SINGAPORE - Some food manufacturers here might not be entirely honest with their product labelling, researchers who conducted DNA testing on fresh and frozen seafood samples taken from supermarkets have discovered.

In one instance, what was sold as premium fish meat turned out to be of cheaper origin.

The team of scientists from the National University of Singapore (NUS) also found, among other mislabelled food items, the DNA of pigs present in five samples of food sold as cuttlefish balls and prawn balls.

The Straits Times understands that the samples were bought at different times from different supermarkets across the island. They were also sold by the same Singaporean company.

In response to queries from The Straits Times, the Singapore Food Agency (SFA) said the presence of porcine DNA in these non-halal products could be due to the manufacturing process.

Said SFA: "For example, a food manufacturing plant may produce a wide range of products, including seafood and pork products. As such, trace levels of porcine DNA may be introduced into the seafood products.

"This, however, is not a food safety issue," it added.

The agency also confirmed that it has detected porcine DNA in fishball products, stressing that these were are not halal-certified.

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Professor Rudolf Meier, the principal investigator of the study, said: "It is possible to get pork DNA in the cuttlefish balls if one takes a piece of pork meat and drag it across a bench, and then afterwards prepares squid balls on the same bench."

The porcine DNA could also be present in the food samples if pork meat and squid meat are minced in the same mincer, he noted. If the mincer is not cleaned properly, pork DNA may be found in the squid meat, pointed out Prof Meier, adding that the factory should be inspected for "potentially unclean" work processes.

He added that the food samples found to contain pigs' DNA were not labelled halal or kosher. But he noted in the academic paper the team published on Thursday (Oct 31) that this was still a serious problem given that many consumers avoid pork not only for religious, but also for ethical or health, reasons, such as allergies.

The NUS team tested 105 food samples - 84 from six supermarkets and 21 from two seafood restaurants - across the island in the first week of May last year.

They discovered 12 of these samples contained DNA of animal species not indicated on the food labels and/or ingredient lists, including the three samples of cuttlefish balls and two samples of prawn balls that contained the DNA of pigs.

All of the samples classified as mislabelled were bought at the supermarkets.

There were cases of food sold at a premium being replaced with cheaper food.

These included the more expensive prawn roe, wild-caught Atlantic salmon and halibut, that turned out to be the cheaper fish roe, farmed Pacific salmon and arrowtooth flounder respectively.

A product that was labelled crab leg without any ingredient list was found to contain only the DNA of fish.

In addition, the team noted in its study that all samples labelled as containing crustaceans - crab, prawn or lobster - yielded only fish DNA. However, the team did not classify these samples as "mislabelled" in this study. Prof Meier emphasised that regular testing and clear regulations on how to label food products are needed to solve this problem of ambiguity in some food products.

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He said: "The study shows that most seafood is correctly labelled but there are a few producers that use loopholes with regard to labelling.

"These loopholes should be closed by requiring precise species information on the list of ingredients. Some products such as seafood balls seem to need more regular testing with methods suitable for analysing highly processed food," he added.

SFA said: "The Singapore Food Regulations stipulate that all pre-packed food for sale in Singapore must be labelled with the name of food, ingredients, net content and source. It is the responsibility of the industry to ensure that any food labelling claim made is accurate and can be substantiated.

"SFA investigates feedback on authenticity of food products. Attempting to pass off one species as another species is an offence (eg beef being sold as mutton) under the Sale of Food Act, and the offender is liable to a maximum fine of $5,000."

Prof Meier noted: "As a consumer, I want to be able to look at the list of ingredients and find out what is inside. As a scientist, I want accuracy to be enforced using regular testing with modern methods."