The principle behind this addition of alcohol is that most bacteria and strains of yeast are rendered impotent, unable to react with sugar or other wine constituents, in solutions containing more than 16–18% alcohol, depending on the strain of yeast.
The stage at which spirit is added has enormous implications for the style of fortified wine produced. The earlier it is added in the fermentation process, the sweeter the resulting wine will be. vins de liqueur such as pineau des charentes, for example, are simply blends of sweet, unfermented, or hardly fermented grape juice with grape spirit, known as mistelles in France (see mistela). An even stronger charge of alcohol is added before fermentation to a significant proportion of the rich grape juice used in the production of Australia’s topaque and muscat. For most port-style wines (including the sweeter styles of madeira) and for all vins doux naturels, fortification takes place during fermentation. Much of the natural grape sugar is retained by arresting fermentation before its completion, thereby boosting alcoholic strength to a pre-ordained level: usually 18–20% in port but 15–16% in most vins doux naturels. In the making of sherry, similar wines such as montilla, and the drier styles of madeira, spirit is added to dry, fully fermented wine only at the end of fermentation. Any sweetness in such wines is usually due to a pre-bottling addition of sweetening agent, often itself a mixture of grape juice and spirit (see mistela, px).