Food Dork Fridays—the Science of Leavening
Simply put, a leavener is something that is either a bubble, or something that blows a bubble. The leavening in a recipe is what makes bread rise, a cake fluffy, and cookies soft.
The bubbles caused by leavening agents are a necessary part of baking because they provide the open spaces that improve the texture inside a baked good. It stands to reason that in order to churn out consistent baked goods out of a home kitchen, its important to understand a little about what leavening agents are, how they work, and why they’re important in different recipe applications.
In baking, gases must be retained until the structure of the baked good is set enough to hold its shape. Unlike liquids and solids, gases expand a great deal when they are heated. Leavening is the incorporation or production of gases in a baked good that increases the volume, shape, and texture in the finished product. If you’ve ever had a cake fall or bread fail to rise, then you have firsthand knowledge what happens when the leaveners stop working before the structure is set.
Leavening agents can be categorized into a few main categories: Air, Steam Chemical Leaveners, and Yeasts.
Air and Steam
You’d be hard pressed to find “air” and “steam” listed in the ingredients of a recipe, but the presence of both is required, and responsible for most of the leavening done in baking. Some products, such as popovers, are leavened entirely by the air whipped into the batter. Others, such as cakes or cookies, rely on a significant amount of air being incorporated into them before baking.
Steam is important to note because it is a universal leavener and natural by-product of many baking ingredients. All baked products contain some form of moisture, usually in the form of milk, water, eggs, or butter. When that moisture is heated in the oven, it evaporates and produces steam; that steam expands to 1,100 times its original volume, thus incorporating a lot of air.
Chemical leaveners are a widely used form of leavening in the home kitchen, as they give quick breads, cakes, cookies, muffins, and biscuits their rise. Chemical leaveners work by releasing gases produced by the chemical reactions between ingredients. The most common chemical leaveners called for in basic baking recipes are baking soda, cream of tartar, and baking powder.
Chemical Leaveners = Gas Producers
So using chemical leavening is all about producing gas!
Baking Soda (sodium bicarbonate) is an alkaline base, so it relies on the acid provided by other ingredients in the recipe to react with and produce carbon dioxide gas bubbles, which in turn leavens the product by giving it a little internal “lift.” We’re not talking a harsh kind of battery acid here, but rather the considerably more mild, naturally occurring acids found in common ingredients–usually in the form of buttermilk, sour cream, yogurt, molasses, or brown sugar.
Precise measuring of baking soda is necessary, because if you use more baking soda in a recipe than can be neutralized by the acid ingredient(s), you’ll wind up with a metallic-tasting baked good. We’ve all been there.
Food Dork Trivia Note: Baking soda is extracted from the trona deposits found in old mines in Montana, or from dry lake beds in California.
Cream of Tartar (potassium hydrogen tartrate) is a dry acid, and when mixed with baking soda and a liquid, produces carbon dioxide bubbles to leaven baked goods. Cream of Tartar is a white powder sold in small bottles in the spice aisle. Cream of Tartar gives a slight acidic “bite” that is desirable in some cookies; it also acts as a stabilizer when whipped into egg whites, which allows them to obtain greater volume than when whipped alone.
Food Dork Trivia Note: Cream of Tartar is an all-natural acid whose crystals are extracted from the inside of wine old barrels.
Baking Powder created by combining a base (baking soda) with a dry acid (cream of tartar) to react with, along with a moisture absorber (usually cornstarch). Since a base and an acid will react and start producing carbon dioxide bubbles upon contact with even the tiniest amount of moisture, cornstarch is added to the mixture to keep the two separate; this is accomplished by absorbing atmospheric moisture during storage, and thus preventing the premature production of gas. Once baking powder is added in a recipe, the cornstarch component acts as a wick to draw moisture to mix with the baking soda and cream of tartar components; thus activating a chemical reaction between them and causing gas to be released.
Useless Information Note: The Sons find the idea of intentionally producing or hindering gas production to be utterly hilarious. Just a little peek into my world.
“Single-Acting” baking powder requires only moisture to release gas. It’s made by combining baking soda with one dry acid and a moisture absorber. Once single-acting baking powder is mixed with moisture, it produces a bunch of carbon dioxide bubbles all at once, before fizzling out—it’s a one and done situation. If you find yourself in the midst of a recipe before discovering that you’re out of baking powder, you can make your own single-acting baking powder by combining:
- 2 parts cream of tartar
- 1 part baking soda
- 1 part cornstarch
The homemade substitution will work well enough in biscuits, however it won’t have the staying power required to produce enough bubbles to give dense batters enough rise. Heavier mixtures, like cake batter, will aerate with the big burst of bubbles created by the moisture reaction to a degree, but it won’t be enough to sustain it until the baked structure is set enough to hold shape its during a long baking time. Such thick batters require the extra-punch provided by double-acting baking powder.
“Double-Acting” Baking Powder is simply single-acting baking powder mixed with cream of tartar (a dry acid), and a second dry acid–typically sodium acid pyrophosphate—that reacts differently than the first. The cream of tartar and baking soda components react immediately with moisture, but the second acid component is slower acting, and only really starts producing gas bubbles upon being heated. The vast majority of baking powder sold in the United States is double-acting.
STORAGE: Chemical leaveners should be kept tightly closed and stored in a cool place when not in use. If left open, they can absorb moisture from the air and lose some of their leavening power. Once opened, chemical leaveners have a lifespan of six months.
Ah, yeast! Oft-maligned and vilified, yeast is the one leavener that, if given the power, can send would-be bread bakers into full-on panic mode. Friends, it doesn’t have to be that way. When handled with care and treated well, yeast can perform wonderfully delicious acts of chemical acrobatics in the kitchen. Yeast contributes both flavor and leavening to such baked goods as breads, Danish pastries, and dinner rolls.
