Unlike the produce section at grocery stores, sushi restaurants don’t actively advertise the freshness of their fish. Yet, as evident by numerous Yelp reviews, plenty of people equate great taste with the freshness of the fish. There’s some notion that the most delicious piece of sushi eaten was just caught off the shore just hours ago, regardless of how far you are from shore.
In actuality, many of the fish you eat at sushi restaurants was frozen at some point. And even when it’s not, flesh was aged for flavor.
Not your grandmother’s freezer
Due to health concerns, a rising demand for sushi, and drastic improvements in refrigeration techniques, 50 to 60 percent of sushi served in the United States have been frozen at some point in its journey from ocean to plate.
Many of the fish in the wild have parasites living in its flesh. By flash freezing the flesh, the parasites are killed, while retaining the texture of the fish. That’s not to say all wild caught fish have parasite in them, as some species are more prone to parasites than others. But you should know your fish before eating fish straight out of the ocean and streams.
For example, maguro (tuna) has relatively clean flesh, compare to say mackerel which is prone to parasites. It’s partially the reason why saba (mackerel) is cured in salt for two hours, then marinated and soaked in vinegar. It’s not only for flavor, preservation, but also to kill parasites. With sake (salmon), it’s usually frozen, since it’s usually susceptible to parasites. The exception to this is that there are some farm-raised parasite-free Salmon, which requires no freezing for raw consumption. But as to which one, you’ll have to ask your local fish-monger.
But despite having clean flesh, even tuna is often flash frozen. Due to refrigerated shipping containers enabling year-round supermarkets, Americans don’t have a sense of seasonality of fruits, vegetables, or fish. Since Americans want to eat toro year round, the toro you’re eating may have been frozen months ago.
Superfreezing is a relatively new technology that was developed in the 1990’s that drops the core temperature of a 500 pound tuna to minus 70 degrees below zero. According to Shin Tsujimura, a chef at Nobu, even he can’t tell the difference between unfrozen and flash frozen fish in a blind taste test.
So don’t shy away from frozen sushi. Chances are, it makes sushi more available and safe to eat, while not losing any texture.
Buckle up for maximum flavor
But freezing isn’t all there is to preparing fish for sushi. There’s a technique called Ikejime (活け締め) used to butcher the fish and maintain the quality of its flesh and prepare it for aging. It’s considered the fastest and most humane way. It involves the quick insertion of a spike directly into the hindbrain.
When spiked correctly, the fish fins flare and the fish relaxes, immediately ceasing all motion. Destroying the brain and the spinal cord of the fish will prevent reflex action from happening; such muscle movements would otherwise consume adenosine triphosphate (ATP) in the muscle, and as a result produce lactic acid, making the fish sour. Furthermore, the blood contained in the fish flesh retracts to the gut cavity, which produces a better coloured and flavoured fillet. — Wikipedia
When butchered in this matter, the fish doesn’t know that it’s dead. The resulting flesh is pristine, and there’s no blood spots in the meat produced by violent flopping. This way of butchering also sets up the fish for aging to develop a deep umami flavor.
If you remember back to HS biology, ATP (Adenosine triphosphate) powers cells. When you butcher a fish using ikejime, it doesn’t use up a lot of ATP floundering about, since it’s quick death. So rather than a lactic acid buildup due to all the flopping, which makes the muscles sour, you conserve the ATP in the muscles. ATP apparently breaks down over time into inosinic acid, which is a kind of nucleotide monophosphate, similar to in structure and taste to monosodium glutamate (MSG), giving you that umami flavor.
Different types of fish are aged different amounts to maximize the flavor. Just because the fish is fresh, doesn’t mean it’s delicious. Often times, minutes after a fish is caught the flesh doesn’t have much flavor and is just chewy.
Maguro and toro needs at least a week of aging, sometimes up to two weeks in refrigeration. Sake (salmon) and Ohyou (halibut) requires around five days of aging. On the other hand, according to Kaz Matsune, fish like binna maguro (albacore tuna) and katsuo (bonito) taste better without aging.
While sushi might look simple — just a piece of fish on a ball of rice — the preparation that comes before it is deep. A sushi chef “cooks” the fish and develops flavor for it by butchering it in a specific way to enable umami flavors to develop over a specific period of time.
So the next time you’re at a sushi restaurant where the chef takes particular care in preparation, don’t drown it in wasabi and soy sauce. Instead pay attention to the flavors and umami swirling around as a result of careful butchering and aging.
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