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The Wonders of Baker’s Yeast

Among life’s simplest joys: smelling freshly baked cinnamon rolls wafting through the kitchen, sliding the tray of artfully coiled pastries from a warm oven, and marveling at their golden crust and fluffy interior. An ideal cinnamon roll features a potent cinnamon-sugar mixture oozing in sticky spirals. It’s often topped with a generous smear of tangy cream cheese icing that’s tempered with notes of orange peel and vanilla, sweet and rich enough to catapult you back to childhood. While the filling and icing are notable qualities, what really makes or break a cinnamon roll is its texture. Cinnamon rolls may simply serve as a vehicle for sugar and icing, but their bready foundation boasts an often-understated value. Imagine greedily lunging for a roll and biting into it, only to discover that it’s a rock-hard spiral of disappointment, instead of an airy and delicate pastry with a tender crumb. The science behind the texture of a perfectly fluffy cinnamon roll lies in the yeast.

Photo credit: Mai Nguyen

When you’re browsing the baking aisle in the grocery store, you may be overwhelmed or confused by the sheer number of different forms of yeast available—you’ll find loose granules in packets and jars, bricks, discs, and fast-rising, instant, or active dry. Despite the multitude of forms the yeasts can come in, they’re all merely purified and processed versions of the same organism. Saccharomyces cerevisiae, or baker’s yeast, is a microorganism used in professional and home kitchens alike primarily as a leavening agent for baked goods (1).

The three most commonly/commercially available forms of yeast are:

  • Caked yeast: This moist block consists of fresh, living cells that are packed tightly together. This form of yeast shows substantially higher leavening activity than its dried forms. Caked yeast is highly perishable and has a shelf life of only one to two weeks. More commonly, you’ll encounter yeast granules in packets or jars, widely available as active dry or instant.
  • Active dry: Active dry is a granular form of yeast that has been dried at high temperatures. These granules are comprised of yeast clusters that are encapsulated in a protective coating of yeast debris that formed on the surface of the granules during the drying process. These yeast cells are dormant and need to be rehydrated in warm water before being used. Simply sprinkle the granules in warm water (around 110°F), stir, and wait five to ten minutes. Water will dissolve the protective coating surrounding the granules, releasing the revived yeast cells from within. As the yeast become active, you should see a foamy layer of bubbles forming at the surface, which is carbon dioxide being released.
  • Instant rapid-rise yeast: Boasting higher viability and increased CO2 production, instant rapid-rise yeast is dried at more gentle temperatures than active dry, so more yeast cells survive this drying step. Bakers can add instant rapid-rise yeast directly to the flour, eliminating the need for prehydration. Because instant rapid-rise yeast produces carbon dioxide more vigorously than active dry yeast, these two forms of yeast should not be used interchangeably.

Granules of active dry yeast
Photo credit: Mai Nguyen

Instant rapid-rise yeast
Photo credit: Mai Nguyen

How does this tiny organism transform a dense blob of dough into a puffy masterpiece? To harness its leavening power, we rely on the phenomenon of fermentation. In the first steps of bread baking, water, yeast, flour, and salt are combined. Kneading hydrates the flour and after just a few minutes of manipulation, the dough becomes noticeably stretchier and more pliable. Water enables individual protein molecules in the dough, glutenin and gliadin, to link together to form long, elastic chains of a protein called gluten. These individual gluten strands combine to form a mesh-like network which gives bread its structure and chewy texture (2). Meanwhile, the addition of water also activates enzymes in the flour known as amylases which break down the flour’s starches into simple sugars, providing food for the yeasts (3).

The yeasts feed on these simple sugars and convert them into ethanol and carbon dioxide gas (CO2). This is where the magic begins. As carbon dioxide is released into the dough, it becomes trapped in the gluten matrix. As more and more CO2 bubbles form, the protein network stretches, inflating the dough. Depending on the recipe, dough can spend between an hour to several days rising and can expand two to four times its original size. This initial rising step is often referred to as bulk fermentation.

Like many other types of yeasted breads, a classic yeast-based recipe for cinnamon rolls calls for two rising steps. After the dough has been kneaded and has undergone bulk fermentation, it’s time to roll out the dough and shape it to prepare it for the second rising step, known as proofing. Many recipes for yeasted breads will instruct you to “punch down” dough after the initial rise. In this step, we turn and fold the dough, fill it with a cinnamon-sugar mixture, shape it into coils, and allow them to rise into bloated versions of their former selves (2). This “punching down” or turning step serves a couple of purposes: it stretches the gluten and expels excess CO2 buildup trapped in the dough from the bulk fermentation step, which can inhibit any further yeast activity. Handling the dough at this stage also redistributes yeast, moisture, heat, and sugars throughout the dough for optimal lift and flavor.

A noteworthy point: while our goal is to encourage yeast proliferation and to optimize the production of CO2 and flavor molecules, bakers should be cautious of overfermentation. If yeast fermentation happens too rapidly or continues for too long, gas bubbles can overinflate and burst, causing our dough to collapse (3). The excess of CO2 can also cause the yeast to leave behind many unwelcome tasting flavor compounds and the bread may end up tasting like alcohol.