It’s alive! Yeast cells are live creatures, and as with most living things, are sensitive to temperature fluctuations and can become less active with age.
For best results, handle yeast with temperature and age sensitivities in mind:
|34°F (1°C)||Inactive (storage temperature)|
|60° to 70°F (15° to 20°C)||Slow Action|
|70° to 90°F (20° to 32°C)||Best Growth (fermentation & proofing temperatures for bread doughs)|
|Above 100°F (38°C)||Reaction Slows|
|140°F (60°C) and Up||Yeast is killed|
|Expiration dates matter||If the expiration date is past, or even close, throw it out!|
Yeast acts as a leavener though the process of fermentation, in which it creates carbon dioxide and alcohol as a byproduct from breaking down the sugars it feeds on. Yes, alcohol is a byproduct of fermentation, but no worries! The alcohol produced during this brief fermentation completely evaporates during or after baking.
Due to the temperatures that yeast begins to be active, I store all yeast in an airtight container in the refrigerator in an effort to suppress that friskiness until I’m ready for the yeast to work for me. I don’t want my yeast to be too exhausted to work because the temperature of my kitchen is warm enough for it to start getting busy on it’s own.
Wild Yeast is an active culture present in sourdough starter. They’re active little buggers, and can sometimes be found floating through the air of prolific bread bakeries.
Fresh Yeast (a.k.a. Compressed Yeast) is moist, perishable, and preferred by professional bakers. Fresh yeast is basically a whole mess o’ live yeast cells that are compressed into a block—feisty and raring to go! When kept tightly wrapped and refrigerated, fresh yeast lasts about 2 weeks. Fresh yeast can be found at specialty baking supply shops, but due to its fragile nature, it is not usually recommend for novice bakers.
Active Dry Yeast is a dry, granular form of yeast, which must be rehydrated in four times its weight of warm water before it can be of any use. (Water between 105F and 115F is the common temperature range called for in bread recipes.) The term “active” is a misnomer because around 25% of the yeast cells found in active dry yeast are already dead, due to the harsh conditions presented in the drying process; the remaining yeast cells are is in a sort of stasis hibernation, needing to be “proofed” to reinvigorate them up before use.
Proofing is the process of soaking active dry yeast in a warm water bath with a little sugar to snack on. Proofing should perk up any dried yeast cells that may still have some life left in them, but no amount of proofing will revive that 25% that is already dead. The dead yeast can change the overall quality of dough.
Instant Dry Yeast (also called rapid-rise or bread machine yeast) is also a dry, granular form of yeast, but it does not have to be proofed in warm water before use. Nearly identical in appearance to active dry yeast, pay close attention to the package labeling that identifies the yeast as Instant.
Instant yeast comes from a hearty strain of yeast which is dried with a more gently process than active dry yeast, so with instant yeast, the little yeasty mortality rate is significantly lower. Due to the higher concentration of living yeast, instant yeast can be mixed directly in to the dry ingredients when making dough. No proofing required. Instant yeast produces more gas, and does it far more quickly than active dry yeast.
Sourdough Starters give bread a wonderful depth of flavor and desirable texture. True sourdough starters are largely wild yeast that has been raised and cultivated. Starter is a robust leavener, needing time and attention to keep from spoiling. A wild sourdough starter can live seemingly forever with good care–in fact, my favorite bakery in the San Francisco area boasts making bread from a Mother starter that is over 200 years old. A good, wild yeast starter can bring rise to bread all on its own.
As wild yeast microbes may be hard to come by in most homes, a decent starter can be created at home with commercial yeast. Such home-grown starters are really more of an active liquid culture than a wild starter, and as such, have a little trouble lifting the dough entirely on its own; therefore a little extra yeast added to the dough is necessary.
Other Yeast-Related Notables
Salt is not a leavener but does play an important role in baking. Salt does more than add flavor to baked goods; it retains moisture, slows down the yeast fermentation, strengthens gluten, and provides structure in baking applications. Inhibiting the yeast fermentation process serves to improve flavor and structure of the bread, making salt is a critical component of bread dough. Salt also acts as a preservative for baked goods, keeping them soft.
Aromatic Spices such as cinnamon, nutmeg, cloves, etc. act as antimicrobial agents and can slow down the yeast fermentation process.
If you come away with any knowledge at all about yeast, it should be this:
- Yeast isn’t scary!
- Just as other living creatures require a little tender loving care in order to thrive, yeast cells are alive and should be handled with care.
- Instant yeast is far superior to active dry yeast.
- Instant yeast can be substituted for active dry yeast in equal parts. Rise time will likely decrease.
- The right amount of salt is critical to bread baking success. Not enough salt, and the yeast will bubble unchecked, causing the bread to lack structure. Too much salt, and the yeast will take a nosedive, quitting well before the job is done.
- Aromatic spices added directly to bread dough can slow down the yeast fermentation process, thus inhibiting the rise.
- Sourdough starter adds flavor and texture to bread.
Slash That Yeast Bread Dough!
Since bread dough bakes from the outside inward, cutting shallow slashes into the top of the risen loaf before baking it serves a useful purpose. As bread bakes, an outer crust forms before the inside of the loaf is done–the interior of the bread continues to rise as the outer crust sets. As the inside of the loaf rises, it will eventually meet resistance from the upper crust, and given enough yeasty energy, the air bubbles in the dough will push the interior though the top crust to escape. If you’ve ever had a loaf of bread with a side burst open, this is why. The slashes allow the internal dough to continue to rise until it has fully baked, thus avoiding the unsightly burst loaf.