Rennet is often the ingredient in a cheesemaking recipe that hangs the new cheesemaker up. You might have asked yourself, “What is it? What does it do? Is it natural? Do I really need it for cheesemaking?”. In this post I’m going to answer all of these questions and help you learn how to coagulate your milk with this amazing ingredient!
Coagulation of your milk is an important step in all pressed cheese recipes. It is the act of adding culture and enzymes to your milk, letting it sit and coagulate into a curd mass, that can then be cut into smaller curds.
What is rennet?
Rennet is a catchall phrase used to describe different enzymes that cause a reaction in milk, that ultimately results in its coagulation.
Rennet that you purchase from cheese supply stores, comes in several different types and forms. Animal Rennet, Vegetable Rennet, and Microbial Rennet are the main types that you will encounter. You will also see that you can purchase them primarily in liquid or tablet form. These different types of rennet’s all use different enzymes to achieve the coagulation of milk.
How it works
Rennet is an enzyme that when added to milk, begins a reaction that allows proteins in the milk, to repel the water and coagulate into a mass.
The science behind it
A protein molecule is a structure comprised of chains of amino acids. Though milk has many different types of proteins, milks main protein, is a molecule called “casein”. In milk there are 4 different types of casein protein, each type presenting with a small genetic variation in their chains of amino acids. These different types of casein molecules use calcium phosphate to bind together (like glue), in the hundreds, to form groups called, micelles.
A casein micelle contains all the different variants of casein protein in milk, and these protein molecules are strategically placed within the micelle, to attract water, and allow the micelle continued suspension in the water, verses separation from the water. One type of casein called kappa casein, protrudes a hair like part of itself, out of the micelle, allowing the micelle to essentially be a hairy ball. This hairy layer is hydrophilic, and negatively charged. It attracts water, and thus allows the micelle the ability to maintain suspended in the water. Negatively charged micelles repels other negatively charged micelles, much like how two negatively charged magnets can not come together, neither can two negatively charged micelles.
Along comes Rennet, and rennet gives that hairy micelle a hair cut. It chops off those protruding casein proteins and they float away. Suddenly a reaction occurs, these casein proteins start binding to each other, using calcium phosphate as their glue.
Rennet works quickly, as soon as it touches milk, it begins this reaction. In fact, it works so quickly and efficiently that if conditions are favourable, a small amount of rennet (a tsp for example) can coagulate 8 gallons of milk in less than an hour.
Calciums role
This coagulation process will only work properly, if there is enough calcium in the milk to “glue” the casein molecules together. If the calcium has been depleted and weakened from the pasteurization and hom*ogenization process, the casein proteins will not bind together properly, this will create a weak curd.
Depending on season and freshness of milk, the calcium content may also be changed. For example, cows that are nearing their next calving date (late lactation) sometimes need calcium chloride added into the milk to achieve good coagulation. You may still be able to make strong curds without adding calcium into your milk, but it is a good idea to have a bottle of calcium chloride, stored in the fridge for these times when you are struggling to get your milk to coagulate.
If you are using milk that has had the calcium content depleted, such as pasteurized milk, you will need to add calcium chloride, before adding rennet, to ensure proper binding of your casein proteins. My rule of thumb for adding it in to cheese is 1/4tsp calcium chloride to 1 gallon of milk. Calcium chloride should always be diluted in 1/4 cup luke warm water before adding it into your milk to help it disperse throughout the cheese pot.
What is rennet and where does it come from?
There are many speculations and legends on how cheese was actually invented. They are all fairly probable, and seeing as cheese has been made all over the world for centuries, they may have all happened in some form.
Some say a calves stomach was used as a container for milk on a long journey, when the owner went to take a swig of milk, it had turned into curds. Others say that when a young calf was butchered, curds where found in its stomach. The farmer decided to taste them, and found them to be good. Certain Cultures found that when certain plants were added to milk, such as figs, the milk coagulated into curds.
The general gist of it all, is that someone realized that when milk comes in contact with the lining of a young ruminants stomach, such as a calf or goat kid, or with certain plants, milk coagulates into curds, these curds can than be formed into cheese and stored for a long time. That milk that was once highly perishable, could now be made into a product that was shelf stable.
It wasn’t until food science made its debut, that it was realized that the lining of ruminants stomachs and extracts from certain plants, such as figs, actually contain different enzymes that cause this reaction to occur.
