Revised Viticulture for Low-Alcohol Wine Production: Strategies and Limitations

Abstract

Interest in the wine sector focusing on no- or low-alcohol wines is growing. De-alcoholation, typically a post-fermentation process, faces restrictions in some countries and is often quite costly. Using raw materials like low-sugar grapes suitable for this purpose seems logical, yet the literature currently lacks contributions in this area. In this review paper, we outline an ideal ripening process where the goal of producing “low-sugar grapes” can be achieved through various methodologies applied at (i) the whole-canopy level (minimal pruning, hedge mechanical pruning with or without hand finishing, cane pruning combined with high bud load and no cluster thinning, applications of exogenous hormones, late irrigation, and double cropping); (ii) the canopy microclimate level, involving changes in the leaf area-to-fruit ratios (netting, apical or basal leaf removal, late shoot trimming, use of antitranspirants); and (iii) through new technologies (high-yield plots from vigor maps and the adoption of agrivoltaics). However, the efforts in this survey extend beyond merely achieving the production of low-sugar grapes in the vineyard, which is indeed primary but not exhaustive. Therefore, we also explore solutions for obtaining low-sugar grapes while simultaneously enhancing features such as lower acidity, increased phenolics, and aroma potential, which might boost consumer appreciation. The review emphasizes that (i) grapes intended for low-alcohol wine production should not be viewed as a low-quality sector but rather as an alternative endeavour, where the concept of grape quality remains firmly intact and (ii) viticulture for low sugar concentration is a primary strategy, rather than merely a support to dealcoholization techniques.

1. Introduction

The fields of viticulture and enology are expected to be highly complementary, with strong connections at academic, scientific, and technical levels. However, when “innovation” collides with the “tradition” of the wine value chain, one discipline sometimes advances more rapidly than the other without any seemingly convincing reason. A pertinent case study is the application of “precision viticulture”, which has seen a continuous increase in publications over the past two decades [1], while “precision enology” remains relatively underdeveloped and often confused with standard winemaking techniques. To address this imbalance, although the concept of “natural wines” has recently garnered significant attention and curiosity among stakeholders [2], we cannot claim that “natural vineyards” have achieved the same level of recognition, likely due to the influence of growing systems such as organic or biodynamic.

An illustrative example can be found in the opportunities and challenges presented by low-alcohol wines, which lack an equivalent in “low-sugar viticulture.” This is somewhat peculiar, especially when considering the shared classification of wines with reduced alcohol content [3], which includes the following: de-alcoholized: <0.5%; low-alcohol: 0.5–2.0%; reduced alcohol: 1.2–4.5%; lower alcohol: 5.5–10.5%. Although this is not the focus of our review, it is worth noting that the ethanol concentration in wine can be adjusted (e.g., reduced) during and after fermentation through various techniques. These include selecting low-ethanol-producing yeasts or physically removing alcohol via distillation or membrane-based technologies such as reverse osmosis, vacuum distillation, spinning cone technology, membrane separation, and evaporative perstraction [4,5,6,7]. In Europe, Commission Regulation No.606/2009, allowing the partial removal of alcohol using physical separation techniques up to a maximum of 2% relative to the original alcohol content, is being replaced by less restrictive national regulations. For instance, in Italy, since 1 January 2025, the production of de-alcoholized (<0.5%) or low-alcohol (0.5–8.5%) wine is now permitted, de facto eliminating the maximum 2° vol de-alcoholization threshold.

Within the above scenario, however, confining the role of viticulture solely to managing total soluble solids (TSS) levels in grapes seems overly restrictive and leads to another critical question: “How do people perceive a low-alcohol beverage?” In the context of wine, there is a general consensus that a “lack of taste or body”, often linked to flavor imbalance, is a significant drawback for low-alcohol beverages [8,9]. People still tend to view wines with reduced alcohol content as lower quality products, which may lead them to be willing to pay less for such wines [3,10,11]. It is also well established that, regardless of the method used for de-alcoholization, removing ethanol from wine can cause notable sensory changes due to a reduction in volatile compounds, particularly esters and terpenes. Additionally, reducing alcohol content may facilitate the binding of aroma compounds to protein-like substances, diminishing their volatility and sensory impact. Moreover, the methods employed to reduce alcohol content can occasionally result in undesirable off-flavors [12,13,14,15,16]. It is crucial to recognize that alcohol itself contributes taste properties, such as sweetness and bitterness, and can significantly enhance both taste and aroma. As de-alcoholization progresses, acidity, bitterness, and astringency often become more pronounced, sometimes reaching a point of imbalance due to the reduced softening and harmonizing effects of alcohol.

While the significant changes in wine flavor and character following de-alcoholization have been outlined, it is overly simplistic to suggest that viticulture’s role is merely to produce berries with lower sugar content. The objective is more ambitious, as achieving low sugar levels should ideally be separated from factors that result in excessive acidity, harsh tannins, overly strong herbaceous notes, and a poor or indistinct flavor profile. In the following paragraphs, we will outline our general perspective on how grape ripening can be exclusively manipulated or guided through pre-harvest interventions. This approach aims to achieve an overall composition that, depending on the final enological target, could (i) make post-harvest de-alcoholization unnecessary or significantly easier and (ii) enhance the perceived quality of low-alcohol wines.

2. Ideotype of Berry Ripening for Low-Alcohol Wines

The dynamics of sugar accumulation in the grapevine berry is a well-documented process [12]. Until the end of the lag phase, the TSS remain relatively stable, ranging around 3.5–4 Brix. However, at veraison, there is an exponential increase that tends to become linear and eventually saturates. Later in the season, TSS may exhibit another abrupt rise, which is typically an artifact resulting from berry water loss. It is also well established that the rapid increase in TSS at the onset of veraison occurs within 24–48 h and is synchronous with berry deformability, whereas other processes, such as color appearance and the resumption of berry enlargement, are delayed by a few days [13,14,15]. If the ultimate goal is to achieve a lower TSS compared to normal-strength ripening, it stands to reason that the factors influencing the timing and rate of the initial TSS increase will also affect the final TSS level at harvest.

