Evaluation of static and flow-through depuration system on depuration of naturally contaminated farmed edible oyster Crassostrea madrasensis (Preston, 1916)

Depuration of naturally contaminated farmed edible oyster Crassostrea madrasensis was evaluated using fill and draw depuration system (FDDS) and flow-through depuration system (FTDS) to ensure food safety in oyster consumption. Depuration conducted at room temperature with seawater salinity ranges 30–35 psu and pH of 7.5–8.0. Prior to depuration, the levels of faecal indicator bacteria (<230 MPN/100 g) and toxic metals including Pb (2 mg/kg) and Cu (3 mg/kg) were above the acceptable limits. Other metals such as Ni, Co, Fe, Mn and Zn were also investigated. The results of the investigation pointed out that, both the depuration methods could significantly (p < 0.01) reduce the bacterial and heavy metal contamination from oyster samples, though there was no significant difference between the two methods with respect to efficacy (FDDS and FTDS). Significant differences were also found in reduction of toxic metals such as Pb and Cu (p < 0.05) as a function of depuration duration. To reduce the toxic Pb levels to the acceptable limit, 16 h of depuration was required in FTDS whereas FDDS oysters required 48 h of depuration. The FTDS and FDDS required 24 and 36 h of depuration respectively to reach the acceptable limit of E. coli. Visualisation by PC plots clearly separated the investigated samples according to depuration method and duration, thus indicating the effect of treatment methods on depuration. We recommend FDDS for bacterial and heavy metal depuration in oysters owing to its comparatively lower cost, even though FTDS depuration was faster in reducing the bacterial and metal contents in oysters