Fortunately, researchers are examining adaptation strategies for sustaining the production of wines and maintaining their typicity. Here, we look at three recently published papers which offer relatively simple strategies for dealing with the increase in temperatures during the vegetative cycle of the grapevine, and the resulting increase in sugar content and pH in grape must.
Blending wines made from ripe and unripe grapesResearchers at Spain’s University of Murcia have shown that blending wine from unripe grapes with wines from technologically mature grapes could be a useful tool for mitigating the problems caused by global warming in a warm and semiarid Mediterranean climate as it reduces alcohol content and pH simultaneously and improves wine colour.
In their study of the red variety Monastrell, published in OENO One (Vol. 57 No. 1, 2023), wines from the 2018 and 2019 vintages were subjected to sensory analysis and most of the ten panellists preferred the blended wine in both years. After 12 months in the bottle, the blended wine scored highest for aroma intensity, aroma quality, fruity aroma and mouthfeel quality. When it came to colour intensity, the panellists scored it on a par with the wine made from ripe grapes.
The blended wine had significantly lower alcohol content and pH and higher titratable acidity than the wine made purely from ripe grapes. It also had similar values of anthocyanin and tannin in 2018.
These results are in line with those obtained in similar studies on red varieties such as Bobal, Merlot and Cabernet Sauvignon.
The researchers commented: “The use of this technique is easy to apply in most wineries and does not involve any additional costs or equipment.”
If you are thinking this could be an interesting technique to try this vintage, here are the winemaking details.
The winemakingMonastrell grapes from the same commercial vineyard in Jumilla were used to obtain three different wines. To obtain the unripe grape wine (URG), 150kg of green clusters were thinned at the grape phenological stage number 33 in the BBCH scale, and taken to the experimental winery at the University of Murcia, where they were crushed and destemmed and put in a hydraulic press to obtain around 50L of grape must.
The must was sulphited with 70mg/L of SO2 and placed in a 50L steel tank. The URG (approximately 5 °Be) was left to settle for 24 hours, racked to another tank and inoculated (40g/hL) with selected yeast (Enartis Ferm Perlage, Ciudad Real, Spain). Alcoholic fermentation (AF) was carried out at about 21°C. When the AF had finished, the wine was racked and sulphited (70mg/L SO2) and stored at 2°C. The URG comprised the following analytical parameters for 2018 and 2019:
Alcohol – 3.9% and 3.4 %
Total acidity – 17.5 and 19.6g/L
pH – 2.98 and 2.85
Grapes from the same vineyard were harvested at two subsequent moments of the ripening stage: the first harvest (H1) and second harvest (H2) were carried out when the potential degree of alcohol was 13% and 14.5% respectively (at 22.6 ºBrix and at 24.4 ºBrix). 150kg of H1 grapes and 300kg of H2 were handpicked and taken to the same winery, where the grapes were crushed, destemmed and randomly distributed into 50L stainless-steel tanks: three tanks of H1 and six tanks of H2. All the tanks were sulphited (70mg/L of SO2) and inoculated with 30g/hL of selected yeast (EnartisFerm Q7, Ciudad Real, Spain). All the vinifications were conducted at 24 ± 1°C until the end of fermentation and monitored daily for temperature and density of the must.
A manual punchdown was carried out daily for seven days during maceration. At the end of maceration, the free-run and pressed wines were combined and returned to clean tanks at room temperature until the end of fermentation (residual sugar <2g/L). After fermentation, three 50L tanks from H1 and H2 were used as the control wine. The other three tanks of H2 were mixed with the elaborated unripe grape wine (URG) to obtain the blended wine (URGH2): specifically, the URG wine was blended with H2 until it had reached the same alcoholic degree as H1 (13%).
Increasing the trunk heightSurface temperature is projected to rise during the 21st century. Heatwaves are predicted to occur more often and last longer. Grapes are expected to ripen earlier in the season and in warmer temperature conditions. This may in turn affect grape composition and balance, and therefore the typicity of the produced wines – while early budburst could lead to more frequent spring frost damage…
Adapting the vine training system is a solution that is relatively easy to implement. But does it work and what are the likely consequences? A study of the vertical thermal gradient in the vine canopy was carried out in Bordeaux from 2016 to 2020 to determine whether increasing the trunk height could be an adaptation strategy for manipulating the microclimate in the bunch zone and limiting the impact of climate change on vine development and grape composition.
This study, published in OENO One (Vol. 57 No. 1, 2023), found that increasing the trunk height had a limited effect on the timing of berry ripeness (delaying it by two or three days), but it could significantly reduce the negative impacts of both spring frost and summer heatwaves.
Temperature, which has a significant impact on grape berry composition and the key phenological stages (such as budbreak, flowering and veraison), was measured at four different heights from the soil (30, 60, 90 and 120cm) in two adjacent vineyard parcels of Merlot. One parcel was managed with cover crop and the other by tilling the soil.
The results of this study in a dry-farmed vineyard in Saint-Émilion show that the type of parcel management is significant: close to the ground, the cover crop parcel generally had lower minimum temperatures and higher maximum temperatures in comparison to the tilled parcel, exposing the vines to an increased risk of both frost and heatwave damage. When investigating the factors driving the vertical thermal gradient, soil moisture was found to have an impact. The drier the soil, the greater the gradient of minimum and maximum temperatures.
An advantage of trunk elongation, according to the study, is the decrease in maximum daily temperature during the growing season and in maximum temperature within the fruit zone during extremely warm days (>35°C).
- The vineyard floor management and the height had an effect on the diurnal temperature range (DTR), with DTR being greater for the cover crop parcel and greater at 30cm than 120cm. “This effect was not significant when comparing 30 and 60cm, and it was more pronounced in the cover crop parcel compared to the tillage parcel,” the study reported.
