Yeasts are among the most common microorganisms on the planet, in the air everywhere. One family of natural yeasts digests the natural sugars in grain and converts them into varying proportions of alcohol and carbon dioxide, depending whether beer or bread is the desired end result. Yeast was first purified in the 19th century, but keeping the tiny organisms alive was sometimes problematic. Cakes of fresh yeast quickly deteriorate and lose their leavening power, especially if they're carelessly handled. Active dry yeast is better suited for long storage, though it also loses potency over time.

Expending the time and effort to mix bread dough, only to find that the yeast was dead, was even more frustrating in the days before powerful stand mixers and bread machines. In self-defense, bakers adopted the habit of testing their yeast to prove its viability. The yeast must be crumbled or stirred into either a cup of warm milk, or a cup of warm water with a small amount of sugar added. If the yeast is fresh, it will foam vigorously in its warm bath as the microorganisms wake from their dormant condition and begin to eat and reproduce. This is proof that the yeast is alive and potent, so the process is often called "proofing."

Milk contains natural sugars such as lactose, so it can support a lively growth of yeast without any added table sugar. Fresh or cake yeast should react and begin to grow immediately, as soon as it's well moistened by the warm milk. Active dry yeast can take 10 to 15 minutes to reach its full potential, because the granules of dormant yeast are surrounded by a protective dry coat of dead yeast cells. That outer layer must be fully hydrated and dissolved by the milk before the living yeast cells inside can begin to feed and reproduce.

Bread bakers are sharply divided over whether it's still necessary to proof yeast. Improved manufacturing and packaging processes have greatly improved the reliability of dry yeast, and most retailers manage coolers and expiration dates well enough for fresh cake yeast to give predictable results. As a result, many bakers skip the proofing step and add the yeast directly to their dough. However, if you don't know the age or origin of the yeast you're using, proofing is a relatively quick and simple step that can save you a measure of frustration later.