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.