I blogged about carbonated strawberries some while ago. Those were made using dry ice which unfortunately is not always easy to get hold of. Last week however I bought a iSi Gourmet Whipper – one of those Ferran Adria uses to make foams/espumas. I plan to experiment with that as well, but the first thing I decided to prepare was carbonated fruit. In fact this is a safe way (the only?) to make carbonated fruit at home using a pressurized container.

The instruction booklet which comes with the iSi Gourmet Whipper only mentions cream chargers (filled with N₂O, dinitrogen oxide), whereas soda chargers (filled with CO₂, carbon dioxide) are not mentioned (I guess the opposite is true for the iSi Siphons?). This is quite amazing actually! Luckily however the cream and soda chargers are exactly the same size and both hold 8 g of gas. So it should be possible to make carbonated fruit with any of the iSi whippers (cream, easy, gourmet, dessert, thermo) or siphons available.

Here’s how you proceed:

1. Fill you iSi whipper (or siphon) with fruit, preferably fruit which has a cut, wet surface to allow the carbon dioxide to dissolve in the water/juice.
2. Screw on top securly
3. Charge with one soda charger (two if you have the 1 L whipper)
4. Leave in fridge over night
5. Release pressure with valve (Important!)
6. Unscrew top and serve immediately!
7. Enjoy!

This is what carbonated grapes look like. As you see, I decided to cut the grapes in to halves.


Notice how they sizzle!

A quick recap of the chemistry: cold water dissolves more CO₂ than tempered water, that’s why we leave it in the fridge. Also, remember that it takes some time for the carbon dioxide to dissolve in water, therefore it’s better not to be in a hurry. A quick calculation of the pressures gives the following: Both gases have molecular weights of 44 g/mol, so 8 g of gas corresponds to 0.1818 moles or 4.1 L at 25 °C and 1 atm pressure. The volume of the chargers is 0.01 L which gives an initial pressure in the chargers of impressive 445 atm! With an approximate volume of 0.7 L this gives a pressure (in an empty whipper) of nearly 6 atm – the same as in a bottle of champagne. However once you add water, the equilibriums will change and the pressure in the head space will drop. Anyone who remembers how to calculate the head space pressure at equilibrium if the container is filled with 0.5 L of water and cooled to 4 °C?

I’ve done some googling and there is also some mention of making carbonated fruit with an iSi whipper over at Ideas in food.

(The word play in the title works better for those with a mother tongue where iSi would be pronounced just like “easy”!)

About.com has a nice guide on how to color eggs, and the list of colors is quite impressive (click for instructions):

Lavender
Small Quantity of Purple Grape Juice
Violet Blossoms plus 2 tsp Lemon Juice

Violet Blue
Violet Blossoms
Small Quantity of Red Onions Skins (boiled)

Blue
Canned Blueberries
Red Cabbage Leaves (boiled)
Purple Grape Juice

Green
Spinach Leaves (boiled)
Liquid Chlorophyll

Greenish Yellow
Yellow Delicious Apple Peels (boiled)

Yellow
Orange or Lemon Peels (boiled)
Carrot Tops (boiled)
Celery Seed (boiled)
Ground Cumin (boiled)
Ground Turmeric (boiled)

Brown
Strong Coffee
Instant Coffee
Black Walnut Shells (boiled)

Orange
Yellow Onion Skins (boiled)

Pink
Beets
Cranberries or Juice
Raspberries
Red Grape Juice
Juice from Pickled Beets

Red
Lots of Red Onions Skins (boiled)

More information about the chemistry behind can be found in the article “Chemistry in the dyeing of eggs” (Journal of Chemical Education, 1987, 291). The article discusses anionic dyes with sulfonate groups. These bond to the cuticle (protein) covering the egg shell forming salt linkages as shown (illustrated using FD&C yellow no. 6):

By lowering the pH (for example by adding vinegar), more amino groups in the proteins covering the egg shell are protonated and thus available for formation of the salt linkages with the anionic dyes.