My ten-year-old just discovered popping candy and declared it as the best thing ever. I tried to convince her that her innards would explode, but she would have none of that myth. That got me wondering what exactly was going on with the stuff.
Back in 1956, food chemist, William A Mitchell invented Pop Rocks for General Foods, while trying to make an instant soft drink. But this little piggy did not go to market until 1975. After all the hassle of convincing people they were not dangerous, Pop Rocks were discontinued in the mid-1980s, but have since been resurrected.
The main ingredients of Pop Rocks, and their knock-offs, are sugars, flavorings and carbon dioxide. Carbon dioxide is a gas we breathe out and is a major greenhouse gas that contributes to climate change, but it has various uses as well. In this case, it is a non-toxic gas readily available, for a reasonable price.
According to patents on the process of making "gasified candy," the mixture is heated up to 138°C (280°F) and carbon dioxide is added under pressure—4,137kPa (kilopascals) or 600psi. This is about half the pressure of some soft drinks and about 40 times higher than atmospheric air pressure. When the concoction is cooled and the pressure is released, the candy shatters into small pieces filled with little bubbles. Batches with “bigger” bubbles are better. We're talking more than about 225μm (μm is the symbol for micrometre—one millionth of a metre), about twice the diameter of a human hair.
Candy makers sieve the pieces, so that they have a more even size distribution. An organoleptic panel then determines the quality of the gasified candy when eaten (orangoleptic refers to experiences that involve your senses). The panel rates batches of pop rocks on a scale of 0 to 14 for popping experience, with 14 being the greatest pop for your penny. Any batch below 7 is out.
The heat and moisture of your mouth dissolve the candy so that the candified bubbles of gas burst, making the popping sound. The pop might even cause the candy to jump off your tongue! If you want to measure how much carbon dioxide comes from the candy, here are instructions and some other excuses for experimenting with candies. But I suppose I should mention that thought experiments, if you can manage them, have fewer calories and won't cause cavities.