The headline above is surprising but true, and you can test it yourself: put 1 L of whipped cream on the left pan of a balance scale and a 1 L brioche on the right. The scale will tip to the left. Whipped cream has a reputation for being light and airy, but it’s about twice as dense as brioche.
The demonstration is hard to believe because it violates our expectation that a foam should be lighter than a solid. But bread is also a foam—it is just a set foam. The brioche’s crust is solid enough, but the crumb inside is mostly air.
This simple experiment illustrates that the density of bread—that is, its mass divided by its volume—is less than that of almost any other kind of food. Ciabatta, baguette, brioche, sandwich bread, and other common yeast breads typically have a density of just 0.22–0.27 g/cm3. Whipped cream, by comparison, has a density of 0.49 g/cm3. A liter of whipped cream thus weighs twice as much as a brioche of equal volume.
Bread seems denser than it is in large part because its mass is not evenly distributed: a crunchy baguette crust, which resists cutting and chewing, is 50%–100% more dense than the crumb. The crust is about as dense as pinewood (and whipped cream), whereas the density of the crumb is more like that of cork.
But if the crust is as dense as whipped cream, why does crust feel heavier? The short answer is that the chemistry of these two foams differs. To bite through bread (a set foam), you have to tear apart strong chemical bonds among adjacent molecules. But to eat whipped cream (a colloidal foam), you merely have to push adjacent particles apart.
Intuitively, you might expect that airier breads, such as a baguette, are less dense than loaves that have a tighter crumb, such as pumpernickel and other rye breads. And, in fact, that’s true, as this chart shows.
As it turns out, brick-like rye breads are more dense than red pine—and less dense a kernel of wheat. Scientific insights like this are why we find bread endlessly fascinating and fun.