All cheeses use coagulation of the milk as an essential cheese making step. This articles discusses how coagulation works and the different types generally used in cheese making.
After acidification, the next step in making cheese is coagulation. Coagulation of milk is the first step towards concentration of milk’s casein and fat and expulsion of whey made up of water and milk’s soluble components. During coagulation the casein micelles form long chains that branch in all directions and bond with themselves forming a three dimensional matrix that encompasses and all the milk including fat and water. Cheese makers call this matrix the curd.
The process of coagulation occurs through two different mechanisms, primarily acid coagulation and primarily enzyme coagulation. Each method results in two very different families of cheese. To be literally correct, rennet is the historical name of the product from animals, but in cheese making rennet is the generic term for all types of enzymes, whether of animal, plant, microbial or fermentation origin, that are used to coagulate milk.
The most common method is enzyme or rennet coagulation as it produces a lower moisture content and longer shelf life curd without excessive hardening. Virtually all hard cheese are made using rennet coagulation.
Lactic Acid Coagulation
Several soft cheeses such as cottage cheese, quark, and traditional cream cheese use lactic acid coagulation which occurs as response to a reduction in pH from production of lactic acid by the starter culture. Pasteurization including Ultra Pasteurization and UHT milks are fine for making Lactic Acid cheeses, they will give a thicker lactic acid set curd than when using raw milk.
The procedure for setting is to add a precise amount of mesophilic starter culture to milk at a temperature of ~21°C / 70°F, much lower than for rennet coagulated cheese, although some recipes – procedures use warmer temperatures.
The starter culture causes lactose to be converted to lactic acid, lowering the pH with full coagulation into a solid curd occurring around a pH of 4.6 – 4.7. This can take 4 to over 24 hours, depending on the temperature and the amount and activity level of the starter culture.
The curd that is formed from lactic acid coagulation is much weaker than from rennet coagulation and the curd more strongly resists the expulsion of whey. Thus the resulting cheese is softer and higher moisture than rennet coagulated cheeses.
As the resultant cheeses are softer and moister, they have a shorter shelf life and are consumed young and thus in some countries such as USA, milk is required to be pasteurized for health reasons.
For lactic acid coagulated cheeses, when cow’s milk is used, it is normally pasteurized or skimmed otherwise cream will separate during the long incubation time resulting in an non-homogeneous curd. Whole non-homogenized cow’s milk is used but the whole curd must be re-worked to form a homogeneous mixture. Some cheese making recipes-procedures use both acid and rennet coagulation, here rennet is normally added in significantly smaller amounts than when rennet coagulating milk as it is added not to cause coagulation but rather to enable better whey separation and better curd formation, and thus less cream separation when using non-homogenized milk.
Conversely non-homogenized sheep and goat’s milk do not easily cream and thus there is less need for rennet addition or homogenization in lactic acid coagulated cheeses, however some recipes still call for the addition of small amounts of rennet when using sheep or goat’s milk for lactic acid coagulated cheeses.
If using rennet in lactic acid coagulated cheeses, the lower setting temperature than for rennet coagulated cheeses still allows the non-enzymatic phase of rennet coagulation but not the second enzymatic phase.
Rennet coagulation originally used enzymes from the lining of the fourth stomach of calves and from the stomachs of kid or lamb as they have these enzymes naturally in their stomach to better digest milk and as that was what was commonly available. There are two prime enzymes in these stomachs that coagulate milk, initially chymosin and later after weaning, pepsin. It is the chymosin enzyme that is the stronger milk coagulant. Since the 1990’s other forms of “rennet” (fermentation, microbial) have been made.
Rennet coagulation is a two stage process involving an initial enzymatic phase during the first ~10 minutes where a chemical change is occurs as preparation for second non-enzymatic phase where the casein micelles start forming linked chains and eventually a full solid curd is formed, if enough calcium is present.
Milk is warmed to optimum rennet coagulation temperature of 30-36°C / 86-96°F. Higher temperatures up to ~40°C / 104°F result in faster coagulation times. Above 40°C / 104°F and the rennet becomes inactivated. Lower temperatures result in slower coagulation, below ~18°C / 65°F coagulation will not occur.
Starter culture, CaCl2, lipase, colourants such as Annatto and flavor additives must be added to the milk before renneting so that they are incorporated evenly in the curd. See Wiki: Ingredients, When To Add.
Other additives such as dill weed, caraway or cumin seeds, or smi dried fruit like apricots can be mixed in later, normally after the curd is cut and whey is drained.
Note, many rennet coagulated cheese making procedures have a pre-ripening time of up to 1 hour after adding the starter culture and before adding rennet.
Adding Rennet To Milk
Rennet is very concentrated, so adding it directly to the milk would cause it to set the milk in just that area and not in the overall milk. Even if one stirred it after adding directly, it would still coagulate in areas resulting in a poor curd formation. Therefore the common method of adding rennet is to first dilute / dissolve in cool non-chlorinated water before adding to the milk. Dilution amounts are discussed in the Animal Based Rennet webpage.
Best practices for rennet preparation and addition are:
- When ready to add rennet, dilute or dissolve rennet in cool un-chlorinated water. Chlorine is a strong oxidizing agent and rapidly destroys the rennet enzymes.
- Trickle the diluted rennet into the milk while stirring the milk with a skimmer for a maximum of 60 seconds in an up and down method without breaking to surface (no splashing). Do not dilute rennet in advance of adding to milk as its strength deteriorates when diluted.
- Stop the swirling of milk after stirring with skimmer to enable better coagulation.