Hi Jen, What a great question! Reading older recipes, I’m often surprised by what was assumed that the cook knew and understood, like proofing yeast, or scalding milk.

Before I explain what proofing is, did you know that yeast is comprised of living organisms? When you buy yeast, it is dehydrated, and the organisms are in a dormant state. But adding warm liquid reactivates the yeast, so it will grow and multiply, and in the process raise (or leaven) dough. Proofing yeast means that you check (or prove) that the yeast is still alive and will do its job of raising the dough properly.

To proof yeast, in a small bowl, place a few tablespoons of warm water, milk or other liquid you'll use in the recipe. The liquid should not be cold or hot—it should feel like a pleasantly warm shower, or about the temperature you'd use for a baby's bottle. If it’s too hot, it may kill the yeast. Sprinkle the yeast over the liquid, along with a bit of sugar—even a small pinch will do, but you can add up to a teaspoon, depending on what you’re making. Let it sit for a few minutes. If the mixture becomes foamy or bubbles, that means that the yeast is alive and working and you can continue with the recipe. If the yeast just sits there on top of the liquid, you’ll have to get some new yeast to use in the recipe.

These days, most active dry yeast (the yeast most commonly called for in recipes) is more reliable than the yeast that was available back in the day when your recipe was probably written. But if you buy yeast in bulk or buy cake yeast, or the package you have is near or past its expiration date, you might want to proof it. (And as for scalding milk? That simply means that you heat it in a pan over medium heat until just before the boiling point, when tiny bubbles form around the edge.)