Soya milk and curdling

July 11, 2007 9:25 AM

Why does soya milk curdle sometimes?

Soya milk separates/curdles with certain coffees. Why? And how can it be stopped?

A couple of points:

1) It's not related to caffeine because it happens with decaff (unless the small amounts of caffeine in decaff still trigger it); it also doesn't always happen with regular coffee

2) It doesn't always happen, and seems to depend on the coffee in use. The decaff I bought yesterday causes it to happen all the time.

3) I read that it's something to do with temperatures (cold milk vs hot coffee); I always put the milk in first, so I briefly microwave-blasted the cup. Same thing happened. But why does the brand of coffee I buy seem to affect it if this is the case?

4) I'm using a cafetiere (aka french press) and am in the UK. I use So Good, Tesco or Morrison's soya milk (in the UHT cartons -- not fresh). Sometimes Alpro if I can't get anything else, because Alpro seems to be particularly prone to this problem.

posted by deeper red to food & drink (8 comments total)

The "curdling" isn't spoiling, but the coagulation of the soy proteins. Making tofu is basically this process.

As for why it happens, I believe the magic formula is cold soy meeting something hot. So, if you nuke your cup and pour cold soy in, it's still hitting something hot. I don't think the temp difference needs to be that much for it to happen. Have you tried putting cold soy in the mug and then nuking them together?
posted by mkultra at 10:03 AM on July 11, 2007

Seconding nuking. I make cocoa out of soy milk, and I combine the powder with the soy milk first, stir vigorously, and then heat the mixture in the microwave last.

Also, soy milk -- at least the brand I drink, Silk -- will curdle of its own accord. It seems to have about one week of shelf life after being opened. (It'll last months in the unopened state, however).

Chemically, the process is different from the curdling of milk, I suppose. But the result (and unpleasantness) is the same.
posted by Gordion Knott at 10:14 AM on July 11, 2007

Maybe it's the acidity of the coffee, different brands may have different acidity...
posted by JPDD at 10:16 AM on July 11, 2007

sounds like you are properly tempering by putting the milk first.

could be, some brands of coffee have trace amounts of nigari or other type of chemical coagulant?
posted by dorian at 10:18 AM on July 11, 2007

I don't think the hot coffee/cold soya milk theory is correct all the time. This might happen, but I've been known to prepare coffee and then forget about it for 10 minutes. During that time both the coffee in the cafetiere has cooled and the soya milk has warmed closer to room temp. It still coagulates when mixed.

Just now I poured some milk in the cup and microwaved it for 10 seconds. After stirring around, I then poured in the coffee, which had been standing for a few minutes, and it still coagulated.

It should be noted that the coffee is still fine to drink once it's stirred. It just looks a bit offensive. After all, we're not talking about nasty bacteria-infected milk here. We're talking about plant proteins.
posted by deeper red at 11:35 AM on July 11, 2007

Interesting- I Googled around on this, and there are a few folks confirming JPDD's idea on the acidity of the coffee.
posted by mkultra at 1:15 PM on July 11, 2007

Hmmm... I might have had a breakthrough in my experiments.

Firstly, I shook the soya milk carton really thoroughly. Really, really thoroughly, for like 60 seconds.

Then I heated the mug by sloshing around some boiling water. Remove boiling water, pour in soya milk.

Leave everything for a few minutes. This gives the coffee chance to brew.

Pour. No coagulation.
posted by deeper red at 12:34 AM on July 12, 2007

OK, I think I've cracked this one (at least for me).

1) Shake the soya milk really vigorously, which you should do anyway, but do it even more

2) Pour milk into cup and slosh around to make less cold, assuming it's come straight from the fridge

3) Pour coffee into cup, preferably not boiling hot. If you use a cafetiere like me, let it stand for a few minutes.

4) POUR SLOWLY!! This appears to key. Dribble the coffee into the cup and stir constantly.

I've just done this with a type of coffee that always caused coagulation, along with a soya milk that's coagulation-prone, and it's worked a treat.
posted by deeper red at 6:52 AM on July 12, 2007