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Snap! Crackle! Pop! What Rice Krispies Has To Do With Glass

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My name is Jennifer, and I am completely addicted to Nature’s Path Flax-Plus brand of pumpkin/flax granola. Seriously addicted. As in, I eat the stuff straight out of the box, no accompanying soy milk, no nothin’.

In fact, I have to exercise considerable restraint not to down the entire box over the course of a single day by taking “just a nibble” here and there. I monitor the “stash” in my pantry very closely, and get a bit jittery when the inventory starts to run low. Occasionally I go “cold turkey” for a few weeks, just to prove to myself that I can quit any time. Jen-Luc Piquant suspects the folks at Nature’s Path have laced their cereal with crack, but if so, it’s Certified Organic (TM) crack, delivered in a tasty, nutritious format.

My addiction might not be the subject of hard-hitting investigative journalism (“Tonight on Hard Copy: people who love their breakfast cereal too much!”), but I’m certainly not alone in my enthusiasm. Americans each consume about 10 pounds — that’s 160 bowls — of cereal per person every year.

In the US, among the most enduring brands is Kellogg’s Rice Krispies, introduced in 1928, which has its very own Website. Kellogg’s ingenious marketing strategy for the cereal certainly helped boost its enduring popularity, particularly the introduction of three cartoon elfin sprites, appropriately named Snap, Crackle and Pop, after the sound made whenever milk is poured over a bowl of the cereal. Check out this vintage 1950s commercial.

Memorable, right? That’s why they are instantly recognizable to any American; in fact, in 2002 a pollster found that most Americans can name the three elves but can’t name any three of the nine sitting Supreme Court Justices, who clearly need a catchy slogan.

Eventually, Kellogg’s went global with its marketing slogan: it’s “Riks! Raks! Poks!” in Finnish; “Piff! Paff! Puff!” in Swedish; “Pim! Pum! Pam!” in Spanish; and “Knisper! Knasper! Knusper!” in German. It’s nice to know that the practice of onomatopoeia is universal.

(The sprites are known affectionately to Jen-Luc Piquant as Cric! Crac! and Croc! She categorically denies those tabloid rumors about one wild drunken night in Vegas with the Krispie Krew that ended with her being briefly married to Crackle. Lies. Vicious lies.)

But what is it about Rice Krispies that makes them go snap, crackle, pop? It’s not microscopic sprites, although research on the topic has admittedly been sparse. Nonetheless, a food scientist named Ted Labuza at the University of Minnesota investigated the matter a few years ago and came up with a decent explanation for why these popular cereal crisps produce such a distinctive sound.

During the cooking process, each piece of rice expands, 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 snap, crackle, or a pop. Eventually, of course, the cereal becomes saturated and soggy, and the signature sounds cease.

It’s quite a bit like how popcorn pops, which depends on the moisture and starch inside the corn kernel, and the hard shell surrounding it. The moisture percentage in particular must be just right. Heating up the kernels causes the starch granules to expand, thereby increasing the pressure inside the hard shell, which eventually explodes when the pressure gets high enough. And the starch granules expand into the fluffy white globs we know and love.

Grains of rice  don’t naturally have sufficient moisture, but this is added (via steaming) during the manufacturing process for Rice Krispies, and the grains are then oven-popped to give them their unique texture. (We also encourage Labuza to extend his research to investigate why excessive consumption of Cap’n Crunch is so harmful to the roof of one’s mouth. Inquiring geek-minds need to know!)

Labuza admits, “It’s not exactly rocket science.” No, it’s materials science, and things start to really get interesting when you take things down to the molecular level. 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.” In fluids, the molecules are disordered rather than rigidly bound, enabling the substance to “flow.”

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.” (Viscosity is a measure of a liquid’s resistance to flow; the higher the viscosity, the greater the resistance.) 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! — has led to the mistaken assumption that glass is actually a fluid. 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.

Yvonne Stokes, a mathematician at the University of Adelaide in Australia, has performed detailed  calculations on old cathedral windows, and estimates that it would take at least 10 million years for the glass at the bottom to grow just 5% thicker. 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 1999 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.” It would be a distinct state of matter, rather than the confused hybrid that is so familiar to us: motionless, with a rigid molecular order like a crystal — except it wouldn’t be a crystal.

Physicists have no idea how to even begin visualizing such a thing. But it could be important. We’ve heard whispers to the effect that discovering an ideal glass transition phase — namely, a point during the supercooling process where the molecules have no choice but to move rapidly from the disordered liquid configuration to a highly-ordered solid configuration — could yield insights into the structure of the early universe, which may have existed in a similar amorphous disordered state.

Alas, the news on that front isn’t encouraging. A paper in the June 9 2006 issue of Physical Review Letters, by Princeton University’s Salvatore Torquato (et al), concluded that such an ideal glass transition phase doesn’t exist. Torquato’s team performed a bunch of computer simulations and couldn’t find any such well-defined transition point. Torquato told Live Science that “You could have this continuous change from most disordered to most ordered, and there are an infinite number of possible transition phases between these points. It puts another nail in the coffin for [the ideal transition] theory.”

Maybe that ideal transition phase is a bit questionable, but the mysterious “Moosino” over at Chi c’e’ in Ascolto reports on a very different kind of “transition phase” from amorphous solid into, well, a million little pieces. Apparently she was driving along one day, when one of the side windows of her car spontaneously shattered. Being such a well-trained scientist, she nosed around until she found some answers.

Basically, the side windows of a car are made of tempered glass, a process that causes the exterior surface to compress while the interior is still expanding a bit. The end result is an exterior compression layer and an interior tension layer — I believe the technical term is an “inclusion.” If a crack develops later on in the compression layer, all the interior tension is released all at once. The window goes snap! Or crackle! Or pop!

Just like a bowl of Rice Krispies.

Images: (top) Rice Krispies cereal box. Source: Wikipedia, under fair use. (bottom) Molecular structure of amorphous silica. Wikimedia Commons/Public domain.

Originally posted in September 2006 at the old Cocktail Party Physics site.


P W Anderson (1995). “Through the Glass Lightly,” Science 267 (5204): 1615. DOI:10.1126/science.267.5204.1615-e

“Do Cathedral Glasses Flow?” (1998) Am. J. Phys. v66, pp 392—396.

A. Donev, F. H. Stillinger and S. Torquato. (2006) “Do Binary Hard Disks Exhibit an Ideal Glass Transition?” Physical Review Letters, 96, 225502.


Jennifer Ouellette About the Author: Jennifer Ouellette is a science writer who loves to indulge her inner geek by finding quirky connections between physics, popular culture, and the world at large. Follow on Twitter @JenLucPiquant.

The views expressed are those of the author and are not necessarily those of Scientific American.

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