Rennet is a mixture of enzymes that mammals produce in the stomach to digest milk, and it is used to make cheese. It contains a proteolytic enzyme, or protease, that causes milk to coagulate and makes it separate into curds and whey, or solids and liquids. For vegetarians, there are non-animal enzymes that can be used to make cheese.
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Clean the stomach of the calf as soon after the animal's death as possible. Do this by using your hands and scrubbing the interior and exterior of the stomach in cold running water and sea salt. Handfuls of salt can be used to scour the entire area prior to adding water, and this will help to preserve the stomach as well.
Scratch away any particles of food or bile that may be stuck to the inner walls of the stomach. This task is much easier if the calf has not been fed 12 hours prior to slaughter.
Tack the stomach on a wooden frame to dry for 24 hours in direct sunlight. This will help to rid the stomach of any bacteria or fungus that will cause health hazards.
Remove the stomach from the frame, and cut it into squares. These squares should be small enough to fit inside a bowl without hanging out. Pack in sea salt and apply the lid for later use.
When you are ready to use the rennet, soak a square in cold water for half an hour and wash gently. Once clean, place it in the milk with a string tied around so it can be pulled out without damaging the curd in the cheese-making process.
You can pack or store the stomach in brandy or wine for added flavor, but most recipes call for packing in salt to preserve for longer periods.