How do they get that crackle sound in breakfast cereal?

[Leo_Bloom]

Also, of course, the snap and p.

A related question: is there a metric or a method of cereal manufacture that can be used to rate when the cereal gets mushy? This is a real question; I would be willing to bet that the food giants’ research departments go into this topic.

[Kimstu]

You would win that bet.

Many people, of a morning, wonder why their breakfast cereal becomes soggy. Thanks to a study published in 1994, the answer can be read over morning coffee.

A Study of the Effects of Water Content on the Compaction Behaviour of Breakfast Cereal Flakes, by DMR Georget, Roger Parker and Andrew Smith of the Institute of Food Research in Norwich, looks at the basic physics of the matter. The scientists rigorously analyse how crunchiness declines in the presence of a soggifying liquid. […]

They discovered that, up to a point, as a flake takes on liquid it retains much of its youthful firmness. Beyond that point, though, the flake goes suddenly limp.

One can express this in formal language: the Heckel deformation stress becomes increasingly sensitive to the particle density as the water content increases - which may seem obvious now, but at the time came as a revelation.

[Nametag]

As for your first question, these guys have a good guess:

During the cooking process, each piece of rice expands and a network of air-filled caves and tunnels form inside.

The high temperature […] creates extremely strong bonds that […] make the rice act like glass.

When you pour milk into your breakfast bowl, the cereal absorbs the milk. As milk flows into the crispy kernel, the liquid puts pressure on the air inside and pushes it around.

The air shoves against each pocket’s walls until they shatter, forcing out a snap, or a crackle, or, as you, know, sometimes a pop.

[Leo_Bloom]

@Kimstu

You would win that bet.

A Study of the Effects of Water Content on the Compaction Behaviour of Breakfast Cereal Flakes, by DMR Georget, Roger Parker and Andrew Smith of the Institute of Food Research in Norwich, looks at the basic physics of the matter. The scientists rigorously analyse how crunchiness declines in the presence of a soggifying liquid. […]

They discovered that, up to a point, as a flake takes on liquid it retains much of its youthful firmness. Beyond that point, though, the flake goes suddenly limp.

One can express this in formal language: the Heckel deformation stress becomes increasingly sensitive to the particle density as the water content increases - which may seem obvious now, but at the time came as a revelation.

I stumbled on the following cite, on my hard drive since before the thread went underground, but somehow with all the fuss after 2/21/2010 it never made it.

According to Kimstu’s source (the JOIR), the test method used a Mettler LP16 moisture balance, an Instron 1122 Universal testing machine, and a piston-driven capillary rheometer.

The techniques used (I am not able to judge the results) are quite different.

INFLUENCE OF WATER ACTIVITY ON THE ACOUSTIC PROPERTIES OF BREAKFAST CEREALS

EWA GONDEK1, PIOTR P. LEWICKI1, and
ZBIGNIEW RANACHOWSKI2
Article first published online: 22 SEP 2006
Journal of Texture Studies
Volume 37, Issue 5, pages 497–515, October 2006

ABSTRACT
Corn and wheat bran flakes were compressed in a plastic cylinder and the generated vibrations were measured by a piezoelectric accelerometer. Amplitude–time records were thoroughly analyzed and the total acoustic energy in arbitrary units, number of acoustic events and energy of a single acoustic event were calculated. Relationships between the time of compression, frequency and sound intensity are presented as acoustograms. All parameters, except the energy of a single acoustic event, were strongly dependent on water activity (Aw). The flakes differed essentially in their ability to propagate vibrations. At low Aw’s, both types of flakes were alike, but at higher Aw’s, corn flakes damped high-frequency waves while wheat bran flakes did not show this property.

[Isilder]

Suffice to say that the quieter your cereal the better for you …

Its a form of murphies law, If it tastes (or looks or sounds) good, it is bad for you.