How to freeze/preserve fresh fish for sushi & sashimi

From my previous post, some people asked me how I could take care of the great amount of fish. The answer is, I just freeze them!

However, the freezing process needs some caring, and I’m going to demonstrate how I freeze my own fish without causing any damage to the fish.

The fish I prepared today is ‘madai’. Wild-caught and 9.7 lb

The process is in 3 steps:
(1) Addition of salt (+Trehalose) and removal of water.
(2) Vacuum packing.
(3) Rapid freezing and low-temperature storage.

(1) Sprinkle some fine salt on the fish fillet. This process is called ‘Shiojime’. I prefer using just the right amount of salt and not rinsing the fillet afterward. If you can find Trehalose, you can use it together along with the salt. The fish fillet has different thicknesses depending on the body parts, so adjust the salt. More salt on the back, less salt on the belly.

Leave the fillet in a fridge for 30-60 minutes (depending on the size of the fish) and wipe off the moisture.

*If you’re not sure about the ‘right’ amount of salt you need to use, just use enough and rinse the fillet with 1% brine afterward. This would lower the risk of failure.

For the ‘shiromi’s (white-fleshed fish), whose consistency is more important, I extract extra water out with ‘Okamoto Pichit sheets’.

Addition of salt (and Trehalose) reduces free water, increases water-holding capacity of the tissues, and prevents the growth of ice crystals during storage (Ostwald ripening).

These effects result in better taste and consistency after the freezing-thawing process.

(2) Divide the fillet into the size you want. Then vacuum-pack the fillets. Vacuum packs prevent the fillets from being dried during storage and keep them away from oxidation.

(3) Flash-freeze the fillets. The way I use is ‘Immersion-freezing’. I have a freezer that runs at -70F. I mixed water and ethanol so that its freezing point is around -70F, the temperature at which my freezer operates.

The thermal conductivity of this mixture is much higher than that of air, and its temperature is very low. Therefore, if you put fillets in it, they freeze extremely fast! The faster the freezing rate, the better the quality of frozen fish.

One thing to notice is that the freezer of your choice might have been made for storage, not for freezing. Putting unfrozen food directly into the freezer can overload the compressor.

Therefore, I first froze some ice with an ordinary freezer and filled about 30% (in volume) of my -70F freezer. I waited till the temperature equilibrates. The next day, I prepared the alcohol-water mixture, whose temperature already dropped with the ordinary freezer. Then I put the mixture into the -70F freezer and waited till the temperature equilibrates again. For a few days from then, I tuned the mixture’s freezing point by adding some water.

I use about 4 times the weight of the fish I want to freeze and let this alcohol-water mixture be in a ‘slurry state’. The heat of fusion from the fish fillets is used as the heat of fusion by the mixture, so the temperature of the mixture does not increase much during the process, which ensures flash-freezing. The heat released from the alcohol-water tank is absorbed by the ice with which I filled the freezer (the 30% of the total volume).

With these ‘a little’ complicated methods combined, I can freeze fish fillets intact without damaging my freezer.

My freezer is usually filled with frozen-rare fish, and I can have them anytime I want. The quality of the fish just remains intact before and after freezing. I don’t think what I do is applicable to everyone but hopefully, it could find someone!

by Fishcook_engineer