December 1, 2006 Food scientists have discovered why Rice Krispies make their characteristic sound when soaked in milk. Rice Krispies contain lots of sugar and are cooked at high temperature, which makes the sugar form crystals and creates air-filled cavities. When a Krispie absorbs milk, the capillary forces push the air to shatter the cavities' walls -- and make a noise. With the exception of pop rocks candy, it is the only food that acts this way.
MINNEAPOLIS -- Snap, crackle, pop! Does your cereal talk to you? Rice Krispies -- the breakfast that talks to you -- has been around for nearly 80 years, but scientists have only recently figured out why it makes that noise.
Food scientist Ted Labuza, says there are a few reasons.
"Rice Krispies has sugar in it. It's the sugar that causes it to interact in the different way with the starch, and that makes a big difference," Labuza, who is professor of Food Science and Engineering at University of Minnesota in Minneapolis, tells DBIS. In fact, sugar is the second ingredient in the Krispies. It's much lower on the list in other cereals.
Another reason? Rice Krispies are cooked at such a high temperature, the sugar forms crystals that behave almost like glass. If you crush them, they'll break into pieces.
During the cooking process, each piece of rice expands, and tiny air-filled caves form inside. The Krispies' bubbles are about 20-times bigger than those in puffed rice cereal.
Here's what really happens inside: The Krispie absorbs the milk and the air-filled caves become filled with liquid. Air is then pushed around until the walls shatter and make a noise. And that's what you hear!
Labuza says the Krispies probably make different sounds because of the different sizes of the air pockets. They stop making noise after they pick up all the milk they can handle. He says this is the only food that acts this way -- with the exception of Pop Rocks candy. But never mind why it happens. Some of us only care how it tastes!
BACKGROUND: Food scientist Ted Labuza at University of Minnsesota has studied his morning bowl of Rice Krispies cereal, and can explain why it snaps, crackles and pops. It's similar to how popcorn pops, but at the molecular level, Labuza finds that the cereal actually behaves like glass. Rice Krispies feature strong molecular bonds to hold the starch molecules together. Just like glass, if you smashed a rice crisp with a hammer, it would crack and shatter.
WHAT'S GOING ON: Labuza says that the signature snap, crackle and pop of Rice Krispies is the result of the cooking process. Grains of rice are steamed and then oven-popped to give them their unique texture. Heating up the rice grains causes the starch granules inside to expand, creating a network of tiny air-filled pockets and tunnels inside the kernel. Add milk, and the cereal starts to absorb the liquid. This puts pressure on the air inside the pockets, causing the "walls" to shatter with a crackling sound. When the cereal becomes saturated and soggy, the crackling sound stops.
ABOUT GLASS: Glass is an unusual substance that straddles the boundary between a solid and a liquid; scientists call it an "amorphous solid." In a solid, molecules are arranged in a precise lattice structure; in liquids the molecules are more disordered rather than rigidly bound, so the substance can "flow." Glass molecules are rigidly bound, as in a solid, but they are still more disordered than the molecules in a crystal. This unusual state arises from how glass is made: by cooling a liquid below its freezing point, then cooling it some more. Cool the liquid fast enough and the molecules don't have time to arrange into a solid lattice structure. Instead, the liquid becomes more "viscous" – resistant to flow. The molecules gradually move more and more slowly, until they are hardly moving at all, giving glass its solid characteristics.