(Animal Rennet, Vegetarian Rennet, Optimization, and more!)
Curds and Whey
Remember Little Miss Muffet, who sat on her tuffet, eating her curds and whey? When we make cheese, our goal is to separate the solid components of milk (casein protein, fat, minerals) into curds and the liquid into whey (water, lactose, whey protein). This separation is possible due to coagulation and usually rennet is the enzyme used to achieve this. There are a few cheeses that use acid to coagulate instead of rennet, like Ricotta, but here we are going to discuss all the wonders of rennet. 
Folklore of the First Cheese
Legend has it that the first cheese was invented by accident. Many ages ago, well before pasteurization, a man journeyed across the desert. Before setting out, he filled his flask, made from a lamb stomach, with milk. It was a long, hot journey. When he got to his destination, he opened his flask and it was no longer milk. There were some soft white chunks (curds) and clear, yellowish liquid (whey). It was all the traveler had, so he consumed it, and it was delicious. If you are familiar with the basic cheesemaking steps, there is a lot to this story that makes sense. He began with raw milk which contains naturally occurring bacteria. These bacteria converted lactose to lactic acid, dropping the pH, and creating the perfect environment for cheese. He also had time and warm temperatures, which are important in any cheesemaking recipe. In the end, he had curds and whey (remember Little Miss Muffet?). The missing ingredient is rennet, which comes from our traveler’s flask.
What is Rennet?
Rennet is made of enzymes that are found in the lining of un-weened mammal stomachs, usually from a cow (calf), sheep (lamb) or goat (kid). Young mammals’ main source of food is milk. The enzymes slow down the digestions of the liquid milk by turning it to a solid in the mammal’s stomach, giving the baby time to absorb the nutrients. As the mammal ages, these enzymes are no longer produced. Unfortunately, this means that rennet must be harvested from young animals, typically veal. Thankfully, a lot of rennet can be made from each animal.
Types of Rennet
Animal Rennet
Animal rennet is traditional and has been used for thousands of years. It contains about 90% chymosin, 10% pepsin and other enzymes. Animal rennet can be purchased in liquid, paste, or tablet form. Our recipes are typically written using single-strength liquid animal rennet, like that sold at cheesemaking.com. We like this product because it is easy to use, and “cheesemaking made easy” is kind of our thing.
Before the manufacture of rennet, cheesemakers would make their own rennet by drying the fourth stomach of calves and cutting off pieces to add to the milk. This practice is still used in some places today. However, manufactured products have become preferred by many cheesemakers because they are controlled for potency and have more predictable results. The manufacture of animal rennet is also tightly controlled, and homemade rennet will likely not be FDA-approved. If you’d like to learn more about traditionally made rennet, I highly recommend The Art of Natural Cheesemaking by David Asher. Because animal rennet in any form is a naturally occurring product, it’s difficult to exactly replicate the ratio of enzymes present.
Vegetarian Rennet
Vegetarian rennet, sometimes labeled microbial rennet, is created in a lab, and is sometimes derived from a mushroom. It is typically 100% chymosin and therefore does not contain the same nuanced balance of enzymes as animal rennet. Some vegetarian rennet is produced using GMO technology, but you can find an organic, non-GMO version at cheesemaking.com. Vegetarian rennet is typically a double-strength liquid, meaning you should use ½ the amount called for in your recipe. It works just as well as animal rennet, and most people can’t tell the difference in a finished cheese, although a few of our students with discerning pallets say they can detect a slight mushroomy flavor in aged cheeses made with vegetarian rennet.
Rennet Vegetables
Traditionally, either due to animal rennet shortages or simply geographic availabilities, rennet vegetables like thistle and cardoon have also been used to coagulate milk for cheese. These typically result in a softer curd that requires you to handle it more gently. It also may give off a bitter flavor if aged.
You can purchase liquid thistle rennet from cheesemaking.com and follow the directions accordingly. For cardoon, David Asher recommends steeping 2 grams dried cardoon petals in ¼ cup warm water for one hour. Strain, then “squeeze the petals of their juice.” This “juice” can then be used as a substitute in your recipe to coagulate one gallon of milk.
Science of Rennet
Below is a brief, and much simplified, explanation of the science of rennet. If you want to learn more, I highly recommend this article from cheesescience.org.
Rennet goes to work on the main protein in milk, casein, which exists in a micelle structure. On the outside of the casein-micelle are little “hairs” called kappa-casein. Kappa-casein is hydrophilic, or water-loving. Rennet gives the casein-micelle a haircut, cutting off the hydrophilic kappa-casein. This changes the casein-micelle from hydrophilic to hydrophobic, or water-repelling. Once this happens, the casein-micelles connect, trapping fat, creating one solid coagulation. Rennet coagulation happens in two phases: the enzymatic phase, or the “haircut”, and the non-enzymatic phase, when the casein structure connects. This is why it is important not to disturb the milk after adding rennet. If you stir or agitate the milk, you may disturb either the enzymatic action or the connection of the protein structure.
How to Use Rennet
Be sure to check the packaging of your rennet for how each is stored. Most must be refrigerated or frozen.
Whatever rennet you choose to use, it’s best to dilute it in water just before adding it to the pot (even liquid rennet). This helps it disperse evenly and provides more consistent coagulation. The longer the rennet sits diluted, the less potent it becomes, so don’t mix it ahead of time. It’s also important to use non-chlorinated water. Chlorine messes with bacteria and enzymes, both of which are important to the cheesemaking process. You can source non-chlorinated water for cheesemaking by purchasing distilled water, using a filter (most remove over 95% of chlorine), boiling and cooling your water, or leaving your water in an open container overnight which allows the chlorine to dissipate off.
It’s important to know that rennet can lose potency over time. If you aren’t sure if your rennet is still good, you can check out our post on how to test rennet.
If you’d like to learn how to make some relatively easy renneted cheeses, some suggestions from our On Demand course line-up include COTTAGE CHEESE, FETA, INTRO TO HARD CHEESE, and 3-DAY-AGED FARMER’S CHEESE. To thank you for reading this article, use the coupon code “rennet” to take $5.00 of any of our On Demand classes!
~ submitted by Kelly Liebrock, April 2022