The remarkably consistent TSS accumulation patterns across a wide range of genotypes, locations, year-to-year variability, and cultural factors suggest a need for strict genetic control. Yet, surprisingly, the nature and mechanism of such control remain largely unexplored. According to the literature [16], water deficit, whether applied pre- or post-veraison, accelerates ripening and induces changes in gene expression regulating flavonoid biosynthesis in grape berries. However, while genes coding for total anthocyanin biosynthesis showed high responsiveness, sugar accumulation and the onset of veraison were much less affected. Recently, a paper may have shed new light on the subject; in the search for a major quantitative trait locus controlling ripening speed, Ref. [17] discovered that a haplotype originating from the American species Vitis riparia [18] can halve the maximum sugar accumulation speed, regardless of crop levels and berry sizes. This finding opens new possibilities for either delayed complete ripening or earlier low-sugar ripening. Regrettably, the same authors note that Vitis vinifera cultivars exhibit limited phenotypic variation in ripening speed. However, this is somewhat contradicted by an intriguing study conducted in Spain [18], which examined several varieties to assess the initial sugar level at the first sign of berry coloring. The study revealed surprisingly high variability; over three years of observations, Alicante Bouchet recorded the lowest value (≈6 °Brix), while Grenache registered the highest (12 °Brix). The study also demonstrated that these thresholds were not tied to any specific chronological date but were instead genetically controlled. Another study, which examined the ripening trends of 24 cultivars grown in Victoria State, Australia, over 20 years [19], showed significant differences in ripening rates among cultivars. It found that (i) later-maturing cultivars ripened more slowly (slope given as Baumè/day) and (ii) higher yields slowed down the ripening rate.

Turning to cultural/endogenous factors, it is widely agreed that the rate of sugar accumulation and, consequently, the final sugar concentration at harvest is closely linked to the source-to-sink vine balance, best expressed by the total leaf area-to-yield (LA/Y) ratio. As demonstrated in numerous studies [20,21,22,23,24], a significant source deficiency (i.e., LA/Y lower than 0.5 m²/kg) at the onset of veraison can lead to severely delayed ripening, resulting in quite low TSS at harvest. While this principle has been generally validated, two considerations must be noted. Firstly, (i) the “working” LA/Y ratio should be synchronized with the onset of veraison rather than estimated at harvest. Assuming no additional leaf area develops between veraison and ripening, extrapolating the LA/Y ratio back to the veraison stage typically provides a more “optimistic” assessment of the source per unit of crop, as a portion of berry growth is yet to occur. Secondly, (ii) the rationale above holds true if no significant main or lateral shoot regrowth occurs before harvest, thus ensuring the “source” assessment remains stable. An exemplary case of an ideal response within the framework of “low-sugar viticulture” is illustrated by Cabernet Sauvignon vines [23]. These vines were shoot-trimmed after fruit set to retain only six main leaves, with any emerging laterals subsequently removed. This approach created a “permanent” source limitation, resulting in the severely pruned vines achieving a TSS of just 14.6 Brix at harvest, compared to 19.1 Brix in the untrimmed plants. The LA/Y ratios were 0.52 m²/kg and 1.73 m²/kg, respectively.

A valuable tool for determining whether a given LA/Y estimated at veraison is limiting, and consequently leads to a slower and less efficient sugar accumulation pattern, is derived from modeling. As noted in [25], a weather-based seasonal model has been developed to dynamically estimate the LA/Y ratio, which is useful for assessing whether a source limitation exists at the critical time of veraison. However, the ripening ideotype illustrated in Figure 1 offers even greater flexibility if the ultimate goal is a reduced TSS. Indeed, regardless of the current LA/Y ratio at the onset of veraison, there are tools and techniques available to impose a calibrated source limitation, some of which will be discussed in a subsequent chapter of this review.

While the ambition to develop a ripening pattern suitable for low-alcohol wines should not be confined to merely limiting the final TSS, it is important to note that reduced alcohol red wines often lack ripe fruit aromas or mouthfeel characteristics [26,27]. In contrast, the effect of de-alcoholization on the flavor profile of white wines is more variable. For example, reducing the alcohol content by up to 2% v/v in oaked Chardonnay wine did not significantly alter its sensory composition [28]. Conversely, in the white semi-aromatic variety cv Falanghina, nearly 50% of higher alcohols, acids, and lactones were retained in dealcoholized wine with a 9.8 vol% alcohol content. However, this percentage dropped to 30% in the sample with a 6.8 vol% alcohol content and was even lower in the dealcoholized wine with an even lower alcohol content.

Considering this scenario, the temptation is to guide grape ripening towards lower TSS while achieving some decoupling in terms of reduced TA and green taste, improved aroma profile, and enhanced phenolic structure. Therefore, it is essential to consider how closely the synthesis of these latter compounds aligns with the dynamics of sugar accumulation and what the chances are of keeping these processes somewhat distinct. Based on the ripening patterns of different groups of grape components from fruit set to harvest reported in [29], a first group, including sugars, anthocyanins, terpenes, nor-isoprenoids, proline, and arginine, shows low amounts from fruit set until veraison, followed by a rapid linear increase before reaching a pre-harvest plateau. Since these compounds share the same accumulation kinetics, achieving selective conditioning for each of them appears particularly challenging. A different scenario is observed for a second group, which includes malic acid, methoxypyrazines, carotenoids, catechins, and tannins. These compounds exhibit a trend opposite to the previous group; they maintain high and constant concentrations until veraison, followed by a rapid decline, resulting in low and stable levels at harvest. Consequently, it is inherently easier to manipulate these compounds to achieve desired kinetic effects.

