Char siu is one of my favorite dishes to prepare sous vide, because it is so easy and delicious. Because the pork is cooked for 24 hours in the marinade, I had always assumed there was no use in marinating before sous vide cooking. But I wasn’t sure. And when I’m not sure, I’d like to do an experiment to find out if I can taste the difference.
For this experiment I used two pieces of pork neck (pork butt) of about 150 grams (5 oz) each. And I used char siu marinade from a jar; one of the few exceptions to my rule of making everything from scratch.
I weighed both pieces of pork accurately and used 50 grams of marinade for each piece. The first piece I vacuum sealed with the marinade and then allowed to marinate for 24 hours in the refrigerator before sous vide cooking. The second pieces I stored in the refrigerator during those 24 hours, and then vacuum sealed it with the marinade just before cooking both pieces sous vide for 24 hours at 57C/135F.
After sous vide cooking I reserved the liquid from the bags in a saucepan to make a sauce, patted the pork dry with paper towels, and then weighed it. The pork that had been marinated for 24 hours before sous vide, had shrunk from 152 grams to 128 grams, a los of juices of 16%.
The pork that was cooked sous vide right after combining it with the marinade, had shrunk from 144 grams to 111 grams: a loss of juices of 23%. So the difference between marinating before or not was clearly measurable and larger than I had expected!
On the inside the difference was clearly visible: the marinated pork (on the left on the photo) had more color and looked more moist. And more importantly, the difference could also be tasted very clearly: the marinated pork was definitely more moist.
I do not know the exact scientific explanation, but apparently the marinate penetrates more easily in the meat when it is still raw, compared to during the cooking process. And so it makes a difference to allow meat to marinate before you cook it sous vide. (Note that many ingredients of the marinade will not penetrate deeply into the meat, but at least the salt definitely does.)
Years ago I performed an experiment with salting meat before sous vide that I should repeat with a lower concentration of salt, because with the experience I have gained since, I now expect that with a lower concentration of salt, I may come to different conclusions then I did back then.