Have you ever asked this question to your sushi chef, "So, what's fresh today?"
From a sushi chef standpoint of view, I must confess that it's rather annoying. In fact, many of the sushi chefs I worked with, disliked being asked this question.
Because the fish supposed to be fresh to begin with - at least fresher than other seafood establishments because sushi restaurants serve them raw.
Secondary, many sushi chefs know that a fresh fish does not always mean "tasty."
How is it so?
For example, Tuna starts to taste good after few days. Halibut is good after two to three days and Ika, squid takes about one week to have "umami" - the fifth flavor many Western chefs are talking about right now.
The Science of "aging."
Just like beef, fish aging increases its umami, or amino acids and savory glutamate. Also, fish has what is called IMP (inosine monophosphate) which has a similar savory taste to that of gultamate. IMP is a transient substance, thus the savoriness of fish increases for some time after its death.
A very fresh fish has "fresh" firm texture, though low in IMP/savory taste.
As the fish ages, it looses texture or the meat becomes softer and gains more savory flavor.
"The timing is important. When to eat is up to individual's taste," says Hirokazu Ito of Tokyo Fish Sellers Association. "Generally speaking, farmed fish needs less aging compared to wild fish."
Many Japanese sushi chefs age their fish including the world famous Jiro Ono.
"Fresh halibut is too chewy and has almost no flavor. I let it age overnight. If the tuna is too young, I will let it age in ice water for a few days."
Usually, the tuna tastes the best when its color becomes musty dull "red," especially for Toro.
Other chefs let Buri (hamachi) age three to four days, sardine for two to three days.
There are even sushi restaurants specialize in "aged" fish.
So, instead of the usual "what's fresh?" or "what's the best fish?", the next time you make your trip to your favorite sushi restaurant, ask your chef about fish aging.
Just because it's fresh or the best, does not mean you'll like it.