A leavening agent (, also leaven agent; ), also known as a raising agent, is any one of a number of substances used in doughs and batters that causes a foaming action that lightens and softens. The leavening agent incorporates gas bubbles into the dough. The alternative or supplement to leavening agents is mechanical leavening by which air is incorporated by mechanical means. Most leavening agents are synthetic chemical compounds, but carbon dioxide can also be produced by biological agents. When a dough or batter is mixed, the starch in the flour mixes with the water in the dough to form a matrix (often supported further by proteins like gluten or polysaccharides like pentosans or xanthan gum), then gelatinizes and "sets"; the holes left by the gas bubbles remain.