A Short Look at a Long History

• Yeast-raised breads have been made for thousands of years, but it was only in the late 19th century that French biologist Louis Pasteur successfully isolated the yeasts responsible for carrying out that task. Manufacturers quickly learned to produce pure strains of yeast, freeing bread bakers from the necessity of maintaining unreliable starters from one batch to the next. Along with high-speed flour milling, this paved the way for modern industrial bread-baking. As a side benefit, it also made life easier for home bakers.

Looking for Proof

• The first type of yeast to be mass-produced was fresh, live yeast, sold refrigerated in small cakes. It makes excellent bread but is very perishable, so it's not well suited to use by casual bakers. Active dry yeast is a more versatile product, consisting of dormant yeast cells rolled into round granules. It must be soaked in liquid before it's used, so the liquid can penetrate the protective coating of dead yeast cells and re-activate those dormant yeasts. This is referred to as "proving" or "proofing" the yeast, because until you've completed this step you can't tell whether the yeast inside is still usable.

Ready in an Instant

• Instant or quick-rising dry yeast is made in much the same way as dry active yeast, but the end result is different. It's made in much finer granules, with only a minimal protective coating. This means it can absorb as much moisture as it needs from your bread dough, without needing the preliminary step of proofing. Your bread will simply begin rising, as the yeast becomes fully rehydrated and begins to reproduce. This rapid activation makes instant yeast a good choice for quick-rising loaves prepared in your bread machine or stand mixer. Slow-rising artisanal loaves are usually best with older-style active dry yeast.

The Exception That Proofs the Rule

• Although proofing instant yeast isn't ordinarily required, you might occasionally choose to do it anyway if you aren't sure of its freshness. A forgotten package of yeast lost in the depths of your pantry, or a bag purchased from the bulk foods store, might not retain its full complement of viable yeasts. To find out for sure, measure a part of the warm liquids for your dough and stir in the sugar. Add the yeast, and wait for five to eight minutes. If your yeast is still good, it will foam lavishly in that time and give off a pleasantly yeasty smell. If it fails to foam vigorously it's old and tired, and shouldn't be used.