Pop Rocks have their place in everyone's childhood. Although the myth that they could make your stomach explode was debunked in that great 1990s documentary Urban Legend, kids still ahve a morbid fascination with them. Perhaps it was because the little explosions made Pop Rocks seem like Nerds that had the capacity to fight back. Actually getting them down was a victory. Whatever the attraction, most of us spent at least some portion of our childhoods with a mouthful of hissing sugar. Let's take a look at how that sugar was made - and then make some ourselves.
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Maybe one of the big attractions of pop rocks is they always sounded like booze. And for good reason. The hissing, fizzing, and popping sound that they both make has the same origin - little bubbles of carbon dioxide. In alcohol, the carbon dioxide is dissolved in the liquid until the release of pressure from a popping cork or opened top brings it out. Pop Rocks don't have any dissolved gas, but they do have gas forced into the candy.
In fact, the gas is forced in at about 600 pounds per square inch. The actual physical candy, according the pop rocks site, is made of corn syrup, lactose, and sugar - for those of you keeping track that's processed sugars, milk sugars, and sugar sugars - with a little 'flavoring' added in. (I can't be sure, but I think the flavoring is sugar.) The sugars are thrown together and boiled. Then they're pressurized and the gas is pushed into the liquid. It forms little bubbles that, as the temperature drops and the mixture solidifies, are trapped in the candy. Once the mixture is solid, the pressure is taken off, and the larger gas bubbles explode, shattering the bar. The smaller bubbles, without enough carbon dioxide in them to overcome the confining walls of the candy, remain trapped.
When the candy hits your tongue, saliva starts dissolving the solid sugar. The hissing sound is the constant escape of little gas bubbles as the walls near them dissolve. The bigger pops and kicks happen when a larger bubble has been trapped and the walls around it get thin enough for it to blow through them at 600 pounds per square inch.
The process sounds too industrial to be reproduced at home, but some people have had success with making their own pop rocks. The Greenists offers a good recipe for those who are willing to get their own candy thermometer. It doesn't take much. All it needs it sugar, naturally, enough liquid to get the sugar into a liquid state, baking soda, and citric acid crystals that can be found at specialty baking shops. Wet the sugar, heat it up, add the soda and crystals, bring the whole thing to 300 degrees on a stove top, and pour it into something to let it harden. It's more a fizz rock than a Pop Rock - no one can compete with 600 pounds per square inch - but sometimes a fizz is all that's needed.