Sold under such brand names as Marmite and Vegemite, yeast extract is a concentrated, thick, dark–brown substance with a salty taste and an intense flavour. Strictly speaking, it is not an extract, but is produced by allowing yeast to digest itself with its own enzymes. Its proteins are broken down by hydrolysis to form amino acids. The bitter yeasty flavour is filtered out and the liquid is boiled to form a thick mass. Other substances are added to flavour it, depending on the manufacturer. Yeast extract can be used to flavour savoury dishes and drinks, as a vitamin supplement and as a sandwich spread. It is an incomplete protein food and also contains amino acids and useful amounts of vitamins B1 and B2.