If makers of cheese and tofu don't want to use animal-derived rennet to separate the solid protein curds from liquid whey, then they can use edible acids such as citric acid.

Coffee is also acidic, generally more so than tea, so the solids and liquids separate in the soya milk as if you were making tofu.

The pH required to make tofu from soya milk is around 5.7 to 6.4. Tea and coffee are even more acidic than this, explaining why soya often curdles in some teas and coffees. When you make cheese, separating cow's milk into curds and whey requires a significantly lower pH, in the region of 4.6. As this pH is more acidic than is usual for tea and coffee, curdling is rarer.

One possible solution to the curdling problem is to use roasts with lower acidity - those from Central America and East Africa are generally regarded as the more acidic. You could also reduce the acidity of your existing brew with a small dash of sodium bicarbonate, although to coffee purists this is no doubt sacrilege. Alternatively, the curdling also depends on temperature and agitation, so allowing the coffee to cool for a few minutes before stirring in the soya milk may also help resolve your curdling issues.

Ben Davis, Beeston, Nottinghamshire, UK