In addition to the type of model that describes the temporal pattern of accumulation or degradation of a given compound, it is crucial to consider the sequence of various events over time. For instance, when field berry tasting is conducted to assess ripening progress, observers often note that the emergence of grape flavor is abrupt and occurs late in the ripening cycle, specifically when berries have already significantly accumulated sugar [30]. This gap was quantified in Riesling samples by evaluating juice aroma from a field experiment in the Barossa Valley, South Australia [31]. The authors discovered that juice from irrigated and thinned grapes, harvested at a total soluble solids concentration of 16.5 °Brix, had a low aroma score of 3.7 (on a scale of 1 to 10). In contrast, juice sampled from the same vines two weeks later, at 19.8 °Brix, achieved a maximum aroma score of 8.1. This sudden and steep rise in aroma may partly be due to the aroma threshold effect, where floral volatiles are not perceived until their concentration surpasses their aroma threshold levels. Further evidence of the late accumulation of aromas in grape berries compared to the sugaring process is provided by Shiraz grapes grown in Riverland, South Australia. These grapes showed that at the already remarkable TSS of 10.5 Brix, the amount of non-anthocyanin glycosides [32] was still minimal, whereas after reaching 20 Brix, there was a sudden and rapid increase.

Overall, it appears feasible to develop a ripening pattern that reduces sugar concentration by primarily influencing the timing and slope of the TSS curve, while simultaneously enhancing the compositional profile of other must components. This approach is likely to significantly increase consumer appreciation of future de- or low-alcohol wines and will be explored in greater detail in the following paragraphs.

3. Aiming at Low Sugar at Harvest: Keep It Simple First

When the primary objective is to achieve a significantly reduced sugar concentration, three “simple” solutions stand out: (i) harvesting early, (ii) increasing yield, and (iii) utilizing late-season vegetative competition. Early harvesting is an obvious approach to producing low-alcohol wine [33,34]; the sugar concentration in berries can be easily monitored throughout the post-veraison season, and harvesting should occur before the TSS exceed a certain threshold (e.g., 15 Brix or 9° alcohol v/v). However, this early harvest can pose logistical challenges, especially in warm regions affected by global warming, where an “early harvest” might occur as early as mid-July. There is a consensus that such grapes would have undesirable acidic and unripe flavors [33,35], which are not well-received by consumers. This issue is difficult to mitigate because, for example, tartaric acid is relatively temperature-insensitive [36], and, as previously discussed, many aromas have yet to be synthesized and accumulated [31]. Our conclusion is that “early” harvesting, as a standalone technique for producing low-alcohol products, does not offer a comprehensive solution.

Another approach, sometimes referred to as “early harvest”, offers a distinct perspective. As noted in [37,38,39], this method involves blending wine made from early harvested grapes with those from later harvests to produce a less alcoholic wine or fermenting them together. In a study on Cabernet Sauvignon [39], the “early harvest” involved picking fruit at 8.1 °Brix to create a wine with very low alcohol and pH levels (4.5% and 2.76, respectively). Essentially, clusters removed through thinning just after veraison are used to produce “unripe wine”, which is then blended with a portion of late harvest wine. Interestingly, in this study, the wines that received the highest appreciation were those where alcohol reduction was achieved through water addition, a practice forbidden in several EU countries. Notably, overripe sensory attributes like ‘hotness’ and ‘port wine’ were preserved in wines where the unripe must was blended with grapes from the commercial harvest date of that vineyard. More promising results were observed in a trial involving Merlot, Cabernet Sauvignon, and Bobal [40]. A portion of the thinned clusters at veraison was used to produce a very acidic must (TSS at 5 °Brix, total acids at 17.8 g/L, and pH = 2.78). After fermentation, the wine was treated with activated carbon and bentonite to remove phenolics and aggressive green flavors, resulting in an odorless and colorless product. To adjust the composition of wines from normal harvests, an aliquot of the green must was added to replace an equivalent amount of standard must for each cultivar. Within batches of 8 kg of grapes and a must yield of 6.4 L, the replaced must fraction was 0.85, 1.50, and 2.0 L for Cabernet Sauvignon, Merlot, and Bobal, respectively. The results were as expected, with the final alcohol content in the blended wines reduced by 0.9% (Cabernet Sauvignon), 1.7% (Merlot), and 3.0% (Bobal) compared to traditional wines. Additionally, due to a significant reduction in wine pH, the blended wines also showed higher total anthocyanin concentrations.

Increasing yield until sugar accumulation becomes limited is another viable option. However, the physiology underlying this practice is far from simple.

The first requirement for making this approach valuable is achieving yield per vine levels high enough to significantly limit berry sugar import. Assuming that this balance can be easily achieved by merely increasing the crop on the vine is somewhat unrealistic. Numerous examples in the literature demonstrate that, for a given site and genotype combination, there is always a yield range that can maintain TSS that are overall unchanged [22,41,42,43]. This can be easily explained by the fact that it is not the total yield itself, but rather the amount of leaf area (LA) per yield (Y) that primarily controls sugar accumulation dynamics [21]. Indeed, at different yield levels, there might be sufficient LA to ripen the fruit. Beyond this point, however, the yield becomes excessive, LA is significantly reduced, and berry sugar accumulation is impaired. Research has indicated that the final TSS may begin to be significantly reduced when the LA/Y ratio falls below 1 m²/kg [20,22,24]. This idea is illustrated in Figure 2, where the red box highlights a range of low LA/Y values that are highly likely to result in low sugar levels at harvest due to an excessive limitation of sources.

Employing the high-cropping method eliminates the need to anticipate the harvest, as the underlying principle involves a reduced amount of sugar available for translocation into the clusters. The primary technical challenge lies in how to establish and sustain a “constantly high cropping system”. It is widely acknowledged that the model correlating the number of nodes per vine retained during winter pruning with the yield per vine is sigmoid in nature, progressively indicating maximum yield at a relatively high bud load level. However, this yield does not increase further, even if winter pruning becomes lighter, due to well-researched yield compensation mechanisms, such as lower budburst and fertility, smaller clusters, and smaller berries [44,45,46].

