UNSW Sydney

Australians consume around 350,000 tonnes of seafood a year. That’s enough to fill 140 Olympic swimming pools. Sydney Fish Market expects more than 100,000 customers when it opens for its annual Christmas rush this year.

But seafood handled poorly can lead very quickly to disaster, contributing to 4.68 million cases of food poisoning across the country each year.

UNSW Associate Professor and food microbiology expert Julian Cox has some handy hints on purchasing seafood, preparing it and preventing it from spoiling your stomach and holiday season.

Point of purchase

When it comes to selecting your first choice of fish, the eyes have it.

“If we start to see a dull or cloudy appearance in the eye, or a dullness on the skin or scales, that’s an indication the fish is not fresh,” A/Prof. Cox says.

“It’s even in the aroma – for example, finfish should smell of the sea, but not very fishy, because we’re then starting to see some kind of activity in the fish itself or through microorganisms, and spoilage.

“Your nose is a reasonable indicator for any kind of seafood. If everything starts to smell just a bit fishy, it probably is fishy, so to speak.”

Bad prawns go grey, slimy and spotty. Unshucked oysters and mussels should be firmly closed at purchase – only open them after preparing or cooking them.

Oysters and mussels are filter feeders, getting nutrients from matter and bacteria in water. If bad weather or a sewage spill has affected where they’re being farmed, your stomach might be up a certain proverbial creek regardless of how you prepare them. It’s worth doing your research on point of origin before purchasing.

Origin is also crucial when avoiding foodborne illnesses like ciguatera and scombroid, as these illnesses depend, respectively, on what the fish ate when it was alive and if it’s been stored correctly since being caught.

Ciguatera causes gastro-like symptoms, but also neurological symptoms, including a ‘reversal of sensation’ (hot versus cold). Scombroid poisoning resembles an allergic reaction.

“The best way to avoid ciguatera is to not consume top-level predatory fish, particularly during the warmer months,” A/Prof. Cox says.

Fish like Spanish Mackerel, coral trout and red emperor concentrate levels of the toxin by eating species that have it in lower doses. It’s a fish best avoided in warmer months.

Scombroid poisoning, named after scombroid fish such as mackerel and tuna, can be avoided by purchasing from reputable suppliers.

Fish fraud

There’s also the problem of seafood fraud to look out for, where expensive species of seafood are substituted with cheaper species somewhere along the supply chain but still sold as the original item for higher profits.

An Australian study last year found that nearly 12 per cent of common seafood products were mislabelled.

“Most people will have no idea…almost anyone can be fooled on face value,” A/Prof. Cox says.

There’s not many options for consumers on fighting seafood fraud, but GoodFish’s Sustainable Seafood Guide can inform you about what’s better for the environment. It’s an easy-to-read guide that’s extensive and free, and even has tips for cooking.

A wiser purchase can reduce the impact of fishing to marine ecosystems and give you a better understanding of what to look out for at the market.

Further up the chain, things are improving with inspection and monitoring.

For example, UNSW scientists have developed a handheld scanner that uses x-ray technology to discover the origins of seafood.

Cheaper seafood from overseas may be substituted for product labelled as Australian, and sophisticated chemical and biological ‘fingerprinting’ with the scanner is helping to verify provenance.

A/Prof. Cox says there will always be people looking to make an easy profit, but improvements to monitoring can have a chilling effect on fraudsters even before they get caught.

“I’m talking to one of the companies involved in this sort of process.

“They were saying they’ve only had one substitution problem that they’ve identified since they started doing testing with the industry…it means people know they’re getting what they paid for.”

Taking it home

If you’ve bought your seafood fresh, take it home immediately.

“Don’t just stick them in the back of your car and think, ‘oh, I just forgot something else’, and do some more shopping,” A/Prof. Cox says.

After being taken off ice, fresh seafood should spend as little time as possible at higher temperatures.

“Maybe make them the last item you purchase. Get them into a cold, insulated container with ice bricks to keep them as cold as possible.”

A rush to buy fresh seafood from a market is a tradition for many Australians at Christmas time, but A/Prof. Cox says buying frozen is fine to save time and hassle.

“It may be best when it comes to things like prawns to buy them snap frozen after they’ve been caught fresh, because that’s probably the best quality you’re going to get [besides fresh].

“Consume them very quickly after thawing so that a cold chain from purchase to transport home to storage to consumption is maintained.”

Another pro tip for buying frozen or freezing your purchase is that it’s a great defence against parasites that may live in fresh fish.

“Advice from one of my very good parasitologist colleagues, Professor Shokoofeh Shamsi, is that freezing for at least three days, hard-frozen, should eliminate the sorts of parasites we’re concerned about in fish.”

Proper preparation

When it comes to raw seafood, A/Prof. Cox says there is ultimately some level of risk that must be considered.

“Viruses can exist for longer in the gut of an oyster and be infectious at very low doses. Oysters are one of the most common ways of transmitting Norovirus.”

Things like raw shellfish, sashimi and even smoked salmon are very tasty but, for some at-risk groups (pregnant people, weakened immune systems and older people especially), it’s best to stick to something that’s been cooked.

“Think about having a nice piece of grilled finfish fillet or maybe oysters kilpatrick instead of raw. Think about how you can mitigate risk through preparation for the [at-risk] consumer.”

Bacteria like Vibrio and Listeria cause gastro and flu-like symptoms respectively, but may be fatal in at-risk people. Luckily, both are destroyed easily by heat.

Listeria has the potential to develop into listeriosis, a rare but deadly disease, with 20-30 cases reported each year in NSW.

“Aside from time and temperature control of fresh product, think about cooking as an obvious intervention to increase safety, and good time-temperature control during and after service.

“During service, serve up small amounts and often, especially under the hot Australian summer sun, and retain leftovers that have been exposed to the heat only for less than two hours.

“Keep any leftovers in the refrigerator for a maximum of two to three days, and consider freezing anything to be retained for longer.”