The thin, tangy liquid remaining after acid-coagulated products such as Greek yogurt, cottage cheese and quark are strained. More acidic and mineral-rich than sweet whey but lower in protein, it is harder to dry into valuable powder, which turned the Greek yogurt boom into a disposal headache for manufacturers. Innovation has followed necessity: acid whey now goes to animal feed, biogas digesters, fertilisers and emerging food-ingredient applications. Its story illustrates how a shift in consumer taste — strained yogurt’s rise — can create a by-product challenge that the industry must engineer its way out of.