Introductory Chapter: Impact of Climate Change on Grapes and Grape Products

1. Introduction

Botanically, grapes are categorized as berries, produced by the plant of the genus Vitis. Grapes (Vitis spp.) are among the most highly prized fruits worldwide. The common grapevine, indigenous to the Mediterranean region and Central Europe, belongs to the species Vitis vinifera, which contains somewhere between 5.000 and 10.000 distinct varieties and is one of the most widely grown and economically important grapevine species in the world, mainly as a consequence of the high quality of its wines [1]. There are approximately 60 grape species, with the majority located in the temperate zones of the Northern Hemisphere, with nearly equal distribution between Asia and America [2]. Grapes constitute a fruit that has been cultivated and consumed in numerous ways since ancient times. The term “grape products” refers to the grape berry itself, which can be consumed either fresh or after drying, as well as the range of products that can be produced from it, such as wine, fortified wine, sparkling wine, grape juice, distillates, vinegar, dried grapes, jams, jellies, and seed oil [3].

Grapes are cultivated all over the world with the world vineyard surface area estimated to be 7.3 million hectares in 2022, with vines for all purposes (wine, juice, table grapes, and dried grapes) [4]. In 2022, the world production of grapes was 80.1 million tons, of which 50% were wine grapes, 42% table grapes, and 8% dried grapes [4]. In the same year, global wine production was estimated at 258 million hectoliters, with the three main wine producers being Italy, France, and Spain [4].

In recent decades, growing attention has been given to global climate change, its associated risks, and the necessary steps for its mitigation. Given the significance of grapes for wine production in many countries and the current circumstances of climate change in regions with a tradition of wine production, it is necessary to identify strategies to maintain the alcoholic concentration of wine at a moderate level to avoid altering the wine composition and freshness concerning consumer preferences. On the other hand, climate change could also increase the incidence of mycotoxins produced by certain fungi in wine. In response, both the wine industry and the scientific community have proposed numerous techniques or methods to mitigate the adverse impacts of global warming on grapes and wines. By those reasons, this introductory chapter aims to provide a brief overview of the impact of climate change on grape quality and the incidence of mycotoxin-producing fungi, as well as strategies to mitigate these occurrences.

2. Impact of climate change on grape and grape product quality

2.1 Reducing the effects of climate changes on grape and wine quality

A primary consequence of rising temperatures due to climate changes is an acceleration in the vegetative and reproductive cycles of grapevines. Grape berries are susceptible to heat stress, affecting both berry composition and wine quality. In addition to influencing primary metabolites such as organic acids, particularly malic acid [5], sugars [6, 7, 8], and amino acids [9], climate change also affects secondary metabolites, including flavonoids and aroma precursor pathways, as evidenced by transcriptomic, proteomic, and metabolomic approaches [9, 10, 11, 12, 13, 14, 15, 16], thereby affecting the balance of berry quality-related compounds at ripeness [7, 17], which are responsible for color, bitterness, mouthfeel, and important sensory attributes. While increased temperature consistently reduces anthocyanins levels [15, 18], moderate increases in berry temperatures, achieved using open-top chambers, can decouple the anthocyanins and sugar contents in red varieties [17] and influence berry sensory characteristics [19]. Therefore, optimal sugar levels are not always correlated with similar maturity in flavor, color, or aroma [17, 19]. This has led to the suggestion that “sugar ripeness” is no longer coordinated with “phenolic” ripeness or phenolic maturation of the grapes as in warmer years, anthocyanins and tannins do not mature to the same extent as they would in cooler years. Berry acidity, another key quality parameter, is likely to decline in response to warming [20]. Warm nights can lead to the respiratory loss of malic acid, affecting the sugar-acid and aroma-acid balance, making it suboptimal [21, 22, 23].

