They are naturally present in the meat. Apparently, that temp inactivates enzymes that, I'm guessing, contribute to shorter shelf life. I'm not sure though. This came up in discussion on curing and smoking bacon I had a while ago with some food microbiologists, as an aside. It wasn't the point of the conversation; it's an assumption I'm making based on my understanding of enzyme activity in meats. It could be, also or alone, an issue of texture.