Ptyalin is the traditional name for salivary amylase, the starch-digesting enzyme secreted by the salivary glands into the mouth. As food is chewed, ptyalin begins breaking long starch chains into shorter dextrins and the disaccharide maltose, which is why a piece of bread held in the mouth gradually tastes sweeter. It works best near neutral pH and is inactivated by stomach acid once swallowed, so its action is brief but it starts carbohydrate digestion before food even leaves the mouth. The enzyme also helps cleanse the oral cavity by acting on food debris. Its discovery in the nineteenth century marked one of the first recognised examples of an enzyme, and it remains the standard illustration of digestion beginning in the mouth.