Rigor Mortis in Sushi Fish
One time we ordered some delicious fatty toro tuna which can frozen from the company. To time its freshness, we thawed it just several hours before serving it to a customer. You could see our chef's surprise when the tuna slice was stiff as cardboard just before the sushi was completed!
Rigor mortis in dead fish starts around 5 hours in. So this Tuna must've been frozen on board the boat as soon as it was caught!
Can we still use it? I tried chewing on it, but it had a very unappealing texture. It was like chewing rubber, you chew and chew but it doesn't break down. Eventually I pulled it out of my mouth and threw it away. That was a waste of prized sushi.
So we waited for another hour and at this point the meat became flaccid again. This time the tuna sushi melted in my mouth.
This only happened once for us, but it gives us a glimpse of the different degree of freshness from different fishing methods. Most fish caught in the UK won't need a fancy on-board freezer as it will be caught close to the coast.