Sugar accumulation in grape berries is an important phenomenon that significantly impacts the amount of alcohol in wine. In addition, in berries, total sugar is a vital fruit quality factor in table grapes. The predominant sugars present in grapes are glucose and fructose, with only trace sucrose content in most cultivars' grape berries. Only a few high-sucrose content cultivars are detected in Vitis rotundifolia and hybrids between Vitis labrusca and Vitis vinifera [22,23,24]. Shiraishi et al. [25] identified two types of grapes based on sugar composition: hexose accumulators, for which the glucose/(fructose + sucrose) ratio was >0.8, and sucrose accumulators, for which this ratio was <0.8. According to Dai et al. [24], most Vitis vinifera cultivars have a glucose/fructose ratio of 1 at maturity, while this ratio varies from 0.47 to 1.12 in wild species. In addition, only a few species (Vitis champinii and Vitis doaniana) accumulate more glucose than fructose. Liu et al. [26] analyzed the sugar concentration of 98 different grape cultivars. They concluded that glucose and fructose were the predominant sugars in grape berries, ranging from 45.86 to 122.89 mg/mL and 47.64 to 131.04 mg/mL, respectively. Additionally, sucrose was present in trace amounts in most of the cultivars studied (except for two cultivars of hybrids between Vitis labrusca and Vitis vinifera, which contained large quantities of sucrose).

The accumulation of sugar in the form of glucose and fructose within the cellular medium, specifically in the vacuoles, is one of the main features of the ripening process in grape berries and is a primary commercial consideration for the grape grower, winemaker and dried grape producer. Thus, sugar content is an indicator often used to assess ripeness and to mark the harvest. Moreover, as most of the sugar is fermented to alcohol during the winemaking process, the measurement of sugar content, the so-called “must weight,” allows the control of alcohol content in the wine.

A schematic representation of grape berry development, sugar uptake, and metabolism during grape maturation is shown in Figure 1. Thus, during grape berry sugar accumulation, sucrose is produced in leaves by photosynthetic carbon assimilation and transported to the phloem berry [27]. Sucrose is loaded into the phloem by a symplastic or apoplastic mechanism [28]. The presence of an apoplastic step requires the involvement of membrane-located sugar transporter proteins (“hexose transporters” in Figure 1) mediating the exit of sucrose from the phloem and the uptake and compartmentation of sugars across the plasma membrane and the tonoplast of flesh cells [29].

In the first phase of berry growth, most of the sugar imported into the fruit is metabolized, and grapes contain relatively low sugar levels. However, at véraison, sugar accumulation begins, and the imported sucrose is converted into hexoses, which are stored in the vacuole. The grape berries accumulate glucose and fructose in equal amounts at a relatively constant rate during ripening [30]. According to several authors [31,32], massive accumulation of glucose and fructose in the vacuoles of mesocarp cells occurs after véraison and, twenty days after this period, the hexose content of the grape berry is close to 1 M, with a glucose/fructose ratio of 1. Since sucrose is the primary translocated sugar in grapevine, the rapid accumulation of hexoses characterizing berry ripening must involve the activity of invertases. Their expression is high at the early stages of berry development but declines considerably when hexose accumulation starts [28]. In addition, Hawker [33] found that invertase enzyme activity in Sultana berries increased immediately after flowering and that the activity peaked 6–7 weeks later, at véraison, when the rapid accumulation of hexoses commenced. According to the same author, another enzyme that might be involved in the breakdown of sucrose is sucrose synthase, which also increases during véraison. Still, their maximal activity is low compared to the level of invertase activity (200–300 times less). Invertases catalyze the hydrolysis of sucrose provided by the phloem conducting complex into glucose and fructose. Different invertase isoforms are localized in the cell wall, cytoplasm, and vacuole. Hydrolysis of sucrose by cell wall invertase may promote unloading by preventing its retrieval by the phloem and maintaining the sucrose concentration gradient.

