Sherry is a special group of fortified wines originally produced in Spain from various white grape varieties [1]. The sherry brand of wine is traditionally made from white grapes that are grown near the city of Jerez de la Frontera in Andalusia (the south of Spain) [2]. Traditional wine brands “Jerez-Xérès-Sherry” and “Manzanilla—Sanlúcar de Barrameda” from Andalusia are controlled by origin ^[3][4][3,4]. Young base wines can be made from grapes grown in the municipal area of nine regions—Jerez de la Frontera, Sanlúcar de Barrameda and El Puerto de Santa Maria and wine cellars in Trebujen, Chipione, Chiclana, Rota, Puerto Real and Lebrija ^[5][6][5,6]. However, the maturation of sherry wines until recently was limited to the so-called “sherry triangle”, that is, the three cities: Jerez de la Frontera, Sanlúcar de Barrameda and El Puerto de Santa Maria [6]. For a long time, there was a confrontation between the main producers of sherry, the cities of Jerez de la Frontera and Sanlúcar, concerning mainly the technical differences between Fino and Manzanilla; as a result, in May 2021, the Consejo Regulador of the Denominations of Origin, a corporation in public law, made amendments and assumptions [7]. The new agreement states that Fino can only be produced in Jerez de la Frontera or El Puerto de Santa Maria (possibly including the new municipalities), while organic wines produced in Sanlúcar can only be called Manzanilla [7]. Additionally, an important assumption in the rules is that sherry wines can no longer be cataloged as fortified wines if they have reached a minimum alcohol content of 15% (v/v ethanol) without the addition of alcohol. The amendments also include the use of a number of obsolete local pre-phylloxera grape varieties for sherry production, notably the archaic varieties Mantúo Castellano, Mantúo de Pilas, Vejeriego, Perruno, Cañocazo and Beba ^[7][8][7,8]. These and other concessions and assumptions will allow winemakers to prepare for the future by innovating in the good, old tradition of sherry manufacturing.

The specific organoleptic properties of sherry wines are acquired as a result of a combination of several factors, the most important of which are: (i) the application of the Solera-Criadera system during the fermentation of wine material in oak barrels [9] and (ii) using the specialized “sherry” (or flor) strains of yeast, Saccharomyces cerevisiae ^[10][11][12][10,11,12]. A unique feature of sherry strains of S. cerevisiae, in comparison with other fermentation strains, is their ability to form a biofilm (or flor) on the surface of alcoholic wine material, in which ethanol is oxidized to acetaldehyde under the action of alcohol dehydrogenase ^[13][14][15][13,14,15]. The ability of sherry yeast to work under conditions of high alcohol content is their adaptive mechanism, which affects the oenological characteristics ^[16][17][18][16,17,18]. There are numerous works devoted to both the mechanism of flotation of sherry yeast strains and the technology of aging sherry wines ^[9][10][13][9,10,13].

2. Worldwide Production of Sherry and Sherry-like Wines

In addition to Spain, Denominación de Origen (DO) sherry wines have been registered in the European Union in Italy (Sardinia), France (Jura), and Hungary (Tokay Hegyal) under the brands Vernaccia di Oristano [19], Vin Jaune [20], and Szamorodni [21], respectively (Figure 1, Table 1). To make these, winemakers use local white grape varieties Vernaccia and Savagnin in Italy and France, as well as Furmint, Harslevelű, and Sárga Muskotály varieties in Hungary. DO sherry wines are produced according to the classical flor technology with biofilm formation [22]. A number of other countries use flor technology as well as submerged fermentation and sherry wine aging technology for sherry-like wine (SLW) production (Figure 1, Table 1). Australia, Southern Africa, and Cyprus produce SLWs of different kinds, both in barrels and using New World winemaking techniques [24]. In the USA (California), a huge range of SLWs are produced, from dry to liqueurs and sweet, both by the film-forming method with flor yeasts and by the “backing” method in steel tanks with a special temperature regime, without yeast ^[8][24][8,24]. Popular in the UK, the SLW Harveys is made from imported must from traditional Spanish grape varieties [25]. A variety of SLWs with the common name Ialoveni are produced in Moldova, using the grape varieties Muscat de Ialoveni, Aligote, Traminer, White Pinot, and Rkatsiteli ^{[33][35][36][37]}[33,35,36,37]. In Ukraine, in the Odessa region, SLW is produced under the name Shabo using the Muscat Ottonel grape variety ^[24][32][24,32]. A lot of SLWs are produced in Yalta, such as Xeres Oreanda (from grape varieties Aligote, Albillo, Kokur) [29], Xeres Massandra (from grape varieties Sersial, Verdelho, Albillo) [30], and Xeres Magarach Massandra (from grape varieties Aligote, Sauvignon, Rkatsiteli) [31]. In Russia, SLW is produced in Krasnodar Krai (under the name Xeres Tamanskii from the Sauvignon Blanc grape variety) [26], in Rostov-on-Don (under the name Xeres Krepki Donskoi from the Aligote, Plavai, and Riesling grape varieties) [27], and in the Republic of Dadestan (under the name Xeres Dagestanskii from grape varieties Rkatsiteli, Narma, Gulyabi, Khatmi, and Ag-izym) [28]. In Armenia, strong white vintage SLW is produced in the regions of Ashtarak, Shaumyan, and Echmiadzin from the local grape varieties Voskeat and Chilar [34].

