In the video of Alice making head cheese the chef didn't know what Activa was made out of. Here is a definition "Activa® transglutaminase, an enzyme with the ability to cross-link proteins. It is produced by a microbial fermentation of a naturally occurring organism. Various forms of transglutaminase are found in animal, plant and microbial sources. Activa is a calcium independent form of the enzyme and this characteristic gives it certain advantages in many food systems." That still doesn't really say what it's made out of but it doesn't sound very good. The FDA says it's safe to make food with up to 100 ppm. Activa. How many ppm. did the chef sprinkle on that head cheese to bind it together?
Anyways there is a tastier way of binding it together. When you simmer the head with all your delicious aromatic vegetables and spices throw in a couple trotters and tongues and let it slowly simmer over night. Then strain it all out and reduce the liquid until it gets thick. Then after you have picked all the meat off the head, and the trotters too, mix it with the gelatin rich stock and let it cool. If you want it to be a cylinder, after it has hardened wrap it in plastic and let it warm up enough to be pliable and roll it back and forth on the counter then rechill it. Slice and enjoy