Introduction: Carbonating Fruit With Dry Ice and a Pressure Cooker
A couple years ago, I wanted to try carbonated fruit. Unfortunately, the standard technique was to use a whipping siphon and CO2 cartridge, neither of which I had. Being unemployed at the time, I didn't want to buy new equipment just for an experiment. I decided to improvise.
The science is pretty simple: dissolve carbon dioxide into the water inside the fruit. This is done by putting the fruit into an atmosphere of pressurized CO2. This is the same process as in carbonated soda, and it has two main effects:
- Adds fizz. This is the CO2 coming out of solution, forming bubbles.
- Creates carbonic acid, which sharpens flavors and adds "bite".
Put those together and you get the distinctive pleasures of a carbonated beverage. Or, in this case, fruit.
Step 1: The Parts
You can get CO2 in pressurized cylinders from industrial gas suppliers, but that's not very convenient. Instead I decided to use dry ice -- frozen CO2. It's easily available at many grocery stores, though they might not sell it to you if you're young.
I also needed a pressure vessel in which to combine the fruit and the CO2. Pressure vessels aren't something you want to fake. I've seen guides that use plastic bottles for this purpose -- do not do this. It is an incredibly dumb thing to do. There is no way to reliably calibrate the amount of CO2 you're putting into the bottle, nor do you know how strong the bottle is. An exploding bottle can easily harm you or others, and you just might end up talking to the cops. (I know someone who very nearly went to prison for this. An exploding bottle is also known as a bomb, and law enforcement takes that kind of thing very seriously these days.) A proper vessel for something like this needs to be pressure rated, it needs a regulator to keep the pressure from getting too high, and it needs a relief valve in case the regulator fails. Unless you really know what you're doing, this simply isn't something you can make for yourself.
Luckily, there is a (fairly) common household tool which meets all of these requirements -- a pressure cooker! They are designed to do exactly what we need. They can hold high (up to 15 PSI, usually) pressure safely, they have a regulator for maintaining that pressure, and they have a relief valve just in case. I used a large canning pressure cooker that I happened to have (I bought it for use as an autoclave when I was investigating the possibility of home bio-engineering) but smaller ones would work as well.
Step 2: Doing the Carbonation
Setup is quite easy. You'll need to put the dry ice in some water to help it sublimate quickly. Don't just dump it directly into the pressure cooker, though, as that will lead to the metal getting very cold, which could compromise its strength. It can also result in a lot more violent splashing of the water as the CO2 sublimates, possibly icing over the regulator port. Instead, put the dry ice and water in a small plastic bowl, and put that in the bottom of the pressure cooker. I found that greatly reduces the splashing, and insulates the vessel bottom sufficiently. You don't need much, either. I didn't take exact measurements, but a few chunks that comfortably go into a small bowl will do it.
Next, place the fruit you want carbonated inside the vessel. I put it on a rack above the CO2, but in smaller cookers it might need to go on the sides. Just don't put it so close to the CO2 that it ends up freezing! You have a lot of choices of fruit to try, so have fun. It works best with fruit that is very juicy and has a lot of surface area. You'll have to experiment to see what you like the best.
Put the lid on the pressure cooker and set the regulator mechanism for its highest setting, which was 15 PSI in my case. The pressure should build fairly quickly. Let it sit for 45 minutes to an hour, then open up the regulator to drop the pressure quickly. Open the cooker up and remove the fruit. If all has gone well, it will be chilled and carbonated.
We had great luck with raspberries, which you could feel vibrating from the fizz as you ate them. The carbonic acid added a delightful bite to the taste, like the best berry soda you've ever had. Pineapple and watermelon also worked well. Oranges were a disappointment, possibly because the membrane prevented absorption. It might work better if you cut the wedges into supremes first. But the best, most surprisingly wonderful result was with green olives. I won't even begin to describe the taste, because I lack the words. Try it and find out!
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