Improved Microbial Safety of Direct Ozone-Depurated Shellstock Eastern Oysters (Crassostrea virginica) by Superchilled Storage

Abstract

The effect of superchilled storage at -1°C on the microbial safety of oyster depurated with 0.2, 0.4, and 0.6 mg/L ozone was studied for 14 days. Fecal coliforms (4,100-16,000 MPN/100 g), <i>Escherichia coli</i> (1,500-3,650 MPN/100 g), <i>Vibrio cholerae</i> non-O1/non-O139 (13.0-102.0 MPN/g), and <i>Salmonella</i> spp. (2.270-3.035 × 10³ CFU/g) were initially present in raw oysters. After 6 h depuration, fecal coliform counts decreased (<i>P</i> < 0.05) to 300, 20 and 20 MPN/100 g for 0.2, 0.4, and 0.6 mg/L treatments, while a 0.3 log decrease in control oysters was observed. Initial <i>E. coli</i> counts decreased (<i>P</i> < 0.05) in oysters to 50, 20, and 20 MPN/100 g for 0.2, 0.4, and 0.6 mg/L treatments, respectively. A 1 log reduction in <i>V. cholerae</i> non-O1/non-139 levels were observed in 0.4 and 0.6 mg/L-treatments after 2 and 4 h depuration. <i>Salmonella</i> spp. was not detected in oyster samples after 6 h depuration in 0.4 and 0.6 mg/L-ozone treatments. Considering the bacterial loads after depuration, at the end of superchilled storage the 0.4 mg/L-ozonated oysters attained lower (<i>P</i> < 0.05) fecal coliform levels (280 MPN/100 g) and <i>E. coli</i> counts in 0.4 and 0.6 mg/L-ozonated oysters (20 and 95 MPN/100 g, respectively). A 2-log decrease in <i>V. cholerae</i> non-O1/non-O139 levels on day 5 in 0.4 and 0.6 mg/L-ozonated oysters (< 0.3 MPN/g) was attained. <i>V. cholerae</i> non-O1/non-O139 counts in control oysters decreased 1 log on day 9 of superchilled storage. <i>Salmonella</i> spp. was not detected in ozonated and superchilled stored oysters. Levels of fecal coliforms, <i>E. coli, Salmonella</i> spp., and <i>V. cholerae</i> non-O1/non-O139 in non-ozone depurated oyster samples were higher than in control, 0.4 and 0.6 mg/L ozonated oyster samples during superchilled storage. The cumulative mortality rates after 14 days of storage for superchilled oysters (22.2%) was higher (<i>P</i> < 0.05) than 0.6 mg/L O₃ (7.2%) and 0.4 mg/L O₃ (5.8%) treatments, and control oysters (5.6%). pH values in control oysters decreased significantly (<i>P</i> < 0.05) throughout the storage period but not in oysters of both ozone treatments, indicating no detrimental effects on oyster survival. The results of this study suggest that superchilled storage enables ozonated shellstock oysters (0.4 mg/L-6 h) stored for 9 days to be safe human consumption.