acid significantly reduced the thermal resistance of both bacteria cultures, with E. faecium serves as an effective Salmonella surrogate.

Significance

The findings suggest that high-acid food products may require less intense heat treatments to achieve microbial safety during drying process. Additionally, incorporating product pH into microbial inactivation models is essential for accurate predictions and process optimization.

Authors: Rajesh Dangal, Tejaswi Boyapati, Xue Sha, Kasiviswanathan Muthukumarappan, and Ren Yang*