Chemistry of Cooking

Chemical Leaveners

Yeast cells produce carbon dioxide slowly, over the course of an hour or more, so the material surrounding them must be elastic enough to contain it for that much time.  Weak doughs and runny batters can’t hold enough gas bubbles for more than a few minutes and are, therefore, usually raised with chemical leavenings. These ingredients are concentrated and small differences in the amount added can cause large variations in the quality of the finished product.  Too little leavening will leave it flat and dense, while too much will cause the batter to overexpand and collapse.

A new method of leavening appeared in the first American cookbook, Amelia Simmons’ 1796 American Cookery. Four recipes, two for cookies and two for gingerbread, include the ingredient “pearlash,” made by soaking the ash produced when plant materials are burned, draining off the liquid, and drying it down to the substances dissolved in it.  Pearlash is mostly alkaline potassium carbonate, which reacts with acids in doughs to generate carbon dioxide gas.  It was the precursor to baking soda and baking powders, which arrived between 1830 and 1850.

Baking soda (sodium bicarbonate, NaHCO₃) is a white, powdery alkaline substance. When baking soda is mixed with acidic ingredients, it reacts and creates bubbles of carbon dioxide (the same gas that yeast would produce.) For instance, yogurt, sour milk, buttermilk and molasses contain acid (lactic acid) that the baking soda can react with. These bubbles are trapped inside a batter and help the baked good to rise. As noted before, the biological way of raising a baked good is by using yeast, and kneading the dough until a gluten develops strong enough to trap indefinitely the carbon dioxide exhaled by the yeast. Chemical leaveners such as baking powder and baking soda are obviously faster than all the kneading; which is why these baked goods are often called "quick breads." Sodium bicarbonate is inexpensive to produce, tasteless, non-toxic and easily purified in production. It comes from soda ash. The soda ash itself can come from a rock named "trona", which is mined, or be made via a method called the "Solvay" process, which involves introducing carbon dioxide and ammonia into a solution of sodium chloride.¹¹

Baking powder is a leavener that consists of a combination of baking soda, cream of tartar (tartaric acid), and a moisture absorber (like cornstarch).  It has the action of yeast but it acts much more quickly because it has its own acid.  It is used in batters where there is no other acid naturally present.

The first modern version of baking powder was discovered and manufactured by Alfred Bird (1811-1878), British chemist and founder of Bird and Sons Ltd.  His improved version of baking powder was created so he could make yeast-free bread for his wife, Elizabeth, who had allergies to eggs and yeast.¹²

Baking powder acts immediately upon addition of water; therefore, a filler (usually cornstarch) is added to absorb the moisture and prevent premature activity.  As soon as the powder gets wet, the two chemicals begin to dissolve and react with each other to produce carbon dioxide.  In most cases, we don’t want the baking powder to release all its gas as soon as we mix the batter, before it is baked enough to trap the CO₂ bubbles in place.  “Double-acting” baking powder releases only a portion of its gas when it gets wet and releases the rest after the oven has reached a high temperature.  Two different chemicals in the powder are responsible for the two separate reactions.

Recipes sometimes call for both baking soda and baking powder.  In this case, the bread, cake or cookie batter is actually being leavened by the baking powder, which contains exactly the right proportions of bicarbonate and acid to react completely with each other.  But if there is another acid ingredient present, such as buttermilk, some extra bicarbonate in the form of baking soda is used to neutralize the excess acid.¹³