After finally being able to purchase seltz or CO2 chargers for our iSi siphons at Williams Sonoma (we had searched everywhere else in the area), we proceeded with our humble efforts to carbonate some grapes. There has been quite a bit written on the subject using the iSi method and also info on using dry ice to achieve fizziness.
My first attempt was with whole red grapes in our 1l siphon with 2 CO2 chargers. I left the grapes sit in the canister at room temp for 30 minutes before removing them to try. They were very fizzy (I'm not equipped to measure fizziness in our kitchen so all of my observations are unscientifically relative) and lasted for about 10 minutes before they became noticeably flat. You cannot really observe the carbonation from the picture, but they were definitely fizzy. Each grape had split its skin from top to bottom.
We referred back to an old posting on Khymos that suggested also cutting the grapes and letting them sit under refrigeration overnight. The explanations for this can be found on Khymos... interesting reading. This time I had used green grapes and for some reason, there wasn't as much as a pronounced fizziness. The carbonation also did not retain for more than a few minutes (although it's easier for CO2 to enter the grapes this way, it's also easier for it to leave the grapes).
I also experimented further with this batch. In order to preserve carbonation in the grapes for future use, what method (besides leaving them in the iSi) could be employed. Cryovac'ing them seems ideal... and also bad at the same time. Although sealed very securely, would the process of 'sucking' all of the gas out of the package be detrimental to the carbonation in the grapes? For that reason, or perhaps another, the grapes did not remain carbonated. Maybe another experiment with whole grapes would yield different results, but as for now... this myth is busted!
One more note... I do not have a device for measuring carbonation in grapes, so all of this is purely observational. In the end, we used whole green grapes, 2 CO2 chargers, 1 night in the refrigerator. They worked out amazingly great... and were still carbonated as much as 20 minutes later. We used them as an intermezzo for a wine dinner for 32 people with a seltzer shot of green grape gazpacho soda. It was a fun break in the meal. The soda base was made from a broth of green grapes and toasted almonds (a play on Spanish white gazpacho)... refreshing and ice cold! Sorry, no picture of the intermezzo.