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 theabomasum, 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 and dried. A small piece would be added to milk in order to turn it into curds and whey. At some point, the Germans began calling this material rennen, meaning to run together, or to coagulate. The technical term for rennin is chymosin.
Until 1990, rennet was produced the old fashioned way (from abomasums), from various "vegetable" rennets (some of which, called microbial coagulant, are made from the microorganism Mucor miehei.) These days, at a cost one tenth of that before 1990, chymosin is produced by genetically engineered bacteria into which the gene for this enzyme has been inserted. When the bacteria are grown in large vats,they secrete rennin, and itis then purified for cheese making.
Rennet is available commercially in tablet or in liquid form. You will find some cheese makers on the web who prefer liquid rennet and disparage the use of rennet tablets. Perhaps, if you are making hundreds of gallons of milk into cheese, buying bulk liquid would make sense, but for making one to ten gallons of milk into cheese, the tablets make sense. I have never had any problems using rennet tablets in making a wide variety of cheeses, and since it is a principle of mine to try to use materials which are readily available locally, I have used tablets for years.
I prefer Junket Rennet tablets because they are readily available and inexpensive. They are easily measured out (1 tablet will coalgulate 5 gallons of inoculated milk) are stable because they are in dry tablets. "Junket Rennet Tablets" come in a packages of 8 (6.5 g) or 12 tablets.
One teaspoon of liquid rennet is reported to be equivalent to one Junket Rennet tablet. Thus, use one teaspoon to coagulate five gallons of inoculated milk, or 4 drops per gallon of inoculated milk. (I have only used tablet rennet, but am assured that liquid rennet works just as well as the tablets.) Liquid rennet can be ordered from various cheese makers' suppliers. I have heard (but have no personal experience) that the liquid rennet looses its potency with age, and one must add more and more to achieve the same degree of coagulating.
A rennet of bacterial origin, called microbial coagulant, is made from Mucor miehei. This tableted rennet should pose no problem for vegetarians.
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