What would you think if your raw sushi had actually been frozen for 15 hours or more before you ate it? If the Food and Drug Administration has its way, that’s how all uncooked fish in the U.S. will be treated.

For several years, the FDA has recommended that any fish that’s served in sushi, crudo, or ceviche be frozen for at least 15 hours or longer in order to kill potential parasites. But since the FDA leaves it to local health departments to enforce the recommendation, few cities actually follow it — until now.

Recently, New York City’s Department of Health decided to enforce the policy and other cities may follow suit. Many cities and states, with the notable exception of California, which has a state code that allows raw fish, have the FDA’s recommendation on the books, but enforcement is patchy.

So what exactly happens to fish when it’s flash frozen? 

In Japan, super-freezing tuna has been commonplace for the last 15 years, according to Mike Kanter, the seafood merchant for FreshDirect, a large online grocer in New York City. Fish are flash frozen to negative 76° F within hours of being caught — sometimes while still on the boat. Sushi kitchens in Japan are often decked out with mini super freezers that keep the fish at the same arctic temperatures until it’s ready to serve.

This is no normal Maytag. If you stick a fillet of fish in your kitchen freezer, the water expands when freezing, forming crystals that break the cell walls of fish. Defrosting it leaves the fish mushy with a pool of water underneath that’s seeped out from the flesh. That doesn’t happen to most fish when you super freeze it, Kanter says. Fish with higher water levels and more delicate flesh, like fluke and sea bass, don’t fare quite as well, though.

The regulation was created to fend off parasites that fish eat in the wild or latch on to the skin. Farmed fish aren’t susceptible in the same way because their food supply is controlled. Experienced chefs can spot most of the parasites while preparing fish because they are visible — but in the larval stage, they can be as small as a pin and ingested.

Exactly how many people get sick is unclear. National agencies don’t track illnesses caused by eating raw fish. Dr. Susan Montgomery, an epidemiologist at the Centers for Disease Control and Prevention, recently told the New York Times that “these infections are rare in the United States and generally aren’t fatal.”

While this technology that kills the parasites is well established in Japan, some are concerned it will put a burden on smaller restaurants and local seafood purveyors in the U.S.

Louis Rozzo, a New York City fish monger whose family has been in the business for more than 100 years, thinks the reward of eating locally caught fish far outweighs the risk of eating a fish with parasites.

While Rozzo says that it’s true that fish frozen at some point in the supply chain is a common practice, and that for many things, the consumer’s experience won’t change that much, but he disagrees that consumers can’t tell if sushi has been frozen.

“Of course there’s going to be a texture difference, even for novice fish eaters,” Rozzo says. “There’s a lot more liquid texture to a fresh piece of tuna fish. It will thaw out to be much drier.”

Brooklyn-based Greenpoint Fish & Lobster created their small fish market and restaurant with a focus on serving local sustainable fish — 95 percent of what they serve is locally caught and wild. Their smaller-scale suppliers won’t be able to super freeze their fish before they sell it.

While the fish they sell at Greenpoint Fish & Lobster for people to cook at home would be exempt, they imagine that unless they invest in a super freezer, which can cost as much as $50,000, that they will have to remove certain menu items from the restaurant.

“I get that you want to keep diners safe,” says Adam Geringer-Dunn, a co-owner of Greenpoint Fish & Lobster. “But they lump everyone in the same category, and we’re really confident in the fish we bring in.”

However, geography dictates that a lot of the country can’t have that same confidence in local fish. Chris Himmel grew up working on fishing boats off of Marblehead, Mass. He’s obsessive about freshness to the point where he’ll sometimes catch and prep the striped bass himself before serving it to a group of Boston restaurants he owns. But he realizes that for most of the nation, same-day access to fish isn’t possible, and that seafood changes hands several times on it’s way to the table. Having it frozen along that distribution line can help keep diners safer, he says.

In land-locked Las Vegas, sushi that’s been frozen first has been the rule for years, says chef Ralph Scamaradella, who heads up the kitchen of Tao there, as well as two locations in New York City. All the kitchens have super freezers, nicknamed “blood freezers” because they are also the same quality that hospitals use to cryogenically store blood.

“If you handle the fish the right way, it really doesn’t affect the texture,” Scamaradella says. “I don’t think anyone in their right mind wants to make someone sick, especially a chef who has worked hard and wants to stay in business.”

But frozen fish is not without its own health worries. Last week, frozen tuna packaged in Indonesia was linked to 62 cases of Salmonella in 11 U.S. states.

The tuna that was recalled, however, was regularly frozen (as opposed to flash frozen) tuna treated with carbon monoxide. It’s a trick that some sushi places use to give frozen tuna, which turns brown when it’s dethawed, an artificially red color again to appear fresh. It never loses that watermelon red color, even when it gets old.

If properly dethawed, tuna that’s been super frozen retains that red color naturally, explains Michael McNicholas, who supplies super frozen fish to YO! Sushi, a U.K.-based sushi chain that has begun opening locations throughout the United States.

“This treatment [with carbon monoxide] removes the ability of the consumer to make a value judgment for themselves as to whether they would purchase a given product or not,” McNicholas says.

If super frozen tuna is placed for one to three minutes in a salt bath, patted dry and then dethawed in a refrigerator, it maintains the natural redness without adding carbon monoxide.

Fresh or frozen, for diners it will be a matter of trust.

Les Barnes, who grew up working in the Fulton Fish Market and now operates London Lennie’s restaurant in Queens and is opening another seafood restaurant in Port Chester, N.Y. in August, doesn’t like to buy frozen fish for his restaurants because he wants to eye the fish himself and make sure the quality is there. He’ll try for a while to see what freezes well before deciding what raw dishes he’ll keep on the menu.

“I just don’t trust anyone else to buy my fish,” he says. “It’s a very loosey goosey industry. So you find people you can trust and keep an eye on them, too.”

This article originally appeared on Yahoo! News.