Chemical leavening created a whole category of baking

Before it, a rise cost either hours or arms

A cook in the early nineteenth century had two ways to make a batter rise. One was fermentation, which meant keeping a yeast culture or obtaining barm from a brewer, and waiting — hours of proving, with a result that depended on the temperature of the room and the health of the culture. The other was mechanical: beating eggs until they held a foam stiff enough to carry a batter, which without a mechanical whisk was long, hard work.

Both routes constrained what could be baked and by whom. A rich egg-foam cake was a substantial undertaking, and a yeasted cake had to be planned around. The idea that a cake could be decided upon and produced within an hour, with no culture to maintain and no prolonged beating, simply did not apply.

Alkaline salts came first and were imperfect

The earliest chemical leaveners in common use were alkaline salts: pearlash, produced from wood ash, and later saleratus, both of which release carbon dioxide when they meet an acid. Sour milk, buttermilk and molasses were all sufficiently acidic to trigger them, and recipes of the period specify these ingredients for that reason rather than for flavour.

The results were unreliable in a way that is easy to underestimate now. The strength of the alkali varied between batches, the acidity of the sour milk varied with how sour it happened to be, and a mismatch left either unreacted acid or, more noticeably, leftover alkali that tasted soapy and turned the crumb yellow. Getting it right was a matter of judgement rather than measurement, and much period advice is about compensating for that.

Packaging the acid with the base was the real innovation

The decisive step, taken in the middle of the nineteenth century, was to combine bicarbonate with a dry acid and a starch buffer in a single tin. Because both reagents were now supplied in known proportions, the cook no longer had to supply the acid or estimate it, and the reaction became predictable. Later formulations added a second acid that responds to heat rather than moisture, which gave the batter a delay and a second release of gas in the oven.

This turned leavening from a skill into an ingredient. It is a genuinely large change, and it is easy to miss because the packet is so ordinary. The reliability is the point: a measured spoonful behaves the same way today as it did last month, which is not something that could be said of a jar of pearlash.

What appeared as a result

A whole family of baked goods followed, most of them still in the repertoire. Quick breads — soda bread, cornbread, banana and other fruit loaves — are defined by the absence of fermentation and could not have existed in that form beforehand. Scones and biscuits in the American sense, muffins in the modern sense, and the great mass of layer cakes all depend on chemical leavening for their texture and their speed.

The category is defined by method rather than by ingredients, which is why it cuts across sweet and savoury and across national traditions. It also changed what a cake was expected to be. The light, even, fine crumb we now consider standard in a cake is achievable partly because gas can be generated reliably in a batter too rich and too heavy for either yeast or egg foam to lift.

The effect on who baked, and where

The domestic consequence was substantial. Baking stopped requiring the maintenance of a culture or a relationship with a brewer or baker, and the time cost fell from a day to under an hour. That coincided with cast-iron ranges and later gas ovens with something approaching controllable heat, and with the wider availability of cheap refined flour and sugar. Written recipes with measured quantities became more useful, because the ingredients had become consistent enough for a measurement to mean the same thing twice.

It is fair to note the trade. Fermentation contributes flavour that chemical leavening does not, since it produces acids and aromatic compounds over hours that no immediate reaction can replicate, and the revival of interest in long-fermented breads is partly a reaction to what was given up. Both things are true at once. The packet made an enormous amount of baking possible, and it made a particular kind of flavour unnecessary, which is generally how a convenience works.