Trade secrets... Curdling of soya milk

Recently I bought a coffee and asked for it with soya milk rather than cow’s milk. At first it looked and tasted good; it was a hot golden brown liquid like every other coffee. However within a few minutes I was shocked to look down and see my lovely coffee had transformed into a watery-looking dark brown liquid with thousands of tiny lumps floating in it. The soya milk had curdled.

But why does soya milk curdle? Jonathan Hare investigates…