Fortified wines are those to which distilled spirits (containing as much as 95% ethanol) are added to raise the total ethanol concentration to 15%.Not only do these wines contain higher concentrations of ethanol, the source of the ethanol (e.g., brandy) is also important since they may contribute unique flavor compounds to the finished product. Aside from this common feature, however, a variety of quite different fortified wines exist. Included are whites and reds, dry and sweet. Fortification usually occurs during or just after the fermentation. In some wines, the added ethanol may inhibit the yeast and prevent the complete fermentation of sugars, resulting in sweet dessert wines.The most well-known of the fortified wines are sherry and port.
Sherry originated in Spain around 200 years ago, and is now produced in the United States and throughout the world. One of the main features used in the traditional manufacture of Spanish sherry (but rarely outside of Spain) is a blending technique known as the solera system. According to this technique, wines of different ages are progressively and uniformly blended to achieve wines of consistent quality. Another unique feature of most types of sherry is the presence of film yeasts that grow on the surface of the wine while it is aged in special casks called "butts." The S. cerevisiae strains that comprise this film, called a flor, are thought to be the same yeasts involved in the primary alcoholic fermentation, although it has also been suggested that a separate group of yeasts actually comprise the flor yeast population. In either case, the secondary yeast growth occurs under aerobic conditions, resulting in oxidation of some of the ethanol and subsequent formation of unique flavor and aroma components (e.g., esters, aldehydes, higher alcohols, and lactones).
In contrast to the traditional (and costly) solera system for manufacture of sherry, other techniques are now widely used in the United States and other countries. In the submerged flor procedure, the wine is aerated and mixed in fer-mentors to enhance growth of the flor yeast. Baked sherries are made by heating fortified but non-aged wine to 50°C to 60°C for ten to twenty weeks. A number of flavor and aroma compounds, including acetaldehyde and furfural, are generated by baking due to carameliza-tion and non-enzymatic browning reactions.
Port, first produced in Portugal, is another fortified wine. It is made by adding distilled wine (i.e.,brandy) to the red base wine, thereby raising the ethanol concentration to nearly 20%. Since fortification occurs well before all of the sugars have been fermented, and this much ethanol inhibits yeast, this step effectively ends the fermentation.Thus, Port contains as much as 10% sugars. Port and other fortified sweet wines made in this manner (e.g., Madeira and Marsala) are often referred to as dessert wines. Because fortification not only arrests the fermentation, it also reduces the time available for extraction of anthocyanin pigments and phenolic flavor compounds. Mixing pressed wine (pomace) with the free run wine, therefore, is necessary to provide adequate color and flavor in the finished wine. In the United States, heat (e.g., steam) is sometimes used to extract pigment from the grapes. Finally, as for sherry, blending and aging are also important steps for port production. Some ports are aged for as many as fifty years.
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