Summary

Objectives

1. To conduct a desktop review of R&D in shellfish purification technology that may be pertinent to the NSW Oyster Industry and the aims and objectives of the NSW Shellfish Quality Assurance Program (NSW SQAP). The main objective of the NSW SQAP is to ensure that shellfish are taken from estuarine waters to be sold for human consumption only if the shellfish and shellfish culture waters meet specified standards.
2. Provide advice to industry, the NSW Shellfish Quality Assurance Committee and the Oyster Research Advisory Committee on the status of shellfish purification R&D to enable industry to make decisions on future directions in purification technology investments and identify areas requiring further R&D in purification.
3. Provide a summary of the current "state of the art" and limitations of purification technology and R&D accomplishments to date.
4. Identify promising and likely future directions in matters relating to purification, particularly in terms of efficiency and issues such as viruses and biotoxins and usefulness of rapid detection techniques.
5. Make recommendations on HACCP quality assurance programs including the use of water quality monitoring programs as an integral part of the overall purification procedure.

Non Technical Summary:

Depuration in the context of this report can be defined as the process by which harvested shellfish are placed in land based plants containing clean estuarine water to permit the purging of their gastrointestinal contents under controlled conditions. Depuration does not include the practice of relaying shellfish to clean estuarine waters for long periods to promote self cleansing.

Shellfish (defined as bivalve molluscs for the purposes of this review) filter large volumes of water and trap particulate matter and dissolved substances suspended in the water as a source of food. Consequently, if the water in which they are grown is polluted, then the shellfish may concentrate microbes or chemicals which may be injurious to the consumer. Because shellfish are often consumed raw or (slightly cooked) and whole, (including their gastrointestinal tract), they are generally classified as a high-risk food group by health authorities worldwide.

Shellfish are depurated in order to reduce the likelihood of transmitting infectious agents to consumers. Depuration has been demonstrated to successfully reduce to low levels the number of bacterial and some viral agents in moderately polluted shellfish. The effectiveness of the depuration process is dependent on a number of variables including the health status of the shellfish, environmental parameters within the depuration plant (salinity, temperature, turbidity), the type of pathogen, and level of contamination.

Depuration has been practiced around the world since early this century. In 1978 the practice was formally introduced in NSW as a response to a food poisoning outbreak involving over 2000 cases of viral gastroenteritis, which was attributed to oysters farmed in the Georges River. Depuration of all oysters harvested in NSW became a statutory requirement in 1983.

After reviewing the literature and consulting industry and relevant experts, two areas of contention have been identified with the practice of depuration in NSW and perhaps elsewhere in the world. The first issue relates to operational parameters and regulation of the process, the second appears to be a simple failure by sections of industry and regulatory authorities to appreciate that depuration alone does not ensure shellfish food safety.

At the time depuration was introduced in NSW, environmental factors which affect the rate of depuration (e.g. temperature, salinity and turbidity) were identified as critical control points in the process and as such required further investigation (Fleet, 1978). Despite the commercial practice of depuration for almost 20 years in NSW many of these questions remain unanswered. The absence of this information has resulted in the adoption of arbitrary environmental parameters for the operation of depuration plants. It appears that these arbitrary standards have not been capable of providing an adequate guide to conditions which will promote optimal oyster function and consequently ensure efficient depuration. This deficiency is illustrated by the continued difficulties experienced by plant operators in successfully depurating product for the required time period whilst avoiding oyster spawning. Appropriate, scientifically valid operating parameters need to be determined for depuration plants throughout NSW. This fundamental information is of paramount importance to permit the efficient, successful and cost effective operation of depuration facilities.

The number of depuration plants currently registered in NSW, relative to shellfish production, is much higher than in most other countries. Regulation and assessment of the performance of this large number of depuration facilities is logistically difficult and a drain on the limited resources of the regulatory agency, NSW Health. In order to promote economic efficiency and ease regulatory supervision the number of depuration plants in NSW should be reduced and independent audit procedures implemented. Larger, professionally managed plants would provide an economy of scale encouraging the development and adherence to industry best practice, whilst being more amenable to regular audit.

Continuing food poisoning outbreaks in NSW despite mandatory depuration demonstrates that the process alone is not sufficient to ensure the food safety of shellfish product. It is now recognised that the success of depuration is intimately associated with an understanding of the farming environment and an appropriate monitoring program to assess growing water and shellfish quality. Depuration is a process which will confer a level of additional food safety assurance to shellfish harvested in accordance with stringent protocols, from areas which may be subject to intermittent pollution. Depuration will not render shellfish grown in heavily polluted waters a safe product. A sanitary survey and continuing bacteriological monitoring program are necessary to identify areas which are inappropriate for harvest under any circumstances and those where depuration will be a useful adjunct to quality assurance.

Viral agents are potentially the major cause of foodborne illness and in recent times have been implicated in several large shellfish associated food poisoning outbreaks. Investigation of such outbreaks has often demonstrated sewage contamination of the shellfish growing area, and human enteric viruses such as Norwalk virus and hepatitis A are common aetiological agents. Standardised molecular techniques for the detection of human enteric viral agents in shellfish and research to ascertain the effectiveness of depuration in the elimination of such agents, would assist in the development of improved shellfish sanitary control procedures and consequently benefit the shellfish industry and consumers.

Apart from the continuing need to eliminate pollution of waterways, additional procedures which would promote improved quality assurance procedures in the shellfish industry include the implementation of HACCP based food safety plans by all producers, formal training of shellfish producers in sanitary control measures, and the development of a labelling system so that product can be reliably identified from the farm to the consumer.