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:
- http://www.thekitchn.com/why-does-soymilk-curdle-in-hot-148329
- How to prevent curdling in a local vegan coffee creamer- aka- Post Apocalypse hippy coffee creamer?
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?
Best Answer
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.