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" ý resistant to flow. The molecules gradually move more and more slowly, until they are hardly moving at all, giving glass its solid characteristics.