Thermal, pressure and shear on the inactivation of Lactobacillus brevis and Bacillus cereus

Research Poster
Jie Xu
Post Doctoral
V.M. (Bala) Balasubramaniam
Department of Food Science and Technology

Ultra-shear technology (UST) exposes bacterial cells to lethal factors including pressure, shear and heat. Limited studies evaluated the sensitivity of bacteria to these lethal factors. The aim of this study is to investigate the inactivation of two bacteria (Lactobacillus brevis and Bacillus cereus) by ultrashear applied at 400MPa and 40°C and 70°C. 
Cultures of L. brevis (1.6×1010 CFU/mL), a non-spore former, and B. cereus (3.2×108 CFU/mL) a spore former, were suspended in HEPES buffer (pH 7.3) and tested in this study. Bacterial suspension was processed using a bench scale ultra-shear equipment at 400MPa at 40°C (temperature contribution to microbial lethal effect ignored) or 70°C (temperature contributes to microbial lethal effect). The product exits through a shear valve at two different shear flow rates (0.41 ± 0.10g/s and 1.05 ± 0.06g/s). UST treatment time was varied with or without addition of 1.5 m long holding tube. The number of survivors was enumerated on lactobacilli MRS agar (L. brevis) and nutrient agar (B. cereus). Additional isothermal (40°C/70°C) and high-pressure processing (HPP; 400MPa, 40°C or 70°C) experiments were carried out to evaluate thermal only, pressure only and combined pressure-thermal effects.
The process come-up time for thermal (70oC, 0.1 MPa), HPP (70oC, 400 MPa) and UST (400 MPa & 70oC exit temperature at the shear valve) were 10, 30, and 3-10 (depending on flow rate) seconds respectively. This resulted in respective maximum reductions of 0.7, 8.4, 7.1-log for L. brevis and 0.6, 2.3, 2.1-log for B. cereus. Within the study conditions, shear flow rate and residence time, did not show significant (P>0.05) influence on microbial inactivation for both strains. 
This study evaluated the influence of pressure, temperature, and shear on two bacterial strain using batch and continuous high-pressure treatment. The study contributes to the development of safe harbor UST process conditions for beverages.