Illustration: Tom Tsang
When reviewers visit high-end sushi restaurants, they often praise the fish as being "fresh". But unless you see it being pulled out of the tank, killed (although some chefs skip this step) and filleted right in front of your eyes, there's a good chance that at least some of the fish you eat during the average sushi meal - even at the most expensive places - has been frozen.
That's not to say they're serving bad fish - we're not talking about frozen fish fingers here. Sometimes fish is frozen to kill any parasites it might be carrying, but often it's done as a simple and efficient way to preserve it.
Commercial boats that go after large deep-sea fish, such as the popular bluefin tuna, can spend days, if not weeks, at sea, and the only way to keep their catch from going bad is to freeze it. Obviously, they're not using wimpy home freezers that are only set to about zero degrees Celsius; the ones on fishing boats go down to minus-60 degrees or colder.
The tuna are bled, gutted and flash-frozen so quickly that the quality doesn't deteriorate. As long as it's frozen, defrosted and handled correctly, it should be almost impossible to tell it apart from fresh fish. The best of these frozen bluefin tunas can sell for more than HK$50,000 per kilo at auction in Tokyo's Tsukiji fish market.
With the best quality fish, whether it's previously frozen tuna or varieties that are eaten only during specific seasons, they are probably then aged - left to mature - after being killed or defrosted.
The stiffening of the muscles after death - known as rigor mortis - will usually dissipate within eight to 24 hours, although it can take longer for larger fish. But chefs at expensive sushi restaurants are going further and waiting to let the fish mature even beyond this stage; they age it for several days (and in some cases, for a week or more), which lets certain en-zymes break down, giving the fish more flavour and a softer texture. This doesn't mean you'll get a better, tastier product if you buy day-old, dried-out sashimi on sale for a reduced price at a cheap takeaway sushi joint. As with dry-aged beef, these sushi chefs age their fish in carefully controlled conditions to make the meat more tender and flavourful. If done correctly, it shouldn't taste "fishy" or "off".
Some of the items served at high-end sushi restaurants are never (or shouldn't be) frozen, such as uni (sea urchin), roe (such as that from salmon or flying fish) and shellfish.
Truc (tryk): noun, masculine, trick, gimmick, device. A French word for a chef's secret.