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Virtually every single egg white recipe will say add some cream of tartar to egg whites to help them whip better because the cream of tartar will lower the pH and make it more stable. But if all it's doing is making it more acidic, why exclusively cream of tartar over the dozens of far more common acids found in the kitchen?

One difference is cream of tartar is the only solid acid so it wouldn't add more water to the egg whites except many books also say that adding a tbsp of water to egg whites increases stability as well. Is there something else in cream of tartar that isn't in other acids?

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I think 'virtually every single egg white recipe' is coming it a bit strong. They're more likely to recommend white wine vinegar or lemon juice. – ElendilTheTall Jul 26 '12 at 11:32
Do you have a couple of examples of recipes saying to add water? I don't think I've ever seen that, except in the context of scrambled eggs/omelettes/etc., which isn't really whipping and is just to increase volume. – Aaronut Jul 28 '12 at 15:45

2 Answers 2

As I understand it, cream of tartar isn't actually an acid, but a salt, albeit one whose pH is fairly acidic (around 3.5). It is substantially less acidic than lemon juice or vinegar (in the 2-2.5 range). Plus, cream of tartar is flavorless.

It is also an acid buffer, meaning that it doesn't just lower the pH of the food, but rather also keeps it at a very specific pH value. Basically, when combined with other acids, it can actually raise the pH.

Meanwhile, egg whites are neutral or alcaline (7.6 to 9.8).

Now I do not know exactly how all this affects whipping egg whites, but my guess is that lemon juice or vinegar simply are both too strong, and not as predictable, as cream of tartar. In addition, lemon juice and vinegar would add a flavor, liquid, and volume to whipped egg whites, all of which could physically interfere with the whippability.

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Cream of tartar is most definitely an acid. It's tartaric acid. The powdered form that you buy is a potassium salt, but only with one of the carboxylic acids. The other end still has an acidic proton. If this weren't true (i.e. if you had a di-potassium salt of tartaric acid), you'd end up with a basic salt rather than an acidic salt. – S. Burt Nov 16 at 21:33
@S.Burt thank you for adding that detail. I think what you said is a more chemically correct version of what I tried to say, along with an explanation of why even the potassium salt form in the powder still is acidic. Much appreciated. – Kevin Keane Nov 20 at 0:45

Ingredient substitution lists say you can use an equal volume of lemon juice or vinegar if you don't have cream of tartar.

Most likely, the assumption has been that a baker will be more likely to have cream of tartar on hand than other acid sources due to the fact that it has multiple uses in the kitchen:

  • Leavening
  • Stabilization of egg whites
  • Prevent crystallization of sugar in things like frostings, syrups, chocolates, etc.

Cream of tartar also has a number of beneficial properties:

  • It is odorless and practically tasteless, unlike lemon juice or vinegar.
  • It acidifies without adding water, which might be detrimental in some applications.
  • Unlike fresh lemons, cream of tartar has a nearly indefinite shelf life.
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It also doesn't add liquid. – Joe Jul 26 '12 at 15:09
Joe, I left that out in the original answer because the questioner already mentioned it. – Didgeridrew Jul 28 '12 at 6:34

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