Mangrove Oysters: Mortality and Microbiota Population Dynamics during Processing and Ambient Storage

Mangrove Oyster (Crassostrea gasar) is a molluscan bivalve shellfish that is a common inhabitant of
the mangrove/estuarine ecosystem of the Niger Delta of Nigeria. Mortality and microbiota population
dynamics of raw, processed and storage of mangrove oysters at ambient temperature were
investigated. The mortality rates of raw oysters were determined during depuration in tap water (TW)
and brackish water (BW) microenvironments for 14 days. Mortalities were observed in the milieu on
the 5th, 11th days and afterwards respectively. The shelf-life of oysters in BW microcosms was
considerably enhanced. Thus, indicating the beneficial impact of depuration of mangrove oysters in
BW than in TW microcosms. The microbial counts of raw (shucked), processed and oyster meat
samples during ambient storage were determined using standard microbiological methods. Aerobic
plate counts (APCs) were 1.36×105 and 3.00x103 CFU/g for raw (shucked) and processed oyster
meat samples on day 0 respectively but increased markedly to 1.55x106 CFU/g during storage for
24h. Fungi counts were 3.6×102 and zero/no growth detected (NGD) CFU/g for raw and process
oyster on day 0 respectively but increased from 0 - 0.8×102 CFU/g after storage. Bacteria were more
predominant in numbers and diversity than fungi. The most frequently isolated microbes from raw
(shucked), processed and stored oyster meat samples were Bacillus, Pseudomonas, Vibrio, Proteus
and Staphylococcus others are Aspergillus, Penicillium and Fusarium. The most dominant genera
during storage were Bacillus (20.8%) and Pseudomonas (16.7%); Aspergillus (52.3%) and Penicillium
(45.4%). However, non-detectability of Escherichia coli and Acinetobacter species as well as
reduction in bioburden underscores the criticality and importance of adequate heat processing
treatment of oysters prior to consumption as some of these organisms are not only potentially
pathogenic but of public health significance. This study highlights the marketability of depurated
mangrove oysters on or before 5days and 24h for the processed samples to ameliorate health risks
and serious postharvest economic loses.