Relaying and depuration are common approaches for reducing bacterial contaminants in shellfish. In the relaying process, shellfish is transferred before harvest from polluted areas to an unpolluted waterway for natural biological purification. However, increased pollution along coastal line due to introduction of animal waste from farmland into marine environment has resulted in reduced clean area for growing shellfish. The lack of clean and unpolluted marine environment for growing shellfish creates a big challenge for the relaying practice.
Depuration is a controlled process that allows shellfish to purge sand and grit from the gut into clean seawater. The process usually leads to a reduction of microbial contaminants in shellfish and therefore increases shelf life of refrigerated products. However, studies have shown that depuration with clean seawater was not effective in reducing certain persistent bacteria including Vibrio spp. in shellfish because of the colonization of those bacteria in the intestinal tracts. In most instances, total aerobic plate counts could be reduced by one log value via the depuration process. Nevertheless, reduction of bacteria to fewer than 104 cells per gram of shellfish is rarely reported. A study conducted by Kelly and Dinuzzo (1985) reported that oysters required 16 days to depurate laboratory- contaminated V. vulnificus to non-detectable level. In a similar study, Eyles and Davey (1984) observed no significant differences in mean counts of naturally occur-
ring V. parahaemolyticus between depurated and non- depurated oysters.
To increase the efficacy in reducing bacterial contamina- tion in oysters, depuration in conjunction with chlorine, ultraviolet light, ozone or iodophors were studied (Fleet,
1978). However, none of them could effectively eliminate V. parahaemolyticus from shellfish. Croci et al. (2002) studied depuration of blue mussels experimentally con- taminated with Escherichia coli, Vibrio cholerae and V. parahaemolyticus in ozonated water and reported a substantially smaller reduction in the numbers of V. cholerae and V. parahaemolyticus in the mussels (approximately 1 log) than of E. coli (approximately
3 log) after 44 h of process. Ren and Su (2006) examined the effects of electrolyzed oxidizing (EO) water depuration on reducing V. parahaemolyticus and V. vulnificus in laboratory-contaminated oysters and found that both species could only be reduced by approximately 1.0-log unit after 8 h at room temperature.