Sweet wines, on the other hand, are produced by interrupting the fermentation before all the grape sugars are converted into alcohol; this is called Mutage or fortification. The methods of enriching wort with sugar are multiple: on-ripening on the vine, passerillage (straining), or the use of noble rot. Sparkling wines, which are mostly white, are wines where the carbon dioxide from the fermentation is kept dissolved in the wine and becomes gas when the bottle is opened.