The most straightforward and cost-effective method to achieve such balance is mechanical winter pruning (Figure 3), which ideally should not be followed by any manual intervention [47,48]. It is well established that hand finishing tends to align the bud load on the vines with that of standard hand pruning, thereby missing the chance for a consistently limited sugar accumulation [43]. Once the desired bud load is reached (for example, doubling the number of nodes compared to the load retained under hand pruning), this pattern should generally be maintained across seasons.

When working with cultivars that exhibit high basal node fruitfulness, it becomes relatively straightforward to adjust yield beyond the threshold where leaf area (LA) becomes limited, leading to a significant restriction in sugar accumulation. This aligns with the ultimate goal of producing low-alcohol wines. The most effective approach to achieve this involves short winter mechanical pruning, with or without gentle hand finishing, on permanent cordon training systems such as spur-pruned permanent cordons, T trellises, GDC trellises, and single high wire trellises (Figure 3). When combined with mechanical grape harvesting, managing these training systems becomes particularly attractive, as the high yield is associated with a relatively limited number of working hours per hectare [49,50,51]. Ultimately, the renewed interest in low-alcohol products may serve as a kind of “vindication” for the use of mechanical pruning in viticulture, which has historically been criticized for disregarding quality, vine health, and longevity [47].

A third, relatively straightforward method to slow sugar accumulation in berries involves leveraging late-season vegetative competition. Previous studies have indicated that if such competition occurs after veraison, it can effectively limit sugar accumulation, even when the LA/Y ratio is not genuinely restrictive [23]. While transitioning to mechanical winter pruning is a relatively simple decision, employing “excessive” vegetative competition presents some practical challenges. Firstly, the vines must be capable of responding with a late-season surge of vegetative growth. If the vines are already bearing a high yield, this reactivity might be reduced, as vegetative growth can be naturally curtailed earlier in the season. Nonetheless, from a physiological perspective, there is always the potential to encourage late-season lateral regrowth, which can indeed exert significant competition during the ripening phase. In a dry-farmed vineyard with adequate soil water availability, late-season shoot trimming (i.e., during the lag phase or onset of veraison) is the simplest way to promote competitive lateral regrowth. However, if the season is too dry to support such a response, as is often the case, late-season irrigation might be a viable alternative [52,53].

4. Aiming at Low Sugar at Harvest: A More Sophisticated Approach

Figure 4 presents a summary of various techniques and practices that could be suitable for achieving a significant reduction in berry sugar accumulation while still allowing the grapes to retain certain characteristics that, paradoxically, are indicative of quite ripe grapes (e.g., lower TA, enhanced phenolics and flavors, reduced green taste, etc.). Achieving this latter aspect is particularly challenging, as it requires sugar accumulation to be relatively decoupled from other processes that are naturally interconnected. A prime example is the dynamics of sugar and anthocyanin accumulation in the berry. A soft, yet still green berry, has already begun its rapid increase in sugars, whereas berry pigmentation will take a few more days to commence; this is a clear example of a close “coupling”. The objective, therefore, is to selectively limit the sugaring process without hindering the pigmentation process. This is commonly referred to as “decoupling”, and in the context of global warming, it is currently one of the most challenging ripening issues for red varieties grown in warm–temperate climates [54,55].

While some of the practices illustrated in Figure 4 have been presented and discussed in the previous paragraph, others involve more specific and targeted approaches. Within the scope of “whole-vine approaches”, minimal pruning is a technique that may regain interest if low-sugar viticulture is the goal [47,56,57,58]. Implementing almost no pruning during winter ensures a high crop level; for an even more stringent limitation, a lateral summer skirt can further reduce leaf area to achieve a low LA/Y ratio. Additionally, some inherent growth and ripening characteristics of minimally pruned vines may enhance, for example, phenolic composition and disease tolerance, the latter also ensuring better preservation of primary aromas. These features typically result in numerous small, loose clusters with small berries, a trait often observed in wild grapes [59]. Small berries, with a higher relative skin ratio, can increase anthocyanin concentration, while cluster looseness is a key factor in limiting cluster rot and unwanted off-odors [60]. It is also worth noting that a single or bilateral high cordon is the ideal training system for a minimal pruning strategy, as it greatly facilitates the summer skirt.

Under the definition of cane pruning combined with a high bud load and no cluster thinning, we identified several traditional expanded training systems, such as Sylvoz, Casarsa, overhead, and various pergola and pergoletta types, which remain prevalent in Italy. These systems could gain renewed interest if the goal is to achieve low sugar levels [61]. A prime example is the “pergola” trellis, where a high bud load is maintained with several long fruiting canes on the same plant, allowing yield per vine to easily surpass 15–20 kg, thus resulting in low sugar potential. However, the specific microclimate of the clusters might help separate sugar composition from other ripening parameters; in red cultivars grown in warm environments, the predominantly shaded clusters might synthesize and preserve color more effectively [62], while in white cultivars, reduced photo-oxidative sunburn incidence could positively influence wine sensory properties [63].

Although spraying grapevine canopies with growth regulators may not be the most appealing solution, detailed research on pre-veraison auxin treatments (1-naphthaleneacetic acid) on cv. Shiraz [64] has confirmed their effectiveness in delaying both sugar and anthocyanin accumulation while significantly enhancing the synchronicity of berry sugar intake. Similar effects, in terms of ripening delay and more uniform TSS and malic acid concentration within the same cluster, were also observed in a trial on Riesling [65]. However, in none of these studies has spraying exogenous auxins proven effective at selectively retarding TSS with a good level of decoupling from total anthocyanins and malic acid, respectively.

At the whole canopy level, the latest and somewhat groundbreaking approach is the protocol recently termed “double cropping” [66], which essentially takes advantage of the extended growing season encouraged by global warming. Even in temperate climates, where a dormant phase typically separates two growing seasons, it is now possible to harvest two crops within the same season (Figure 5). To accomplish this, dormant buds, which undergo induction and partial differentiation in their first year, must be forced to break dormancy and develop within the same year they are formed [67]. Essentially, their behavior mirrors that of a prompt bud. This system is regulated by a phenomenon known as “correlative inhibition”, which suggests that once a dormant bud has entered an eco-dormancy stage, it can only develop if young organs and meristems are appropriately removed [67,68].