3. Sherry Wine Manufacturing Process

The history of the production of Spanish sherry goes deep into the past and is an integral part of the culture of the Andalusian region, which became possible due to the mild climatic conditions (3000 h of sunshine per year, an average annual temperature of 25 °C) [3]. Palomino Fino, Pedro Ximenez, and Moscatel grape varieties are traditionally used to produce high-quality sherry wines. The main types of classic dry sherry wines include the so-called Vinos Generosos (brands: Fino, Oloroso, Amontillado, Palo Cortado, and Manzanilla), Table 1. These dry wines are made from the same base wine but subjected to different aging procedures [38]. There are also blended fortified liqueur sweet wines in the category Vinos Generosos de Licor (brands: Dry, Pale Cream, Medium and Cream) and natural sweet wines in the category Vinos Dulces Naturales (brands: Pedro Ximénez, Moscatel and Dulce).

3.1. Production of Fino Sherry Wines

In Spain, the process of making sherry wines of various types has a strict gradation by stages (Table 2, Figure 2). Fino brand wines are biologically aged dry sherries and go through successive stages of preparation. For Fino, first-pressed Palomino grapes must enter the stage of alcoholic fermentation in order to obtain a young wine (sobretablas stage, Spanish). At this stage, in addition to S. cerevisiae, it is possible to detect other wine microorganisms (Figure 2), such as lactic acid bacteria and non-Saccharomyces yeasts, whose growth may lead to organoleptic deviations and deterioration of the wine [39]. To prevent lactic acid bacteria development, lysozyme could be used, as its muramidase activity has an antibacterial action [39]. Concerning non-Saccharomyces yeasts, these could be Candida stellata, Dekkera anomala, Hanseniaspora guilliermondii, Hanseniaspora uvarum, Issatchenkia terricola, Starmerella bacillaris, Lachancea thermotolerans, and Torulaspora delbrueckii, which are often found in nature on the surface of grapes ^{[40][41][42][43][44]}[40,41,42,43,44]. Further, the wine material is subjected to fortification (encabezado stage, Spanish) by adding ethanol at 15.0–15.5% (v/v ethanol), while the composition of the microflora changes dramatically—ethanol-sensitive microorganisms are almost completely replaced by sherry strains of S. cerevisiae, a distinctive feature of which is the ability to form a biofilm on the surface of the wine material—flor 1–3 cm thick [45]. Additionally, an important role in this process is played by such adaptive advantages of Saccharomycetes as high fermentation rate, resistance to alcohol, ability of anaerobic growth and rapid switching of metabolism from enzymatic to oxidative ^[42][46][47][42,46,47]. This sherry stage takes place in the criadera-solera system in oak barrels with a volume of 500–600 L ^[5][40][5,40]. It was shown recently that, in addition to Saccharomycetes, the wine microbiome at this stage may contain small amounts, of less than 0.5%, of the yeasts W. anomalus, P. membranaefaciens, P. kudriavzevii, and P. manshurica [5]. At present, their role in the maturation of sherry wines has not been elucidated [5]. In order for the flor to grow on the surface of the wine, the barrels are filled to 4/5 of their volume and arranged in pyramidal rows called “criaderas” ^[1][5][1,5]. There may be several such tiers, which are numbered from bottom to top, for example, for three tiers: first criadera, second criadera, and third criadera. The wine material is initially loaded into the uppermost tier, kept for a set time, and then enters the lower ones, until it reaches the very last, lower, “solera”, containing the oldest wine in the system. From here, the wine is sent for bottling. The bottom row of barrels is emptied by no more than 40% per year. Thus, the amount of wine withdrawn from the lower tier is compensated by the same amount from the upper tier [48]. The process of wine renewal is carried out 3–4 times a year; there is a constant circulation of wine material, and its organoleptic properties remain unchanged. The biological aging of wine material during its aging under the flor largely determines the properties of Fino brand wines and lasts at least 5 years ^[49][50][49,50]. The distinctive fruit and floral notes are formed in taste and aroma due to due to the formation of such compounds as farnesol, β-citronellol, β-ionone, and 1,1-diethoxyethane [49]. Additional characteristics are cheesy notes, such as rancidity and pungency due to the appearance of butanoic acid and acetaldehyde, respectively [49]. Fino brand wines have been shown to contain very little or no phenolic aldehydes [51]. This brand of sherry wine is also characterized by a pale shade, which is explained by the presence of a biological aging stage. It is assumed that the flor on the surface prevents the diffusion of atmospheric oxygen into the wine material, thereby preventing the appearance of brown pigments in it [51].

