What makes sourdough bread rise?

Why, yeast – of course.

But how does sourdough bread rise even when there's no yeast in the recipe?

Wild yeast – the stuff that's floating in the air all around us.

After all, our frontier-settling ancestors weren't packing Fleischmann's RapidRise in their Conestogas.

They had to rely on their own homemade “brew” of fermenting flour and water, and the wild yeast it attracted: sourdough.

If you're a bread baker, you're familiar with all kinds of yeast. Your mom probably learned to bake bread with compressed yeast, a crumbly, moist yeast that comes wrapped in individual squares. Due to its perishable nature, compressed yeast has pretty much fallen out of favor with home bakers.

You yourself probably grew up with the aforementioned Fleischmann's - either RapidRise, or their classic active dry, in the bright yellow packet (or brown glass jar).

Or perhaps you learned to bake with Red Star, another active dry yeast that's been around for decades – since 1887, to be exact.

Nowadays, instant yeast is all the rage. So within the space of about two generations, we’ve moved from compressed yeast, to active dry, to instant – a category that includes bread machine yeast, and “rapid” yeast.

So, what’s the difference? Say, between between active dry and instant yeast? Or among RapidRise, instant, and bread machine yeasts?

Well, they all start with Saccharomyces cerevisiae, just one strain of the more than 1,500 identified species of yeast.

1,500 strains of yeast? But wait, there’s more – literally. Those 1,500 identified yeasts are just an estimated 1% of the yeast population in the world; most species remain as yet unnamed.

And what exactly is yeast? It’s a single-cell organism, part of the fungi kingdom. The yeast we use most often today, Saccharomyces cerevisiae, is one of the oldest domesticated organisms known to mankind: it's been helping humans bake bread and brew alcohol for thousands of years.

Used to be, there was quite a difference between instant yeast and active dry; active dry contained a greater percentage of dead cells, which "cocooned" around the live ones, making it necessary to “proof” the yeast – dissolve it in warm water – before using. This water bath dissolved the dead cells, and freed the live ones for use.

These days, active dry and instant yeasts have just about the same number of live cells. So, active dry yeast no longer needs to be dissolved before use; simply mix it into your bread dough along with the rest of the dry ingredients, just as you do instant.

SAF leads the way among instant yeast brands. Produced by France's LeSaffre company, largest yeast producer in the world, SAF Red is widely used by professionals everywhere - including the bakers in the King Arthur Bakery and test kitchens.

SAF Gold, another SAF variety, is an “osmotolerant” yeast, perfect for sweet breads, and any dough with a high amount of sugar.

How does it work? Sugar likes to absorb water; and when sugar’s in bread dough, it pulls water away from yeast, leaving the yeast thirsty. The yeast cells in SAF Gold are bred to require less liquid to live and reproduce; so they’re better able to withstand sugar’s greedy ways with water.

Next up:  RapidRise, instant, and  bread machine yeasts. Is there truly any difference?

It's widely agreed that instant yeast and bread machine yeast are the same beast. But then, the plot thickens...

We've spoken at length to representatives from Lallemand (another large yeast company), Fleischmann's, and SAF/Red Star (both brands now owned by LeSaffre). And there's no agreement, even among folks from the same company, as to whether RapidRise and instant yeast are the exact same yeast, save for their names (RapidRise is Fleischmann's trademarked name).

Having beat our collective heads against this brick wall long enough, we decided to... well, remain undecided, for now. Personally, I find RapidRise is faster out of the gate than SAF, but gives out sooner. And since I like to give my loaves leisurely rises (a long rise brings out bread's flavor), I like SAF.

That's my yeast story, and I'm sticking with it!

Now, back to our wild yeast, and the bread it produces: sourdough. Saccharomyces exiguus, one of the most common wild yeasts, flourishes in a simple flour/water medium. Put flour and water on the counter, and you'll probably see your liquid begin to bubble in a few days. That's wild yeast at work.

Unfortunately, Saccharomyces exiguus alone isn't the most effective yeast for raising bread dough. Saccharomyces cerevisiae is better at converting flour's native sugars into an easily digestible yeast food. So, while you can make bread with sourdough alone – no dry yeast – adding a dash of SAF instant or another processed yeast will certainly help things along.

These days, with all kinds of dry yeast widely available, we don't need to rely on sourdough for its leavening power. Most folks use sourdough for its rich, tangy flavor, rather than its leavening power.

Still, it's fun to make a loaf of bread using just sourdough every now and then; our Extra-Tangy Sourdough is such a loaf, if you'd like to experiment. It takes about 24 hours to make, start to finish; but it's well worth it, if you're a true sourdough aficionado.

