Coagulation
This webpage is divided into the following topics:
Introduction
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
General
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.
The procedure for setting is to add a precise amount of mesophilic starter culture to milk at a temperature of ~21C / 70F, 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.
Milk Type
For lactic acid coagulated cheeses, when cow’s milk is used, it is normally pasteurized or skim cow’s milk otherwise cream will separate during the long incubation time resulting in an non-homogeneous curd. Whole non-homogenized cow’s milk can still be 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 small amounts 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
General
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 better milk
coagulant. Many cheese makers still prefer animal based rennet for
rennet coagulation, but also available are plant, microbial and
fermentation based rennets. Please see rennet page for further
information.
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 Preparation
Milk is warmed to optimum rennet coagulation temperature of 30-36C / 86-96F. Higher temperatures up to ~43C / 110F result in faster coagulation times. Above 40C / 104F and the rennet becomes inactivated. Lower temperatures result in slower coagulation, below ~18C / 65F 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.
Other additives such as dill weed, caraway seeds or cumin seeds can be mixed in later, normally after the curd is cut and whey is drained.
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
Rennets are available in liquid, paste, powder and tablet form and in different concentrations. 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 for a maximum of 60 seconds. Do not wait as rennet's strength deteriorates when diluted.
- Stopping the swirling of milk after stirring.
