Inactivation of viruses in oysters by high pressure processing
Hepatitis A is a food borne virus that is transmitted via the faecal-oral route and infection is passed from person to person via foods and beverages contaminated with faeces or by direct personal contact.
Foods most susceptible include ready to eat foods that does not receive any further heat treatment. They include those eaten without cooking such as fresh produce which may be irrigated with contaminated water (salads), and bivalve shellfish (such as oysters) which may be grown in waters contaminated by human faecal effluent.
Oysters feed by filtering particulate matter such as algae from the surrounding water. They can accumulate viruses and other pathogenic microorganisms if they are present in the water during feeding. Once accumulated, viruses may take weeks to purge from an oyster, long after faecal coliform counts of the harvest waters have declined to permitted levels. The common practice of consuming oysters raw or mildly cooked means contaminating viruses within the oyster will not be inactivated prior to consumption, potentially resulting in food borne infection.
In Australia during 1997 there were more than 400 hepatitis A cases, including one death that was linked to consumption of contaminated oysters harvested from the Wallis Lakes area in NSW.
The source of the virus was probably untreated human sewage effluent which flowed from upstream of the oyster lease areas because of unusually high rainfalls. As a result of this outbreak, NSW adopted the internationally recognized Australian Shellfish Quality Assurance Program (ASQAP). The Program has been highly effective in the control of oyster-borne illness. In May 2006 Food Standards Australia New Zealand’s Primary Production and Processing Standard for Seafood was introduced. The Standard sets out food safety and suitability requirements for seafood generally from pre-harvesting production of the seafood up to, but not including manufacturing operations.
Research funded by the Australian Food Safety Centre of Excellence (AFSCoE) on the effects of High Pressure Processing (HPP) on hepatitis A virus in oysters was completed by a PhD student, Stephen Grove, in 2006.
The non-thermal technique of HPP is currently used in the United States to extend the refrigerated shelf life of oysters by inactivating spoilage microorganisms without altering sensory or nutritional qualities. Pressure is applied to oysters submerged in water within a pressure vessel and varies between 250 and 450 megapascal (MPa). Oysters are also shucked by the process, releasing the adductor muscles that hold the oyster shells tightly closed, enabling easy removal of meat without shell damage. One such company is Goose Point Oysters.
The AFSCoE PhD research investigated the effect of a range of pressure and processing times at different salt concentrations, on the viability of hepatitis A virus in artificially contaminated oyster meat. At a pressure of 450 MPa, a 100 fold reduction of hepatitis A in oyster meat was achieved by a 300 second treatment time. The sensitivity of hepatitis A virus to HPP was increased by lower salt concentrations within the oyster meat. A predictive model of the data was developed, and may be used by oyster processors to predict the inactivation of hepatitis A resulting from a particular treatment.