Complex mechanisms regulate berry sugar accumulation. For example, the expression of disaccharide transporter genes (DSTs) and monosaccharide transporter genes (MSTs), sugar transporter proteins that mediate the exit of sucrose from the phloem and the uptake of sugars across the plasma membrane and the tonoplast of flesh cells, may be affected by various parameters, including light, water, and ion status, wounding, fungal and bacterial attacks, and hormones [34,35,36]. According to several authors [24,37], sugar composition is mainly determined by genotype, and sugar concentration is strongly affected by factors such as environment and cultural management. For example, irrigation has a variable effect on sugar accumulation in the grape berries. Thus, according to several studies [38,39,40,41,42], there is a variation in sugar concentration (increase, decrease, or no changes) due to irrigation practice. Esteban et al. [39] analyzed the impact of water availability on the yield and must composition of Vitis vinifera L. cv. During the three-year period, Tempranillo grapes concluded that total soluble solids and the concentration of glucose and fructose were significantly higher in the irrigated vines than in the non-irrigated vines, mainly towards the end of ripening. On the other hand, Intrigliolo et al. [41] consider that the effects of irrigation on must and wine composition largely depend on the climatic characteristics of each year, namely by the different rainfall amounts and crop levels.

For several researchers [43,44], temperature is an important environmental factor affecting grape sugar accumulation. For temperatures above 25 °C, net photosynthesis decreases even at constant sun exposure [45]. In addition, for temperatures above 30 °C, several authors [46,47] have reported a reduction of berry size and weight, and metabolic processes and sugar accumulation may completely stop. However, although high temperatures accelerate grape maturation, according to Coombe [47], temperature effects on final sugar accumulation are reported to be relatively small. Higher temperatures (30 °C) may lead to higher suspended solid concentrations, but Brix levels higher than 24–25 Brix (238.2 g/L of sugar to 249.7 g/L of sugar; 14.15% (v/v) estimated alcohol to 14.84% (v/v) estimated alcohol) are likely not due to photosynthesis and sugar transport from leaves and wood, but to concentration by evaporation [48,49]. In recent years, the alcohol content of wines has increased due to different factors. One of them is the sugar increase in grapes, which must be attributed to climate change [50]. However, according to [44], the exceptionally high sugar concentrations reached at harvest today, especially in warm climates, may be associated with the desire to optimize technical or polyphenolic and aromatic maturity. Finally, moderate water deficit, UV-B radiation, and low temperatures (below 30 °C) have a positive effect during grape ripening by increasing sugar content in grape berries [51,52]. Duchêne and Schneider [53] showed that, over the last 30 years, the estimated alcohol level of Riesling grapes in Alsace increased by 2.5% (v/v) due to warmer ripening periods and earlier phenology. Additionally, Godden and Gishen [54] observed in Australian wines an increase in the alcohol content from 12.3% (v/v) to 13.9% (v/v) for red wines and from 12.2% (v/v) to 13.2% (v/v) for white wines, between 1984 and 2004.

Taste strongly influences food intake [55], including alcohol consumption [56,57]. Alcohol activates olfactory, taste, and chem-esthetic receptors, and each modality is carried centrally by different nerves; these inputs affect the perception evoked by alcohol. Chemesthesis is defined as the chemical sensibility of the skin and mucous membranes. Chemesthetic sensations arise when chemical compounds activate receptors associated with other senses that mediate pain, touch, and thermal perception. Examples of chemesthetic feelings include the burn-like irritation from chili pepper, the coolness of menthol in mouthwashes and topical analgesic creams, the stinging or tingling of carbonation in the nose and mouth, and the tear-induction of onions. The oral consumption of alcohol by humans is accompanied by the chemosensory perception of flavor, which plays a vital role in its acceptance or rejection. Three independent sensory systems, taste, olfaction, and chemosensory irritation, are involved in the perception of flavor in food and wine in particular [58].

As Allen et al. [60] reported, humans perceive alcohol as a combination of sweet and bitter tastes, odors, and oral irritation (burning sensation). However, several researchers like Lanier et al. [58] found that some people describe experiences of more bitterness and less sweetness when drinking alcohol, and this directly relates to the genes they have inherited and individual differences in bitterness and sweetness as predictors of alcohol liking and intake in young adults. In addition, the perception of bitterness and sweetness also varies as a function of alcohol concentration [61,62].