A consequence of climate change for the wine industry is a clear increase in the sugar content of grapes and the corresponding alcohol content in wines, observed in many wine regions [24]. In Bordeaux (France), wines typically 12.5% alcohol in the 1980s have now risen to 16%, and these wines will also lack freshness and aromatic complexity [25]. For instance, in Languedoc (France), alcohol content has risen from 11 to 14%, pH has increased from 3.50 to 3.75, and total acidity has dropped from 6.0 to 4.5 g/L [26]. Wines with elevated alcohol levels are generally considered less healthy [27, 28]. Given societal concerns related to high alcohol content and the growing trend for healthier lifestyles, consumers are increasingly interested in wines with lower alcohol levels [29]. However, lowering alcohol content in wine presents challenges due to legal constraints. Regulations typically permit the addition of water only for the preparation of enzymes, yeast, or other enological products, such as fining agents, not for diluting the sugar content of grape must [30]. Alternative techniques are being explored to achieve alcohol reduction, which could be grouped as viticultural or winemaking practices [31]. Viticultural practices could involve harvesting grapes at an early stage of maturity [32], double-pruning, to encourage bud growth during spring and summer, and shifting berry ripening to cooler periods of the season. This approach enhances the phenolic composition of the berries and ensures a more balanced sugar/phenolic content [33]. Introducing new grape varieties with delayed sugar accumulation and acid degradation [34], as well as implementing and adjusting cultural practices, such as post-veraison leaf removal [33]. In the winery, various enological techniques have been suggested to produce wines with lower alcohol content. For instance, utilizing yeast strains (Saccharomyces or non-Saccharomyces) with reduced ethanol production during winemaking [35] is one approach. Additionally, biotechnological solutions, mostly relying on the selection and improvement of yeast strains capable of metabolizing sugar into pyruvic acid, glycerol, and other compounds besides alcohol, are being investigated as a potential solution [36, 37]. Production of L-lactic acid by non-Saccharomyces as Lachancea thermotolerans through the enzymatic activity of lactate dehydrogenase (LDH) is an interesting approach to increase wine freshness and decrease the wine pH [38]. Alternative winemaking techniques to address the rising alcohol levels in red wines attributed to global warming have been developed, such as the practice of blending unripe grapes or must with fully ripened counterparts. Numerous studies involving various Vitis vinifera grape varieties, such as Shiraz, Cabernet Sauvignon, Pinot Noir, Malbec, Grenache, and Tannat, have explored the effects of blending unripe grapes or must with those from fully ripened grapes [32, 39, 40, 41]. These studies have demonstrated that blending unripe grapes or must with well-ripened counterparts results in wines with lower alcohol and pH levels and higher total acidity. Furthermore, the blending process positively impacts the phenolic composition, particularly tannins and anthocyanins, leading to enhancements in color and mouthfeel. Various physical methods are commonly employed in the removal of ethanol from finished wines, including membrane filtration, vacuum distillation, and supercritical fluid extraction [42, 43]. Among these, the spinning cone column and reverse osmosis system are frequently used [44, 45]. However, these methods require expensive equipment, and their impact on wine quality is still not well understood [42].

2.2 Adjustments in viticulture practices to reduce grapes temperatures

A wide array of techniques exists to delay grape ripening, which can be broadly categorized into two groups: alterations in viticulture practices and adjustments in plant material.

Shading, a technique used to mitigate high temperatures, has been examined in Sangiovese and Semillon grapevines [46, 47]. This method involves partially or completely covering grapevine canopies from budburst to harvest [48, 49]. The effects of this technique on canopy growth and berry development were studied, and it was found to decrease berry mass and increase the accumulation of phenolic substances, increasing total acidity [50].

Another mitigation strategy for climate change consists of the clonal selection of grape varieties as clonal variability, or genetic diversity within a grape variety, is a well-known phenomenon. Traditionally, clones have been chosen based on characteristics such as early ripening, high productivity, and elevated grapes sugar concentration. However, with the changing climate, new clones with contrasting characteristics may be more desirable. For example, sugar accumulation patterns differ significantly among clones, as demonstrated by a clonal selection trial with Cabernet Franc. The variation in grape sugar concentration among clones can be as high as 17 g/L (equivalent to 1% potential alcohol) at ripeness [51].

As temperatures continue to rise, traditional grape varieties may no longer ripen within the optimal period, potentially impacting the quality of wine produced. One potential strategy for adaptation is to shift toward planting later-ripening grape varieties. In European wine appellations, grape variety selection is usually regulated to ensure the best quality and typicity under local climatic conditions. However, as the climate changes, these regulations may need to be adjusted. A broader selection of grapevine varieties could serve as an important tool for adapting to climate change. As a result, grape varieties currently cultivated, especially those that ripen early, may no longer grow in their current locations under altered environmental conditions in the future [52]. This may be particularly relevant for regions already experiencing warm climates, where climate change may threaten the balanced ripening of grapes and the sustainability of existing varieties and wine styles [53, 54]. However, future warming in cooler climate regions may improve the suitability for producing high-quality wines [55].

It is crucial to explore mitigation alternatives to maintain grape quality [56]. One such technique involves the exogenous application of minerals, which can help reduce water usage while preserving or even enhancing wine quality. Silicon dioxide (SiO₂) particles, when applied as an aqueous suspension, form a physical or mechanical barrier (as precipitated amorphous silica) in the skin. Additionally, under conditions of water stress, the presence of Si may improve potassium uptake by plants [57]. Potassium plays a significant role in wine pH, which, in turn, affects chemical and microbiological stability, as well as the perception of wine flavor [58]. Numerous authors have concentrated their research on the advancement of eco-friendly practices, such as the application of kaolin, to maintain quality in a challenging climate. This requires the reduction of leaf and fruit berry surface temperature [59]. Indeed, the foliar application of solar protectants has already demonstrated promising results concerning the general fruit quality potential within the climate change context at a local scale [60, 61].

