Effects of Preservatives on the Bacteriological, Chemical and Sensory Qualities of Mangrove Oyster (Crassostrea gasar)

Aims: This work was undertaken to investigate the quality changes in mangrove oysters (Crassostrea gasar) exposed to various preservative treatments (PTs) including sodium benzoate (NaB), sodium chloride (NaCl), potassium aluminum sulphate (PAS) and green lime juice filtrate (LJF) during ambient temperature storage (30±2ºC) to enhance the shelf-life.
Study Design: Oyster samples were subjected to various preservative treatments to enhance the shelf-life and the bacteriological, chemical and sensory qualities determined and the data obtained were analyzed.
Place and Duration of Study: Department of Microbiology (Ofirima Complex), University of Port Harcourt, Port Harcourt and Department of Science Laboratory Technology, Rivers State Polytechnic, Bori, Nigeria during the dry and rainy seasons between June, 2008 and May, 2009.
Methodology: Freshly harvested oysters (200) were steamed for 5 min and manually shucked. The oyster meat samples were then subjected to four PTs as follows: 0.1% (w/v) NaB, 1.0% (w/v) NaCl, 1.0% (w/v) PAS, 10% (v/v) LJF while the control samples were subjected to sterilized distilled water and analyzed for 3days.
Results: Bacterial flora isolated varied; with control samples showing nine bacterial genera which included Bacillus spp., Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus spp., Vibrio spp., Proteus spp., Micrococcus spp., Lactobacillus spp. and Corynebacterium spp. but fewer (five) bacterial genera were isolated from PAS-preserved oysters. The bacterial population of control and preservative-treated samples increased with storage time but minimal increase occurred in PAS-preserved samples. The pH of the samples differed with treatment but the control and NaB-preserved samples had the highest (4.72- 5.03) while PAS- and NaCl-preserved samples showed the lowest (3.20 -4.05).
The sensory attributes of all samples decreased significantly (p<0.05) and became unacceptable after one day but PAS-preserved samples remained highly acceptable throughout the storage.
Conclusion: Of all the samples, the PAS-preserved samples presented the best bacteriological and organoleptic qualities during the storage. Thus, the PAS-preservative treatment is highly recommended for shelf-life extension of oysters.