The practice of “double cropping” has been successfully tested for two years on Pinot noir in Northern Italy [66] and on Grenache, Tempranillo, and Maturana tinta in Spain [69,70]. This approach retains the primary crop, leading to a partial overlap between the primary (P) and forced (F) crops. It involves drastically trimming the main shoots around the fruit set while simultaneously removing any lateral shoots. This treatment typically allows dormant buds to be released, initiating a second reproductive cycle on the same vine. In the Pinot noir trial [66], the crop from the forced primary shoots was about 40–50% of that from the regular primary shoots. Interestingly, the second crop’s grape quality showed higher TSS, total anthocyanins, phenolics, and total acidity. While the standard crop reached ripeness by the second week of August—a typical pattern for an early-ripening variety grown in an environment providing 1800–1900 GDD—the forced crop was harvested on 7 and 8 October. An appealing aspect of this bold technique is not only its capacity to achieve two harvests within the same season but also its flexibility in producing batches of grapes with such varied compositions that different market segments can be profitably targeted. If the goal is to obtain grapes with maturity suitable for low-alcohol wines, the primary crop obtained in the forced treatment might achieve the desirable trait of low TSS, primarily due to two overlapping effects: (i) early harvest and (ii) a limiting source due to the quite severe trimming of the main shoot and removal of the developing laterals. It is enticing that, on the same vine, two crops with almost diametrically different grape compositions can be obtained (Figure 5).

5. Acting on Microclimate or Source–Sink Balance

Figure 4 reports four techniques which might be useful to achieve final low TSS through a finer modulation of canopy/cluster microclimate or modification of the vine source-to-sink balance at specific growth and ripening stages.

While vineyard netting is known to be a multi-purpose strategy [71], its specific effects due to a certain degree and duration of shading that is cast usually on a sector of the canopy might be functional to a reduced grape sugar content. While trying to find commonalities among the various works nicely summarized by [72], it is quite clear that it is possible to separate “consistent” from “variable” effects; among the former we include (i) a general reduction in vigor and/or vegetative capacity, (ii) a postponement of ripening, (iii) lower sugar concentration and higher concentration in total acidity and malic acid, and (iv) reduction in pH. Variable ones certainly include effects on anthocyanins and the volatile aromatic fraction. They can be defined overall as mild or nil as well as those on yield and some of its main components [73].

One operational conclusion from this analysis is that, for now, excluding the use of photo-sensitive nets (i.e., those of colors other than black and white), the most “solid” effect we can anticipate from nets with a prominent shading function is, besides the expected reduction in sunburn damage, a general slowdown in technological ripening. This suggests a preferential use of nets in environments where sparkling or semi-sparkling base grapes are produced, especially if a low-sugar product is desired. The use of netting is also intriguing as it may act as a decoupling factor in the ripening of red cultivars. Sugar levels can be limited due to generally lower total canopy photosynthesis, while the cooling effect of the net at the cluster level might positively influence anthocyanin accumulation, as demonstrated in a trial conducted on Cabernet Sauvignon [74].

Depending on the timing and method of leaf removal, it can become a crucial practice for shifting vine balance towards lower sugar production. The technique known as “apical to the cluster leaf removal” [75] aims ambitiously to delay or slow sugar accumulation without adversely affecting phenolic and aromatic ripening. This method is notable for its consistent effects and, importantly, its simplicity and ease of implementation. It involves a specific type of defoliation, performed mechanically and later in the season, on the upper part of the canopy. This approach is grounded in an understanding of leaf functionality physiology, which, although somewhat dated, remains valuable [71,72,73]. It is well-established that from veraison onward [74], the leaves in the middle and apical sections of the canopy are the most functional, having reached maturity but not yet senescent [74]. Thus, the concept of creating a “window” of defoliation in this area emerges, aiming to induce a deliberate and calibrated photosynthetic limitation to slow sugar accumulation. Two pioneering studies conducted over two years on Sangiovese vines in Umbria and Emilia-Romagna demonstrated that high defoliation performed post-veraison [71,75] effectively delayed the target sugar level by about two weeks without hindering the accumulation of anthocyanins and polyphenols. Subsequently, the technique, applied in a similar fashion, proved effective for both Montepulciano [76] and Ortrugo [72].

For those planning to utilize this technique, it is generally recommended to ensure its effectiveness by performing the operation before the average sugar level surpasses 12 °Brix, and by removing no less than 30–35% of the leaf area present. Additionally, this intervention is particularly favored by winegrowers, as it can be fully mechanized using a standard leaf-plucker machine. Unlike traditional defoliation, which is conducted at the cluster band level, this method operates in an area free of clusters, alleviating the operator’s concern about damaging the fruits. However, to create a “window” of vegetation approximately 50–60 cm high, two quick passes per row are necessary. Some authors [71] have sought to explore the reasons why this technique, as observed so far, can delay sugar accumulation without simultaneously postponing phenolic ripening. The explanation lies in the fact that, due to the “high” level of defoliation, the microclimate at the cluster level remains unchanged compared to defoliated vines. As previously discussed [77], the synthesis and degradation of anthocyanins primarily depend on the “local” thermal microclimate at the cluster level, so there is no reason to expect significant variation in pigment synthesis compared to a non-defoliated scenario. On the other hand, it is well established [78] that a canopy’s overall potential to accumulate sugars largely depends on its ability to intercept incident radiation and perform effective photosynthesis. This capability is “intentionally” and “partially” reduced by the technique, thereby achieving the desired effect in this context.

