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:
- 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?