How to inject carbon dioxide into hard candy, to make popping rocks?

There are recipes on the internet for making home made pop rocks.

Some of them call for using baking soda plus an acid, so that you generate carbon dioxide in the syrup, rather than injecting it as a pressurized gas.

I would assume that you could also use baking powder if you can't find powdered acids.

Looks like it takes about 600psi CO2 to treat the liquid candy. Patent search should reveal more detail: US patent 3012893

Patent links are notorious for decaying over a short time frame. If the second link is dead go http://www.uspto.gov/patents-application-process/search-patents or here to find find out how the inventors did it. Patents before 1976, are in image format, and are hard to look through, sometimes Google is a better choice than USPTO on these.

See also US Patent 4289794 (1981)

Gasified candy which produces a more pronounced popping sensation is prepared by maintaining a sugar melt at a temperature of below about 280° F. ... Such a candy is made by a process which comprises melting crystalline sugar, contacting such sugar with gas at a pressure of 50 to 1,000 psig for a time sufficient to permit incorporation in said sugar of 0.5 to 15 cm3 of gas per gram of sugar, maintaining the temperature of said sugar during said absorption above the solidification temperature of the melted sugar, and cooling said sugar under pressure to produce a solid amorphous sugar containing the gas. Upon the release of the pressure, the solid gasified candy fractures into granules of assorted sizes.

High temperature, high pressure. Even as a hard candy, the stuff has a short shelf life. Baking soda/acid mixture candies are just Fizzies in disguise. Feel weird when they bubble in your mouth, but they do not explode like pop rocks.

Applied Science, a YouTube channel, had a video about making carbonated candy a la Pop Rocks: https://www.youtube.com/watch?v=qsSwvmNEr0Q

Here's a broad outline:

1. Craft a mixing chamber that can hold 600 psi (40 bar), has an inlet for CO₂, and a rotary pressure seal with a mixing/whisk attachment.
2. Make a hard candy (280 °F/137 °C) on the stove
3. Preheat the chamber to avoid the candy solidifying too quickly
4. Pour molten candy into the chamber
5. Close and pressurize to 600 psi.
6. Mix (with drill or whatever) for 3-4 minutes
7. Let cool while under pressure
8. Depressurize and open chamber, remove candy with mallet, chisel, and/or hammer.
9. Enjoy!

600 PSI sounds dangerous. Water will absorb carbon dioxide. The colder the water, the more it will absorb. I make beer. I usually do in the bottle conditioning but when I do forced carbonation the highest PSI I will typically use is 30PSI.

If your candy is a syrup at some point, and not too hot, that would be where to add the CO2. The trick would be how to carbonate it before it cools and solidifies.

Perhaps you could buy a pressure cooker and drill/tap the lid so you can add a valve and attach a CO2 cylinder. You can get a small 5 pound cylinder at home brew stores but you'll also need a regulator and probably some other fittings. The cylinders come empty so you also need a place to fill the cylinder. Look for a place that services fire extinguishers.

There are also smaller versions of this that are mainly used for keeping kegs of beer pressurized at parties. They typically use 72 gram CO2 cylinders and the mini-regulator I have maxes out at 20PSI...just enough to push the beer out of the keg.