Graduate Thesis Or Dissertation
 

Application of probiotics and green tea extract in post-harvest processes of Pacific oysters (Crassostrea gigas) for reducing Vibrio parahaemolyticus and extending shelf life

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  • Vibrio parahaemolyticus is a human pathogen which is prevalent in marine environment. Consumption of raw or undercooked seafood contaminated with V. parahaemolyticus can cause foodborne illness. This study investigated the application of probiotics in depuration for reducing V. parahaemolyticus in raw Pacific oysters (Crassostrea gigas) and the utilization of green tea extract on inactivating V. parahaemolyticus and extending shelf life of oyster meats during refrigeration storage. Lactobacillus plantarum ATCC 8014, which exhibited strong bactericidal effects against mixture of five clinical V. parahaemolyticus strains mainly due to the production of organic acids, was utilized in the study. Populations of lactic acid bacteria in oysters increased from 1.83 to 4.66 log CFU/g after exposing oysters to artificial seawater containing L. plantarum at a level of 6.41 log CFU/ml after 20 h accumulation at room temperature. Levels of L. plantarum colonized in oysters decreased gradually when oysters were depurated at 20°C, but remained moderately colonized in oysters (3.10 log CFU/g) after 4 days of depuration. Adding cells of L. plantarum (10⁷ CFU/ml) to seawater for depuration of raw oysters inoculated with V. parahaemolyticus BE 98-2029 (O3:K6) at levels of 10⁴ MPN/g did not enhance V. parahaemolyticus reductions in oysters depurated at 15±1ºC but significantly (p<0.05) decreased levels of V. parahaemolyticus in oysters depurated at 10±1°C after 5 days without mortality compared with controls. Lactic acid bacteria other than L. plantarum ATCC 8014 can be explored for potential application in post-harvest treatments of raw oysters for reducing V. parahaemolyticus contamination. Longjing Tea extract containing 4.594 g/l total phenolic contents (TPC) as gallic acid equivalents (GAE) or higher determined by Folin-Ciocalteau reduced a mixture of five clinical V. parahaemolyticus strains in tryptic soy broth plus 1.5% NaCl from around 4.5 log CFU/ml to non-detectable level (<10 CFU/ml) within 8 h. Shucked oysters treated with tea extract containing 9.054 g/l TPC as GAE for 2 h at 23±1ºC at oyster/tea extract ratio of approximate 0.9 g/ml resulted in greater (p<0.05) V. parahaemolyticus reductions (0.75 log MPN/g) compared to controls (0.15 log MPN/g). Storing oysters treated with tea extract at 5±1°C in containers filled with tea extract at oyster/tea extract ratio of approximate 0.7 g/ml enhanced reductions of V. parahaemolyticus and extended the shelf life of oyster meats from 8 days for controls to 18 days for oysters stored in tea extract. Further studies need to investigate the effects of green tea extract treatments on the sensory attributes of oyster meats.
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