If melon tastes fizzy, it means that the fruit is overripe and beginning to spoil. The fizziness is caused by carbon dioxide gas escaping from the fruit. When fruits and vegetables are ripening, they release ethylene gas. This gas helps the fruit to ripen and soften. However, if the fruit continues to produce ethylene gas after it has ripened, this can cause the fruit to become overripe and begin to rot. The rotting process causes bacteria to produce carbon dioxide gas, which escapes from the fruit and gives it a fizzy taste. Overripe fruits and vegetables should be avoided as they can cause stomach upsets.