What you're hearing is the sound of those toasted bubbles breaking from the pressure of the milk as it pushes air against those fragile baked grain walls that subsequently shatter. The cereal may have begun as a breakfast food, but about 15 years later Rice Krispie treats, possibly equally as iconic as the cereal itself, got their start.
The sweet snack was originally created for a Camp Fire Girls fundraiser by a cook in the Kellogg Cereal Company test kitchen.
The recipe was published in and has become an instant classic and beloved treat thanks to the crunchy rice cereal and gooey marshmallow that holds it all together.
The original recipe is solid, but variations have kept the treat moving with the times. Give your next batch a grown-up twist with aromatic coffee and the tart fruity bite of dried cherries.
For a simple variation, substitute chopped apricots for the cherries and omit the coffee. Or combine classic Rice Krispie cereal with colorful Fruity Pebbles for a confetti effect.
Skewering each treat on a lollipop adds to the overall whimsy of this sweet. You can bring smoky-sweet flavor to the marshmallow-rice square with a sprinkle of cooked and crumbled bacon and a drizzle of maple syrup. Or as an alternative, omit the bacon and add a pinch of smoked paprika for a vegetarian variation. One of the world's most innovative chefs, Ferran Adria , used Rice Krispies for a mock paella that won rave reviews and demonstrated that the oven-puffed cereal's properties have a place in savory dishes just as much as they do in desserts.
And another recipe employs crisped rice cereal as a coating for super crunchy, tempura-like shrimp. For a spiced topping on a vegetable side dish, toast Rice Krispies in a skillet along with spices and a little oil then sprinkle over vegetables just before serving. The early days of cereal Getty Images. Inquiring geek-minds need to know! Labuza admits, "It's not exactly rocket science. That's when you realize that Rice Krispies essentially behave like glass. Rice Krispies feature strong molecular bonds holding the starch molecules together, and, like glass, if you smashed a rice crisp with a hammer, it would crack and shatter.
The fine folks at Molecular Expressions include close-ups of the structure of Rice Krispies at various magnifications in their extensive image gallery; you can see them here.
Unlike breakfast cereals, glass is an intensive topic of scientific research, because glass is one of those substances known as "amorphous solids," straddling the boundary between solid and liquid phases of matter. No less a luminary than Philip W. Anderson has observed, "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition.
To wit: In a solid, the molecules arrange themselves in a very precise lattice-type structure, earning them the moniker "crystalline. Glass falls somewhere in between: the molecules are still rigidly bound, but they are also more disordered than in a pure crystalline solid. So glass is neither, or both: it has its own distinct molecular structure that exhibits properties of both liquids and solids.
These structural properties stem from how glass is made. These days, windows are made by pouring molten glass onto molten tin, and letting it naturally spread out and solidify into a perfectly flat sheet. Older methods were less precise; a few artisans still practice them. You may have seen it at arts and crafts fairs: the glass-blower gets a all of molten glass on the end of a pipe, then blows it into a long, wooden tube-shaped mold.
Once the glass has cooled, it's removed from the mold, reheated, and ironed into a single pane. Windows made this way usually contain air bubbles and "waves," and aren't always of perfect thickness throughout. But what's actually happening as the glass goes from a liquid to an amorphous solid?
In a straightforward phase transition, like when water freezes into ice, the transition is dependent on well-defined temperature and pressure points. The glass transition is different: it also depends on the rate at which the heating or cooling takes place.
Glass is formed by cooling a liquid below its freezing point, then cooling it some more. Cool it fast enough, in a process known as "super-cooling," and the molecules don't have sufficient time to organize themselves into the rigid crystalline lattice structure of a solid. Instead, as the temperature drops the liquid becomes much more "viscous. As this happens, the molecules gradually move more and more slowly, until they are hardly moving at all.
This indecisiveness on the part of glass -- choose a state of matter already! There is an enduring urban legend that the glass windows in medieval cathedrals are thicker at the bottom because over hundreds of years, the glass has "flowed" downward and pooled at the bottom.
There is a tiny bit of truth to the legend. At the molecular level, glass does "flow", it just does so very verrry sloooowly. She emphasizes that this is a conservative estimate; it might take much longer. So there's frankly no way in hell that the irregularities in medieval cathedral windows are due to the flowing properties of glass. Instead, the observed anomalies are probably due to inherent flaws resulting from the manufacturing process.
For more detailed information on the molecular structure of glass, whether or not it can be said to truly "flow," and some fascinating early history, see this excellent discussion. In a article in Discover magazine on the physics of glass, Robert Kunzig discussed the possibility of an "ideal glass": "what you would produce if you could cool a liquid with geologic slowness while somehow preventing it from crystallizing.
Physicists have no idea how to even begin visualizing such a thing. But it could be important. Ted Labuza is a professor of food science and engineering at The University of Minnesota. He says that even though Rice Krispies have been making noise at breakfast for more than 80 years, science has been mute about the reasons why.
So, he looked at the mechanics of mixing them with milk. Here's what he found: Each Rice Krispy is actually a series of air-filled chambers.
The chambers form because the rice is cooked at very high temperatures. When you pour milk onto them, it pushes the air out - increasing pressure on the walls of each chamber. They shatter, and that's what you hear!
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