In a young ruminant (such as a calf) this enzyme is called chymosin. Chymosin’s original purpose is to slow the digestion of milk by coagulating it in the young animals stomach. If it was not for this coagulation, the young animal would not absorb all the nutrients that the milk had to offer, before it was passed on in digestion.
Producing your own rennet
Not many cheesemakers these days produce their own rennet. Factors such as cost, ethics and efficiency all come into play. Vegetable rennets can be tricky to achieve timely coagulation with and animal rennet involves the butchering of young ruminants (preferably 2 weeks old).
Check out David Ashers book ‘The Art of Natural Cheesemaking’, for an in-depth description of producing your own animal rennet. This is also a topic that we have covered in my monthly membership club, The Milkmaid Society.
Purchasing rennet
Rennet can be purchased from most online cheesemaking shops. Choosing what type of rennet to purchase is a personal decision. I prefer to use pure calf rennet, but I have used vegetable rennet with similar results. I find liquid rennet to be slightly more convenient, but tablet form works just as well.
Regardless of the form of rennet you decide to use, you will need to dilute it in a bit of non chlorinated water, before adding it to your milk. This action of diluting it, makes more of the rennet enzyme available to a bigger surface area of milk. This is important because rennet begins its work as soon as it comes in contact with the milk. If you directly added a ½ tsp of liquid rennet to 4 gallons of milk, it would be nearly impossible to distribute it evenly throughout the milk.
Troubleshooting
Even though rennet, when stored properly, keeps for a long time, there are several reasons you may notice differences in coagulation.
Rennet comes in different strengths
For starters, not all rennet is equal, though you should not see much variation between two different brands or batches of animal rennet, you may notice a difference when you switch between type of rennet, such as switching from vegetable to animal.
Also keep in mind that different rennets can be different strengths. For example, you can purchase double strength calf rennet or regular strength. For this reason, I always recommend using the package directions on your rennet bottle to determine how much to add to your milk, verses the recipe directions.
Available calcium affects how well rennet can coagulate
As I mentioned above, rennet needs an adequate amount of calcium, to bind the proteins together, if the calcium in the milk has been diminished or altered in anyway, you may notice a difference in achieving a stable curd. Adding Calcium Chloride back, will usually fix this problem. It keeps indefinitely and can be a valuable tool in a cheesemakers fridge.
Temperature
Temperature plays a big role in the coagulation of milk. Rennet is heat sensitive and anything over 120F will probably deactivate it. On the other hand, rennet requires milk to be warm to coagulate properly. A pot of milk that has cooled down significantly since adding in the rennet, will produce a weak curd. For this reason, maintain the warmth in your cheese pot during coagulation as best you can. My favourite trick is a piece of aluminum foil wrapped overtop of the pot.
How long does rennet last?
Standardly, Rennet’s shelf life is about a year. Though I have used rennet, well past its best before date, with good results, if you continue to experience issues with coagulation of your milk, try a new batch, it can’t hurt!
What happens if I add too much rennet?
Adding more rennet than you need into a cheese pot can result in rubbery, and sometimes bitter cheese.
What should properly coagulated milk look like?
When you open the lid on your cheese pot, after waiting the required amount of time for your milk to set. The milk should be coagulated, and it should look like junket or milk jello, if you will. Undisturbed, you should not notice whey, but a smooth surface of milk.
To check if your milk is ready for the next step of cheesemaking, you need to check for something called “a clean break”. This means that when a finger or a knife is inserted under the surface of the milk, and lifted ever so slightly upwards (being careful not to break the surface), a clean crack will appear in your milk. If instead your milk splits off into wiggly curds, or has very little structure, your cheese is not ready for the next step. A curd that is too weak, will loose valuable solids during the stirring process, resulting in a smaller cheese yield.
Conclusion
Rennet is an important ingredient in many cheese recipes. Proper coagulation of your milk means that you will have a strong curd, that can withstand some manipulation during cheesemaking. Rennet can easily be purchased from cheesemaking supply shops, and will keep in your fridge for the duration of your cheesemaking season. Choosing the type of rennet you will use, is a personal preference. A preference built on your morals, values and cheesemaking technique. Experiment with different types and forms, until you find a product that works for your lifestyle and milk.
Always remember, cheesemaking should be fun! Treat every mishap as a learning experience, and before you know it, you will be coagulating your cheese like a boss (until the next mishap anyway)!
Additional Resources
- The Art of Natural Cheesemaking by David Asher
- Homestead Cheesemaking 101 Course
- The Milkmaid Society
