Why Does Baking Bread Smell So Good?

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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.

Cooking Under Pressure: Pressure Caramelized Sweet Potato Soup

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Just in time for winter, we decided to develop a new seasonal variation of one of our Modernist Cuisine traditions: Pressure Caramelized Sweet Potato Soup. The recipe for this magical soup incorporates black peppercorns to give it a nice zip, and hints of sweet onion and Makrud leaves complement the caramelized sweet potato stock.

 

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The charm of this soup is twofold: the elevated temperature of pressure-cooking coupled with an alkaline environment ensure that caramelization reactions will flourish.

Vegetables are made up of cells with strong walls that soften at higher temperatures than the cells in meat do. Vegetables are composed mostly of water, however, and their temperature normally won’t exceed the boiling point of water (100˚C/212˚F) until they are dried out. Vegetables in a fully pressurized cooker don’t dry out as they quickly become tender under higher temperatures (120˚C/250˚F). And because the air is sealed in, you don’t need to add much water, so juices are extracted without becoming diluted.

Add to this a pinch of baking soda to bring the soup to a more alkaline pH of about 7.5 and you have ideal conditions for Maillard reactions to commence. The result is a gorgeously colored soup that is the concentrated essence of caramelized sweet potato.

 

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We like to finish our soup with purple sweet potato confit, roasted chestnuts, and toasted marshmallows. The purple sweet potatoes add a brilliant dash of color, and toasted marshmallows add a touch of tradition and whimsy. This soup is the perfect way to begin special dinners this holiday season.

Pressure-Caramelized Sweet Potato Soup

The Maillard Reaction

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One of the most important flavor-producing reactions in cooking is the Maillard reaction. It is sometimes called the “browning reaction” in discussions of cooking, but that description is incomplete at best. Cooked meats, seafood, and other protein-laden foods that undergo the Maillard reaction do turn brown, but there are other reactions that also cause browning. The Maillard reaction creates brown pigments in cooked meat in a very specific way: by rearranging amino acids and certain simple sugars, which then arrange themselves in rings and collections of rings that reflect light in such a way as to give the meat a brown color.

The important thing about the Maillard reaction isn’t the color, it’s the flavors and aromas. Indeed, it should be called “the flavor reaction,” not the “browning reaction.” The molecules it produces provide the potent aromas responsible for the characteristic smells of roasting, baking, and frying. What begins as a simple reaction between amino acids and sugars quickly becomes very complicated: the molecules produced keep reacting in ever more complex ways that generate literally hundreds of various molecules. Most of these new molecules are produced in incredibly minute quantities, but that doesn’t mean they’re unimportant.

The Maillard reaction occurs in cooking of almost all kinds of foods, although the simple sugars and amino acids present produce distinctly different aromas. This is why baking bread doesn’t smell like roasting meat or frying fish, even though all these foods depend on Maillard reactions for flavor. The Maillard reaction, or its absence, distinguishes the flavors of boiled, poached, or steamed foods from the flavors of the same foods that have been grilled, roasted, or otherwise cooked at temperatures high enough to dehydrate the surface rapidly — in other words, at temperatures above the boiling point of water. These two factors, dryness and temperature, are the key controls for the rate of the Maillard reaction.

High-temperature cooking speeds up the Maillard reaction because heat both increases the rate of chemical reactions and accelerates the evaporation of water. As the food dries, the concentration of reactant compounds increases and the temperature climbs more rapidly.

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Temperatures need to be high to bring about the Maillard reaction, but as long as the food is very wet, its temperature won’t climb above the boiling point of water. At atmospheric pressure, only high-heat cooking techniques can dry out the food enough to raise the temperature sufficiently. It’s not the water that stops the reaction, but rather the low boiling point at normal, sea-level pressure. In the sealed environment of a pressure cooker, the Maillard reaction can, and does, occur. This is something we exploit when making soups, like in our Caramelized Carrot Soup, or purees, like the broccoli puree in our Brassicas recipe. Adding baking soda to the pressure cooker raises the food’s pH (making it more alkaline), which also helps. Chinese cooks often marinate meat or seafood in mixtures containing egg white or baking soda just before stir-frying.

So, in boiled, poached, and steamed muscle foods, an entirely different set of aromas dominates the flavor. Drying and browning the surface first will, however, allow the reaction to proceed slowly at temperatures below the boiling point of water. This is why we sear frozen steak before cooking it in a low-temperature oven. Searing food before vacuum sealing and cooking sous vide can add depth to the flavor of sous vide dishes. This step should be avoided for lamb, other meats from grass-fed animals, and a few other foods in which presearing can trigger unwanted reactions that cause off-flavors and warmed-over flavors to form when the food is later cooked sous vide. We recommend searing those foods after cooking them sous vide.

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One of the challenges to getting the Maillard reaction going is getting the surface hot and dry enough without overcooking the underlying flesh, or at least overcooking it as little as possible. Cooks have developed several strategies to this end, some simple and some fairly baroque.

One strategy that works well is to remove as much water from the surface of the meat as possible before cooking it (via blotting or drying at low temperature). Fast heating using deep fryers, super-hot griddles and grills, and even blowtorches are also helpful tactics, such as when we deep-fry chicken wings.

You might think that raising the temperature even higher would enhance the Maillard reaction. It does up to a point, but above 180 °C / 355 °F a different set of reactions occur: pyrolysis, also known as burning. People typically like foods a little charred, but with too much pyrolysis comes bitterness. The black compounds that pyrolysis creates also may be carcinogenic, so go easy on charring your foods for visual appeal.

Adapted from Modernist Cuisine