Week 11: Leavening by chemistry

Bicarbonate is a base looking for an acid, baking powder arrives with its own, and gluten has stopped being the goal

3 May 2027

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Yeast is a population. You feed it, keep it warm, and read it like a gauge, which is what the last six weeks have been about. Bicarbonate of soda is a white powder that does one thing, once, the instant it gets wet, and then it is finished. There is no culture to judge, no schedule to hold to, and no window in which the batter improves. The whole method is arranged around a reaction you cannot slow down.

One reaction, and it has already started#

Bicarbonate of soda is sodium bicarbonate, NaHCO₃, and it is a base. Put it in water with an acid and it gives up carbon dioxide:

NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂

The arithmetic is worth carrying around. Sodium bicarbonate has a molar mass of 84 and carbon dioxide 44, so a gram of soda that finds enough acid yields 0.52 g of gas — 0.012 moles, which is about 290 mL at room temperature and around 360 mL by the time the centre of the loaf is approaching 100 °C. A quarter teaspoon in a small loaf is a great deal of carbon dioxide.

Note what is not in that equation: time, warmth, food, or anything alive. The reaction runs at room temperature in a bowl, and a good part of it is over inside two or three minutes. That single fact sets the shape of the whole method. You cannot mix the batter in the morning and bake it at lunchtime, because the gas will have left through the surface while you were away. Mix, fill, bake — ten minutes, not an hour.

Soda needs an acid; powder brings its own#

With no acid to react with, soda does very little that you want. It will decompose thermally above about 80 °C into sodium carbonate, water and carbon dioxide, which does raise the batter a little, but sodium carbonate is strongly alkaline and tastes of it. A recipe calling for soda is telling you there is acid elsewhere in the ingredient list. Buttermilk, yoghurt and sour cream; brown sugar, which is acidic because of its molasses; honey; citrus juice; natural cocoa powder — but not Dutch-processed cocoa, which has been alkalised and will leave your soda with nothing to do. And very ripe fruit.

Baking powder is the same base with the problem solved in the tin. It is sodium bicarbonate, one or two powdered acids, and cornflour as a dry filler that keeps them apart and takes up stray moisture so the tin does not fizz on the shelf. Add liquid and it is self-sufficient. A single-acting powder uses monocalcium phosphate, which is fast and cold-reacting: roughly two thirds of its gas is gone within two minutes of wetting. A double-acting powder adds a second acid, usually sodium acid pyrophosphate or sodium aluminium sulfate, that needs heat before it does anything, so only a small fraction leaves at the bench and most of the gas waits for the oven. That is the margin that forgives slow bench work, and it is why almost every powder on a supermarket shelf is double-acting.

Because most of a teaspoon of powder is acid and starch, it carries far less soda than a teaspoon of soda does, and the ratios differ accordingly.

Treat both as starting points rather than limits. The soda figure scales with the acid in the batter, not only with the flour: a loaf carrying 400 g of very ripe banana and 150 g of brown sugar has far more acid in it than that baseline assumes, and will take half as much soda again before any is left unreacted.

Many recipes use both agents, and that is not hedging. The soda neutralises acid that would otherwise leave the crumb tight and pale, and the powder tops up the gas without shifting the pH any further.

What too much tastes like#

Overdosing the soda is the commonest fault in a quick bread, and it announces itself three ways at once. The flavour first: soapy, faintly metallic, bitter on the finish, from the sodium carbonate left over once every available acid has been consumed. Then the colour, because raising the pH accelerates Maillard browning — the crumb comes out darker than its bake time should give, sometimes with a grey-green cast where the alkali has met the pigments in cocoa or fruit.

Then the shape. Too much gas arrives too early, before the starch has gelatinised or the egg has set, so the loaf inflates fast, domes hard, splits wide, and then sinks through the middle as it cools because nothing had solidified to hold the volume. A crater down the centre of a banana bread is almost always soda, not the oven.

