HISTORY OF CHEESE RENNET: Presumably, the first cheese was produced by accident when the ancients stored milk in a bag made from the stomach of a young goat, sheep or cow. They found that the day-old milk would curdle in the bag (stomach), yielding solid chunks (curds) and liquid (whey). Once they discovered that the curd-chunks could be separated out and dried, they had discovered a means by which milk, an extremely perishable food, could be preserved for later use. The addition of salt was found to preserve these dried curds for long periods of time.
At some point, someone discovered that the most active portion of the young animal's stomach to cause curdling was the abomasum, the last of the four chambers of the stomach of a ruminant animal. (In sequence, the four chambers are rumen, reticulum, omasum and abomasum.) In particular, the abomasum from a suckling kid or calf was especially active. The abomasum was cut it into strips, salted and dried. A small piece would be added to milk in order to turn it into curds and whey.
