Post-harvest storage conditions are informed by recommendations to keep oysters as cool as possible to limit pathogen growth, while keeping the animals alive since dead seafood may lead to rapid spoilage and adversely affect microbiological safety. In Australia, guidelines for temperate Pacific oysters (POs) (Magallana gigas) and Sydney Rock Oysters (SROs) (Saccostrea glomerata) are based on their different responses to spoilage at different temperatures, however these temperatures may not be relevant for tropical species such as Blacklip rock oysters (BROs) (Saccostrea echinata/ lineage J). BROs grow in warm waters which support many species of indigenous aquatic microbes including Vibrio spp., that are potentially pathogenic to humans. In Australia, Vibrio spp. are recognised as an emerging food safety risk. The objective of this study was to determine the effect of postharvest storage temperature on the growth rate of tropical Vibrio parahaemolyticus strains in artificially inoculated BROs to provide the necessary foundation for postharvest temperature control plans.
A mixture of four V. parahaemolyticus strains isolated from tropical rock oysters were injected into BROs, the oysters placed in open plastic bags and stored at 4°C, 13°C, 18°C and 25°C. Three oysters were pooled per sample, with 5 replicates per time point at each temperature. The level of V. parahaemolyticus was measured at different time-points in the stored oysters by plating diluted oyster homogenates on CHROMagarÔ Vibrio. Counts were transformed to log10 values, and lines or curves fitted to the data. Growth rates (log10 CFU/h) were calculated from best fit lines at 4°C and 13°C. Specific growth rates and maximum population densities (log10 CFU/g) at 18°C and 25°C, data were imported into https://foodmicrowur.shinyapps.io/biogrowth/ and fitted using a modified Gompertz model.
V. parahaemolyticus did not grow at 4°C and low growth occurred at 13°C. V. parahaemolyticus growth at 18°C and 25°C was not significantly different, but was higher than at 13°C. Results support BRO storage at 4°C and 13°C. While these V. parahaemolyticus growth data provide important food safety information for the industry, other shelf-life quality characteristics will also need to be assessed to develop more definitive guidelines for BRO storage and transport to maximize product quality and safety.