NaHCO3 + H+ -----> Na+ + H20 + CO2 (soda) (acid) (sodium) (water) (gas)Sure, you've seen this- when you mix baking soda with vinegar or lemon juice it fizzes. The fizzing is release of the CO2 bubbles. The same thing happens when you add baking soda to a recipe. If you notice carefully, recipes that use baking soda for leavening always have an acid somewhere. It might be obvious, such as vinegar (in muffins? bleah), lemon juice, sour milk or buttermilk. The acid might be hidden- for example honey and molasses are acidic. Cream of tartar is a DRY acid that might be called for in a recipe; it cannot react with the baking soda until liquid is added.
The problem with baking soda is that it releases the gas all at once! So if the cake batter sits around for a while before you get it in the oven or it you beat the batter too much, the leavening will be lost and your baked goods will be flat. You don't want to add too much, either, because the taste is rather salty and you'd have to add more acid too. If you don't have enough acid to react with the baking soda you won't release the gas, plus your cake or muffins will have a bitter or soapy taste because of the unreacted bicarbonate.
There are some times, though, when an fast rate of gas release is desired. In that case, ammonium bicarbonate or ammonium carbonate can be used. This is advantageous for eclairs, cream puffs, and some cookies, espcially where a quick spring is needed before the product spreads in the oven. The reaction with ammonium bicarbonate is:
NH4HCO3 -----> NH3 + H20 + CO2Here, ammonia (NH3) is produced. This produces a distinctive smell during baking, which dissipates so is mostly gone from the finished product. Ammonium bicarbonate or carbonate are rarely used in the home because they don't store well and lose their action quickly.
Baking powder is a combination of baking soda plus a few other things, most importantly a dry acid.
When the baking powder is mixed in a batter with the wet ingredients, the dry acid and the baking soda can then react together and release carbon dioxide.
There are different types of baking powders.
Single-acting baking powders are characterized by the type of acid they include. Tartrate baking powders contain both cream of tartar (potassium acid tartrate) and tartaric acid. These create gas quickly when combined with baking soda in the presence of liquid, so the batter must be cooked quickly or it will go flat. Phosphate baking powders contain either calcium phosphate or disodium pyrophosphate (source of sodium pyrophosphate). They work a little slower than the tartrate baking powders, but most of the gas is still created outside of the oven and therefore can be lost. S.A.S. baking powders have sodium aluminum sulfate (alum) as the acid. S.A.S. baking powders react slowly at room temperature and release more of the gas when heated. The phosphate and tartrate baking powders react rapidly at room temperature to release the leavening gas, which means that the batter has to be cooked quickly after the liquid ingredients have been added. On the other hand, the S.A.S. baking powders are better for products that will sit a while before being cooked. The problem with S.A.S. powders is that they have a bitter taste. They are used in combination with other leavening agents so not as much is needed. S.A.S. is often used in D.A. powders.
Double-acting (D.A.) baking powders are the most common type of baking powder in US supermarkets. The first "action" refers to the release of gas when the baking soda in the powder reacts with an acidic liquid. D.A. baking powders contain a dry acid which does not react with the baking soda in the powder until water is added; at that point the baking soda dissolves, the acid dissolves, and the two can now mix and the reaction shown above occurs.
The second "action" refers to the release of gas when the batter is heated in the oven or on a griddle. This relies on the presence of the slower acting acid, S.A.S. which only combines with soda when the temperature increases.
I had some fun testing the gas-forming behavior of these powders with some simple experiments, which would probably bore most of you so I put it in a separate link!
When you read the container of powder, cornstarch is listed as the major ingredient. The cornstarch has three purposes: 1) it helps keep the product dry and free-flowing, 2) it helps keep the bicarbonate and acid dry (and therefore separate) so they don't react during storage, and 3) it helps bulk up the powder for easier measuring and standardization.
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