After many persistent years I have resigned myself to the fact that there are some things, like wine names, that my brain is fundamentally incapable of remembering. But then there are those things I really ought to know. That go in and rattle around, but rather than getting filed somewhere useful they just mull about in some kind of vague, peripheral haze. Take, for example, chemical leavenings. I know they react quickly and need acids, that baking powder is a more complex, all-in-one preparation. But press me for details and I become worried that my inadequate depth of understanding will soon be exposed. I shouldn't, of course. After all it's just basic chemistry, which I get. The problem so far has been finding time to work through things properly. But with a shift in career focus and the realisation that this weakness is of absolutely no use when it comes to dabbling in recipe adaptation, I decided I really should take some time to review the situation…

BAKING SODA (aka sodium bicarbonate, bicarbonate of soda, sodium bicarb, bicarb soda) is an alkali that, when combined with an acid partner, reacts to produce carbon dioxide (gas). In foods, this reaction is used to leaven the batters of biscuits, pancakes and quick breads, the last of which are more tender (as opposed to chewy) and stale quicker than those prepared using yeast. Common acids used to complement this basic ingredient include: buttermilk, lemon juice and vinegar; and ideally these would be added in sufficient quantities to perfectly balance the base. Although this can be controlled for easily when it comes to standardised products like vinegar, it is less achievable for those produced agriculturally, as these can vary greatly by season, the animal's diet or how the company chooses to process the raw preparation. In general, it is important to get batters in the oven quickly as the acid-base reaction begins as soon as the raising agent gets wet, although release of carbon dioxide from baking soda is primarily driven by heat, with production starting at temperatures above 80°C.

Sodium bicarbonate reacts with acetic acid (vinegar) to produce sodium acetate, water & carbon dioxide gas:

NaHCO₃ + CH₃COOH → CH₃CONa + H₂O + CO₂

Thermal decomposition of sodium bicarbonate occurs gradually at temperatures above 50°C, and rapidly at 200°C:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

BAKING POWDER is a more sophisticated preparation that already contains the acid ingredients together with the matching alkali. By weight, a typical preparation will contain 30% sodium bicarbonate, 5-12% mono calcium phosphate, and 21-26% sodium aluminium sulfate, with the remainder made up by cornstarch, which serves to absorb moisture and help with the consistency, storage and stability of the mixture. The majority of domestic powders are designed to be double-acting, containing two acids that allow gas production over two separate stages. The first, controlled by low temperature acid salts (cream of tartar, mono calcium phosphate), occurs at room temperature and provides the batter with body before baking begins. The second stage, which is dependent on high temperature acid salts (sodium aluminium sulfate, sodium aluminium phosphate, sodium acid pyrophosphate), occurs during baking after the crust has permanently set, with the aim being to create even more bubbles so to produce a fluffier finished product. This double action also increases the reliability of the baked goods, as the time elapsed between mixing and baking becomes less critical.

What should be clear by now is these two chemical leavenings are not interchangeable, although sodium bicarbonate can be substituted for baking powder provided sufficient acid reagent is also added. However, it's not just the raising effects that must be considered, as both agents also have an important effect on a product's appearance and flavour. If the high pH of baking soda is not neutralised by the addition of sufficient acid ingredients, it will enhance browning via the Maillard reaction and produce something that appears burnt and is inedibly bitter. On the other hand, in situations where the caramel notes of Maillard browning are highly desirable (for example, a quick-baking choc-chip cookie) using baking powder instead of soda will make for an unsatisfyingly pale biscuit, and mean you're more likely to taste the raw ingredients, rendering your baked treat unappetisingly doughy and a little bit bland. To complicate matters even further, if baking powder is used in a mixture that is already acidic, then the additional acids within the powder will remain unconsumed, also lending an unpleasant taste to the food.

So there you have it; a quick introduction into the world of chemical leavenings. We've confirmed that baking powder and baking soda are not interchangeable, and learnt that while the reactions occur quickly, these agents are handy for making goods where the flavours of yeast fermentation are undesirable, or where a batter lacks the elastic structure to hold gas bubbles for more than a few minutes. Most importantly, we should remember that baking soda always requires an acid partner, and that pH balance must be considered in all cases if the desired flavour and appearance is to be achieved.

A few extra tidbits that may come in handy:

To make quick breads that stale less quickly use recipes that include some oil/fat, sugar, egg yolk or whole grains.
Use 1 tsp baking powder to raise a batter containing 1 cup flour, 1 cup liquid and 1 egg.
Cream of tartar, an acid ingredient of baking powder, is a byproduct of winemaking that crystallises in wine casks during fermentation. It can also precipitate out of wine, sometimes forming on the underside of the cork when bottles are stored below 10°C (yet another good excuse for pairing bread & wine?).
Heat and moisture can cause both baking powder and bicarb soda lose effectiveness over time. Test the activity of chemical leavenings by placing a teaspoonful into a small cup of hot water. If it fizzes energetically, the agent is still useable.
To avoid unpleasant tastes in an already acidic mixture, replace some of the baking powder with bicarb soda.

Further reading:

Greene, A. (2013) The difference between baking soda and being powder. Decoding Delicious. Published online 14 June 2013.
McGee H. (2010) Keys to good cooking: A guide to making the best of foods and recipes. Penguin Books, reprint edition (October 31, 2012)