Inactivation of Lactobacillus brevis cells and Bacillus cereus spores as influenced by pressure, shear, thermal, and valve geometry

Abstract

This study investigated the influence of pressure, shear, temperature, and their interactions on the inactivation of two bacteria–Lactobacillus brevis and Bacillus cereus spores. Cell suspension of L. brevis (1.6 × 1010 CFU/mL) and spore suspension of B. cereus (3.2 × 108 CFU/mL) were suspended in HEPES buffer (pH 7.3) and subjected to ultra-shear treatment (UST) at 400 MPa, process temperatures of 40 and 70°C, and flow rate of 0.0012 ± 0.0003 kg/s. In separate equipment, thermal (0.1 MPa-70°C-0/5 min) and high-pressure processing (HPP-400 MPa-40°C and 70°C-0/5 min) experiments were carried out to evaluate thermal only, pressure only, and combined pressure-thermal effects. Pressure-only treatment at 400 MPa for 0 min (come-up time) at 70°C resulted in 8.4 and 2.3 log reductions of L. brevis cells and B. cereus spores, respectively. After UST treatment at 70°C, L. brevis and B. cereus spores were reduced by 7.1 and 1.6 logs, respectively. Both strains showed different sensitivities to thermal, pressure, shear, and their combinations. B. cereus spores were relatively more resistant to HPP and UST treatments. Three valve geometries, namely, ultra-shear valve, needle valve, and tubular valve, were used to compare UST treatment effect on L. brevis and B. cereus spores at 40 and 70°C. UST treatment using the ultra-shear valve produced inactivation of 2.0 and 7.1 log cycles for L. brevis during 40 and 70°C process temperatures, respectively, which was relatively higher than needle valve and tubular valve. This higher inactivation could be attributed to the smaller valve gap and higher shear rate in the ultra-shear valve. Further, the comparatively higher inactivation at 70°C UST (7.1 log) emphasized the role of temperature on inactivation during UST. For both L. brevis and B. cereus, the inactivation was least in the tubular valve with relatively lesser shear rate, which indicated the desirable contribution of high shear rate for microbial inactivation. The information from this study contributes to the development of safe harbor UST process conditions for liquid foods.