Why Does Baking Bread Smell So Good?

Here’s a fun thing to try: stand outside a bakery on an early summer morning, and watch how people react to the smell of baking bread wafting out the door as they walk by. Their heads turn, their noses lift, their eyes close . . . It’s only a matter of time until someone says, “Oh my God—that smells good!”

What is it about the aroma of bread in the oven that is so irresistible? Yes, for many people, the odors evoke powerful, pleasant memories of childhood. But even people who grew up on plastic-wrapped, essentially aroma-free Wonder Bread break into contented smiles when they enter a bakery while the ovens are going. The reason has as much to do with chemistry as it does with psychology.

We can get some clues as to where the aromas originate by considering wheat products that don’t smell quite as good. Wheat pasta, for example, has essentially no odor when boiled, and not much even when baked—that heartwarming aroma from a baked lasagna comes mainly from the sauce, cheese, and meat, not the noodles. Most unleavened crackers don’t do much for the nose, either.

But chemically leavened baked goods such as biscuits and muffins (made with baking soda and baking powder rather than yeast) can smell very tempting once they start to brown. The color change is a sure tip-off that Maillard reactions are happening. These reactions—in which sugars combine with amino acids to form tasty golden and umber complexes— throw off lots of volatile aromatic compounds that float through the kitchen air and into your nostrils.

Recipes for biscuits and muffins almost always call for added sugar of some kind: the lactose in buttermilk, the fructose in fruit, the dextrose in corn, or even crystals of sucrose sprinkled into the mix. Added sugars help kick-start Maillard reactions.

Another, even better way to generate pleasant aromatic compounds such as ethyl esters (ethyl acetate, hexanoate, and octanoate) is to leaven the flour with yeast. As a by-product of the microbes’ metabolic processes, the yeast cells produce chemicals that break down during baking into delicious-smelling aromatics. The longer the fermentation, the more pronounced the yeast flavors become since the microbes have more time to produce these compounds.

We have tried baking the same bread recipe with and without yeast, and the yeast bread develops a far more complex flavor profile. A big part of the difference is how much better yeast bread smells. The unleavened bread also doesn’t brown nearly as well. Thanks to yeast, your dough is stocked with amino acids that are an integral component of Maillard and other browning reactions.

So the next time you have a loaf in the oven and your kitchen smells like heaven, you have the tiny yeasts to thank.

Is Fresh Yeast Best?

Yeast—living, single-celled fungi—is one of the main reasons bread is so complex and special. These microbes behave like miniscule factories that specialize in the production of bubbles and booze by way of a process called fermentation. In addition to leavening dough, fermentation makes important contributions to the aroma, flavor, and texture of bread.

The yeast used to create bread can be commercially derived, or it can be cultivated from the environment around us in the form of a levain. Using a levain is considered to be the very definition of fermentation by some bakers who dismiss commercial yeast (also known as baker’s yeast) as not producing “real” fermentation. We reject that view; fermentation is fermentation, whether it involves levains or commercial yeast. One method is not more legitimate than the other. The fermentation method you choose depends heavily on your schedule, ability to plan ahead, and yeast preference.

Commercial Yeast

When it comes to commercial yeast, there’s an ongoing debate as to which type of yeast is best for baking bread: active dry, instant, or fresh. The main issue doesn’t seem to be about the “power” of the yeast but rather an unspoken stigma that persists for each kind. You might have heard that “fresh is best,” but in truth, yeast is yeast is yeast—Saccharomyces cerevisiae to be specific.

S. cerevisiae is a fermentation superstar—the species is used by bakers, brewers, and vinters, although the strains that they work with differ. Strains are often isolated, grown, and stored in tightly controlled conditions so that they are best adapted for particular situations, such as making a sourdough, a French bread, an ale, or a champagne. That means that you probably won’t get great results if you try to make bread using a strain developed for brewing beer or winemaking.

At this point you might still be wondering what type of commercial baker’s yeast you should use. If a baker uses the right techniques, there is no reason to use fresh yeast over instant yeast—in a lineup of baked loaves, you’d be hard-pressed to distinguish one from the other in terms of the yeast used. We like working with instant yeast rather than fresh yeast or active dry yeast. After you read our explanations of the differences between the three forms, you will be better equipped to make your own choices.

Fresh Yeast

Developed in the mid-19th century, fresh yeast is the oldest commercial form of yeast. It was originally sold as a cream of yeast mixed with a mash, which served as a growth medium. Fresh yeast is more commonly sold today in blocks of cake or compressed yeast that resemble crumbly, cream-colored modeling clay. Fresh yeast must be dissolved into a liquid but easily does so, dispersing efficiently throughout the dough, which is a plus.

