LEAVENING AGENT

substance causing expansion of doughs and batters by the release of gases within such mixtures, producing baked products with porous structure. Such agents include air, steam, yeast, baking powder, and baking soda. Leavening of baked foods with air is achieved by vigorous mixing that incorporates air bubbles, producing foam. Egg white is well suited to this purpose because it produces voluminous and strong foams that retain their expanded structure when dried by the baking process. Egg white is used in such baked products as angel food and chiffon cakes and sponge cakes. Gluten, the elastic protein of flour, may also be whipped to produce a foam, as in beaten biscuits. Puff pastes, which are used for light, flaky pastries, are expanded by water-vapour (steam) pressure. During baking, as the interior of the product nears the boiling point, the vapour exerts pressure within bubbles that have been incorporated earlier by other means, producing swelling. Leavening may be achieved by the process of fermentation, which releases carbon dioxide gas. Bakers' yeast, composed of living cells of the yeast strain Saccharomyces cerevisiae, is available as a pressed cake and in a powdered form. When added to doughs, yeast initiates fermentation by acting upon certain sugars contributed by other dough ingredients, releasing both carbon dioxide and substances that affect the flavour and aroma of the baked product. Yeast-leavened products include most types of breads and rolls and such sweet-dough products as coffee cakes, raised doughnuts, and Danish pastries. The sourdough method, used for rye breads, employs a small portion of dough, or sponge, in which sugar-fermenting bacteria have been allowed to develop. When added to a fresh dough mixture, the sponge produces fermentation. Commercial sour cultures are sometimes used as substitutes for naturally fermented sourdoughs. Chemical leavening agents also produce expansion by the release of carbon dioxide. Modern baking powders are combinations of baking soda (sodium bicarbonate) and dry acids or acid salts, usually with starch added for stability in storage. Single-acting baking powders, containing tartaric acid or cream of tartar, release carbon dioxide at room temperature, and mixtures in which they are used must be baked immediately to avoid loss of most of the gas. Slow-acting baking powders, containing phosphates, release part of their gas at room temperature and part when heated. Double-acting baking powder, the most widely used type, contains sodium aluminum sulfate and calcium acid phosphate and releases a small amount of gas when mixed and the balance when heated. Baking soda is added to doughs and batters in which acid is provided by other ingredients, such as honey, sour cream, molasses, or cocoa. If used without acid ingredients, baking soda may produce yellowing and undesirable odours and flavours in the finished product. Mixtures leavened with baking soda require quick handling to avoid release of most of the gas before baking.