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Chaptalization is the process of adding sugar to grape juice or must before fermentation, which will boost the alcohol level in the finished wines. This process is practiced in cool weather regions, where under ripeness are a problem and the sugar level of the grapes is too low to rich the desired alcohol level. Control of chaptalization is fairly strict in many countries. The legality of chaptalization process varies by country, region, forms of sugar and even wine type. After sugar is added to the grape juice or must, naturally-occurring enzymes convert the sucrose molecules in sugar into glucose and fructose, which are then fermented by the yeast and producing alcohol and carbon dioxide.
Grapes with too much sugar means a lot of alcohol will be produced and this can cause problems with the primary and secondary fermentations. High alcohol level is becoming inhibitor for most yeast strains and malolactic bacteria and actually kills them, which can cause a sluggish or stuck fermentation. To prevent that some water need to be added to lower the sugar levels in grape juice/must. Adding water will not only dilute your sugar concentration, but you will dilute the total acidity of the juice/must as well. For that reason, unless the must/juice already has high acid, it is important to check and adjust your acidity and pH to a proper level. However this practice could cause a dilution in flavors as well, so winemakers need be very carefully how much water to add to their grape must/ juice prior fermentation.