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Food Safety Focus (151st Issue, February 2019) – Food Incident Highlight

Consumption of Raw Oysters Carries Inherent Microbiological Risk

According to Agriculture, Fisheries and Conservation Department, oysters harvested in Hong Kong should be cooked before consumption.  Oysters are able to accumulate contaminants including bacteria and viruses from surrounding seawater.  In other places such as EU, only oysters collected from approved harvesting areas with specified water quality can be used for raw consumption.

In farming of some oysters, there is a process called depuration (purification) to reduce contamination and increase shelf-life of oysters.  It is done by holding oysters in tanks of clean seawater for several days to allow bacterial contaminants such as Escherichia coli in the oysters to be purged.  According to Food and Agriculture Organization of the United Nations, depuration enables the removal of microbial contaminants from light or moderately contaminated bivalves and thus increases the availability and supply of safe and nutritious bivalves.  Furthermore, it enables the industry to meet the legal requirements of many countries on depuration of bivalves mandatory under specific circumstances.  However, depuration is less effective in removing naturally occurring marine vibrios such as Vibrio parahaemolyticus and Vibrio vulnificus and viral contaminants such as norovirus and hepatitis A.  Since heat treatment can slay those microorganisms, it is therefore safer to consume oysters after thorough cooking.

Even though effective measures are in place, there is no guarantee that imported raw oysters are completely free from microbiological risks.  From time to time, imported raw oysters harvested from areas monitored for faecal contamination were still found to be tested positive for norovirus by overseas authorities or linked to local and overseas food poisoning cases.  As such, it is always best to cook oysters thoroughly before consumption.