I kept procrastinating posting my frozen fish findings, but a question that Matthew Amster-Burton, a columnist on Culinate.com has just posted on my How to store fish story has inspired me to finally get off my lazy butt and write up my frozen fish experiments.
I used to be of the conviction that frozen fish was ALWAYS worse than fresh. I know, I know -- Whole Foods and many fish cookbooks like to tell you that previously frozen fish can be even fresher than not previously frozen fish because it was frozen at the peak of freshness. Just so that I don't have to use the "not previously frozen" terminology (that just takes too long to type), I'll use the word "fresh" to refer to fish that did not undergo the freezing process. The question I'll try to answer is whether previously frozen fish can taste as good as fresh, not whether it's as safe to eat.
Why would I care? I can get plenty of excellent fresh fish here in Boston. The problem is that when it comes to eating fish raw, freshness is not the only thing you have to worry about. Parasites are another hazard. Depending on the fish, they might pose an extremely small risk (to read all about them, see my posts on parasites, parts 1 and part 2). But if you want to eliminate that risk completely by killing the parasites, the only way to do it is to freeze the fish for at least 7 days. Cooking kills them too, of course, but that doesn't help you much with sushi.
My attitude to raw fish is pretty laid back. But when I teach sushi classes, I want to give my students an extra precaution option. Some people don't want to take a risk of food-born illness, no matter how minuscule. When I am serving fin fish raw, I only use tuna, farm-raised salmon, farm-raised branzino, and farm-raised yellowtail. The only way to get yellowtail in Boston is flash frozen and shipped from Japan so parasites are not an issue for that one at all. The other fish are fresh, but the odds of them having parasites are practically zero, so I just buy them from a reputable market (The New Deal in Cambridge) and eat them. As far as the freshness of the fish goes, freezing does nothing for you. It doesn't kill bacteria, just temporarily stops it's growth, so freezing inferior fish does not make it "safe".
I used to think that all fish would be damaged by freezing. Fish is mostly made of water, and water expands during freezing. This tears the flesh of the fish and makes it mushy. To prepare for my sushi class, I froze piece after piece of different fish, and here are my findings -- different fish react to freezing differently. Fatty fish freeze relatively well, and their texture is barely affected. Lean fish turn to mush in their defrosted raw state and rubber when cooked.
In these pictures, farm-raised salmon (very fatty) and fluke (very lean) were frozen the same way for the same amount of time. After defrosting, the fluke was so soft, I could turn it into a puree with a chop stick.
The reason I was so surprised was that I've had previously frozen salmon before that was terrible, so I concluded that salmon doesn't freeze well. What I didn't take into account was that it was wild Coho and Sockeye Salmon that tasted awful. They are extremely lean compared to farm-raised Atlantic or King salmon and do turn to mush when frozen. So when you choose your salmon for freezing, go with Atlantic (always farm-raised) or King (farm-raised or wild).
Even if you choose a freezer-friendly fish for your sushi, you have to freeze it and defrost it properly. Remove the skin from the fillet, wrap it as tightly as possible in plastic wrap without bending it, then put it in a zip lock bag to make sure it's completely sealed. Freeze for 7 days, then move it to the fridge 24 hours before using. Do not defrost on the counter or in water. Fast defrosting can damage your fish. Also, do not keep it in the freezer for months. Couple of weeks is the longest I've tried. Somewhere between 7 days and a few months, fish texture starts to change. It happens at different times for different fish. I try not to test my luck and don't keep fish frozen longer than obligatory 7 days. Also, don't freeze whole fish. The smaller the piece, the faster it will freeze, and the less trauma your fish will undergo.
What about FAS (frozen at sea) or flash frozen fish? FAS is a nice marketing term and no more than that. At sea or not, frozen is frozen. "Flash frozen" is a very fast freezing methodology that is an optimal way to freeze fish -- supposedly, much better than your home freezer. I've never had flash frozen and home frozen fish side by side, so I can't comment on how big of a difference it makes taste wise. I have a feeling that flash frozen stuff can just stay in that state longer than home frozen stuff without affecting the taste, but I've never tested that theory. From my experience the type of fish makes a much bigger difference. I've had excellent fatty fish like Chilean Sea Bass and fatty salmon that have been flash frozen and home frozen and they tasted fine. But I've never had any luck with previously frozen lean fish even if it was flash frozen.
Does this mean that all Trader Joe's fish is sushi grade? It's all previously frozen, right? There is much more to sushi grade than lack of parasites. The quality of the fish, the fat content, freshness at the time it was frozen, and storage conditions all make a difference. The stuff that sat in Trader Joe's freezer for 6 months is not an option for eating raw (at least not a tasty option).
Even if you choose a freezer-friendly fish for your sushi, you have to freeze it and defrost it properly. Remove the skin from the fillet, wrap it as tightly as possible in plastic wrap without bending it, then put it in a zip lock bag to make sure it's completely sealed. Freeze for 7 days, then move it to the fridge 24 hours before using. Do not defrost on the counter or in water. Fast defrosting can damage your fish. Also, do not keep it in the freezer for months. Couple of weeks is the longest I've tried. Somewhere between 7 days and a few months, fish texture starts to change. It happens at different times for different fish. I try not to test my luck and don't keep fish frozen longer than obligatory 7 days. Also, don't freeze whole fish. The smaller the piece, the faster it will freeze, and the less trauma your fish will undergo.
What about FAS (frozen at sea) or flash frozen fish? FAS is a nice marketing term and no more than that. At sea or not, frozen is frozen. "Flash frozen" is a very fast freezing methodology that is an optimal way to freeze fish -- supposedly, much better than your home freezer. I've never had flash frozen and home frozen fish side by side, so I can't comment on how big of a difference it makes taste wise. I have a feeling that flash frozen stuff can just stay in that state longer than home frozen stuff without affecting the taste, but I've never tested that theory. From my experience the type of fish makes a much bigger difference. I've had excellent fatty fish like Chilean Sea Bass and fatty salmon that have been flash frozen and home frozen and they tasted fine. But I've never had any luck with previously frozen lean fish even if it was flash frozen.
Does this mean that all Trader Joe's fish is sushi grade? It's all previously frozen, right? There is much more to sushi grade than lack of parasites. The quality of the fish, the fat content, freshness at the time it was frozen, and storage conditions all make a difference. The stuff that sat in Trader Joe's freezer for 6 months is not an option for eating raw (at least not a tasty option).