Multiple studies [64,65] have linked variation in TAS2R (taste receptor, type 2) bitter receptor genes to alcohol intake. An important gene contributing to PTC (the ability to taste the bitterness of phenylthiocarbamide) perception has been identified [66]. The gene (TAS2R38—taste receptor, type 2, member 38), located on chromosome 7q36, is a member of the bitter taste receptor family. There are two common molecular forms (proline-alanine-valine (PAV) and alanine-valine-isoleucine (AVI)) of this receptor defined by three nucleotide polymorphisms that result in three amino acid substitutions: Pro49Ala, Ala262Val, and Val296Ile. Duffy et al. [67] reported that TAS2R38 haplotypes are associated with alcoholic intake, with AVI homozygotes, who perceive less bitterness from the bitter compound propylthiouracil (6-n-propylthiouracil (PROP) is a thiouracil-derived drug used to treat hyperthyroidism, including Graves’ disease, by decreasing the amount of thyroid hormone produced by the thyroid gland) consuming, significantly, more alcoholic drinks than heterozygotes or PAV homozygotes. More recently, Dotson et al. [68] reported associations between TAS2R38 and TAS2R13 polymorphisms and alcohol intake derived from the Alcohol Use Disorders Identification Test (AUDIT) in head and neck cancer patients.

In addition to bitter and sweet sensations, as we mentioned before, alcohol also irritates, commonly described as burning or stinging [58]. Burning sensations in the mouth are due, in part, to the activation of the transient receptor potential vanilloid receptor 1 (TRPV1) that is activated by noxious heat, capsaicin [69,70], and alcohol even at relatively low concentrations (0.1% to 3% v/v) [71]. When the TRPV1 gene is knocked out in mice, knockouts have a higher preference for alcohol and consume more than wild-type mice [72]. Collectively, these data suggest the TRPV1 receptor likely plays a role in the perception and acceptability of alcohol.

Many factors underlie the role that alcohol flavor plays in the development of alcohol preference and consumption patterns. Such factors include the activation of peripheral chemoreceptors by alcohol [70]; central mechanisms that mediate the hedonic responses to alcohol flavor [73]; learned associations of alcohol’s sensory attributes and its post-digestive effects and early postnatal exposure to alcohol flavor [74,75]; and genetically determined individual variation in chemosensation [21,61]. The study of the role of chemosensory factors in alcohol intake and preferences is of particular interest because the past decade has witnessed significant technical and scientific advances, which include the identification of receptors and other critical molecules involved in the transduction mechanisms of olfaction [76,77], chemosensory irritation [78], and taste [79,80,81,82].

The terms “body” and “fullness” are wine attributes frequently used to describe the in-mouth impression of both red and white table wines [83]. Wines are regularly classified as being light, medium, or full-bodied. Presumably, wines of different styles appeal to different market segments and are consumed in various social and culinary contexts. However, despite its widespread use and application, there appears to be a lack of common understanding within the wine trade of what sensory aspects contribute to the wine body. Most importantly, there seems to be no agreed position on the conditions for “fullness” in wine or other alcoholic beverages. Despite the apparent lack of agreement on what constitutes body in wine, Gawel [84] showed that experienced wine tasters, with extensive practical training, had an equivalent understanding of “body” in white wines and considered the feature important in distinguishing between the wines. It has long been speculated that alcohol strongly contributes to palate fullness in white wine [85]. Pickering et al. [4] were the first to test this premise formally. They found that the perceived density of a de-alcoholized wine generally increased with increasing alcohol over a 14% (v/v) range, while its perceived viscosity was highest at 10% (v/v) ethanol. Later work [86] using model wine solutions showed a positive monotonic effect of alcohol content on perceived viscosity and density over the same alcohol range, further supporting a positive relationship between alcohol content and fullness in white wine.

The contribution of ethanol to wine sensory properties extends beyond that of possibly enhancing fullness. Ethanol affects the headspace concentrations of many wine volatiles [87] and contributes to sweetness [88]. Furthermore, ethanol-induced palate warmth and perceived viscosity may indirectly affect aroma and flavor perception. Moreover, according to the work of Joshi and Sandhu [89], the sensory evaluation results of different vermouths prepared with varying concentrations of ethanol, sugar levels, and spices extract showed significant differences for various sensory quality parameters. The data obtained revealed that for color and appearance, 12% (v/v) and 15% (v/v) of alcohol with 2.5% (w/v) spices extract scored better, but for aroma, virtually all the treatments were comparable. However, in total acidity, vermouth with 18% (v/v) ethanol scored lower than those with 12% (v/v) and 15% (v/v). In bitterness and astringency, vermouths of all the treatments were comparable. In overall quality, apple vermouth with 15% (v/v) ethanol, 2.5% (w/v) spices extract, and 4% (v/v) sugar content scored the highest. So, bitterness, astringency, and total acidity are influenced by the alcohol vermouth concentration. However, for Noble [90], the higher concentrations of alcohol in wines enhance bitterness intensity but do not affect the perception of astringency.