Is fresh fish better for sushi? Why Sushi Den and Izakaya Den dry-age.

Yasu Kizaki knows most sushi fanatics are looking for the freshest cuts of fish. That’s why his youngest brother Koichi visits the Nagahama fish market in Japan every day at 2 a.m. to box up tuna, mackerel and scallops in dry ice and ship them to Sushi Denver in less than 24 hours.

But fresher isn’t always better.

“You all think that the freshest fish has to be eaten on the spot right away,” Yasu said. “That’s the case for many, but for some of the fish you just can’t.”

To help spur conversation about the centuries-old Japanese tradition, Sushi Den and Izakaya Den installed three dry-aging coolers earlier this year at the entrance they share at 1487 S. Pearl St. There, customers can peer through the glass at cuts of bluefin tuna and red snapper.

“If we said we’re serving seven-day-old fish people would think, ‘Oh my god,’” Yasu said. “But if I say seven-day-aged tuna, it sounds better, right?”

Sushi Den has been dry-aging snapper, tuna and mackerel since Yasu and his other brother, Toshi, first opened in 1984. To do that, they keep whole and large cuts of seafood at 1-degree Centigrade (33.8 degrees Fahrenheit). The three- to seven-day aging process develops the purest form of umami, acting as a natural tenderizer and sweetening the meat, Yasu said. “It gets sweeter because it starts to produce glutamate, which is what we call umami in Japanese.”

When fishermen catch tuna, the biggest fish at the sushi bar, the fish swims around aggressively trying to escape. “The issue is that the fish’s body temperature starts to go up, and the blood is heated up, making the whole body warm,” Kizaki said.

For example, say two fishermen spend three hours on a boat catching 300 lbs of bluefin tuna. “When the fish temperatures go up, it starts to decompose and the meat gets soggy and sour,” Kizaki said. To stop this, the fishermen flush out blood using seawater. “And [fishermen] try to cool it down by putting it in an ice bucket [on the boat]. But the meat is still contracted and hard, plus it’s bloody, so you can’t eat it like that.” Hence, the dry-aging coolers.

Mackerel, on the other hand, gets spoiled quickly, so the sushi masters cure them with sea salt and rice vinegar for normally two to three days.

Sushi Den and Izakaya Den’s “Catch of the Day” menus feature the dry aging seafood in cuts of sushi, sashimi and nigiri.

And dry-aging isn’t the only thing the Kizaki brothers teach to customers. They pride themselves on guiding guests through a proper Japanese sushi experience, including etiquette tips. For instance, silence is encouraged between bites at a traditional omakase restaurant, and when a customer rubs their chopsticks together, it’s considered an offense to Japanese restaurant owners, who receive a message that their chopsticks are cheap.