The following recipe combines the best of both worlds: sourdough's flavor, and dry yeast's leavening power. The resulting loaves are typical crusty baguettes, with a pleasing hint of tang from their sourdough starter. Enjoy!

First, let's get our sourdough ready.

Uh-oh... looks like it's been awhile since I've fed the poor thing.

No worries. I'll just pour off most of that dark liquid...

...stir it up...

...and it's ready for a meal.

Note: If the liquid atop your sourdough is pinkish; or if it smells bad – “off,” rather than fresh, tangy, and alcohol-like – it may have become infected with harmful bacteria. Best to discard it, and build yourself a new starter.

Transfer the starter to a bowl, so you can wash out its container.

If you like, discard 1 cup of starter; this will control the amount of starter you're dealing with. If your starter is scanty and you're trying to build up the amount, there's no need to discard.

Or, if you do discard - yet don't want to simply “discard” - use that extra cup of starter to make Sourdough Waffles, the best waffles you'll ever taste.

Add 1 cup flour (King Arthur Unbleached All-Purpose, of course) and 1/2 cup lukewarm water to the remaining starter. Notice this is equal parts flour and water, BY WEIGHT.

Stir to combine.

Cover, and let rest for a minimum of 4 hours, or for as long as 12 hours.

Your goal is an actively bubbling starter, so give it as long as it needs.

Remove 1 cup of starter for your baguette recipe, and put the remainder back in its crock. Store it in the fridge till next time.

At last! Let's make baguette dough. Put the following in a bowl:

1 cup fed sourdough starter
1 1/2 cups lukewarm water
1 teaspoon instant yeast
1 tablespoon sugar
2 1/2 teaspoons salt
5 cups (21 1/4 ounces) King Arthur Unbleached All-Purpose Flour

Mix to make a rough dough...

Then knead till smooth. You'll find this dough is particularly silken.

And look at that gluten development!

Allow the dough to rise, in a covered container, for 1 hour.

You should see it gain a bit of volume.

Refrigerate overnight, or for up to about 18 hours.

Whoa! Now THAT'S a nice rise!

And look how beautiful the dough is – still so silky smooth.

Next, divide the dough into six equal pieces; a scale makes the task easy.

Shape each piece into a rough cylinder. Cover the pieces of dough, and let them rest for about 10 minutes. This will relax the gluten, making them easier to shape.

Working with one piece of dough at a time, gently flatten it.

Fold in half lengthwise...

...and seal the seam, using the side of your hand.

Repeat the process, flattening the dough, folding over, and sealing.

You'll have a loaf that's already about 10” long. Gently roll it under your cupped fingers to a loaf about 12” long. Put the loaf on a parchment-lined or lightly greased baking sheet.

Repeat with the remaining pieces of dough, using two baking sheets.

So OK, they're not perfect, looks-wise. But beauty is only crust-deep!

Cover the pan, and let the loaves rise until they're very puffy, about 3 hours.

Towards the end of the rising time, preheat the oven to 425°F.

The risen loaves should look about like this.

Spray the loaves with lukewarm water...

...and, holding a very sharp chef's knife or heavy serrated knife at a 45° angle to the dough, make three diagonal slashes.

Be aggressive enough to make a deep cut.

Once you've slashed the loaves, don't fool around. See how they're deflating? You want to get them into the oven immediately.

After just a minute or so in the oven, you can see them picking right up.

Bake the loaves for 25 to 30 minutes, until they're a deep golden brown.

Like this. They'll probably be slightly flat, rather than perfectly cylindrical.

For rounder, more shapely loaves, use a baguette pan.

Nicely risen...

...slashed, and into the oven they go.

30 minute later - fini!

Six lovely baguettes.

Look at the top vs. bottom crust; love that blistering, don't you?

Here's the difference in shape between baking baguettes freeform, on a baking sheet (left); and using a baguette pan (right).

Can't you just hear that crust crackle as you tear into a hot baguette?

Nice crumb, eh?

Read, bake, and review (please) our recipe for Wild Yeast Baguettes.

One final word – our yeast video is a great live-action comparison of the various yeasts discussed here.

Filed Under: Recipes
PJ Hamel
The Author

About PJ Hamel

PJ Hamel grew up in New England, graduated from Brown University, and was a Maine journalist before joining King Arthur Flour in 1990. PJ bakes and writes from her home on Cape Cod, where she enjoys beach-walking, her husband, two dogs, and really good food!