I use this gypsum powder to make douhua (tofu fa, soft tofu custard).
At this point, I like the results with 3/4 tsp of this gypsum powder per 4.5 cups of soy milk made from soaking 140 gm of dry soy beans. The strange 4.5 cup quantity is because I fill the Joyoung CTS1098 soy milk maker to just below the max fill line.
1. I have tried simultaneously pouring 85 degree F soy milk and gypsum solution (diluted with 2 TBS of hot water).
2. I have tried quickly stirring in the gypsum solution into 85 deg. F soy milk in a thick container.
3. I have tried stirring gypsum solution into cold soy milk and simmer the container of soy milk and gypsum mixture in a pot of water at just below boiling temperature for 10 minutes or so until set.
In all of the above three methods, I just have to cover the soy milk container with a towel and leave it undisturbed for about 15 minutes. They all have worked for me with slight differences in the firmness of the douhua.
Adding 1 TBS of corn starch/potato starch to the gypsum solution in all these situations works as far as coagulation goes. This increases the firmness and smoothness of the douhua, especially for Method no. 1.
