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Word of the Week: Glass

By Paul Adams Published

There’s more to glass than meets the eye.

Working at the intersection of food and science, we come across a LOT of scientific jargon and terminology. We’ll be sharing many of these fun words with you in this ongoing series, Word of the Week. Get ready to impress at your next cocktail party.

Pop quiz: What do all these items have in common? The crackly crust on fresh-baked bread. Lollipops and other hard candies. The crisp skin of roast chicken. Crunchy, puffy Cheez Doodles. Crackly breakfast cereal. Your kitchen window.

If you said “they’re all delicious,” you just might be this kitten I used to know who loved licking windowpanes. Partial credit for that! But the answer we’re looking for is: On a molecular level, those are all forms of glass. That’s why they crunch and shatter.

A glass, by definition, is a solid substance whose molecules are amorphous, all pushed together indiscriminately. In that way, it has more in common with a liquid, full of disordered swirling molecules, than with a crystalline solid such as salt, in which all the molecules are neatly ordered, each facing the same direction and tucked closely together like bricks in a wall.

The world—including the edible world—has many different glasses. Silicate glass, the stuff we drink wine from, wear to correct our eyesight, and hurl bad guys through in action movies, is just one of them.

Where do glasses come from? A soft substance, or a viscous liquid, will form into a glass under particular circumstances; typically if it’s heated—which removes moisture and mobilizes the molecules into an amorphous state—then cooled rapidly.

Hard candies are made by heating a sugar syrup above 300 degrees F/148 degrees C, to remove all of the water; then quickly cooling it till it solidifies, molded into whatever shape you please. The key is for it to cool quickly. If the sucrose molecules have time to carefully arrange themselves into crystals by the time the candy solidifies, you’ve got a crumbly candy filled with crystalline grains (think rock candy). But if they’re still disorganized when it becomes solid, you get a perfect glossy glass. For this reason, also, candymakers often use a variety of different types of sugar—sucrose, fructose, glucose—in a single recipe. In a candy made of all identical sugar molecules, the molecules nestle sweetly together into gritty crystals very readily, but if the molecules have slightly different shapes, they don’t crystallize as easily.

Other edible glasses are made in the same basic way, as far as physics is concerned. Raw chicken skin has a texture that science calls “rubbery.” As it’s heated, it loses water, and the proteins in it unravel and form a gel. Once out of the oven or fryer, the molecules quickly cool and set into, yes, a glass.

The same gel-setting process happens with other things we like to crunch, like starchy snack foods and crusty bread. The water in a bread dough keeps it soft and pliable, but as it’s heated, some of the water evaporates from the outside of a loaf. At the same time, the starch molecules loosen their holds on each other, a process called gelation. These jumbled molecules are ready to change to a crackly, glassy crust as they cool.

The amorphous nature of glassy materials, with random gaps between the molecules, means that they’re inherently not as stable as other states. Over time, especially in the presence of warmth and humidity, a glass tends to either grow internal crystals—which is why those butterscotch candies that have been in your purse for months are sticky and crumbly—or absorb water and lose its rigidity, which is why the crust of last week’s rustic loaf has a leathery chew.

The next time you crunch into a hot, crisp French fry, snap a ginger snap, or whack peanut brittle till it shatters into manageable pieces, take a moment to appreciate its special, delectable texture. Raise a glass!

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Graphics by Jay Layman

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