When you pour soy milk into a hot cup of coffee, the soy milk seems to curdle. What's puzzling is that it does not happen with every cup of coffee or every brand of soy milk. And it never happens with tea. The reason for the inconsistency is simple: the soy milk is not actually curdling. The acid in the coffee actually separates the protein in some soy milk formulas into tiny tofu lumps. To stop your soy milk from curdling, try changing the way you pour. If that does not work, the problem may lie in your soy milk brand or coffee variety.
Shake the soy milk container thoroughly, for 10 seconds or so.
Pour the soy milk into the coffee mug.
Pour in a splash of coffee, then stir the contents of the cup. Keep pouring splashes at a time until you have more coffee than soy milk. Then pour the rest of the coffee into the cup slowly.
Switch brands of soy milk if yours keeps curdling. Cheap soy milk brands, like the kind you find unrefrigerated on the grocery store shelf, are more likely to curdle than more expensive brands. Try out a few different brands of soy milk until you find one that is not so prone to separation.
Cold brew your coffee. Cold brewing is a great way to reduce the acid content of your coffee and reduce the likelihood that your soy milk will curdle. Cold brewing takes a little bit of time, but you can cold brew pounds of coffee at a time and store it in your refrigerator for quite sometime. The low acid content is kinder on your stomach and your soy milk and the continual supply of fresh coffee will save you time in the morning.