Both still and sparkling wines normally continue fermentation until the yeast has completely converted all the fruit sugars to alcohol, resulting in a dry wine. To create a sweet wine, three methods can be used by the winemaker: the sugar levels in the grapes to begin as so high that the yeasts die from the alcohol long before they convert all the sugars, the fermentation can be stopped early (leaving residual sugars) or the least interesting, additional non-fermented grape juice can be added after fermentation to dilute the alcohol and raise the sugar level.
To get the highest sugar content in the juice the techniques are fully natural, either by air-drying the grapes so that they loose water content before pressing, or allowing Botrytis Cinerea, the noble-rot to take hold on the vine. Air-drying is performed in well-ventilated, drying rooms, fruit packed into individual layered trays to encourage evaporation yet protect the delicate grapes. The Noble Rot is a difficult technique that relies on perfect timing, the mould weakens the grape skin and draws out moisture causing shrivelling. Too long and the vine itself can be risked, so meticulous hand-picking is required to harvest the delicate grapes.
Fermentation can be interrupted by either the addition of additional alcohol (fortification) which kills the yeast, or they can be removed from the fermenting wine by fine filtration. Muscat de Beaumes de Venise and other Mediterranean dessert wines tend to use fortification, where time in oak barrels allows some oxidisation and the development of complex flavours common with dried figs, prune apricot or raisins.
Anthony Byrne Fine Wines Ltd. Registered in England and Wales with company number 01713692.