What to use instead of cream of tartar
It depends entirely on what it's doing in the dish. Here's the job, what else does it, and what to skip.
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Acid that stabilises a whipped egg white
It lowers the pH so the white proteins link up more evenly, which makes the foam harder to overwhip and slower to weep liquid back out.
Lemon juiceRan out
1/2 tsp lemon juice per 1/4 tsp cream of tartar
Same stability. It adds liquid, so the foam takes a minute longer to climb to stiff peaks.
White vinegarRan out
1/2 tsp vinegar per 1/4 tsp cream of tartar
Same effect and the smell blows off in the oven. A meringue made with it weeps a little more on a humid day.
What doesn't work: Baking powder. It contains an acid but it also contains soda and starch, so it fizzes on contact with the wet whites, leaves a metallic taste, and the foam collapses as the gas escapes.
Dry acid that turns baking soda into a leavener
Soda needs an acid to release gas, and this delivers it as a powder, so the dough stays inert until liquid arrives. It is also what gives a snickerdoodle its tang.
Baking powder, replacing both the tartar and the sodaRan out
1 1/2 tsp baking powder per 1 tsp cream of tartar plus 1/2 tsp soda
The same lift with none of the tang, so the cookie tastes flat and bakes a shade browner.
Lemon juice, keeping the soda as writtenRan out
1 tsp lemon juice per 1/2 tsp cream of tartar
The dough goes wetter and spreads more in the oven, and the cookie loses its dry, sandy snap.
White vinegar, keeping the soda as writtenRan out
1 tsp vinegar per 1/2 tsp cream of tartar
Fires immediately in the bowl, so the tray has to go into the oven within about ten minutes or the lift is spent.
What doesn't work: Dropping it and leaving the soda in. Without an acid the soda has nothing to react with, so the cookies stay flat, go yellow at the edges and taste faintly of soap.
Stops a sugar syrup crystallising
It splits some of the sucrose into glucose and fructose, and those get in the way of crystals forming, so the syrup stays smooth as it cooks down.
Lemon juiceRan out
1/4 tsp lemon juice per cup of sugar
Does the same job. Go much past that and the candy will not set firm at all.
Corn syrupRan out
1 tbsp per cup of sugar
The most reliable of the three, and the finished candy stays soft and chewy for far longer.
Glucose syrupRan out
1 tbsp per cup of sugar
Same protection as corn syrup with less sweetness, so the caramel flavour comes through cleaner.
What doesn't work: Stirring the pan to keep it smooth. Agitation is what starts crystals, and scraping the sides drags dry sugar back into the syrup, so the whole pan seizes into grit.
In a pavlova, cornstarch does a different job from the acid. It soaks up loose moisture so the middle stays marshmallowy, and neither one replaces the other.