Cream of tartar
An acid powder that does two unrelated jobs — half of a baking powder, and the stabiliser that makes an egg-white foam forgiving.
Also called tartar (INTERNATIONAL); potassium bitartrate (TRADE); tartaric acid (INTERNATIONAL) — Related and not the same compound; recipes occasionally confuse them.; cream of tartar substitute (INTERNATIONAL)
Cream of tartar is a dry acid. Everything it does follows from that, and it does two things that have almost nothing to do with each other.
AS A LEAVENING COMPONENT it is the acid half of the acid-plus-base pair. Bicarbonate of soda needs an acid to react with and produce carbon dioxide; cream of tartar supplies one in powder form, which is why a mixture of the two is effectively home-made baking powder. It reacts as soon as it is wetted, so anything relying on it goes into the oven promptly.
AS A FOAM STABILISER it does something quite different. Lowering the pH of egg white makes the protein network more tolerant of over-beating — the foam takes longer to reach the point where the proteins tighten and squeeze water out. It does not make a foam rise higher; it widens the window in which a good foam exists. That is why meringue and angel food recipes call for a small quantity and why leaving it out makes them less forgiving rather than impossible.
It also discourages sugar from crystallising in boiled syrups, by a third mechanism again.
BakeHQ publishes no quantities for any of these. The useful knowledge is which job the recipe was asking it to do, because that decides what can replace it.
What changes if it is left out
It depends entirely on the job. Where it was the acid in a leavening pair, removing it removes the gas and another acid — buttermilk, yoghurt, lemon juice, treacle — can take its place, though each brings liquid and flavour with it. Where it was stabilising a foam, removing it leaves the foam achievable and less forgiving; a little acid from lemon juice does the same work. Where it was preventing crystallisation in a syrup, nothing else in an ordinary kitchen does that job.
Follows from the acid-base leavening model and the egg-foam account recorded elsewhere on BakeHQ, both of which carry their own support. No substitution ratio is published, because the correct one differs by job.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Aeration
Only as the acid half of a pair. On its own it produces no gas at all — it is bicarbonate of soda that does that, and cream of tartar is what lets it.
Can stand in for
With bicarbonate of soda, and not alone. The pair is effectively home-made baking powder and reacts as soon as it is wetted, so it has no delayed second action. BakeHQ publishes no ratio.
The science behind it
The acid-base reaction it takes part in, and the reason it does nothing without a base.
Its completely separate second job: lowering the pH of egg white widens the window in which a good foam exists. It does not make a foam rise higher.
Used in
Acid that stabilises the whites and makes them tolerant of over-whipping.