What is Alum?

Reader Amy writes to say that some of her old family recipes call for alum, but what is it and is it really necessary? Great questions. Anyone who’s every watched a Warner Brothers cartoon has probably wondered the something similar. You know those scenes: Tweety Bird somehow manages to pour a box of powdered alum down Sylvester’s the Cat’s throat and his head shrinks up to the size of a golf ball. But what the heck is that stuff and what did people use it for?

Alum is short for aluminum potassium sulfate. It was once a common household item here in the U.S., especially during the war years when people did a lot of home pickling. A pinch of it in a jar of kosher dills or watermelon rinds kept the pickles firm and crispy. Too much and the result was a serious pucker, since alum is both an acid and an astringent (which is to say, a compound that causes shrinking or constricting of blood vessels and/or mucous membranes).

How ’bout an answer to the question, digression king? Right. Alum has another household use: as a chemical leavener. It’s commonly formulated into baking powders as a reactant along with baking soda. This is probably why it appears in your family’s recipe, Amy. Is it strictly necessary? No. You can use some other form of kitchen acid to create your reaction if you like: a little cream or tartar or a teaspoon or two of vinegar or fresh lemon juice will do the trick. Then again baking powder is a lot easier, which is the reason I prefer it.

Of course any baking powder you buy probably has alum or some close variant of alum in it, so if you’re specifically looking to avoid alum or aluminum itself, there are some non-aluminum baking powders on the market that work pretty well.

7 thoughts on “What is Alum?”

  1. When I was a kid a little rock of alum was what you dropped into a canker sore. It would dry those puppies up in a matter of hours.

    I went looking for it when my kids were little to discover pharmacies don’t have it anymore. Bummer!

    1. Huh, that’s really cool. I honestly haven’t seen alum on a shelf in forever. Thanks, Rainey!

      – Joe

      1. However, you can still get alum as a shaving accessory, to my understanding both alum blocks and styptic pencils are both made of alum! I’m not sure what else might be in them or how good they might be repurposed for canning purposes, but for Rainey’s case it might be useful!

  2. My sister kept a jar of powdered alum when her kids were younger, to discourage bad language or back talking. A little bit on the tip of the tongue – tasted awful enough to work, and safer than washing out their mouths with soap.

  3. Both powdered alum and blocks of alum (which presumably could be chipped to use on cancer sores) are available on Amazon.

  4. I read a recipe for Nine Day Pickles recently that called for “a pea-sized piece of alum per jar” of pickles… Alum powder is also available in our local Bulk Food store.

  5. I was recently making Lemon Chiffon Cake from the Tartine Cookbook (an astoundingly good cake) and the recipe recommends “Alum-Free” baking powder saying it results in a finer crumb. I found aluminum-free at my local market and assumed they must be the same thing. Now I know they are. Thanks Joe.

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