Why Does Milk Curdle?

To understand why curdling happens, we need to learn a little about protein chemistry.

(Or you can just skip ahead to the good stuff in our “Soy vs Dairy: Will it Curdle?” experiment.)

First of all, it is the delicate protein structures that are to blame for the curdling effect.

Proteins are long, folded up chains of amino acids, which are their basic building blocks. When we digest food, our stomach acids and enzymes break proteins down into amino acid blocks, to be built up again into our own. Just like Lego, they can form a huge variety of structures: muscles, hair, enzymes and hormones, just to name a few. They achieve this variation by the sequence in which they sit after one another in the chain, and, importantly, by the 3D structures this chain folds into.

A lot of proteins are highly specific to the species that synthesises them. There are several types of proteins found in dairy milk, their exact structure specific to cows (1). Soy milk, on the other hand, has its own unique set of proteins (2).

Curdling is Protein Denaturation

As long as proteins are sitting in their optimal environment, they can retain their regular structures. However, there are a few things we can do to destroy this original build-up. This process is called denaturation, during which the weak bonds holding the 3D structure in place break, and the chain unravels. The amino acids within the chain are held together by strong covalent bonds, which can’t be broken by denaturation reactions (4).

The best ways to denature a protein…

3D structure of a protein found in dairy milk (3).


Heating something, on a physical level, means increasing the speed at which its atoms bounce around. The same goes for proteins: at higher temperature, the atoms in this complex structure vibrate so much, that the weak bonds break and the orderly sheets and helices begin to unfold. They will ultimately form other structures that can look completely different to the original: think of frying an egg, where both the colour and consistency of the proteins change- for the better on the plate!


The addition of acids or bases disrupts the ionic bonds between the charged side branches of the amino acid chain. Some ends (the “acid ends” ) are negatively, others (the “amine ends”) are positively charged. The so called salt bridges that form between these help maintain the 3D structure. When an acid (or base) is added, it binds to these side chain ends more strongly than the other amino acid, once again ultimately unfolding of the protein.


Other than salt bridges, hydrogen bonds between amino acid side chains also help hold chains together. Similarly to acids, alcohol molecules bind to the sidechains, thus denaturing the protein. Have you tried adding vodka to milk…?

Heavy Metal Salts

They form insoluble salts with the protein… hopefully we don’t have to worry about led or mercury in coffee.

Bottom Line: Soy milk proteins are different to milk proteins. Milk curdling is due to the denaturing of its proteins. Proteins in general can be denatured by heat or a change in pH, but their tolerance depends on the type of the protein.

Which is exactly what we test in the Soy vs. Dairy: Will it curdle? experiment!


1) The majority are beta-lactoglobulins, alpha-lactalbumins and caseins. In addition, caseins (only found in animal milk) like to form micelles, large ball-like aggregates that help with the emulsion forming and foaming properties of milk. (Thompson et al, Milk proteins, 2009 p36-37)

2) Such as the soybean 2S albumin (however, they are still built of the same set of amino acid blocks).

3) Bovine beta lactoglobulin. Source: Research Collaboratory for Structural Bioinformatics Protein Data Bankhttp://www.rcsb.org/pdb/explore.do?structureId=1BEB

4) Our digestive system needs to work hard to break these strong bonds. That’s why we have strong stomach acids (~pH1) and special enzymes that cut the chain like scissors.

Tags: #coffee #protein #coffeesciencelab #milk #barista #denaturation #science

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