Autumn is the best time to enjoy Chinese mitten crabs. It is the season when these aquatic creatures fatten and are rich in buttery orange roe, which is delicious and loved by crab eaters. But owing to their habitats, they are susceptible to be contaminated with dioxins, which are a group of environmental pollutants and the excessive levels detected in food has been a public concern.
How do Chinese mitten crabs become contaminated with dioxin?
Dioxins are environmental pollutants produced in natural disasters (e.g. volcanic eruptions and forest fires) and during industrial processes and waste incineration. Once released into the air, dioxins enter the soil, rivers, lakes, etc., and pollute the environment, then pass along the food chain. These pollutants endure a long time in the environment and are resistant to degradation. They tend to bioaccumulate in plants and animals along the food chain, mainly in the fatty tissues of animals. Human exposure to dioxins is generally through food (e.g. meat, poultry, seafood and dairy products) and they can remain in the human body for years.
As these pollutants tend to deposit in the sediment at the bottom of lakes, where water movement is slow, Chinese mitten crabs live in the benthic zone of lakes; so, these creatures are exposed to dioxin in their habitats. Despite every stage in the aquaculture of Chinese mitten crabs being rigorously monitored, it is still hard to ensure they are free from contamination with environmental pollutants. Thus, it is advisable to consume a moderate amount of Chinese mitten crabs to avoid excessive cholesterol intake and minimise exposure to environmental pollutants.
Is the supply of Chinese mitten crabs to Macao subject to source supervision?
Chinese mitten crabs sold in Macao are mainly imported from China, coming from approved aquaculture farms that are under the supervision of competent authorities in the mainland. Source supervision is conducted throughout the entire aquaculture process in the farms, with respect to water quality, sediment, crab larvae and crab feed, to ensure the quality of mitten crab supply to Macao that meet the specific requirements and are safe for consumption. Moreover, the Municipal Affairs Bureau (IAM) implements pre-inspection measures on Chinese mitten crabs cultivated in the mainland that are the first-time supply to Macao in every calendar year. Under this pre-inspection system, IAM also regularly conducts on-the-spot investigations and studies at these mitten crab farms which are the suppliers to Macau. Under the pre-inspection system, staffs from the Municipal Affairs Bureau (IAM) conduct regular visits to aquaculture farms that supply Chinese mitten crabs to Macao, so as to study the supervision and control of their farming environment, water quality and management, paying particular attention to the monitoring system adopted throughout the production process of Chinese mitten crabs. The staff will also look into the safety indices for dioxins and nitrofurans so that ensuring the sources of Chinese mitten crabs are safe and their products are of good quality, minimising the potential food safety risks associated with the Chinese mitten crabs.
Inspection and quarantine measures taken by Macao on Chinese mitten crabs
According to existing laws and regulations of Macao’s import system, all imported Chinese mitten crabs that are available for sale and supply through any means, including via online shopping, buying agents and physical stores, importers are required to submit an import declaration with the competent authorities. The imported Chinese mitten crabs are subject to routine sampling for mandatory sanitary inspection. Importers have to submit health certificate of the Chinese mitten crabs issued by the official authorities of their country of origin, and provide supporting information on their health inspection for application review. These mitten crabs must be shipped directly to Macao and subject to mandatory inspection before releasing into the market. As for members of the local food traders, such as food importers, wholesalers and retailers, food establishments and stall owners in municipal markets, the stakeholders must select their source of goods cautiously, and must not supply or sell those Chinese mitten crabs from unknown sources. They are obliged to keep the transaction documents of incoming and outgoing products so that the competent authorities can trace the sources and distribution flows of any questionable products when necessary. IAM continues to step up food inspection. Any person who is suspected of producing, operating, selling or smuggling of food materials, which have not been subject to statutory inspection, shall be prosecuted in accordance with the “Food Safety Law”.
In terms of dioxins testing in Chinese mitten crabs, the chemical analysis is a complicated and time-consuming process. The sampled Chinese mitten crabs are required adequate pre-treatment. Since dioxins are fat-soluble substances, a Soxhlet extractor is used to remove the fat substances from the samples, then following by liquid-liquid extraction. Samples are purified (e.g., purification through acidic silica gel, mixed silica gel column, alumina column, gel permeation column, activated carbon column, Florisil column, and/or separation by automated purification system), concentrated the extracts by solvent exchange, and eventually taken HRGC-HRMS (high resolution gas chromatography - high resolution mass spectrometry) analysis, accompanied by isotopic calibration, internal calibration, calculation of recovery rate, as well as qualitative and quantitative determination of toxicity equivalent of dioxins and dioxin-like compounds. The entire testing process would take at least half a month or more, while the analytic equipment are expensive and professional technicians are needed to run the various experiments. Hence, it is difficult to determine the safety of Chinese mitten crabs merely by testing. Source supervision is a more effective way to monitor the quality of Chinese mitten crabs so as to minimise potential food hazards.
What are the things to note when eating Chinese mitten crabs?
Although the whole supply chain of Chinese mitten crabs is thoroughly monitored by competent authorities from the source of farming, their export, distribution and sales in Macao, many environmental factors come into play; so, it is difficult to keep their production free from environmental pollution. Even if rigorous monitoring is in the place, neither is it possible to reduce the associated food risks to zero. Consumers should buy Chinese mitten crabs from reputable and reliable shops in good hygiene condition, and never make a purchase decision based on photos shown in online advertisements alone. Moreover, residents should be well-informed about the source of the Chinese mitten crabs and make sure that those are imported through legal channels to ensure their safety and minimise potential food hazards. Chinese mitten crabs are susceptible to contamination with dioxins, which are fat-soluble environmental pollutants and often accumulate in the cholesterol-rich crab roe. Therefore, it is necessary to limit the consumption of Chinese mitten crabs and eat a balanced diet to avoid excessive exposure to environmental pollutants from a single type of food. In addition, Chinese mitten crabs should be thoroughly cooked before eating because these may contain pathogens, such as bacteria and parasites. If consumers harbour doubts about the source of Chinese mitten crabs, or suspect that the crabs are imported into Macao for sale through illegal channels, please call the Food Safety Hotline (Tel: 2833 8181) to report your concerns.