Leavening agent

Incooking, aleavening agent(/ˈlɛvənɪŋ/) orraising agent, also called aleaven(/ˈlɛvən/) orleavener, is any one of a number of substances used indoughsandbattersthat cause afoaming action(gas bubbles) that lightens and softens the mixture. An alternative or supplement to leavening agents is mechanical action by which air is incorporated (i.e.kneading). Leavening agents can be biological or syntheticchemical compounds. The gas produced is oftencarbon dioxide, or occasionallyhydrogen.
When a dough or batter is mixed, thestarchin the flour and the water in the dough form amatrix(often supported further byproteinslikeglutenorpolysaccharides, such aspentosansorxanthan gum). The starch thengelatinizesand sets, leaving gas bubbles that remain.
Chemical leavens are mixtures or compounds that release gases when they react with each other, with moisture, or with heat.  Most are based on a combination of acid (usually a low molecular weight organic acid) and asaltofbicarbonate(HCO3).  After they act, these compounds leave behind a chemical salt.  Chemical leavens are used inquick breadsandcakes, as well ascookiesand numerous other applications where a long biologicalfermentationis impractical or undesirable.
Chemical leavening usingpearl ashas a leavening agent was mentioned byAmelia Simmonsin herAmerican Cookery,published in 1796.
Since chemical expertise is required to create a functional chemical leaven without producing off-flavors from the chemical precursors involved, such substances are often mixed into premeasured combinations for maximum results. These are generally referred to asbaking powders.  Sour milk and carbonates were used in the 1800s.  The breakthrough in chemical leavening agents occurred in the 1930s with the introduction ofmonocalcium phosphates(Ca(H2PO4)2). Other leavening agents developed includesodium aluminium sulfate(NaAl(SO4)2·12H2O),disodium pyrophosphate(Na2H2P2O7), andsodium aluminium phosphates(NaH14Al3(PO4)8·4H2O and Na3H15Al2(PO4)8).  These compounds combine with sodium bicarbonate to givecarbon dioxidein a predictable manner.
Steamandairare used as leavening agents when they expand upon heating.To take advantage of this style of leavening, the baking must be done at high enough temperatures to flash the water to steam, with a batter that is capable of holding the steam in until set. This effect is typically used in products having one large cavity, such aspopovers,Yorkshire puddings,pita, and most preparations made fromchoux pastry. The effect is also seen to a lesser extent intempura.
Using awhiskon certain liquids, notablycreamoregg whites, can also create foams through mechanical action. This is the method employed in the making ofsponge cakes, where an egg protein matrix produced by vigorous whipping provides almost all the structure of the finished product.
TheChorleywood bread processuses a mix of biological and mechanical leavening to produce bread; while it is considered by food processorsto be an effective way to deal with the soft wheat flours characteristic of British Isles agriculture, it is controversialdue to a perceived lack of quality in the final product. The process has nevertheless been adapted by industrial bakers in other parts of the world.