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What is Alum?

Reader Jess writes to say that some of her old family recipes call for alum, but what is it? And is it really necessary? Great questions, Jess. Anyone who’s every watched old Warner Brothers cartoons has probably wondered something similar. You know, when Tweety Bird pours a box of alum down Sylvester’s throat and his head shrinks down to the size of a golf ball. What is that alum stuff and why did people keep it around?

“Alum” is short for aluminum potassium sulfate. It was once a common household item, especially during the war years when people did a lot of home pickling. A pinch of alum in a jar of kosher dills or watermelon rinds kept them 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 the constricting of mucous membranes and blood vessels).

How ’bout an answer to the question, digression king? 

Ah 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.

Thanks Jess!

4 thoughts on “What is Alum?”

  1. When I was a kid my mom would always give us a small “pebble” of alum to put in a cancer sore. Dried them up SUPER fast!

    Today I wouldn’t even know where to get any alum…

    1. Ah yes! People used “alum stones” for all sorts of things. Since alum causes blood vessels to contract, they were used to stop bleeding from small nicks and cuts (they were used a lot in barber shops). And because they caused mucus membranes to shrink up, they were good for canker sores. The same action would also shrink pores, making them a good acne treatment, and sweat glands, so there were also used as antiperspirants. Amazing they ever went away!

      Thanks Rainey!

      – Joe

  2. My homemade play dough (salt dough) recipe has alum in it. I thought maybe it was acting as a preservative or something? The play dough does last and stay soft a long time! (And I can still find alum at the grocery store here!)

    1. Very interesting, Marilyn. I don’t know why that would be, myself, but then who knows when I might need some play dough around here? Many thanks!

      – Joe

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