Growing interest in the science of food, as well as a better appreciation of preparation techniques, has brought to light the practice of ageing fish for raw consumption.

In Hong Kong, Okra’s Max Levy is at the forefront of this culinary practice. The chef was raised in Louisiana, worked in New York, trained in Japan and made his name in Beijing, so he’s criss-crossed enough boundaries and traditions to respect and defy them.

At Okra, he has openly come out with the fact that he ages some of his produce to create specific flavours and textures better than “fresh” fish can offer.

“People love what I call ‘Japan’s biggest prank’ – the idea of having sushi at a fish market,” says the American. “People line up and say how great the sushi tastes but in reality, it’s pretty bad for the price. But people think eating in the market, it’s super fresh.

“If you go to a sushi place and the fish is really soft and flavourful, then most likely they are ageing it or their supplier is doing it for them. But the whole concept of ageing and dry-ageing fish only started being thrown around four or five years ago. Before, suppliers and fishermen aged fish naturally in the pre-air-freight transport process.”

The process adds flavour, Levy says. “Ageing fish also adds natural MSG. When people come and say ‘We don’t want MSG in our food’, I say, sorry, you can just drink water. We [age fish] because we create the dishes around stronger flavours.”

Levy’s revelation came after he left the United States to work in Asia. Although he was working with fresher produce at restaurants, it wasn’t tasting better.

“When I first started working in sushi restaurants and being around fish, the fish was different. This was in the early ’90s. Tuna, when it arrived in the US, was two or three weeks old because most of it was caught way out at sea and kept on ice, and it definitely had a stronger fish smell. Now [in Asia] when the fish shows up, it’s one to two days out of the water, and it has no smell at all.

“Anyway, I noticed the fish didn’t taste as good as when I first started working. I assumed being near Japan and Taiwan, it should be the best fish in the world and should have more flavour. When I started talking to fish suppliers back in America, they suggested maybe their fish was a bit older. That was my first connection.”

When proteins ferment they release MSG and develop umami and depth, even if no salt, soy or mirin is used.

Levy is not the only chef to age his sashimi, he’s just been the most vocal. The process is also done quietly at some of the best Japanese restaurants in Hong Kong. Sushi Ta-ke’s head chef, Seiji Adachi, is one of the few willing to admit to this sort of “don’t ask, don’t tell” discretion.

“Yes – some of the fish has to be aged before being served, such as tuna,” Adachi says. “Fresh tuna has a tough texture. It would take about one to three days in the refrigerator until it acquires the umami and tenderness. We also age some of the flounder and golden eye snapper. On the other hand, some fish is better served fresh such as horse mackerel, sardine and pacific saury.

“The fish ageing technique was first popularised in Tokyo. Nowadays, the technique is used in almost every part of Japan. Sushi chefs started ageing their fish in the ancient times when fish would spoil easily. To keep the fish longer, they would marinate or age the fish, and it has now become a popular technique.”

Adachi has worked in Hong Kong for more than 11 years and is aware of the Cantonese preference for freshness. For steaming or cooking, fresh-out-of-the-water ingredients do work the best, but not so much for sashimi. Still, unless a customer asks, he is not likely to reveal the deeper, richer flavour of his snapper comes from lightly salting it and wrapping it in kombu seaweed for a few days before serving.

Nor will chef Kojima Takao, of the Kowloon Shangri-La hotel’s restaurant Nadaman, openly advertise that he washes his mackerel in mirin to remove some of the fishiness, and marinates and ages his tuna.

“Hong Kong chefs might serve a fish they receive right away, but for us, the water content is too high,” Takao says. “The taste is not mature enough so we will put it in the fridge for a few hours. If it’s really too fresh or the water content too high, we might wait a day or two. The chefs that age and marinate their tuna for longer usually receive their fish in entire slabs that can age for about a week; the flesh becomes much darker so all the edges have to be removed and you end up with only the edible middle. In those cases, the wastage is very high for something so expensive.

“In Japan, some people like longer aged and mature sushi because they like to test the chef’s skill and knowledge in handling the fish and controlling the marinade and flavour technique.”

Most sushi chefs agree on the general length of ageing for different fish. Seafood with a heavy oil content such as mackerel, anchovy and sardines spoil the fastest so they are aged the shortest time. White flesh fish – sea bream, red snapper and flounder – can be rested a couple of days in the fridge.

Large fish with big muscles such as salmon or bluefin tuna can be aged from a week to two weeks.

The exact length of time depends on whether the flesh is skinned, if the fish is cleaned and bled, the weather conditions and the desired strength of flavour.

Adachi says: “There’s no book to say how long it should be aged or when it’s best for flavour. It’s something each chef has to learn by his own touch, feel and sight. Like buying a shoe or clothes, it’s all about feel. Hong Kong people usually don’t want something too strong or fishy, but in Japan more people like a stronger flavour.”

At Okra, Levy learned first-hand that some Hongkongers prefer milder flavours.

“We do have to explain to patrons what we do because they are expecting a very mild-tasting tuna or fish to dunk in soy sauce and wasabi,” he says. “When you tell them the flavour is strong, they associate that strength with bad or old tuna.

“There’s a dish we had to take off the menu because I got so tired of complaints. We take baby tuna, wrap it in a salted shiso and age it for about a week and a half, then lightly fry it so it’s still raw in the middle. Every time we serve it we asked the customer, ‘Do you like strong tasting fish?’ and people would say, ‘Of course I do, I’m Chinese.’ Then we serve it and you see the look on their face that says it’s too strong for them.”

Okra 110 Queen’s Road West, Sai Ying Pun, tel: 2806 1038

Sushi Ta-ke 12/F Cubus, 1 Hoi Ping Road, Causeway Bay, tel: 2577 0611

Nadaman Kowloon Shangri-La, 64 Mody Road, Tsim Sha Tsui, tel: 2733 8751