This idea is based on fresh fruit drinks known as "aguas frescas" in Mexico and some Latin American countries. They are typically fruit juices that are lightly sweetened, and in some cases watered down to improve their texture (if the fruit juice is more of a puree, it needs the addition of water to become more fluid and therefore easier to drink through a straw).
These are "aguas frescas" with carbonation. The actual carbonation does not come from a seltzer siphon, it is from a powdered mixture of baking soda, citric acid and sugar. We made three different flavors, hibiscus (not technically a fruit juice, it is made by infusing hibiscus leaves in water, but it is generally accepted in the very loose category of "aguas frescas"; it is also the soda photographed above and below), tamarind and pineapple.
Why are they instant? Because they have to be made to order. This is another item we will be serving in Chicago in a couple of weeks. It doesn't really fall in the category of dessert, I know, but it helps wash down a dessert, I think.
The principle is very simple: pour about 3 g of the powder mix into a chilled glass, and then put crushed ice on top of the powder. When ready to drink, pour the liquid (about 60 ml) into the glass and stir with a straw. And there you have instant soda. It needs to be drunk within a few minutes, otherwise the fizz runs out.
What is important is to keep the balance right with the powder mix, since too much baking soda can taste terrible, but you also have to mind how much sugar you add to the liquid, since there is sugar in the powdered mix too.I get the feeling this would not be very popular in Mexico.