From Spanish Sherries to Australian Rutherglen Muscats, fortified wines vary in color, flavor, origin and sweetness. But all have one thing in common: fortification.\r\n\r\nFortification, the addition of grape spirit to wine either during or after fermentation, is a technique used to increase alcohol content and stop fermentation. The process was popularized by the English in the late 17th century to stabilize and preserve wines for long sea voyages. Prior to the development of fortification, many of these wines were made originally as still, unfortified wines.\r\n\r\nHowever, many decisions, like at what point during fermentation a wine is fortified and how it\u2019s matured, create a diverse array of bottlings.\r\n\r\n\r\nSherry\r\nAll Sherry hails from hot, dry southern Spain, centered in the towns of Jerez de la Frontera, Sanl\u00facar de Barrameda and El Puerto de Santa Mar\u00eda.\r\n\r\nWhile the low-acid, white Palomino grape dominates the region, it\u2019s often supplemented by aromatic Moscatel (also known as Muscat of Alexandria) and robust Pedro Xim\u00e9nez. In general, fermentation takes place in neutral stainless-steel tanks, followed by extended aging in neutral barrels.\r\n\r\nBecause Palomino is such a neutral grape, the aging process is crucial to the style of the finished wine. Sherry wines are aged in rows of barrels called criaderas using the solera system, whereby fresh wine is added to barrels holding multiple back years of wine, thus leading to many vintages being blended over time.\r\n\r\nThe process works like this: Winemakers take a percentage of wine from the oldest section below of a solera for bottling. Next, they top up the solera with wine from the first criadera (the next-oldest section), and then they fill the first criadera with wine from the second criadera, and so forth. Each style of Sherry has its own solera system within a bodega, some of which may be decades old.\r\n\r\n\r\n\r\nThere are various styles of Sherry, but dry Sherries can be classified largely into two categories: those aged under a veil of yeast called flor, which includes fino and Manzanilla, and ones matured with oxygen contact, like oloroso. Some, like amontillado and Palo Cortado, are \u201chybrid\u201d styles that undergo both aging techniques.\r\n\r\nGenerally, free-run and first-press juice is used for fino and Palo Cortado, while second-press juice is used for oloroso.\r\n\r\n\u201cFree-run and first-press must generally has a more elegant, soft and neutral character,\u201d says Antonio Flores, winemaker and master blender for Gonz\u00e1lez Byass. \u201cThis allows the flor to leave a dominant yeast character. For the oloroso style, we are looking for a must with more structure, body and complexity.\u201d\r\n\r\nFlor-aged Sherries are fortified with a grape spirit until the wine reaches between 15% and 15.5% abv. This encourages flor to grow, which protects the wine from oxygen and imbues it with almond-like, yeasty notes and a dry, refreshing texture.\r\n\r\nSherries aged through oxidation are fortified to about 17% abv. Since flor cannot survive at those levels, oxygen can interact with the wine. This creates nutty, caramel-like notes and develops a round, viscous texture.\r\n\r\nAfter a few months in barrel, the wines are assessed and can be reclassified. If a wine is too robust and hasn\u2019t developed a strong layer of flor, it may be fortified again to 17% abv and aged as amontillado or slightly richer Palo Cortado solera systems. Both of these styles of wine have fresh, citrusy qualities and nutty, oxidative aging characteristics.\r\n\r\nSweet Sherries are the result of different winemaking decisions, though they\u2019re also aged in solera. Naturally sweet Sherry like Pedro Xim\u00e9nez and Moscatel are made from super-concentrated, dried grapes with sugar levels so high that fermentation doesn\u2019t finish before the alcohol is added. They\u2019re fortified to 15 or 16% abv.\r\n\r\nPale Cream and Cream Sherries are usually fermented until dry, then fortified and sweetened.\r\n\r\n\r\nPort\r\nPort is always a sweet, fortified wine made from grapes grown on the steep slopes of Portugal\u2019s Douro Valley. The warm, dry conditions make powerful, ripe red wines, though white grapes are grown as well.\r\n\r\nUnlike Sherry, Port is often the result of a blend of multiple grape varieties, often from different vineyard sites. The most prominent ones used to make Port include Touriga Nacional, Touriga Franca, Tinta Barroca, Tinto C\u00e3o and Tinta Roriz.\r\n\r\n\u201cWe make around 300 separate lots of our top grapes and many more of lesser-quality grapes at each vintage,\u201d says Rupert Symington, CEO of Symington Family Estates. The grapes are macerated extensively for concentration and structure before they\u2019re fermented in stainless steel or open granite lagares.\r\n\r\nFortification with a 77% abv grape spirit takes place before fermentation is complete. This is why Port is always sweet, though the exact level of sweetness depends on a house\u2019s style. The quality and flavor of the grape spirit that\u2019s added also matters, since a fair amount needs to be blended in to achieve Port\u2019s typical 19\u201322% abv.\r\n\r\nIn addition to white and ros\u00e9 styles, Port can be split into two categories. Tawny Ports are aged with oxygen, while ruby Ports are aged for 2\u20133 years in wood, cement or stainless steel prior to bottling.\r\n\r\nThe first step for most Ports is barrel aging.\r\n\r\n\u201cAfter a preliminary sorting at harvest time, the wines are put into wood,\u201d says Symington. An exhaustive tasting is conducted the following spring to identify the wines that are structured and concentrated enough for vintage Port production.\r\n\r\nThe wines are then assessed to find those suitable for long aging in wood to create high-quality tawny Ports, where they will develop nutty, dried fruit notes. Others are determined best for bottling after a few years as fresh, fruit-forward late-bottled vintages or ruby reserve wines. The remaining wines are blended into basic ruby Ports.