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I like to make a coffee with soya milk. Sometimes when I add the milk to the coffee, it curdles. ie it separates into lumps floating in translucent liquid. So this tastes and looks a bit strange (but is still drinkable).

So how to prevent this from happening? Does it depend on the type of coffee, or the type of soya milk? Or how and when they are mixed together?

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It seems that the cause of the situation of curdling soya milk in coffee is that the acid from the coffee curldes the proteins and/or fats in the (soya or other) milk, being catalysed by the heat of the coffee; so that's what needs to be mitigated.

There are a couple related questions on Cooking.SE, including this one that suggests adding salt to the coffee will help a little (table salt, or even bicarb / baking soda). I have tried adding table salt to the coffee first, then adding the coffee to the soya milk, and it seems to help (a little, sometimes; YMMV). Another Cooking.SE question also suggests waiting for coffee to cool.

The linked TheKitchn article (linked from the aforementioned previous question and @PythonMaster's answer) suggests that cooling the coffee, or pouring coffee into the soya milk (rather than the other way around) helps via similar mechanisms. In my experience in messing with this, I have not found the order of adding ingredients makes much difference (i.e., putting coffee into the soya milk). It absolutely depends on the type of coffee, also, because different coffees will have different degrees of acidity. See also this table about relative acidity levels.

However, it is certainly the case that adding coffee into the milk will be helpful in general (compare also to tempering of eggs or adding acid into water rather than the other way around). In this order, these allow temperature and acid (respectively) to more slowly come into balance between the two substances as they are being combined.

IMHO, I have found the brand (i.e., the ingredients and production methods) of the soya milk to be the biggest factor. I (personally, with not-entirely-scientific -- yet! -- trials) find that soya milk brands that contain emulsifiers (or other additives) are less likely to curdle in coffee. My only guesses are that

  • these compounds (emulsifiers, thickeners, or stabilisers, such as carrageenan or xanthan gum) interfere with the coagulation, or
  • something else in the processing (perhaps, e.g., pasteurisation) has denatured the proteins or otherwise chemically changed the nutrients such that they are less likely (cf., UHT or ultra-pasteurised milk)

I generally prefer to avoid the additives, so this is an unfortunate conclusion for me personally.

Freshness of the milk is also an issue; milk of all types will spoil and slowly start to curdle due to natural processes (fermentation, bacteria, other contamination). There's another question about storing soya milk in the freezer, where we reached similar conclusions.

At the risk of making this an even longer answer, here are some other things to try:

  • Add cold coffee into soya milk. Does it curdle? If so, it seems temperature is not your only enemy.
  • Check your label; does it have carrageenan or so? If so, this might not be the metaphorical silver bullet.

I'm interested (fascinated, even; obsessed, maybe?:) ) with this, so I'd really like to hear feedback. Of the dozens of articles I've read on this, I've still not found a proper solution.

Happy experimenting; comments welcomed.

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Well actually, it depends on how hot your coffee is, not the type of coffee. The acidity in black coffee is higher than the acidity in soy milk, though, and it can act like a coagulant, making like loose tofu in your cup of coffee. The heat of your soy milk can actually HELP curdle the soy milk even more, rather than preventing it. In this food science website, which contains the info I gave, it says:

It seems like the best way to prevent curdling is to warm the soy milk slowly by pouring it into the cup first, then gradually add the coffee. Letting the coffee cool a bit before adding soy milk and avoiding more acidic coffee beans may also help.

This should answer your questions and prevent the curdling in your coffee. I hope this helps you!

  • Do you find that order of combination or temperature works? No doubt that it helps a little, but this doesn't help much, in my experience. Also, type of coffee absolutely has an effect on the situation; different coffees have different acidity and this makes a big difference, in my (admittedly non-scientific) experience. – hoc_age Feb 11 '15 at 14:03
  • Oh. Well at least this still helps the person asking! :-) – Anthony Pham Feb 11 '15 at 22:51
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a tiny bit of bicarbonate soda into the black coffee ...then stir fast as you add the soya milk removes the acidity & stops the curdle ...enjoy !

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    Interesting. Is it the pH of the bicarb (baking soda) that makes a difference? Or something else? About how much is necessary -- just a pinch, 1mL (1/4 tsp), ...? Does it not taste salty? – hoc_age Mar 19 '16 at 20:30
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    This is my solution as well. baking soda will bring down the acidity of the coffee which will help prevent curdling. A small pinch is all you need. – meskarune Mar 27 '16 at 20:26
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I agree wholeheartedly that the brand of Soya makes all the difference. With Sainsbury's own it doesn't matter if I heat the Soya, cool the coffee, add before or after - it always curdles. At the other end of the scale Lidl's own never curdles. Looking at the ingredients I see Lidl have something called "acid regulator" - heaven knows what that is, but if the acid in the coffee is the problem then the regulator (doing I suppose what the suggested pinch of salt or bicarb does) is the solution.

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I had the problem with soya milk curdling but now find that by adding a little cold water to the coffee before adding the milk generally helps. OK, the coffee's a little cooler but more palatable than with lumpy soya.

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(1) According to a Kitchn article Why Does Soy Milk Curdle In Hot Coffee?

The two factors behind the curdling of soy milk are acidity and temperature.

You can reduce the acidity via the choice of coffee beans, grind size, and other brewing variations.

You can lower the temperature by letting the coffee cool before adding soy milk or gradually adding the coffee to the soy milk. The latter cools the coffee with air and with the total mass of cold soy milk, and also reduces the number of soy milk molecules that get heated to the critical temperature by hot coffee.

(2) Froth the soy milk before adding the coffee. Frothing in air makes it a nice thermal insulator, increases its surface area, and changes the surface tension. As a result, less if any soy milk will curdle. Frothing has additional benefits. A hand-held frother works for me.

According to this Milk Frothing Guide,

Properly prepared milk is always foamed. Even if you don’t want any foam in the drink you want to foam the milk just slightly. Incorporating air into the milk improves and sweetens the taste. Milk that has not been foamed at all tends to taste flat and dull by comparison.

(3) Experiment with different kinds of milk substitutes. If I recall correctly, coconut milk didn't curdle while hemp milk curdled easily. There's also almond milk, almond-cashew milk, rice milk, hazelnut milk, and oat milk. Hemp milk doesn't froth up very well, at least not the brand I tried.

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1ml or more of dark/dutched (alkalised) cocoa powder mixed in with the coffee before adding the milk stops it curdling. 0.5ml just about works enough to make it drinkable for me without really affecting the taste although I'm not a coffee expert, don't pay much attention to the taste and there is still a little bit of curdling. I guess either make a mocha or go for the sodium bicarb/less acidic coffee tricks

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I have encountered this problem with Soya Milk for some years, my wife is milk - intolerant. I make up the coffee and allow it to cool. I store it in the fridge, then add cold Soya Milk to the desired colour, I warm up the ready made drink (NOT TOO HOT) End of curdling problem provided i don't get it too hot.

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I had 2 cups of coffee with soy milk the other day. The first cup was delicious. The second cup curdled. I was baffled at first, then realized the 2nd cup was decaf, which is weird, because decaf has less acid. So far the soy milk has never curdled with regular coffee, always does with the decaf.

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I saw a demonstration where this person added half sqeezed lemon juice before adding the soy milk, and that balanced the acid levels. Someone one here wrote to add dark cocoa powder mix, prior to adding the soy milk.

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I use Alpro chocolate soy milk. I just add it to hot coffee as usual. Works like a chocolate charm :)

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