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As many people have experienced, soy milk will often curdle in hot coffee. I've experienced this myself with both instant and fresh coffee, and with my homemade soy milk (not my favorite soy milk brand from the store, which has a few extra things).

There are many people out there with the same problem, which is typically blamed on the acidity of the coffee, acid being well-known as a coagulant:

One day, I tried adding salt to my coffee, knowing that it is said to reduce bitterness (ref). That day, my soy milk didn't curdle. On each day thereafter, if I added a pinch of salt to my coffee before adding the soy milk, it wouldn't curdle. My heating procedure has remained the same: mix instant coffee with boiling water to dissolve, add cold soy milk, stir, then microwave until hot again.

What chemical interaction could plausibly result in salt preventing the curdling of soy milk?

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As a side note: Some brands offer soy (or rice) milk with added calcium. I recently observed that these do not curdle when pouring them into hot coffee. Might be a similar effect (e.g. pH buffering). –  Christoph Grimmer-Dietrich Dec 1 '13 at 9:05

4 Answers 4

up vote 2 down vote accepted

I can't find any definitive answers to this question online. All the discussion I could find are speculation.

Factors that cause soy protein to coagulate are heat, acidity, and the presence of magnesium or calcium salts. Additionally the proteins are more likely to curdle if they are heated very quickly.

I will assume that you heated your coffee and milk identically with and without the salt. If you had changed the order of adding boiling water, for example, it could have prevented your curdling.

If you have hard water then there will be a good amount of calcium salts in your water. Coffee itself also has a good bit of magnesium. These salts will coagulate soy proteins more readily than acidity.

Sodium chloride does not cause soy proteins to coagulate. Sodium chloride ions will replace calcium ions- this is used in water softeners and soaking beans.

My suspicion (which to prove would require more experimenting than I have time for right now) is that the sodium ions are preventing the calcium and magnesium ions from coagulating your soy proteins.

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I don't think my heating procedure has changed - added description above. I think you might be on the right track with the ions - if curdling is associated with ion activity, then any "neutral" ions (sodium) that may be interfering with active ions (calcium, magnesium) might be the root cause. –  Jonathan Jan 14 '13 at 18:10
    
Are you a soy-and-coffee drinker? can you confirm that I'm not crazy and this actually works? –  Jonathan Jan 14 '13 at 18:10
    
I don't drink coffee and can't confirm that you aren't crazy. I've just experimented with soy milk a fair bit –  Sobachatina Jan 14 '13 at 20:07
    
Accepted as the most plausible answer - although we will need a chemist to tell us for sure! –  Jonathan Mar 20 '13 at 18:39

Sodium chloride is mildly chaotropic with respect to most proteins. That is, the charge interactions between Na+, Cl- and charged amino acids in soy protein make it moderately more soluble, even in a slightly denatured state.

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Add a little bicarb to your soy milk, no more curdling :-)

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Acid curdles soya: that's part of tofu making. Anything in the coffee that neutralizes acid below a given threshold will prevent it. Black coffee is in the range of 4.3 - 5

According to New Scientist: The pH required to make tofu from soya milk is around 5.7 to 6.4

I'm reckoning that there may be tiny flakes of coagulation in the cup but not noticeably so. You could test this by dissolving in salt first then pouring soya without stirring at all: see what 2min gives. Finally, stir it up and see if it's your regular cup.

Now someone else say why a supposedly neutral salt has that effect...?

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Serving as a buffering solution (see en.wikipedia.org/wiki/Buffer_solution )? Just speculating, I am no chemist! –  SAJ14SAJ Jan 14 '13 at 16:30
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I think your guess is plausible but FYI- most tofu is not acidified at all. It is coagulated with magnesium (nigari) or calcium (gypsum) salts. –  Sobachatina Jan 14 '13 at 16:50
    
I'm not able to detect any tiny flakes of coagulation - just the usual soy grits due to casual straining with a metal strainer. –  Jonathan Jan 15 '13 at 21:00
    
interesting. different brands of soya and coffee I guess –  Pat Sommer Jan 17 '13 at 22:51
    
Thanks, Sobachtina. Lemon juice is listed as a sub for Nigari so I just assumed... –  Pat Sommer Jan 17 '13 at 22:54

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