- Analysis of the night of frost (April 27, 2017) revealed that independently of the vineyard floor treatment, minimum temperatures near the ground were coldest. Compared to the tilled parcel, the cover crop parcel showed a greater vertical gradient (1.7°C compared to 0.9°C) and lower temperatures (by 0.5°C) in the fruit zone (height of 45cm).
- During the heatwave of July 23, 2019, regardless of vineyard soil management, the warmest maximum temperatures in the afternoon occurred close to the ground. The tilled parcel was 0.8°C cooler than the cover crop parcel in the bunch zone.
- Looking at all the days of extreme cold or heat from 2016 to 2020, the minimum temperature was almost always colder in the cover crop parcel and the highest temperatures were in the cover crop parcel at a height of 30cm.
They added: “In view of the results of the extreme temperature analysis, it could be useful for wine-growing estates to increase trunk height; for instance, from an original height of 45cm to one of 90cm. The consequences would be a reduction in maximum temperatures during heatwaves by 0.8°C on average, and an increase in minimum temperatures during nights of frost by 0.4°C. The potential reduction in canopy height could be compensated for by increasing the trimming height by 40cm (eg. from 160cm to 200cm) without changing farm equipment or the leaf area-to-fruit weight ratio. A reduction in the leaf area-to-fruit weight ratio may also further limit the impact of climate change on grape composition by decreasing sugar content without increasing total acidity.”
Using intermittent shading to delay grape ripeningResearchers in Montpellier, France, have been putting potted Syrah vines under shading panels – rather than permanent shading nets – to see if they delay berry ripening and could counter the impact of global warming on yield and berry quality. As the report, published in OENO One (Vol. 57 No. 1, 2023), says: “By accelerating the phenological development of the vines, global warming results in the ripening period shifting to hotter and drier conditions in midsummer. This, in turn, can cause yield loss due to growth arrest or dehydration of berries, resulting in high sugar concentration and wines that are too high in alcohol and bland.”
The two-year trial, repeated on two batches of young potted vines, involved 2m-wide horizontal panels placed 2.4m above the ground. They reduced the available radiation by an average of 55%. In the real world, these could be photovoltaic panels – meeting the increasing demand for green energy while reducing the temperature at crop level and protecting the fruit from getting burnt by direct sunlight during periods of high temperature and excess solar radiation.
The researchers also compared the shading effects under two soil water conditions: well-watered and moderate water deficit.
Although the shading did not significantly modify air temperature within the canopy when cumulated over the growing season, the panels substantially delayed veraison by up to more than 30 days under well-watered conditions. The most marked phenological shifts were noted in the second year of treatment between flowering and veraison when carbon demand sharply increased during berry formation, suggesting there was a carry-over effect likely due to limited carbon assimilation. This was accompanied by sharp decreases in berry diameter and sugar content per berry at harvest. Higher berry growth and sugar loading were maintained when shading was combined with water deficit. However, the trigger effect of water deficit on veraison almost halved the phenological delay caused by the panels. Overall, a cooler period for ripening could be achieved with panels over the vines but at the expense of berry size and sugar amount in berries.
The researchers concluded: “The panels only slightly affected the budburst and flowering stages, while the most marked phenological changes were observed between flowering and veraison, at the beginning of berry formation and onwards until harvest. A substantial delay was observed when the plant water status was preserved by heavy shading and well-watered soil. However, this was associated with a sharp decrease in berry sugar content at harvest due to both a decrease in berry diameter and a blockage of sugar loading at cluster level. All these trends were amplified in the second year of treatment, highlighting the negative cumulative effects. The water deficit shortened the delay caused by the panels, thus partly cancelling out the expected benefits of a cooler period, but it helped maintain the yield components and sugar load by triggering an earlier veraison. Finally, the shading ratio would need to be finely tuned to evaporative and soil water conditions in order to be able to benefit from the phenological delay caused by panels, without altering production in the long term. In this way, the use of tiltable panels, like those used in dynamic agrivoltaics, is a promising tool for adaptive shading strategies.”
- The treatments had much more pronounced effects on veraison than on budburst or flowering date – with a shift in veraison of up to 33 days that was partly maintained at harvest. This delay was longer when shading was repeated for two years in well-watered conditions (about 30 days) than for one year (between 4 and 12 days) or in water deficit conditions (between 4 to 13 days). This may be related to the progressive depletion of carbon stored in the perennial parts of the vine.
- The panels had a much lower effect on vine phenology when combined with a water deficit. Water stress is known to trigger berry colouring and ripening by enhancing abscisic acid biosynthesis. Even a moderate water deficit accelerates ripening. In this study, the latest veraison was observed when the vines were abundantly watered several times a day and shaded with panels. Shading itself decreases water stress by lowering evapotranspiration. The combination of well-irrigated and shaded conditions can therefore be considered to release any water stress and block abscisic acid production, which is required to trigger veraison.
- While a slight drop in daytime temperature under the fixed panels compared to full sun conditions was observed, a slightly higher nighttime temperature was recorded, which compensated for the daytime effect.
- Due to the delay in the ripening phase caused by the panels, the mean temperature during the ripening of the shaded plants decreased by 2.5°C (2019) to 3°C (2020). Similar results have been observed with practices such as kaolin spraying and shading nets, which have proved to be effective for controlling sugar accumulation and decreasing the sugar to acid ratio at harvest. The results from this study confirm the potential for intermittent shading to decrease alcohol level in wine. A cooler ripening period could also preserve berry acidity and anthocyanin content.
- The phenological delay in ripening caused by the panels is quite limited under water deficit conditions. “It can therefore be assumed that the main benefit of the panels in arid areas is not the modification of the phenological cycle, but the water saved as a result of the shading,” the report says.