Food Safety
Pacific oysters (Crassostrea gigas) are filter feeders and filter plankton (small free-floating plants and animals) from the water column. This feeding system is not unique to oysters but is used by many bivalve molluscan shellfish, including mussels and clams.
Filter feeders, however, can filter and accumulate hazardous levels of disease-causing micro organisms, viruses, protozoa, marine biotoxins or toxic substances, if these are present in the water.
The potential food safety problem with bivalve molluscan shellfish is recognised internationally and most countries have regulations that control where and how these shellfish can be collected for sale.
Oysters are generally farmed in shallow estuarine waters which means that they are particularly vulnerable to pollution from land-run off. And as oysters are often eaten raw, this can be a food safety problem.
New Zealand operates a mandatory shellfish quality assurance programme for all Pacific oyster harvesting areas. This is recognised as one of the best in the world and is based on international best practice. The programme is managed by the New Zealand Food Safety Authority (NZFSA), in cooperation with the District Health Board Public Health Units and the shellfish industry. For full details of this programme, refer to the Animal Products (Regulated Control Scheme-Bivalve molluscan Shellfish) Regulations 2006 and the Animal Products (Specifications for Bivalve Molluscan Shellfish) Notice 2006 (both referred to as the BMSRCS).
Before any area can be used to farm commercial Pacific oysters, public health officials survey the water catchment area to identify any potential pollution from humans, wild and domesticated animals or birds, or other environmental effects. This is accompanied by microbiological sampling of water and shellfish over a 12-month period, so all seasonal influences are explored. All the information is evaluated and a risk profile established for the catchment, which determines its suitability as a food production area.
Once an area is classified and listed by NZFSA for harvest, there are requirements for regular monitoring of the water and shellfish flesh to verify environmental conditions. The samples are analysed for microbiological and chemical contaminants.
New Zealand shellfish areas are affected by microbiological contamination from run-off during rainfall events. This is managed under the BMSRCS by classifying the areas as ‘Conditional’ and closing them for a number of days after rainfall to ensure that the oysters have cleansed themselves before harvest.
Marine biotoxins can be produced by some species of microalgae under certain environmental conditions. This is a natural phenomenon, but if oysters accumulate these and are later consumed some of the toxins can cause neurotoxic, diarrhetic, paralytic or amnesic health effects on the human body. To manage this potential health risk, the BMSRCS has a specific requirement that each shellfish growing area be tested on a weekly, fortnightly or monthly frequency for each of the toxin groups - in approved laboratories using approved methods. If toxins are detected above the permissible level the oyster farms are closed until the levels fall below the permissible level
The BMSRCS has strict food safety requirements regarding transport and temperature management during and after harvest to ensure that the oysters remain wholesome and fresh until they reach the processing factories. All bags must be labelled with harvesting details so that traceback to the farm and time of harvest can occur if there any consumer complaints.
Oysters are very hardy and can be transferred around different growing areas. This means that farmers can stock new areas from other farms or if there are pollution problems in a region oysters can be shifted to cleaner waters where they can filter out any pollutants. This process is called 'relaying'. In such circumstances the transit of the product is traced and fully assessed before harvest to ensure that no contaminants remain.
Many overseas countries have problems with Vibrio organisms in oysters. The Vibrio family are bacteria that are naturally present in the marine environment, living in the sediments or free-floating in the seawater. They are not in any way related to pollution, but even so can cause adverse health effects (diarrhoea and blood poisoning) in some individuals. New Zealand has never had Vibrio illnesses associated with commercial oysters . The unique environment in New Zealand means that our waters are normally too cold and salty for the Vibrios to survive well, but we regularly monitor to ensure that they are not in the oysters.
New Zealand sells to more than 50 overseas countries so we have regular food safety audits by overseas agencies, for example the US Food & Drug Administration and European Commission officers. These audits have always confirmed we produce oysters which have the confidence of the international food safety experts.
