A leavening agent (/ˈlɛvənɪŋ/, also leaven agent; /ˈlɛvən/), 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 it, by incorporating gas bubbles into it. The alternative or supplement to leavening agents is mechanical leavening by which air is incorporated by mechanical means. Leavening agents can be synthetic chemical compounds, or consist of biological agents. The gas produced is often carbon dioxide, but other gases such as hydrogen are also used. 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.