Gluten is now the fault#

Week 7 took the gluten away from you: rye has almost none, so there was nothing to knead and nothing to look for in a windowpane. This week is the harder case. There is plenty of gluten-forming protein in the bowl, it will form a network the moment you give it enough agitation, and you have to leave it alone.

The reason is timing. In a yeasted dough you want a continuous elastic network, because it has to retain gas over hours. In a quick bread the gas arrives in minutes and the loaf is set within the hour, and an elastic network does two harmful things to it: it resists expansion, giving a tough, close, rubbery crumb, and it channels the gas that does move into long vertical tunnels instead of letting it spread as small round cells.

So the method suppresses gluten at every step. Plain flour at around 9% protein rather than the bakers’ flour you have used since week 1. Fat and sugar coating the flour particles so the proteins hydrate less readily and any strands that do form stay short. And above all the muffin method: whisk all the dry ingredients in one bowl, combine all the wet in another, then pour the wet into the dry and fold only until the last of the dry flour has disappeared. A dozen folds. The batter should still be lumpy when you stop. Those lumps hydrate in the oven and you will never find them; the tunnels left by beating the batter smooth will be in the loaf for as long as it lasts.

Why the bananas have to be black#

The fruit is a leavening ingredient, not a flavouring. As a banana ripens its own amylase converts starch to sugar — from roughly 20% starch and 2% sugar in green fruit to the reverse once the skin has blackened. Pectin in the cell walls degrades, so the flesh mashes to a purée instead of staying in nuggets that bake into wet pockets. The pH falls to around 4.8, which is the acid your soda is counting on. And the fruit is about three-quarters water, which is why this batter needs no milk.

Ripening is enzymatic, so heat cannot rush it. An hour in a 150 °C oven will blacken a green banana’s skin and soften its flesh, and it will still be starchy, bland and short of acid, because the enzymes were denatured long before they had done any work.

Sugar is the reason it keeps#

A quick bread runs 50 to 70% sugar on flour weight, well past anything in the lean doughs. Sugar sweetens, but it is also hygroscopic and it competes with the flour for water, which limits gluten development, raises the temperature at which the starch gelatinises, and tenderises the crumb. The consequence you will notice is shelf life. Staling is largely starch retrogradation — amylose and amylopectin recrystallising and pushing water out of the crumb — and sugar bound to that water slows the process considerably. An unsweetened tin loaf is past its best the morning after you bake it. A banana bread is at its best on day two and perfectly good on day four.

A moderate oven, and the crack#

170 to 180 °C for 50 to 60 minutes in a 900 g tin. The restraint is geometric. The loaf is deep, it is close to half water, and heat has to travel to the centre before the crust is set and coloured. Bake it at 200 °C and the outside is dark at twenty minutes with the middle still batter — pull it then and it runs; leave it until the skewer is clean and the crust is bitter.

Test the centre with a thin skewer: moist crumbs are done, wet batter is not, and a completely dry skewer means you have gone a few minutes too far. The centre reads about 96 °C.

The split down the top is not a fault to design out. The crust sets after fifteen or twenty minutes while the interior is still expanding, and the surface tears along its weakest line. A quick bread with a flat, unbroken top usually did not get enough lift.

Outline#

• chemical leavening as a reaction rather than an organism, and what that costs you in working time
• NaHCO₃ plus acid, and the 0.52 g of carbon dioxide per gram of soda
• where the acid comes from: buttermilk, brown sugar, natural cocoa, ripe fruit
• baking powder as soda plus powdered acid plus starch, and how much double-acting powder holds back for the oven
• 1 tsp powder per 120 g flour, ¼ tsp soda per 120 g flour, and why the soda figure scales with the acid rather than the flour
• soapy, dark, domed, cracked and sunk: the signature of too much soda
• the muffin method, a dozen folds, and the tunnels that beating puts in the crumb
• ripeness as enzymatic starch conversion, and why forced ripening fails
• sugar as humectant and tenderiser, and retrogradation slowed for days
• 175 °C, 55 minutes, 96 °C in the centre, and the crack as evidence of lift