Contents   Index

Handling and Processing Oysters

Accompanying Notes
Table of Contents

By G. D. Stroud

MINISTRY OF AGRICULTURE, FISHERIES AND FOOD
TORRY RESEARCH STATION

TORRY ADVISORY NOTE NO. 84

Crown copyright material is reproduced with the permission of the Controller of Her Majesty's Stationery Office.

This electronic document has been scanned using optical character recognition (OCR) software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.

Accompanying Notes

Describes the species of oyster marketed in the UK, and gives distinguishing features, common and scientific names, and some brief background information on production, geographical distribution, cultivation, grading for size and transport and storage of live oysters. The total amount of oysters harvested in the UK in 1998 was about 2100 tonnes, 50% of which was farmed production. The Note provides information on handling and processing, including shucking, yield and composition of meat, chilling, freezing in the shell, half shell and as shucked meat, cold storage and shelf life, and other oyster products including smoked and canned meats. Briefly considers utilisation of waste shell. Shell can be deposited at sea as culch to encourage the settlement of spat, but it is essential that any disposal at sea complies with local bye-laws and with fish health regulations. Disposal onshore must be licensed.

The risks of food poisoning are primarily from sewage contamination of harvesting areas and the unpredictable occurrence of toxin producing microalgae. As filter-feeders, oysters can accumulate pathogenic and other bacteria as well as viruses from sewage contamination, and can accumulate potent toxins from toxic microalgae present among the phytoplankton on which they feed. Illnesses such as gastro-enteritis and hepatitis can result from sewage contamination. Briefly, regulations require that bivalve molluscs for human consumption can only be obtained from the areas officially classified for harvesting. The regulations also require compliance with the expressed limits for bacteria, toxins and contaminants and producers are expected to meet the end-product standards. The category of contamination, or classification, officially assigned to the growing waters, or harvesting area, determines whether the bivalves can be marketed directly or, before marketing, must be purified, relayed in a designated area for a required period to purge them of bacteria, or subjected to an approved heat process in approved premises. It is important to note that heat processing may be needed simply to produce a cooked product from molluscs that are already safe to eat or it may be an approved treatment required to ensure that microbiologically contaminated molluscs are rendered safe to eat; different regulations cover these two situations. Briefly, the approved heat processes to inhibit the development of pathogenic microorganisms in bivalve molluscs are:

• Sterilisation in hermetically sealed containers, i.e. canning.


• Animals in the shell and not frozen are immersed in boiling water such that the internal temperature of the flesh is raised to at least 90°C, and this minimum temperature is maintained for at least 90 seconds.


• Animals in the shell and not frozen are cooked for 3 to 5 minutes in an enclosed space in which the temperature is 120 to 160°C and the pressure is 2 to 4 kg/cm2, followed by shelling and freezing of the flesh to a core temperature of -20°C.

Table of Contents

Introduction
Species in the UK
Foreign names
UK oyster production
Geographical distribution
Cultivation
Fishing rights
Raw oysters
Grading for size
Transport and storage of live oysters
Shucking
Carriage of chilled meats
Yield and composition of meals
Freezing oysters
Oyster products
Oyster shells

Contents   Index