\r\n\r\n\r\nMadeira\r\nMade on the subtropical Portuguese island of the same name, Madeira can vary in style based on grape variety and intended quality, but one characteristic rises above all: It is virtually indestructible.\r\n\r\nAccording to the Instituto do Vinho do Bordado e do Artesanato da Madeira (IVBAM), about 85% of Madeira is made using the high-yielding red grape Tinta Negra. But the best Madeira wines are made generally from the island\u2019s four white varieties: Sercial, Verdelho, Boal and Malvasia.\r\n\r\nWhen a wine is labeled by variety, it can indicate the sweetness level. For example, high-acid Sercial tends to be fortified later in the fermentation process, which makes a relatively drier wine. Meanwhile, high-sugar Malvasia tends to be fortified earlier to make a sweet wine. The added spirit is 96% abv, so just a little is needed to reach Madeira\u2019s 17\u201318% abv.\r\n\r\nMadeira\u2019s aging process is rooted in its history. To mimic the conditions Madeira endured as it crossed the oceans during the Age of Exploration, the wine is heated and oxidized.\r\n\r\n\u201cThe maturation of Madeira usually involves exposure to relatively high temperatures, which affects the aroma and flavor composition of these wines,\u201d says Rubina Vieira, a Madeira wine educator with IVBAM.\r\n\r\nThe wines may either be heated rapidly in tanks through the estufagem process, or they may be aged over time in barrels through the canteiro process. The latter, while more expensive and time-consuming, tends to create more complex wines, as they slowly heat and cool in a warm, humid environment.\r\n\r\n\u201cOak casks lose water, and the resulting wine increases in acidity, sugar levels and alcohol content,\u201d says Vieira. Frasqueira, or vintage, Madeira undergoes this superior maturation process, as will most age-indicated Madeira that has aged for at least 20 years.\r\n\r\nThe estufagem process is cheaper and quicker, so it\u2019s used generally for entry-level, youthful wines made from Tinta Negra. \u201cThe descriptors \u2018baked,\u2019 \u2018brown sugar\u2019 and \u2018nutty\u2019 are typical descriptors for these wines,\u201d says Vieira.\r\n\r\n\r\nMarsala\r\nMarsala is among the world\u2019s historic wines, first fortified in 1773. Though commercialization in the past century resulted in a decline in quality, some producers in western Sicily have revived traditional, high-caliber Marsala.\r\n\r\nWith the exception of the less common rubino style, which uses red grapes, oro (golden) or lower-quality ambra (amber) Marsala is typically made from Grillo, Inzolia and Catarratto. The wine may be fortified to 17% or 18% abv at any point during fermentation and ranges from dry secco with up to 40 grams per liter (g/L) of residual sugar, to sweet dolce, with more than 100 g/L sugar.\r\n\r\nLike Sherry, quality Marsala is aged in a solera system, called in perpetuum, made of oak or cherry wood barrels. The wines are largely nonvintage and may be classified according to length of maturation. The range spans from fine, which must be aged for just one year, to vergine, which is dry and aged for at least five years. Vergine, and the 10 year-aged vergine stravecchio, show marked signs of this oxidative aging, with aromas of nuts, caramel and baking spice.\r\n\r\n\r\nVins Doux Naturels\r\nFrance\u2019s fortified wines, made in Languedoc-Roussillon\u00a0and the Southern Rh\u00f4ne, are made largely from Muscat Blanc \u00e0 Petits Grains, Muscat of Alexandria or Grenache. Though they vary stylistically by region, vins doux naturels (VDN) are always sweet and fortified with a 95\u201396% abv grape spirit before fermentation is halted. White wines can be aged oxidatively in barrels or glass demijohns. More often, they\u2019re unaged and youthful like Muscat de Rivesaltes, which features fresh stone, citrus and tropical fruit with floral and honeyed characteristics.\r\n\r\nRed VDNs continue maceration on skins even after fortification, which is why they may be deep in color and well-structured. But depending on whether they\u2019re aged oxidatively to create tuil\u00e9 or traditionnel wines, or made in a youthful style called grenat or rimage, they may range from juicy and deeply fruited to complex with dried fruit notes. Banyuls, Maury and Rivesaltes are all common regions for red VDNs.\r\n\r\n\r\nRutherglen Muscat\r\nWhile most traditional fortified wines hail from the Old World, Rutherglen Muscat is a gem of the New World. In Australia\u2019s inland Rutherglen region, a red-skinned variant of Muscat called Muscat \u00e0 Petits Grains Rouge is grown in warm vineyards.\r\n\r\n\u201cThe aim is to maximize the natural sugar content of the juice, fortify [it] with a neutral grape spirit and intensify the flavor character through long-term aging,\u201d says Ian Diver, winery operations manager for Campbells of Rutherglen. Ripeness at harvest affects the richness and concentration of the finished wine. Some producers pick earlier for freshness, while others wait until the grapes have shriveled on the vine.\r\n\r\nThe Muscat juice is fortified with a 96% abv spirit to make a very sweet wine of around 17.5% abv. Old barrels are used to promote oxidative aging, which produces nutty, savory, caramelized notes. Additionally, as water evaporates, the wines develop a luscious, viscous texture.\r\n\r\nRutherglen Muscat is typically a nonvintage wine, and some producers, like Campbells, use a solera system to age their wines. There are four classifications: Rutherglen Muscat, which averages three to five years of aging; Classic Rutherglen, which ages from six to 10 years; Grand Rutherglen, which averages 11\u201319 years; and Rare Rutherglen, with a minimum age of 20 years. The longer a Rutherglen Muscat ages in these barrels, the richer and more complex it becomes.