Proteins with purpose: why milk curdles and how to avoid it

Dr. Monika Fekete digs deep into protein chemistry to understand why curdling happens in dairy and soy milk and how to avoid it.
Coffee science

If you regularly make soy coffees for your customers, you have probably noticed that soy milk is often harder to work with than dairy milk – it tends to curdle more in coffee, especially when steaming hot. With some coffees it behaves well, with some it doesn’t. 

The delicate protein structures in milk are to blame for the curdling effect. 

Proteins are long, folded chains of amino acids. Humans only use about 20 different amino acids, and they can be linked after one another in any order. Most proteins contain hundreds of them.

When we digest food, our stomach acids and enzymes break proteins down into amino acid blocks. We then build them up again into human proteins. Human proteins are different  to soy proteins and different to dairy milk proteins. Just like Lego, these human proteins can form a huge variety of structures, such as muscles, hair, or enzymes, 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. The order of the amino acids and 3D structure is what makes one protein different to another. 

Proteins are specific to the species that makes them. There are several types of proteins found in dairy milk. Soy milk, on the other hand, has its own unique set of proteins.

Protein Denaturation

As long as proteins are sitting in their optimal environment, they can retain their regular structure. 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. Here are best ways to denature a protein.

1) Heat

Heating something, on a physical level, means increasing the speed at which its atoms bounce around. The same goes for proteins: at higher temperature, their atoms vibrate so much that the weak bonds break and the orderly structure unfolds. They will ultimately form other structures that can look completely different to its original shape. Think of frying an egg where both the colour and consistency of the proteins change with the application of heat over time.

2) pH

The addition of acids or bases disrupts the bonds between the charged branches of the amino acid chain, breaking the original internal links and once again, unfolding the protein. Similarly to acids, alcohol molecules can also bind to the amino acid branches, denaturing the protein. Ever tried adding vodka to milk?

Milk curdling is due to the denaturing of its proteins. Soy milk proteins are different to milk proteins, which is why they behave differently in coffee. 

The effect of coffee

Even if you find that your soy milk steams just fine, curdling often happens when you mix the milk with the coffee. Lighter roasts cause problems more often than darker ones. Darker espresso roasts are generally less acidic than lighter roasts. If acidity is the answer, it would explain why soy tends to curdle more in lighter roasts. Let’s do an experiment to test this theory.

Experiment 

We can test what the most acidic coffee pH that soy milk and dairy milk (both regular supermarket brands) can tolerate. I expect that dairy milk will tolerate more acid (lower pH) than soy. We can also try the same idea with cold and hot milk and see how temperature affects the balance. 

The pH of coffee broadly sits in the pH 4 to pH 5.5 range. A pH of 4 is sharply acidic, while a pH of 5 to 5.5 would be a milder acid coffee. Our model coffee is a mild acid instant at pH 5. 

We will use drops of vinegar to adjust pH in small steps. At room temperature, both milks mix smoothly with a pH 5 coffee. However, when the coffee is adjusted to pH 4, both dairy and soy curdle. 

This means that there has to be a crucial pH value between 4 and 5, which allows the milk to mix smoothly with the coffee without curdling: a lowest safe pH limit. But where is it, and is it different for soy and dairy?

When we adjust the pH in smaller steps, we start to get a picture of the conditions that cause each milk to curdle. Cold soy milk curdles at a rather mild coffee pH of 4.6. Meanwhile dairy milk is much hardier. You need a very acidic pH 4.1 to curdle it. In conclusion, soy milk is much more sensitive to acidity than dairy.

To leave it there would only be half the story because we usually like to enjoy coffee hot. So, how do things change at higher temperature? 

When repeating the experiment with hot milk and hot coffee, the minimum safe limit for soy is pH 4.7, while it is pH 4.5 for dairy. It doesn’t look like a big difference, but it might just be crucial difference in terms of the coffees that would normally be used in milky drinks. Lighter roasts could quite easily cross the pH 4.7 boundary, where soy becomes prone to curdling.

A chemical quick fix 

We see the real power of science when we can use what we’ve learned to propose a solution to our problem. 

Our problem is that soy milk curdles too easily in acidic coffees.
So, quickly adjusting the pH of the coffee before mixing it with the milk might just help.

You could try adding a tiny pinch of bicarbonate soda to the coffee until the pH is above the safe limit – this could affect the taste of a real espresso so best try this just as an experiment. Indeed, the soy milk does not curdle anymore after the coffee pH is adjusted above pH 4.7. 

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Solutions to the curdling problem

Measure the pH of the coffee and if it is below the safe limit, it’s always going to be an issue. The simple solution – avoid using acidic coffees with soy milk.