Leavening agent or leavener is a substance used to leaven (to make light or to rise) a dough or batter. It may be natural such as air or steam or chemicals like baking powder or baking soda or biological agent like yeast. These produce or stimulate production of carbon dioxide in baked goods to impart a light texture. Leavening agents are responsible for the volume in most baked food products.They expand the air bubbles that have been mixed, creamed, kneaded or whipped into the batter. This results in increasing the volume resulting in better texture, taste and aroma. The sensory properties of leavened products depend on the amount of leavening gas in the mixture, the rate and time of gas formation. The three principal leavening gases are
- Air: Air is introduced mechanically into ingredients by heating, creaming and sifting; and into mixtures by beating and folding.
- Carbon dioxide: Carbon dioxide is released by either chemical leavener or by biological agents such as yeast cells.
- Water vapor: Water vapor or steam is formed in any batter or dough on heating. It is the principal leavening agent to cream puffs and popovers.
Yeast Saccharomyces cerevisiae is a microscopic, single celled plant which under suitable conditions of moisture, nutrients and temperature it produces carbon dioxide from simple sugars derived from starch and or sugar. In addition to glucose, baker yeast ferments hexose, fructose and mannose, disaccharides sucrose, maltose, the trisaccharides raffinose and tetrasacharides stachyose. Lactose (milk sugar) is not attacked by baker yeast. Yeast exists active in air as well as in absence of air. In presence of air it grows rapidly and forms little alcohol. In absence of air it grows slowly but alcohol formation increases. Fermentation is an important process in baking. Most favorable temperature for baker yeast growth and fermentation is in the range of 84º to 90º F depending on strain used. Yeast grows and ferment best in acidic environment, tolerating acidities as low as pH 2. In bread making yeasts raise the dough improving palatability by producing several compounds including alcohol, aldehydes, ketones and acids contributing to aroma and taste. However, some times the yeast activity cannot be controlled in some items, fermentation flavour can be undesirable. It is available as,
- Biological leaveners
- Active dry yeast is that which has the yeast and filler mixture dried and then packaged in granular form
- Compressed yeast is a moist mixture of yeast and starch. The yeast is in an active state and due to the presence of moisture is perishable. It needs to be stored in refrigerator.
- Chemical leaveners
- Commonly used leavening acids