Rennet tablets are small tablets made from powdered and packed rennet. They are designed for convenient use, being easier to handle than some more traditional forms of rennet, and they are used for a variety of cooking projects. Several companies make tablets; Junket Rennet, named after a pudding made with rennet, is probably one of the most famous, and many recipes give proportions in terms of these popular products.
An enzymatic extract, rennet is made from the lining of the stomachs of young ruminants, such as cows, sheep, and goats. These animals produce enzymes to help them digest the milk produced by their parents, breaking down it so that the maximum nutrition can be extracted from it in the stomach and intestinal tract. These same enzymes can be used to curdle or thicken milk to make a variety of foods.
People have been using rennet for centuries; it's the foundation of traditional cheesemaking, for example. Initially, people were forced to make their own rennet, which was a messy and unpleasant process, as the stomach must be removed, scraped, cleaned, and dried. To use rennet in this form, cooks had to cut off a piece of the dried stomach and soak it in water to encourage it to dissolve before adding it to the recipe. Over time, butchers started offering rennet as a useful byproduct of their business, and eventually companies seized upon the idea of making rennet in tablet and liquid form for convenience.
A typical rennet tablet is white, and around the size of a thumbnail. The tablet is scored, dividing it into four sections. Depending on the recipe, a whole tablet may be needed, or cooks may be asked to split it, using only part. Before the rennet can be added to a recipe, it must be dissolved in cool water; dissolution is critical for the enzyme to work.
One important thing to know when working with rennet is that high heat will deactivate the enzymes, which is why cool water is required for dissolution, and why temperature control of the milk used for cheese, junket, and other foods that call for rennet is very important. When working with rennet, it's a good idea for a cook to invest in an accurate thermometer, along with tools such as pans and stirrers that can be fully sterilized to ensure that no harmful bacteria enters the milk.
When a dissolved rennet tablet is added to milk, it will slowly start to act on the milk, causing it to thicken. As a general rule, the milk must be left alone at room temperature for six to 24 hours to thicken; if it is disturbed, the action of the enzyme may be disrupted, and the milk will fail to thicken.