This Is Why Soy Milk Goes Funny In Your Coffee

Stop taking your coffee black.

It is very annoying when you add soy milk to your coffee and it goes funny.

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This reaction is called "curdling" and it is not ideal.

There are two reasons why the soy milk curdles:

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1. The acidity of the coffee.

2. The temperature of the drink.


If you favour a more acidic coffee (like many African coffees), it's more likely to curdle.

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According to the head of coffee at Pact ( fresh roasted coffee delivery service) and award-winning barista Will Corby: "There are a few things to watch out for. Firstly, don't put the milk in the coffee when it is boiling hot. And if you are heating up the milk first, don't let it go above 60ºC. Try to avoid using naturally acidic coffees. A lot of African coffees can be acidic, so you would do better aiming for a Brazilian or Colombian. Something like the Planalto coffee would be perfect."

So soy milk + a hot light-roasted coffee = cheesy-looking soy milk.

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Jeremy Challender, co-director of Prufrock Coffee, head judge of the UK Barista Championship, and authorised Speciality Coffee Association of Europe trainer, explains: "The curdle point for soy is a pH of 5.5 if I recall. So soy will curdle below that. And pretty much all light-roasted arabica coffee will have a pH below 5.5."

(According to The Little Coffee Know-It-All, the proteins in soy will maintain their shape up to a pH of around 4.9, with black coffee having a pH of around 5.)

Don't worry, though – there's a solution.

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You just need to either warm the soy milk, or cool the coffee slightly.

Nicole from Climpson & Sons (a roasters and café in East London) explains:

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"At Climpson's Café we steam the coffee and soy milk together on our espresso machines – first adding soy milk, then a shot of espresso, to create a sweet coffee with no curdling. With filter or water-based espresso we try and use warm soy to the same effect."

And if that's too much for you, you could also buy pH-corrected soy milk too. If you want to buy the beans pictured, btw, you can do so here.

And if you're making a big pot of coffee at home, dilution is your friend.

Jeremy Challender says: "Curdling's less likely to happen in a more diluted beverage, so if you did a big 60g/litre pour-over, the pH will be a bit higher than if it was a super strong stove-top espresso. So slightly more dilution should help with your coffee at home."

Problem solved!

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