Reducing oyster-associated bacteria levels using supercritical fluid CO2 as an agent of warm pasteurization

An innovative approach to Post-Harvest Processing (PHP) of oysters is introduced focusing on the effects of supercritical carbon dioxide (scCO2) on bacterial contaminants trapped in the digestive system of oysters. Oysters were exposed to scCO2 under two conditions: (1) 100 bar and 37 °C for 30 min and (2) 172 bar and 60 °C for 60 min. Using FDA standard guidelines for food analysis, variations in the Aerobic Plate Count (APC) were assessed. It was established that exposing oysters to CO2 at 100 bar and 37 °C for 30 min and at 172 bar and 60 °C for 60 min induced 2-log and 3-log reductions in the APC respectively. The decrease in the microbial load as a result of treatment with scCO2 was found to be significant (P = 0.002). A release of adductor muscles from the shell was noted in oysters treated at 172 bar and 60 °C for 60 min; this was not the case for oysters treated at 100 bar and 37 °C for 30 min. A blind study allowing sensory analysis of treated vs. untreated oysters was also completed and no significant change in the physical appearance, smell, or texture was recorded. In this paper, we also report the effect of scCO2 on several bacterial isolates, including a referenced ATCC strain of a non-pathogenic Vibrio (Vibrio fischeri) as well as several other bacterial isolates cultured from oyster' tissues and found to share biochemical features common to pathogenic Vibrio strains. A complete inactivation (minimum 7-log reduction) was achieved with these latter bacterial isolates. A 6-log reduction was observed with V. fischeri.