3.2. Production of Oloroso Sherry Wines

The Oloroso brand includes dry sherry wines obtained by oxidative aging. For the production of the base wine, which is subsequently turned into a finished Oloroso, the must of the second extraction is used (Table 2, Figure 2). Unlike Fino, Oloroso sherry fortifies up to 17–22% (v/v ethanol) ^[23][38][51][23,38,51]. Such fortification kills the flor of S. cerevisiae, that is formed during the settling of young wine, and eliminates the further presence of any yeast in the wine. Oloroso is aged like Fino for a minimum of 5 years [50]. However, the older the wine, the more worthy its blending organoleptic properties become. Oloroso has distinctive notes of tobacco and ether, dry fallen leaves, spices and truffles, associated with components such as ethylguayacol, ethyl acetate, sotolon, and others [52]. The wine of this brand can eventually change its color from amber to red-brown due to the formation of quinones during the oxidation of phenolic components, the type and amount of which depends on the grape variety and production technology ^[38][53][38,53]. Wooden barrels allow oxygen to pass through [54], and as a result of the absence of a yeast film, some polyphenols condense, in particular flavan-3-ol monomers with acetaldehyde [38]. Additionally, progression in browning is due to the condensation–oxidation reaction involving glyoxylic acid, which is formed by the oxidation of tartaric acid in the presence of certain metals [38]. Additionally, when Oloroso ages, a number of components are extracted from oak barrels, and its acidity, SO₂, Fe ions and O₂ content, temperature also change, which form a special organoleptic bouquet [38]. Unlike biologically aged Fino sherry, Oloroso sherry produces phenolic aldehydes of the benzoic type (vanillin and lilac aldehyde), cinnamon type (coniferyl and lilac aldehydes) and coumarins (scopoletin and esculetin) due to oxidative aging in oak barrels [51].

3.3. Production of Amontillado Sherry Wines

Amontillado is a dry sherry that has undergone a long biological aging period followed by full oxidative aging (Table 2, Figure 2). This means the Amontillado wine made from the same Palomino grape is first biologically aged like Fino type sherry, then subjected to fortification to a minimum 16.0% (v/v ethanol), which avoids the feasibility of S. cerevisiae cell development on the surface of wine, and afterwards it is transferred into an oxidative aging system [55]. Chemometric data analysis of Fino and Amontillado has marked great differences between two sherry wines [55]. Technologically, this is the most complex sherry, the taste and color qualities of which are between Fino and Oloroso ^[51][56][57][51,56,57] and depend on many factors, including average aging time, the time it has been in contact with oak wood, etc.