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Baking Soda: Baking soda (bicarbonate of soda), another chemical leavener, is used when there is an acidic ingredient in the recipe. The acid might be hidden such as in honey and molasses. When baking soda comes in contact with an acidic ingredient and is moistened, the alkali/acid combination creates carbon dioxide (CO2), water and a neutral salt. It also renders a neutral, tasteless residue.
SARAH SAYS: Acidic ingredients include: applesauce, soured milk or buttermilk, honey, brown sugar, molasses, cream of tartar, lemon juice or vinegar, chocolate and cocoa powder (only regular cocoa, not Dutch-process).
If the level of baking soda is too high in the recipe, it creates soapy off-notes. If the level is too low, it will allow the acidic flavors to come through. Excess levels also result in over-browning
Baking soda is four times as strong as baking powder. The general rule is to use 1 to 1-1/4 teaspoons baking powder per cup of flour. On the other hand, baking soda should be added at 1/4 teaspoon per cup of flour.
Note: However, if you have a recipe that works and the above ratios are not followed, DO NOT adjust the recipe.
If you add an acidic ingredient (Chocolate, honey, molasses, citrus juice, sour cream, buttermilk and brown sugar, natural cocoa powder etc.) to a recipe, adjust the leaveners:
Adding buttermilk (an acid) instead of milk: if the recipes uses 2 teaspoons baking powder, and you add 1/2 cup buttermilk, instead use 1 teaspoon baking powder + 1/4 teaspoon baking soda or 1 teaspoon baking powder = 1/2 teaspoon baking powder + 1/8 teaspoon baking soda
Note: Many times, other ingredients have to be changed as well when substituting ingredients, such as sugar, flour, fat, etc. It is not as simple as it looks.
QUESTION: Why is hot water used in some chocolate cakes?
SARAH SAYS: The use of hot water is utilized in chocolate cakes in two ways. Sometimes you will see the cocoa powder dissolved in warm water before it's added to the recipe. This helps dissolve it better for more cocoa flavor.
Then, there are recipes where the leavener is added to the hot water at the end of the recipe, ie: add the baking soda to the hot water, and add to the rest of the batter. That is done just for color. Baking soda is added not only to a recipe for leavening, but will also enhance color. When added to water, it expenses the leavener, but changes the pH of the recipe, enhancing/darkening the color of the cocoa powder in the recipe. If there is baking powder in the recipe, as well, that's what in fact, leavens the recipe, plus any left-over baking soda not expensed.