Rennet is an enzyme used to coagulate milk: to form a soft mass of solid curd in your pot. It can be animal or plant-based, and available in liquid or tablet forms. You can even make your own rennet! Dilute ALL rennet in filtered water and use in your cheese recipe within 30 minutes.
The traditional rennet, animal rennet is an enzyme derived from the stomachs of calves, lambs or goats – while they consume only milk (nothing else). Calf rennet is 80-90% chymosin. Animal rennet is the best option for hard, long-aged cheeses. 
Vegetable rennet is derived from a type of mould (mucur miehei) and is not animal-based. No mould is present in the rennet and it contains the same amount of chymosin as calf rennet. Vegetarian rennet develops a slightly bitter flavour after 6 months or so of aging hard cheeses.
Liquid rennet is available as liquid calf rennet or as liquid vegetarian rennet. It is stored in the refrigerator – liquid animal rennet will keep for up to one year and liquid vegetable rennet will keep for around 3-4 months. Some brands of liquid vegetable rennet are double-strength, so use half the quantity stated in the recipe – check the label before use to confirm the quantity of rennet required. 1/4 teaspoon liquid rennet = 1/4 rennet tablet.
Vegetable rennet tablets will last for 12 months in the pantry, around 2 years in the refrigerator and at least 5 years if frozen. Some brands of vegetable rennet tablets are weaker, and may require double the quantity stated in the recipe – check the rennet tablet label before use to confirm the quantity of rennet required for the amount of milk you are using. Use a pill cutter or sharp knife to cut rennet tablets in half. 1/4 rennet tablet = 1/4 teaspoon liquid rennet.
Cheese rennet is made of 20% pepsin and 80% chymosin. Junket is made of 80% pepsin. 
Junket was originally used when cheese-making supplies were hard to find, however junket is not the same as cheese rennet. It is much weaker, contains lots of additives and is also more expensive than cheese rennet. Even if you added extra junket to get the right curd consistency, the increased amount of pepsin in the cheese would break down too many proteins, causing problems as the cheese aged. Save junket for making dessert, not cheese!
Some large-scale home cheese-makers make their own calf or vegetable rennet. The smaller home cheese-maker can make homemade rennet from nettles or fig sap.

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