The traditional practice of basal leaf removal, usually carried out from fruit set to veraison, is intended to decrease canopy density and improve light penetration and air circulation. However, its ability to affect the berry sugaring process seems quite limited [76]. This is because the older basal leaves, which are already experiencing reduced photosynthetic activity, are the ones removed. Moreover, defoliation may prompt photosynthetic compensation from the younger leaves situated higher up [23,77,78], thereby lessening the impact on sugar accumulation.

Nonetheless, late basal leaf removal can still offer advantages within a low-sugar viticulture framework. By enhancing light exposure for high-yielding grapevines, it can significantly influence the synthesis and/or degradation of volatile and bound chemicals. These chemicals primarily encompass monoterpenes (abundant in “floral” grapes), norisoprenoids, benzenoids, aliphatics, and methoxypyrazines [31,79]. While the seasonal accumulation pattern of terpenes and nor-isoprenoids mirrors that of sugars, methoxypyrazines exhibit an opposite behavior, peaking at veraison and then declining in concentration depending on the level of canopy shading [29].

The timing of basal leaf removal and its impact on the final levels of methoxypyrazines is intriguing. According to [80], exposing clusters reduced the accumulation of 3-isobutyl-2-methoxypyrazine (IBMP) by 21−44% at all pre-veraison stages, without enhancing post-veraison degradation. A survey of 13 sites in New York state revealed that IBMP concentrations measured two weeks before veraison were highly correlated (R² = 0.936, p < 0.0001) with levels at harvest, whereas traditional grape maturity indices at harvest showed no correlation with IBMP levels. Therefore, if the goal is to achieve low-sugar grapes with low methoxypyrazine concentration, pre-veraison basal leaf removal is preferable to post-veraison removal. Conversely, the expression of terpene synthase genes and monoterpene content were significantly reduced in shaded bunches but increased upon reillumination [81,82,83]. It is important to note that the positive effect on terpene synthesis achieved by re-exposing clusters to light through leaf removal is largely independent of the timing of the removal. In fact, even early leaf removal (pre-flowering) has been effective in enhancing thiol precursors in Sauvignon blanc [84] and terpene precursors in Ribolla Gialla [85]. However, employing early (i.e., pre-flowering) leaf removal within a low sugar commitment seems illogical; a well-documented and consistent effect of such early intervention is that fruit set, and consequently yield, are typically limited. As a result, there is a tendency for higher TSS compared to a non-defoliated control [76].

Another approach to slowing or hindering berry sugar accumulation is late shoot trimming, performed from the lag phase onward [86,87,88]. This method may be effective if one or both of the following conditions occur. If the trimming is delayed sufficiently, the most functional apical leaves are removed, resulting in significant photosynthetic limitation. If the trimming is not delayed enough, or if late-season conditions promote some vegetative regrowth, the response could be compounded by late-season lateral competition. However, it is believed that if late shoot trimming is considered as part of a strategy to achieve low sugar levels at harvest, it might prove to be somewhat ineffective. This is because if a high yield is essential for achieving low sugar at harvest, it is likely that by the time of late trimming, the shoots may not have elongated sufficiently, and even more likely, the response to trimming may be diminished due to an overcropping condition.

Furthermore, the physiological rationale must always accommodate any operational constraints that may emerge when implementing the technique. Specifically, the topping of shoots must consistently be mechanizable. When working with a traditional espalier using a cutter bar topping machine, it becomes evident that the maximum severity of topping is limited to what can be performed on the shoots extending from the highest support wire. Consequently, topping is never excessively severe, as the average number of leaves retained is at least 12–14. In this scenario, the physiological premise for delaying ripening—leaving an average population of older, less functional leaves when ripening is well underway—is abandoned. Two potential solutions are (i) delaying the first topping until the shoots have begun to fall outward, causing shading to the cluster band, a situation that can present challenging vineyard management issues in environments that promote high vine vigor, or (ii) employing a counter-rotating disk coulter-rooter, which can navigate rigid obstacles during its forward motion and approach the bearing wire closely enough to cut at the height of 6–8 main leaves. However, this method is slower, more costly, and less flexible than a frame with cutter bars.

To significantly limit sugar accumulation in berries, three primary methods can be employed: leaf shading, leaf removal, and inhibition of leaf function. The latter can be effectively achieved by applying antitranspirants to the canopy, which typically form a thin, transparent coating on the leaves, thereby inhibiting gas exchange [89]. To fine-tune this effect, the size of the sprayed canopy section and the concentration of the chemical are the most readily adjustable parameters. The commercial antitranspirant VaporGard^®, which uses the active ingredient pinolene (di-1-p-menthene), has been widely used in viticulture to limit both photosynthesis and transpiration [90,91]. Studies have shown that, provided the chemical is not washed off by heavy rain, leaf net photosynthesis can be reduced by about 40–50% for a period ranging from 30 to 45 days post-spraying, ensuring a consistent source limitation [90,92,93]. In a two-year study on Sangiovese, which examined the interaction between crop level (approximately 9 and 3 kg/vine to define high and medium cropping) and the presence or absence of a VG spray applied at veraison without wetting the fruiting area, the results were quite revealing; the treatment combination TH (high crop-treated) achieved 19.4 °Brix, compared to the highest 24.9 °Brix recorded by the non-sprayed medium crop level combination (CM). Clearly, there was a synergistic effect between the VG spray and crop load, as the unsprayed high crop remained at 22.1 °Brix. Notably, pH, total acidity, malic acid, and tartaric acid concentrations did not differ between TH and CM, demonstrating a clear decoupling among technological maturity parameters. However, in this trial, as observed by others [94], the TH treatment combination also resulted in significantly limited total anthocyanins and phenolics.

6. The Varietal Choice for Low/No-Alcohol Wines

With the stringent and now clearly defined requirements for fruit composition aimed at low- or no-alcohol wines, it is clear that the ideal vineyard profile will depend on selecting the most appropriate grape varieties. In this context, several key factors emerge as crucial. The first is a high yield capacity, and even more importantly, adequate bud fruitfulness, especially in the basal nodes. This is vital because many of the previously mentioned canopy management techniques, such as severe shoot trimming or calibrated yield increases by retaining more buds during pruning, directly depend on the productivity of basal buds.

