Years ago I performed an experiment to find out whether I liked dry curing or wet curing pork tenderloin before cooking it sous vide. The conclusion was that for meat I preferred to just season with salt right before cooking sous vide. And that is what I have done ever since. But for fish I sometimes briefly cure before cooking sous vide, and sometimes I don’t. So it was time to do another experiment to decide whether briefly curing fish before cooking it is worth the effort. The experiment started with the piece of halibut fillet on the photo above. Note that this experiment is also useful if you are going to cook the fish without sous vide, as long as you keep it moist on the inside. If you are going to overcook the fish, curing is not going to help.
I divided it into two pieces of equal size, 120 grams (4 oz).
The very simple curing mix I use for fish (as recommended by Modernist Cuisine) is 1,4% salt, so in this case 1.7 grams…
…and 0,6% sugar, so in this case 0.7 grams. You can already see why I don’t always bother to do this, because you need precision scales to get the right amount of cure.
Note: you can approximate using measuring spoons.
Using regular fine table salt, 1 tsp = 7.5 grams, 1/2 tsp = 3.75 grams, 1/4 tsp = 1.88 grams, 1/8 tsp = 0.94 grams.
Regular granulated sugar, 1 tsp = 4.17 grams, 1/2 tsp = 2.08 grams, 1/4 tsp = 1.04 grams, 1/8 tsp = 0.52 grams.
So for the halibut we would need a heaped 1/8 teaspoon of sugar and a scarse 1/4 teaspoon of salt.
I mixed salt and sugar in a small bowl, and applied it evenly to the halibut.
Then I vacuum sealed both pieces of halibut: one piece with cure, the other without. (Just to rule out the effect of the vacuum sealing in the experiment.)
After 1 hour of refrigeration I took the fish out of the vacuum bags. I rinsed both pieces with cold water and patted them dry with paper towels. The cured fish had firmed up slightly.
Then I vacuum sealed the cured piece again, and salted the uncured piece and also vacuum sealed it.
I then cooked both of them sous vide for 15 minutes at 50C/122F. This time is enough because the fillets are so thin (1 cm, 1/2 inch). For example for 2.5 cm (1 inch) thickness you would need 35 minutes.
After sous vide I chilled them in cold water (no ice needed this time of the year).
The cured piece felt more firm and slightly less fragile, but both pieces had to be handled with care to avoid breaking them.
Note: I should have added some olive oil to the fish before vacuum sealing, as the fish got a bit stuck to the bag. With meat there is no need to use olive oil when cooking sous vide, but fish is so tender that it may otherwise get stuck.
I dusted the halibut with flour and pan fried it in vegetable oil.
These fillets are so thin that pan frying them briefly, just to give them color, will bring them to serving temperature. The fish is already cooked, so it is not necessary to worry if the inside is cooked.
If you look carefully, you can see the difference in the photo. The cured fish has a better texture. Flaky and tender, but firm and juicy. The uncured fish is a bit wet and soft on the inside and seems undercooked.
So my conclusion is clear: briefly dry curing fish before cooking is a good idea. As a follow up experiment I will try if you can also just put salt on the fish, vacuum seal it, refrigerate for 1 hour, and then cook sous vide. That would take away most of the additional effort.