In our final phase, our twice-risen dough is placed into the oven. Once inside, the dough experiences one last rise thanks to the high heat. The heat causes CO2 present in the dough to expand and for about the first ten minutes in the oven, the rising temperatures stimulate a rapid burst of activity in the yeast, causing them to produce even more CO2. Water and ethanol byproducts in the dough will also expand during heating. This causes the bread to rise dramatically in the oven a phenomenon known as oven spring (3). Eventually, the CO2 and alcohol are expelled from the bread and the yeast cells succumb to a dry, hot death once temperatures exceed 140°F (2).

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Photo credit: Mai Nguyen

Behind a cinnamon roll—or any kind of yeast bread —lies an intricate chemistry involved in its creation. Without the wonders of yeast and fermentation, bread wouldn’t exist as we know it today.

References Cited

  1. McGee, Harold. On food and cooking: the science and lore of the kitchen. New York: Simon & Schuster, 1997. Print.
  2. Crosby, Guy. The Science of Good Cooking. Brookline, MA: Cook’s Illustrated, 2012. Print.
  3. Bernstein, Max. “The Science of Baking Bread (And How to Do It Right).”Serious Eats. 1 Oct. 2014. Web. 11 Aug. 2015.

Mai NguyenAbout the author: Mai Nguyen is an aspiring food scientist who received her B.S. in biochemistry from the University of Virginia. She hopes to soon escape the bench in pursuit of a more creative and fulfilling career.

Read more by Mai Nguyen


Zoe Nathan

Zoe Nathan is the co-owner of several Los Angeles restaurants, including Huckleberry Bakery and Café and Milo and Olive. An avid baker, Zoe honed her craft at Tartine in San Francisco where she learned the value of using color as a flavor. At her own restaurants, she has received widespread acclaim for her pastries.

See Zoe Nathan speak at our next 2013 Science & Food public lecture!

The Science of Pie
Featuring Chefs Christina Tosi and Zoe Nathan
Sunday, May 19 @ 2:00pm
Covel Commons Grand Horizon Room (map)
BUY TICKETS

Image credit: Emily Hart Roth

Image credit: Emily Hart Roth

What hooked you on cooking?
I wanted to do something with my hands and was searching for a way to express myself in a way that I could connect with people around me instantly. I loved being able to make something and have someone eat it right away and hopefully enjoy it and understand where I’m coming from. Plus it just makes me really happy!
The coolest example of science in your food?
For me it’s the process of baking. Working with so few ingredients, and then deciding on different processes that will create totally different things to eat. That’s why I never worry about someone stealing a recipe from me, because at the end of the day, it’s not knowing what goes into baking something that makes it special, it’s how you bake it.
The food you find most fascinating?
Bread. For exactly the same reason as above. It’s all about process. That’s why I laugh when people say, “You can only make great Sourdough in San Francisco, or Bagels in New York,” but then I see people try with bad ingredients and a sloppy process. If you care enough you can make great bread anywhere.
One kitchen tool you could not live without?
Mixing bowls!
What scientific concept–food related or otherwise–do you find most fascinating?
The concept I find the most important in baking is the process of caramelization. You can use all the right ingredients and even the right process, but if you don’t get the right caramelization and color on a bake good it simply doesn’t look or taste good.
Five things most likely to be found in your fridge?
Eggs, kale, milk (I have a 2 year old), Dijon mustard, cream cheese.
Your best example of a food that is better because of science?
I’m not a big fan of modern science in cooking, but I’m super happy to have freezers so that I can freeze my scones and biscuits so that I can put the maximum amount of butter inside without having it leak out. I’m happy for convection ovens so my baked goods get that extra little jump. I’m happy for steam on my bread oven so my bread gets that nice shine. I’m also happy for bright lights so my bakers can come in at 3 a.m. and still feel safe!
Your all-time favorite ingredient? Favorite cookbook?
Salt is my all-time favorite ingredient. I have so many cookbooks that I love I can’t choose one.
How do you think science will impact your world of food in the next 5 years?
Honestly, I think it’s mostly negative. I think a lot of people eat processed foods because they’re easier to get because science has made them taste good and last a lot longer than it actually should. Because of all the big advancements in technology people are also used to getting what they want quickly, but good cooking is a patient thing, so I think fewer and fewer people know how to cook. I also think young cooks who are obsessed with immersion circulators and cvap machines often don’t know how to cook a piece of meat on a grill or in a pan which is a shame because that’s how it tastes best.
Your standard breakfast?
Leftovers from whatever my son hasn’t eaten and he eats pretty well. When I actually take the time to make it for myself it’s oatmeal cooked in homemade almond milk.

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Scientists study a vaccine to cure celiac disease, while Mark Bittman shares his tips and tricks for making whole-grain breads at home. (Hint: you’ll want to get out your food processor!) Read more