The second factor involves balancing key ripening parameters, a concept that extends beyond merely accumulating sugar as previously discussed. For instance, in white grape varieties, each one displays a distinctive relationship between TSS and TA from veraison to harvest [95]. Previous research on producing standard sparkling wines in the context of rising temperatures identified varieties that maintained higher acidity at different TSS levels as the most suitable [96]. In the case of low-alcohol wines, this paradigm can be reversed. Since de-alcoholization tends to concentrate acidity, grape varieties that naturally have lower acidity at low TSS levels become particularly advantageous [97]. If this is the objective, a review of the available literature indicates that these traits are often present in certain table grape cultivars [98]. For decades, the breeding objectives for table grapes have prioritized achieving low acidity and a TSS/TA ratio greater than 20, in contrast to the 2.5 to 4 ratio typical of wine grapes. Additionally, while wine grapes generally exhibit a significant decrease in TA only as they near 20 °Brix, certain table grape varieties, such as Red Globe and Queen of the Vineyard, maintain TA values of 5 to 8 g/L, which are optimal for white and sparkling wine production, even at very low sugar concentrations (12–15 °Brix) [99,100]. While dealcoholized wines will not be directly produced from table grape cultivars in the future, these varieties can still offer valuable genetic traits for specific breeding programs. Alternatively, they can be used more straightforwardly as components for blending musts or as a general target.

When it comes to red grapes and wines, the situation becomes notably more complex. Although maintaining low TSS and TA levels is still crucial, the wine’s body and structure are often intricately tied to its alcohol content. If alcohol is removed or significantly reduced, the focus shifts to phenolic maturity, particularly key secondary metabolites like anthocyanins and other flavonoids. The challenge lies in the fact that these compounds are largely governed by carbohydrate metabolism, with sugars acting as vital substrates for their biosynthesis. Consequently, efforts to lower sugar content inevitably result in a decrease in phenolic compounds [101]. In this context, several key criteria are crucial when choosing red grape varieties for low- or no-alcohol vineyards. Primarily, the grapes should be capable of accumulating significant amounts of anthocyanins and flavonols at relatively low TSS levels, allowing for good color and body in red wines, even when harvested early to limit potential alcohol content. Simultaneously, it is vital to avoid undesirable aromas at low sugar levels, which necessitates avoiding varieties that produce high levels of methoxypyrazines at low TSS concentrations. Another challenge is the increased tannin accumulation in seeds, which can render early harvest impractical due to the risk of excessive astringency [102]. One effective way to completely bypass this issue is to use seedless cultivars, which have primarily been developed for fresh grape consumption and only rarely for winemaking but could gain a significant interest for producing grapes prone to early harvest, with low TSS and no astringency. A final trait of interest is the adaptability to mechanized harvest, specifically low resistance to berry detachment from the pedicel at early ripening stages. Apart from the standard benefits of mechanization, this facilitates early harvest at low TSS levels and allows for musts that are free of rachis and bunch stems during maceration, other components that often contribute unwanted tannins and astringency to the finished wine [103].

7. Relationship Between “Low-Sugar Viticulture“ and New Technologies

While viticulture is an ancient practice steeped in tradition and often linked to concepts like “terroir” [104,105], it paradoxically embraces innovation through Information and Communication Technologies (ICT), giving rise to smart viticulture [105]. Within this realm, “precision viticulture (PV)” practices are currently attracting substantial funding and research interest [1,106,107,108]. Wheelan and Mc Bratney [109] offer a widely accepted definition of PV, describing precision agriculture as “the site-specific management of the spatial-temporal variability of a crop to increase input efficiency and enhance yield, quality, and sustainability of production.”

Vineyards, whether young or mature, are inherently variable. Although research has significantly reduced this variability—through increased availability of certified, virus-free clonal material, standardization of certain winter and summer pruning techniques, optimization of pre-planting soil preparation, and heightened attention to excessive land clearing—substantial variations in vine vigor within a plot, regardless of its size, are still common. These variations are largely attributed to soil heterogeneity, including differences in texture, soil depth, and organic matter, which in turn affect the so-called available soil water [97,98,99].

Under the assumption that objective natural variability exists, the perspective opened up by precision approaches is revolutionary, to say the least; in fact, such variability, normally seen as a burden or obstacle, can be transformed, thanks to the hawk’s eye of new sensors [97,98,99], into a factor that significantly enhances the sustainability (environmental and economic) of the winemaking enterprise [100].

As is well known, a vigor map of a given vineyard can be easily obtained by processing images acquired by sensors mounted on various platforms (satellite, on unmanned vehicles, proximal) with restitution of various classes of the NDVI [110] index which, at least theoretically, represents different levels of vegetative vigor [97]. The case study reported in

Table 1. Effects on different vigor levels (high, medium, low) determined within the same Barbera vineyard on vegetative growth, yield components and final grape composition. LA = leaf area; PW = pruning weight; N = nitrogen; TA = titratable acidity. Data are given on a per vine basis. Redrawn from Gatti et al., 2017 [111].

