One of the most crucial steps of cheese making: coagulation is the step that transforms liquid milk into solid curd.


This step of the cheese making process is where the chemical magic is visible to the naked eye (and hand). Coagulation is the push-off-the-cliff that turns milk into cheese. Liquid milk is converted into a solid mass. This solid mass is often called “curd”, “gel” or the “coagulum”. Coagulation can occur in a few different ways: enzyme action, acid addition, or acid/heat addition. These three processes will be the foci of this post.

Cheese Structure

Coagulation of milk: before (left) and after (right)


Milk Chemistry Review

If you haven’t already, check out the previous post on milk chemistry. Specifically the section on protein. That information is crucial to understanding the rest of this post. As mentioned in the milk chemistry post, the protein of most interest in cheese making is casein. Casein micelles are covered with a negatively-charged “hairy” layer of κ-casein.

In milk, these casein micelles float around and bounce off each other. Those κ-casein hairs get in the way and prevent the casein from sticking and aggregating. Our goal in cheese making is to make those casein micelles stick together somehow. Once they stick together, a domino effect occurs, and eventually you form a mesh of casein micelles that form the structure/body of the cheese.

κ-casein hairs cause casein micelles to bounce off each other in milk

How we get those micelles to stick together is what coagulation is all about! The coagulation (or “clotting”) process is done to encourage those casein micelles to stick together somehow. Enzymes (rennets), acid, and acid/heat can all be used to encourage this process. The exact mechanism of each differs and will be discussed below.

Coagulation is getting those casein micelles to stick together
(This is an example of micelles after having the hairs clipped off)


Acid Coagulation

Acid coagulated, acid-set, lactic curd, and lactic-set are all monikers that refer to using acid to coagulate milk. That acid can either be added directly or can be produced by starter cultures. A few examples of acid coagulated cheeses include cottage cheese, quark, and chèvre.

In this case, the goal is to neutralize the negative charge that is surrounding the casein micelles. In milk, all those negatively charged micelles bounce off each other due to them all having a negative charge. We often call caseins “polar” due to all this charge. Think about how magnets repel if you try to push together two of the same poles. Adding acid, in effect, is like adding positive charge. The addition of acid neutralizes the micelle surface and this allows them to bump into each other and stick. This effect is the most prominent at the isoelectric point of casein, pH = 4.6. If you recall from our last post about cheese texture, acid dissolves the calcium “glue” from the casein micelles. This means acid-set cheese is usually softer.

Acid Coagulation Diagram

Acid neutralizes casein micelles and allows them to clot


Rennet Coagulation

Rennet coagulation refers to the addition of enzymes to milk in order to make it clot. Many cheeses fall into this category: cheddar, gouda, queso fresco, and many others.

Rennet enzymes act like a razor and shave off the κ-casein hairs. Without the hairs, the micelles can now stick, aggregate, and form the backbone of cheese structure. An interesting property of enzymes is that they are re-used in chemical reactions. This means a little bit of rennet goes a long way in the coagulation process. At home, a ¼ to ½ teaspoon is usually sufficient to clot 2 gallons of milk. The aggregation occurs when about 80-90% of the κ-casein hairs are clipped off. Not shown in the picture below is the required calcium that acts as a "glue" between the caseins.

Rennet Coagulation Diagram

Rennet clips off the hairy layer and allows the casein micelles to attach


Acid & Heat Coagulation

The last regime of coagulation on the docket could probably be considered a subset of acid coagulation. In this process, acid and heat are used to clot milk. Examples of this include ricotta, mascarpone, and paneer.

We’ve already discussed how acid affects casein, but what about heat? In this case, heat affects the other main type of milk protein we haven’t discussed yet, whey. Whey proteins are denatured (unraveled) by heat exposing “sticky” portions of their structure. These sticky ends can bond to each other across whey proteins or bond to casein proteins. Acid can now also contribute to whey coagulation now that they have been denatured by heat. The result is a matrix of coagulated whey protein, and if casein is present, a matrix of coagulated whey/casein.

Acid & Heat Coagulation Diagram

Heat causes whey proteins to participate in the coagulation fun!
(Not to scale, like every other diagram on the site)


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