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Rennet (pronounced /ˈrɛnɪt/) is a natural complex of enzymes produced in any mammalian stomach to digest the mother's milk, and is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). The active enzyme in rennet is called chymosin or rennin (EC but there are also other important enzymes in it, e.g., pepsin or lipase. There are non-animal sources for rennet that are suitable for vegetarian consumption.

Traditional method

Dried and cleaned stomachs of young calves are sliced into small pieces and then put into saltwater or whey, together with some vinegar or wine to lower the pH of the solution. After some time (overnight or several days), the solution is filtered. The crude rennet that remains in the filtered solution can then be used to coagulate milk. About 1 gram of this solution can normally coagulate 2 to 4 litres of milk.

The fat referred to in Lev 3 & 7 is Chelev - the choicest fats used in making offerings to YHWH. These fats are not to be eaten, the fat of the innards and the fat of the tail.
Digestive system fats
There are many fats around the digestive organs such as the stomach and intestines, and one must be highly educated and trained in order to identify them.

Leviticus Chapter 3
16 And the priest shall make them smoke upon the altar; it is the food of the offering made by fire, for a sweet savour; all the fat is YHWH's.
17 It shall be a perpetual statute throughout your generations in all your dwellings, that ye shall eat neither fat nor blood.

Leviticus Chapter 7
22 And YHWH spoke unto Moses, saying:
23 Speak unto the children of Israel, saying: Ye shall eat no fat, of ox, or sheep, or goat.
24 And the fat of that which dieth of itself, and the fat of that which is torn of beasts, may be used for any other service; but ye shall in no wise eat of it.
25 For whosoever eateth the fat of the beast, of which men present an offering made by fire unto YHWH, even the soul that eateth it shall be cut off from his people.

Both Karaites and Rabbinical Jews agree chelev fats should not be consumed. Should it be consumed, the consumer risks "the soul that eateth it shall be cut off from his people". For most cheeses, animal rennet is used to coagulate the milk turning it eventually into cheese. Animal rennet is made from an animal's stomach which contains chelev fats. Yet, some kosher cheeses are made with animal rennet. So, how can cheese made with animal rennet be kosher?

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