# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 5 Fortified Wines and Fortified Vinegar  
Fortified wines usually present a high alcohol content (15-22%, \(v/v\)), resulting from the addition of distilled spirits, and an oxidative character once they are usually produced under oxidative conditions. These wines can be produced using fermented and unfermented grape must. According to Council Regulation (EC) No. 491/2009 of 25 May, port and sherry are the most common styles of fortified wines. All of them can be used to produce sour-sweet vinegar that maintains some of the original wine's characteristics. Port wine is produced in the Douro Demarcated Region (DDR), Portugal, namely in Douro sub-regions \"Baixo Corgo\", \"Cima Corgo\", and \"Douro Superior\". Among the many authorized grape cultivars of DDR, Mourisco Tinto, Tinta Amarela, Tinta Barroca, Tinta Francisca, Tinta Roriz, Tinto Cao, Touriga Nacional, and Touriga Franca are most commonly used to produce port wine [86]. Considering the wine-making process, two major groups of wines can be differentiated: The bottle-aged ports and the wood-aged ports. During the fermentation of the grapes, once the concentration of remaining sugars reaches the desired degree of sweetness, the wines are fortified by the addition of grape spirit (77.0 \(\pm\) 0.5%, \(\upsilon/\upsilon\) ethanol at 20 \({}^{\circ}\)C) to the fermenting must. This is an important step in port production. The average ratio of grape spirit added is 115 L:435 L (grape spirit and must, respectively) [86]. Due to ethanol toxicity, fortification stops yeast metabolism. Grape spirit also favors polyphenol solubilization and triggers the precipitation of insoluble matter [87]. According to the aging method, oxidative or reductive, wines can be designated as \"ruby\" or \"tawny\" wines. Ruby ages in large wooden vats for 3-5 years, followed by bottling. They are wines with a red color, a full-bodied structure, and fruity notes. Tawny wines age in smaller wooden vessels (about 500-600 L) and usually show spicy, nutty, and woody notes, as well as explicit brown and yellow colors [88]. Sherry wine is a fortified wine made from white grapes that are primarily grown in the Sherry Triangle, located in southwestern Spain. Grape harvesting typically takes place in late summer or early fall, when the grapes are fully ripe. The most commonly used grape varieties for sherry production are Pedro Ximenez (PX), Palomino, and Muscat of Alexandria. The fermentation process of sherry wine involves the use of yeast species such as _S. cerevisiae beticus_, _cheresiensis_, _mottuliensis_, and _Zygossacharomyces rouzii_[89], also known as \"flor\" yeast. These yeasts grow on the surface of the wine and produce a layer of foam and yeasts known as the \"flor\", which gives sherry its distinctive nutty and oxidized flavor [14]. The veil of \"flor\" is formed of multicellular masses composed of fungi, bacteria, and yeasts, which are the later key microorganisms [90]. This veil is primarily responsible for the sensory features of sherry wine and its oxidative metabolism; however, the functional utilization of oxygen and the separation effect exercised by the yeast veil help to prevent excessive wine oxidation. Acetaldehyde inhibits veil formation at concentrations above the yeast's tolerance threshold [90]. During fermentation, the wine is kept at a constant temperature of around 20-22 \({}^{\circ}\)C to promote the growth of the veil. After fermentation, sherry wine is fortified with distilled grape spirits, typically to around 15-18% alcohol by volume. This fortification process helps stabilize the wine and prevent spoilage. The wine is then aged in oak barrels, where it undergoes a complex aging process that can last for several years or even decades [14]. During the biological aging of sherry wines, microorganisms, such as LAB (_Leuconostoc_, _Pediococcus_, and _Lactobacillus_) and fungi (_Botrytis cinerea_), can cooperate and coexist with the \"flor\" yeast strains mentioned above [91].