Sugar in the wine making process is vital primarily because it is what the yeast use to create the alcohol. Sugar in the wine is also important for other reasons but how does it get in the grapes in the first place? I’m glad you asked.
When grapes ripen they go through two parallel stages of ripening: Physiological and Sugar Ripening. Physiological ripening creates most of the flavours and smells in wine. Sugar ripening creates the sugar levels from Carbon Dioxide and Water the grape vines absorb. Deny the grapes enough sunlight, heat or water and they may not develop the required amount of ripeness resulting in unpleasant wine and not enough alcohol.
This is not normally a problem in Australia where sunlight and heat are generally in good supply but can be a problem in Europe that needs to be carefully managed. But more of this next time and in the meantime…enjoy some wine.