Why Does Your Soy Milk Keep Curdling in Your Coffee?
Anyone who’s switched over to plant based milks know the pain: you lovingly brew your filter coffee, pour the hot black liquid into your favourite mug, then unthinkingly splash in a nice big dollop of soy milk … only to raise it to your lips and be confronted with a great swirling mess of lumpy curds floating in your mug.
The drinking of the coffee, no longer a pleasure, now becomes a torture to be endured, as you swirl the lumpy mixture with a teaspoon until it becomes smooth again for a few seconds and you have to swallow it as fast as you can, wincing as you go. It tastes disgusting but you’ll be damned if you’re going to pour it down the drain and start again.
Just what is going on?
Word on the vegan wire was always that it’s just the temperature difference between the cold soy milk and the hot coffee that’s the issue. And that to solve it all you have to do is heat the milk up in the microwave for 30 seconds or so.
Except that whenever I’ve tried that in the past it it doesn’t seem to make any difference at all. I still end up with a mug full of lumpy, bitter coffee.
And so, to settle the confusion once and for all, I set out on a quest to discover the real reason for this mysterious phenomenon.
A quick detour to a tofu factory
In order to understand what’s going on in your curdled coffee, we first have to take a quick detour to a tofu factory.
Tofu — or bean curd if you want to use the more unappetizing name for it — is made by first soaking dried soy beans and then blending them before separating the mixture into the milk and bean pulp. The milk is then heated and sent to a coagulation tank where a coagulant (the substance that causes the chemical reaction that turns the liquid soy milk into a solid state) is added.
Note: If you’re making tofu at home you might get a sachet of something called nigari, which is a substance that’s extracted from seawater and contains magnesium chloride, some magnesium sulfate and other trace elements.
Curdling is a natural process that happens when milk (dairy or non-dairy) goes bad — the bacteria in the milk eat the sugars and produce lactic acid as a waste product, thereby lowering the pH of the milk and making the proteins clump together while also giving it that sour taste. Heat accelerates this process, which is why if you forget to put your milk in the fridge it goes off far quicker than if it’s been properly refrigerated.
It takes about fifteen minutes for the soy milk/coagulant mixture to form the solid curds. After that, the still relatively sloppy mixture is drained of most of its water before being pressed into the familiar blocks that you buy at the supermarket.
But what’s tofu got to do with my coffee?
Well, it turns out that your coffee cup is basically a mini tofu factory. The exact same processes that turn soy milk into tofu in a factory setting are at work in miniature in your coffee mug.
Coffee is acidic, with a pH of about 5 and the ‘curdle point’ of soy milk is around pH 5.5 depending on factors such as the protein level of the milk and the temperature. So when you mix the two, the pH of the soy milk drops and coagulation occurs. The more acidic your coffee, the more likely the milk is to curdle.
Again, heat acts as an accelerant to the coagulation process, which is why the hotter the water in your coffee the quicker the soy milk curdles. It’s also the reason why heating your soy milk in the microwave does nothing to stop the curdling process — in fact you could just be making it worse.
What about tea?
Tea isn’t as acidic as coffee and so there is generally no risk of curdling. The only time it might is if you add lemon to your tea, which is why the norm is to only take lemon with black tea. Interestingly this can also happen with dairy milk, especially if it is unpasteurized (more bacteria = more lactic acid).
So what is the solution?
If you want to tackle the root of the problem, you could try to find a coffee that’s lower in acidity. Now, you could go around the supermarket armed with strips of litmus paper and breaking open coffee packets. Or you could just remember a few basic pointers:
Coffees grown in different soils will have different levels and types of acidity. So if you usually go for Kenyan grown, try switching to South American or vice versa. Arabica beans tend to be less acidic than Robusta. And coffee beans grown in cooler climates or higher altitudes tend to be higher in acidity.
There is no difference in acidity between instant coffee and ground coffee. I’ve always found that soy milk is far more likely to curdle in a cup of instant than a freshly brewed filter, but this is probably down to my own lack of patience than anything. I have a tendency when brewing filter to leave it for at least five minutes (a combination of liking strong coffee and my own forgetfulness) by which time the water has cooled considerably. With instant I get impatient and forget to wait, often pouring cold milk straight from the fridge into water that’s only a few degrees off boiling. The result is not pretty.
The method I tend to use these days as I find it is the most reliable, is pouring the coffee into the milk rather than the other way around. By doing it this way around and pouring the coffee slowly, you’re gently heating the milk which makes it less likely to curdle.
Or try some other plant milks. I don’t think I’ve ever had oat milk curdle in coffee before, though I have heard people claim that it does. Personally, I’m a cheapskate and refuse to buy anything other than own-brand soy milk but barista editions of various milks are becoming more easily available in supermarkets now if you don’t mind forking out extra 💰 — these milks are specially formulated to contain acidity regulators that neutralize the coffee.
Also if you’re using unfortified milks, try switching to a brand fortified with calcium as it can often act as an acidity regulator.
Apparently adding a pinch of salt to the coffee before brewing can lower the acidity, but I’ve never tried this so can’t comment on its effectiveness.
If none of the above seem to work for you, it might be down to how “hard” or “soft” your water is. If you live in an area with soft water, the pH level of your water is likely to be lower as opposed to hard water which has a higher pH because it’s filled with calcium and magnesium and other alkalizing minerals.
Or if all else fails, you could always switch to iced coffee!
