Chemical Leaveners.
A teaspoon of baking soda in a cake with no acid gives a little gas and a lot of soapy, metallic taste. Add buttermilk and the same teaspoon gives a full rise.
Baking soda, sodium bicarbonate, is a base. With an acid it reacts at once to give carbon dioxide, water and a salt; with no acid, heat alone decomposes only part of it, slowly, leaving sodium carbonate behind — a strong alkali that tastes soapy and metallic, turns the crumb yellow and dark (VIII.3) and blueberries green (XX.4). So soda is used with an acid ingredient: buttermilk, yoghurt, sour cream, lemon juice, vinegar, brown sugar, honey, cocoa (natural, not Dutch-processed), chocolate. The proportions are stoichiometry: each acid neutralises a known weight of soda.
Baking powder packages soda with dry acids and starch so it needs only water. "Double-acting" powder has one fast acid (monocalcium phosphate) that releases gas when wetted and one slow acid (sodium acid pyrophosphate or an aluminium salt) that releases more in the oven heat, so the batter can wait a little. Typical use is 1 teaspoon (4 g) of powder per 125 g of flour, or about ¼ teaspoon (1.2 g) of soda per 125 g with enough acid. Too much of either gives coarse, collapsing crumb and off-tastes. The workshop balances soda and acid.
Below: the carbon dioxide released by the baking soda you choose with the acid ingredient and amount you choose (stoichiometry with published neutralising values), the share of soda left unneutralised, and the acid needed to neutralise it all.
- Explain how soda and powder release gas.
- Balance soda with an acid ingredient.
- Recognise too much soda.
- Choose powder or soda.
Match the soda to the acid
Three grams of soda (a scant teaspoon) with 240 g of buttermilk leaves about 33 % of the soda unneutralised — enough to taste and to darken the crumb; neutralising it all would take about 357 g of buttermilk. Two grams with the same buttermilk leave 0 %. That is why buttermilk recipes typically use about ½ teaspoon of soda per cup and add baking powder for the rest of the lift: the powder brings its own acid.
Practical rules: measure leaveners exactly, with level spoons or a 0.1 g scale; whisk them evenly through the flour so no pockets form; use soda when the recipe has an acid, powder when it does not, and both when it has some acid but needs more lift; bake batters with fast-acting leaveners soon after mixing; replace baking powder after about a year, and test it in hot water — it should fizz. In Chinese steamed buns, baking powder or a little soda sometimes supplements yeast for whiteness and lift (XXVI.3); a pinch of soda in 炸 batters gives extra crispness.
Make three small pancakes from one batter: with soda only, with soda and buttermilk, and with baking powder. Compare their rise, colour and taste.
Terms
- Baking soda
- Sodium bicarbonate; needs an acid to release gas.
- Baking powder
- Soda with dry acids and starch; needs only water.
- Double-acting
- Releases gas once when wet and again in the oven.
- Neutralising value
- Grams of soda an acid neutralises per 100 g.
Sources
- Exact stoichiometry: glucose → 2 ethanol + 2 CO2 (180 g → 92 g + 88 g); ethanol + O2 → acetic acid + water (46 g → 60 g); ideal-gas molar volume 24.5 L at 25 °C.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
- H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.