Jeremy Challender, co-director of Prufrock Coffee, head judge of the UK Barista Championship, and authorised Speciality Coffee Association of Europe trainer, explains: "The curdle point for soy is a pH of 5.5 if I recall. So soy will curdle below that. And pretty much all light-roasted arabica coffee will have a pH below 5.5."

(According to The Little Coffee Know-It-All, the proteins in soy will maintain their shape up to a pH of around 4.9, with black coffee having a pH of around 5.)