Treatments	Leaf N (Veraison) %	Main PW (g)	Lateral PW (g)	Total PW (g)	Single Cane Weight (g)	Main LA (m2)	Lateral LA (m2)	Total LA (m2)
High	1.59a	706a	189a	895a	77.3a	3.214a	1.321a	4.535a
Medium	1.50a	607b	147b	754b	59.2b	3.247a	1.098b	4.345a
Low	1.36b	420c	65c	485c	44.9c	2.846b	0.605c	3.451b
Sig.	**	**	**	**	**	**	**	**
	Yield (kg)	Cluster Number	Cluster Weight (g)	Berry Weight (g)	Cluster Compactness (g/cm)	Rachis Length (cm)
High	5.9a	19.9a	291a	3.0a	23.5a	12.5a
Medium	5.3a	19.8a	265b	2.6b	21.5b	11.8b
Low	3.2b	17.0b	181c	2.3c	17.5c	11.4b
Sig.	**	**	**	**	**	**
	TSS (°Brix)	TA (g L−1)	pH	K+ (ppm)	Tartrate (g L−1)	Malate (g L−1)	Total Anthocyanins (g kg−1)	Total Phenolics (g kg−1)
High	22.0b	11.3a	3.11a	1805a	6.4b	4.7a	0.893c	1.674c
Medium	22.5b	11.2a	3.07a	1622b	5.9c	4.5a	1.060b	1.888b
Low	24.9a	10.1b	3.01b	1470c	7.2a	3.3b	1.560a	2.656a
Sig.	**	**	**	**	**	**	**	**

Note: Lowercase letters in the same column indicate statistically significant differences between the different management practices detected with the Student–Newman–Keuls (SNK) test at p < 0.01 (**).

refers to an image acquired around veraison by satellite flight with ground resolution of 5 m, on a vineyard of very modest size (0.64 ha). The vigor map made it possible to identify, on an NDVI basis, three differently colored areas attributable to areas of high (green), medium (yellow), and low vigor (red) [98]. So far, so relatively simple; however, the mechanism becomes complicated when we need to give those three different colors an agronomic meaning. That is, for the three vigor classes identified on a spectral basis, to what extent are they really different from each other? And furthermore, is the interclass variability basically attributable to normal heterogeneity within the same vineyard, or are the differences consistent, stable and, above all, related to different vine behavior in terms of productivity and grape quality parameters?

This crucial question can only be addressed by conducting what might be termed a “ground calibration” of the differences observed from above. In the trial in question [111], the comparison between high vigor (HV) and low vigor (LV) revealed differences that were not only significant but, in some respects, unexpected (Table 1). The vines sampled from the area classified as high vigor by the satellite survey showed, compared to LV and averaged over the two survey years, higher values in all the main vigor parameters, all the primary components of production (notably, HV exhibited a grape production per vine of 5.9 kg compared to 3.2 kg in low vigor), and all the main quality parameters of the grapes at harvest. Specifically, HV showed, compared to LV, higher levels of TA, pH, K⁺, total acidity, tartaric acid, and malic acid, while having lower sugar degree and concentrations of anthocyanins and total polyphenols. If the focus is on low-sugar viticulture, selective harvesting in HV plots would isolate a batch of grapes suitable for this purpose, while the remaining grapes would be reserved for producing stronger wines.

A viable alternative is the use of agro-voltaics (AV) covers on vineyards [112,113] (Figure 6). While a comprehensive discussion on the dual use of land for energy and agro-food production is beyond this review’s scope, it is fairly intuitive that the seasonal and daily shading provided by AV panels on grapevine canopies might act as a limiting factor. This could result in a grape composition ideal for producing low-alcohol wines. An optimal outcome under AV, considering the low-sugar strategy, can be outlined as follows. (i) Yield should be maintained, which likely means that AV shading should commence at the onset of veraison and not earlier. Shading too early might compromise bud induction for the next season’s crop and excessively restrict cell division during the initial stage of berry growth, a primary determinant of final berry size and thus total yield. (ii) To achieve maximum sugar limitation, a theoretically optimal setup would involve maintaining the panels in a solar tracking mode (e.g., covers are orthogonal to the sunrays, casting maximum shade on the canopies) throughout the period from the beginning of the lag phase to 2–3 weeks post-veraison. (iii) To help decouple sugar accumulation from other ripening processes, a calibrated light transmission to the fruiting area during the final part of the ripening season might be beneficial for lowering TA and enhancing aromatic potential. Currently, the results available on the effects of implementing agro-voltaics on vineyards are too sporadic to assess the feasibility of the above [113,114,115]; however, given that the low sugar goal is closely linked to significant shading, the agro-voltaic option might inherently be very suitable, as it also fulfills the goal of producing a substantial amount of energy.

8. Conclusions

In the main section of our text, we have presented various strategies that, whether applied to the entire canopy or just a segment, could aid in producing low-sugar grapes suitable for the low-alcohol market. The ultimate composition of the grapes, influenced by the vineyard method and the wine’s intended profile, might eliminate the need for any post-fermentation de-alcoholization. If such a process is required, it could be more straightforward and less expensive than starting with a very high-alcohol base. It is important to note that our approach differs from the usual strategy of delaying ripening to a cooler period in the season. In that case, the main objective is “postponement”, but achieving full ripeness remains the goal.

In contrast, “low sugar” viticulture is distinguished by at least two specific features compared to a simple “delay.” First, achieving a notable reduction in TSS requires a state of “high cropping”, which involves revisiting practices such as minimal or hedge pruning. These methods have seen limited global adoption, mainly due to their association with unsatisfactory grape quality. Second, it is essential to focus on the physiological separation between sugar accumulation and the development of various secondary metabolites. The primary challenge lies in harmonizing low TSS with desirable traits like moderate acidity, good pigmentation, and adequate wine body. Addressing this challenge involves exploring several possibilities. (i) The seasonal accumulation patterns of key grape constituents (e.g., potassium, acids, quercetin, calcium, methoxypyrazines, carotenoids, catechins, and tannins) differ from those of sugar accumulation, allowing for more targeted manipulation. (ii) While sugar accumulation is mainly governed by whole canopy photosynthesis, the synthesis of secondary metabolites often relies on the microclimate conditions of specific canopy zones, such as the fruiting zone. (iii) Additionally, the temporal shift between the accumulation peaks of sugar and other compounds (e.g., flavor) provides further opportunities to differentiate the dynamics of each component.

We believe that “low-sugar viticulture” should not be seen as a secondary option or a makeshift variation compared to traditional “high-strength” products. Instead, it should be recognized that the low- or no-alcohol wine sector is present and expanding, necessitating viticulture feedback to optimize this approach. Indeed, pre-fermentation viticultural practices for low-sugar grapes must always adapt to local environmental and varietal conditions.