2.3 Vine pests and diseases associated with climate changes and their impact on wine quality

Climate change also poses a threat to grape production through the increased prevalence of pests and diseases along with the vectors that transmit these diseases. Grapes are susceptible to mycotoxigenic fungi, with Aspergillus being of particular concern due to its production of ochratoxin A (OTA). Mycotoxins are secondary metabolites produced by certain fungi and are concerning for human health due to their carcinogenic properties [62]. As a result, the levels of mycotoxins in certain food products intended for human consumption are strictly regulated, with specific maximum thresholds established. In the European Union, OTA is the only regulated mycotoxin for wine and other grape products, with a maximum allowable limit of 2 μg/kg [63].

The temperature ranges that result in high levels of OTA on grapes depend on the fungal species: Aspergillus niger aggregate strains and individual Aspergillus carbonarius have optimal ranges of 30–35°C and 25–30°C, respectively [64, 65]. The temperature’s impact is most significant during infestation [66]. A temperature of 21°C appears to be the lower limit, below which fungal growth and OTA production are insufficient to reach critical levels in wine during the susceptible berry period [67]. However, it is widely recognized that climate change is leading to a redistribution of fungi, with a trend correlated with increasing temperatures and more frequent droughts in southern Europe [68]. Consequently, climate plays a crucial role in determining contamination levels once these fungi are established, with elevated temperatures being a key factor in the presence of OTA. Wines from warmer regions in southern Europe tend to have higher concentrations of OTA compared to those from cooler northern regions. Generally, OTA concentration in wine detected at 30°C exceeds that at 20°C in most cases [69]. Research suggests a correlation between mycotoxin occurrence and warm winemaking regions, likely to expand with global temperature increases. A review indicated such a correlation between climate and grape and wine OTA levels [70]. A survey of 942 wines by a regulatory laboratory found higher OTA concentrations in wines from southern European countries than from northern European countries [71]. It is suggested that as climate change advances, better-adapted species, such as A. niger, could thrive in southern Spain, potentially leading to a rise in fumonisins as OTA levels decrease [72]. The shift toward drier and hotter climates may favor the prevalence of A. tubingensis and A. niger over A. carbonarius as these species are better suited to extreme heat and aridity [73]. The current climate change scenario is expected to alter the geographical distribution of Aspergillus flavus, a concerning fungus. As higher temperatures may promote the dominance of more hazardous mycotoxins, such as aflatoxins, there is a possibility that they might surpass OTA as the main mycotoxin due to increased suitability for thermotolerant Aspergilli that produce aflatoxins [74, 75]. Recent reports have highlighted A. flavus as an emerging contaminant in vineyards, leading to discussions about the necessity of regulating aflatoxin levels in grapes. This widespread occurrence raises concerns about the potential contamination of grapes with aflatoxin.

These findings underscore the impact of climate change on the prevalence of mycotoxigenic fungi, such as A. flavus in vineyards, prompting a reevaluation of the risk associated with grape contamination by aflatoxin [76].

Establishing dependable models to evaluate the impact of climate change on vine health is essential, given its direct correlation with mycotoxin contamination. Although strides have been made in mitigating OTA levels in wine, the implications of climate change on other mycotoxins cannot be overlooked and will become increasingly critical [75].

3. Conclusions

Various strategies are being implemented at different stages of grape and wine production to tackle the significant challenges presented by rapid climate change in recent decades. To address rising temperatures, adjustments in viticulture practices and plant material are essential to ensure that actual harvest dates align with the optimal period. These adjustments involve selecting grapevine clones and adopting new approaches to manage vineyards, aiming to avoid grapes with elevated sugar levels, low acidity, and high pH, all of which significantly impact the sensory balance and microbiological stability of wine. In the winery, it will be necessary to adapt existing winemaking practices and develop novel ones. This includes incorporating a blend of unripe grapes with fully matured grapes, utilizing yeast strains with reduced ethanol production, and enhanced acid production during alcoholic fermentation. Moreover, implementing partial dealcoholization of alcoholic wines through gentle physical techniques, such as reverse osmosis, nanofiltration, spinning cone columns, and others, is essential. The impact of these techniques on wine quality should be thoroughly studied, including their effects on the wine aging process.

Acknowledgments

FCT-Portugal, CQ-VR (UIDB/00616/2020 and UIDP/00616/2020). (https://doi.org/10.54499/UIDP/00616/2020) and Project Vine&Wine Portugal—Driving Sustainable Growth Through Smart Innovation, Application n. ° C644866286-00000011, co-financed in the scope of the Mobilizing Agendas for Business Innovation, under Reg. (EU) 2021/241, in the Plano de Recuperação e Resiliência (PRR) to Portugal, na sua componente 5 - Capitalização e Inovação Empresarial.

Conflict of interest

The authors declare no conflict of interest.