Rennet is a key ingredient in cheesemaking, but it can be tricky to find.
Most people don't realize that the majority of rennet is not vegetarian as it is an enzyme extracted from the stomachs of goats, lambs and calves. It is comprised of chymosin, an enzyme that curdles milk as part of digestion, which is why it is found in the stomachs of dairy-consuming young animals.
It is possible to find vegetarian rennet, which is derived from plants or microbes, but the default type of rennet is animal-based. Plant rennet is derived from sources such as artichokes or thistle. Microbial rennet is often derived from Mucur Miehei, a type of mold. Although microbial rennet is obtained from the mold, it is still vegetarian as there is no mold in rennet itself. Vegetarian rennet works just as well well for most cheeses as rennet sourced from the stomachs of goats, lambs and calves.
Rennet Types and Properties
The default type of rennet is calf rennet because some of its residual components aid in the breakdown of proteins, making it the ideal rennet for cheeses which are aged longer. Cheeses made with microbial rennet which are aged longer than 6 months can have a mildly bitter taste due to some of the more complex proteins in the vegetarian rennet. Certain properties of plant rennet can also impact the flavor of the end result cheese. Whether this impact is positive or negative depends on your personal cheese flavor preferences.
Where and How to Buy Rennet
Rennet is available in powder, tablet or liquid form. They all work equally well to coagulate the milk. Liquid rennet is the easiest type to work with because it can be the most precisely measured. However, the rennet tablets and powders will keep better under non-ideal storage conditions and are often preferred by large scale cheesemakers. If you aren't using it regularly, you should be sure to test your rennet to ensure it has not lost its strength or gone bad prior to making your cheese.
You can often find rennet at health food stores. If you don't have a health food store near you, or if you can't find one that carries it, you can also order your rennet online. The most common rennet brand is Junket. It's the one you are likely to find in stores.
Do All Cheeses Need Rennet?
While it is possible to make certain fresh cheeses like ricotta without using rennet, most fresh and aged cheese recipes require you to use rennet. This is because rennet causes the milk to separate into cheese curds, which are then used in the cheesemaking process, and whey. Without some type of rennet, whether animal, plant or microbe-based, the milk will not coagulate. The amount of rennet you will need to use will depend on the type of cheese you are planning to make. Soft cheeses like mozzarella require much less rennet than hard cheeses like cheddar.