Acids, including vinegar and lemon juice, cause the proteins in milk to coagulate and form curds, which are solid pieces of protein. The curds trap the fat and other solids in the milk, forming a semi-solid substance known as curdled milk. The acid also causes the milk sugar (lactose) to break down into glucose and galactose, which can give the curdled milk a slightly sweet taste. Additionally, the acid can cause the milk to become more viscous, or thicker.