Each gram of compressed yeast contains roughly six billion active yeast cells. Fresh yeast has the highest moisture content of any form of baker’s yeast, but also the shortest shelf life. Blocks require refrigeration and last for only 2–3 weeks after opening. Fresh yeast is highly perishable, a considerable drawback that can cause issues in bakeries as well as home kitchens. At the bakeshop, fresh yeast is likely to sit on the bakers’ worktable for hours while they mix many doughs. The warmth of the bakery will activate the yeast, and it will eventually die because it has nothing to eat. The home baker who buys a pound of fresh yeast must bake frequently to use it all up before it dies. The challenges that come with fresh yeast eventually sparked the next wave of yeast innovation: dried yeast.

Active Dry Yeast

Dried yeast was developed during the Second World War by Fleischmann Laboratories so that United States field infantrymen could bake fresh bread in their camps. The new active dry yeast was not as perishable as fresh yeast and therefore did not require refrigeration and had a longer shelf-life.

Dried yeast is an inert substance when you purchase it, but it becomes a living, thriving colony of microorganisms with the addition of some water and food. During the production process, water is removed from the yeast cells, reducing the moisture content from around 82% to 8% in the case of active dry yeast. The desiccation sends the cells into a state of dormancy. Particles of dormant yeast are coated with a protective layer of dead yeast cells to form tiny granules, which are then packaged for sale. Unlike fresh yeast, unopened packages of dormant, active dry yeast can be frozen for months.

Active dry yeast is more convenient than fresh yeast, but it still requires some additional work and comes with its own set of drawbacks. The dormant cells must be reactivated before use, which can be done by stirring the granules in lukewarm (40–43 °C / 104–109 °F) water. Active dry yeast is temperature sensitive—water that is too hot or too cold can damage or kill the cells, reducing the fermentation power of the yeast.

Around 25% of the yeast cells are killed during the production process, which means that active yeast has, ironically, the lowest amount of active yeast (by weight) of either fresh or dry varieties. Thus, more of it must be added to a recipe than other types of yeast. Dead yeast cells also leach a self-produced chemical called glutathione that relaxes dough. Small quantities of glutathione can be beneficial, depending on the dough, but it can quickly make dough become so relaxed that it’s difficult to handle. Active dry yeast is slower to ferment than both instant and fresh yeast. It needs to proof longer to achieve the same results as the other forms of commercial yeast; the time required will depend on the environment and amount of yeast in the dough. Still, smaller quantities of active dry yeast are often the only option available at supermarkets, which is likely why it’s still commonly used in home baking.

Instant Yeast

Instant yeast, also called quick yeast, was developed in the 1970s by French manufacturer Lesaffre. Like active dry yeast, instant yeast is sold as desiccated granules; it is even drier than active yeast, having a moisture content of just 5% or so. The difference is that instant yeast ferments faster, does not require activation, and is less sensitive to water temperature.

So why do we prefer instant yeast over all other forms of commercial yeast? For starters, instant yeast is truly instant—it does not need to be activated; although we prefer to bloom it, you can add it directly to your dough—and, as soon as it comes into contact with moisture, it will begin the fermentation process.

Instant yeast is made with a fast-acting strain of S. cerevisiae, and the noodle-shaped granules are finer than those of active dry yeast. The surface layer of dead cells is more porous than that of active dry yeast, which allows the granules to rehydrate more rapidly. During production, instant yeast is quick-dried, a process that produces significantly more living yeast cells. As a result its leavening power more closely resembles that of fresh yeast. Manufacturers add salts of fatty acids to the yeast to control rehydration and boost the yeast’s gassing power. The moisture content is lower, which increases the shelf life to 2 years in its vacuum pouch, or even longer when refrigerated. Once the package is opened and exposed to oxygen, instant yeast remains active for 1 year if it’s refrigerated after being opened—it’s the trade-off of the more porous surface. Compared with the active dry form, the instant variety produces more gas during fermentation.

Instant yeast is also available in a number of forms; the one you choose will depend on the type of dough you make. For example, enriched doughs with larger proportions of sugar require osmotolerant yeast; osmotolerant instant yeast requires less water than the instant yeast used in lean doughs. So instant yeast offers options you don’t have with active dry yeast, along with added convenience.

You’re likely to encounter an occasional bump in the road in the road when your local grocery store or purveyor only has one type of yeast on hand. Accidents happen and it’s all too easy to get the wrong package of yeast in a rush. Fortunately, this is one bread-making problem that’s easy to fix. By giving the yeast proper care and employing some basic math, you can use any form of yeast successfully. You’ll find our own conversion table on page 10 (volume 3) of Modernist Bread.

Visit modernistbread.com to learn more about yeast and bread-making.