'It's cold now, so it's fine?' The hidden contaminants in your oysters

Raw oysters served with bossam. [Herald DB]

Is it safe to eat raw oysters again?

October marks the start of autumn in earnest, and with the cold winds come cravings for one seasonal delicacy in particular: oysters.

Most people know to watch out for norovirus when eating raw oysters.

But there is another hazard that many consumers overlook — a substance lurking inside oysters that does not disappear even when the shellfish is cooked.

That substance is microplastics. A growing body of research shows that domestically farmed oysters consumed in South Korea contain significant quantities of microplastics.

The oysters themselves have not changed. The plastic that humans have discarded into the sea has broken down into tiny fragments and found its way back into the bodies of the very shellfish now in season.

Oysters are particularly vulnerable among seafood species because they are filter feeders, drawing in seawater through their gills and straining out plankton and organic matter — along with whatever else is suspended in the water.

Shoppers browse a supermarket in Seoul. [Yonhap]

Experts say greater attention to marine plastic pollution is needed if people are to continue eating seafood sustainably.

According to the Ministry of Oceans and Fisheries, full-scale production of farmed oysters in South Korea begins in October, with shipments concentrated through May of the following year. Demand is especially heavy from October through February, driven by kimchi-making season and winter appetite for raw oysters.

Oysters, often called the "milk of the sea," are filter feeders. They draw water in through their gills, strain out plankton and organic matter, and consume what remains. The problem is that the food particles oysters ingest are similar in size to microplastic particles.

Research published Sept. 15 in the international journal Aquaculture Reports by scientists from the Korea Research Institute of Ships and Ocean Engineering and the National Institute of Fisheries Science found microplastics in farmed Pacific oysters from both the South Sea and the West Sea of South Korea, regardless of farming region.

Fragments of microplastics. [Getty Images Bank]

The research team compared oysters raised at farms in the South Sea and the West Sea — which use different aquaculture methods — examining the quantity, size, shape and composition of microplastics found inside them. Across all samples, an average of 15.07 microplastic particles were detected per individual oyster.

Neither region's oysters showed a clearly higher accumulation. South Sea oysters did record higher microplastic counts and averages than those from the West Sea, but the difference was not statistically significant.

A photo for illustrative purposes. [Getty Images Bank]

Where the two regions did differ markedly was in the type of plastic found. A striking finding emerged from South Sea oysters, which come from one of the country's best-known oyster-producing areas: 68.5 percent of the microplastics detected there were polystyrene (PS), the material used to make expanded polystyrene foam — commonly known as Styrofoam.

Farmed oysters consumed domestically are known to carry particularly high concentrations of microplastics. Experts point to Styrofoam buoys used at aquaculture farms as a primary cause. Because the buoys shed white particles with even minor friction, they are considered a major source of microplastic contamination.

Styrofoam buoys have long been used in large quantities at oyster farms. They are cheap, lightweight and highly buoyant, making them convenient for keeping the lines from which oysters hang afloat. Their critical drawback, however, is that they break apart easily.

After prolonged exposure to sunlight and waves, and repeated collisions with other equipment, the surface of the buoys begins to flake away. What starts as a large buoy gradually crumbles into tiny beads that scatter across the ocean.

Data released by the Ministry of Oceans and Fisheries in 2020 showed that in 2019, about 41 million of the approximately 55 million buoys in use at farms nationwide — roughly 75 percent — were Styrofoam. The ministry itself had flagged the ease with which Styrofoam buoys break down into microplastics during use as a serious problem.

Authorities have been steadily replacing buoys with eco-certified alternatives and prohibiting the installation of new Styrofoam buoys. But simply swapping out buoys is unlikely to fully resolve the microplastic problem.

Decades' worth of discarded buoys and fishing gear remain in the ocean. Aquaculture farms also use other plastic materials. On top of that, plastic bottles, vinyl, synthetic fibers and other everyday plastics discarded on land continue to flow into rivers and the sea.

According to a World Wildlife Fund report, 60 percent of all plastic ever produced ends up as waste. As of 2015, an estimated 150 million tons of plastic waste had already entered the ocean.

Over time, plastic breaks down into smaller and smaller fragments under sunlight, waves and friction. Once plastic reaches a microscopic size, it is nearly impossible to collect. That plastic, drifting through the ocean, ends up accumulating inside filter-feeding oysters.

Oysters are not the only concern. Microplastics have already been detected in a large proportion of the seafood people consume.

A study titled "From Sea to Table," published by research teams at Oregon State University and Portland State University, found that 180 of 182 seafood specimens collected from the Oregon coast contained microplastics or other anthropogenic particles — a detection rate of 99 percent.

Oregon has a large commercial fishing industry, landing about 135,600 tons of seafood annually. The study focused on species most commonly consumed commercially, meaning the vast majority of specimens sampled were seafood that regularly ends up on dinner tables.

The situation in South Korea is no different. A study conducted by the Ministry of Food and Drug Safety in 2020, examining microplastic contamination levels in domestically distributed seafood from 2017 to 2019, detected an average of about 0.47 microplastic particles per gram across 66 items spanning 14 species.

Fragments of microplastics found on a beach. [Getty Images Bank]

Does this mean people should stop eating oysters in season? The research available so far does not support such a definitive conclusion. Studies on the effects of microplastics on the human body are ongoing.

However, it would be a stretch to read the current evidence as meaning there is no risk. More precisely, the data needed to confirm specific health harms have not yet accumulated. The latest research indicates that microplastics can enter various human tissues and may contribute to cardiovascular disease, inflammation and reproductive health problems.

Meanwhile, microplastics found in seafood accumulate mostly in the digestive organs. Removing the innards before eating seafood is therefore the safer approach. For shellfish such as oysters and mussels, where the organs cannot easily be separated, thoroughly purging them in clean water can help reduce microplastic concentrations.

woo@heraldcorp.com