AGRICULTURAL AND FOOD SCIENCE Agricultural and Food Science (2025) 34: 202–212 202 https://doi.org/10.23986/afsci.160985 Assessing Estonia’s viticultural potential based on the compositional analysis of sugars and acids of wine grape cultivars Reelika Rätsep1,2, Mariana Maante-Kuljus1, Kadri Karp1, Priit Põldma1, Angela Koort1, Leila Mainla1 and Ulvi Moor1 1 Chair of Horticulture, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Fr. R. Kreutzwaldi 5, Tartu, 51006, Estonia 2 Polli Horticultural Research Centre, Chair of Horticulture, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Uus 2, Polli, 69108 Mulgi Parish, Estonia e-mail: reelika.ratsep@emu.ee The primary objective of this study was to assess Estonia’s potential for viticulture through the calculation of the Heliothermal Index and analysis of its 20-year dynamics. The secondary objective was to determine the variability in grape sugars and acids composition depending on vintage (2023, 2024) and cultivar characteristics in ‘Solaris’, ‘Regent’, ‘Leon Millot’, ‘Cabernet Cortis’, ‘Marquette’, ‘Hasansky Sladky’, ‘Zilga’, and ‘Rondo’. Over the twenty years, the Heliothermal Index ranged from 872 to 1622, showing a warming trend with implications for viticultural poten- tial. Fructose content in grapes ranged from 72 to 98 g l⁻¹, with ‘Marquette’ having the highest and ‘Zilga’ the lowest. Glucose content was lowest in ‘Zilga’ (64 g l⁻¹) and highest in ‘Marquette’ (98 g l⁻¹). ‘Zilga’ had the highest tartaric acid content (5.9 g l⁻¹), while ‘Leon Millot’ (3.5 g l⁻¹), and ‘Regent’ (3.7 g l-1) had the lowest. ‘Hasansky Sladky’ had the highest malic acid content (5.5 g l⁻¹), while ‘Regent’ (2.1 g l⁻¹) and ‘Solaris’ (2.5 g l⁻¹) had the lowest. The study con- firmed that the tested cultivars are suitable for producing dry wine in Estonia, which belongs to EU viticulture zone A. Key words: total sugars, fructose, glucose, total acid, tartaric acid, malic acid Introduction Climate and weather are key factors influencing wine grape growing conditions. Various viticultural climatic indices, formulated from these variables, have been used in many regions to describe vineyard climates. Based on different weather data, studies indicate that it is now possible to cultivate vines in the Baltic Sea region using several early-ripening Vitis vinifera and hybrid cultivars (Karvonen 2014a, 2014b). In cool-climate regions, the most significant positive trends have been observed in the Huglin and Winkler indices, while less significant trends appear in the Cool Night, Latitude-Temperature, and Temperature-Variability indices (Jones 2018). Estonia has been recognized as an emerging very cold-climate region, where climatic conditions, particularly rising tempera- tures, are gradually altering growing conditions (Maante-Kuljus et al. 2019a). In Estonia, the Heliothermal Index (HI) has been ≤1500 in most years. Under such climatic conditions, certain regions cultivate interspecific hybrids or American Vitis species, which are more cold-resistant than Vitis vinifera, and only very early or early-ripening cultivars are capable of reaching full maturity. Although Estonia is not widely known for grape cultivation due to its northern climate, interest in viticulture is increasing as part of a niche industry. Since 2021, Estonia has been included in EU wine-growing zone A (Appendix I of Annex VII to Regulation (EU) No 1308/2013). However, Estonian wines remain largely unknown internationally due to the smallness of the country, and the viticulture for wine is at the very beginning of its development. Producers of commercial wine in Estonia are organized under the Estonian Wine Trail association (https://veinitee. ee/en/home/), which currently includes 26 wineries. The majority of production focuses on fruit and berry wines. Seven producers operate their own vineyards, while several others purchase wine grapes from growers who do not produce wine themselves. The annual wine (from grapes) production by members of the Estonian Wine Trail has typically been around 6000 liters. However, due to spring frosts, the production in 2024 dropped to just 3500 liters. New vineyards are being established, and a significant increase in production is expected in the coming years. The grape cultivars grown in Estonia are typically winter-hardy cultivars capable of withstanding harsh winters and a short growing season. These are often hybrids specifically bred for cooler climates. In production areas, the most common cultivar is ‘Solaris’ (43% of the total number of plants), which is primarily grown in high plastic tunnels. Received 17 April 2025 / Accepted 7 August 2025 The Scientific Agricultural Society of Finland ©This is an open access article under the CC BY 4.0 R. Rätsep et al. 203 Other common cultivars include ‘Rondo’ (21%), ‘Zilga’ (10%), and ‘Hasansky Sladky’ (7%). Less commonly grown cultivars include ‘Frühburgunder,’ ‘Regent,’ ‘Cabernet Cortis,’ ‘Cabernet Noir,’ ‘Leon Millot,’ and ‘Müller-Thurgau’. Among these, only ‘Zilga’ and ‘Hasansky Sladky’ are commonly grown in open fields, as they have proven to be the most winter-hardy. Winter-hardy grapevines are primarily interspecific hybrids of V. vinifera with V. labrus- ca and V. riparia, giving them a chemical composition distinct from that of V. vinifera cultivars traditionally used for winemaking. This difference limits the applicability of existing knowledge in assessing grape maturity in very cool-climate conditions. Technological parameters—such as total soluble solids (estimated sugar content), pH, and total acidity—serve as essential indicators for maturity assessment in cool climates. Grape’s ripening is characterized by a rapid accumulation of sugars and a decline in malic acid levels (Colak and Bengu 2024), with the timing and extent of these changes varying considerably depending on cultivar, climate, soil, and viticultural practices (van Leeuwen et al. 2023). Tartaric and malic acids are the most abundant organic acids in grapes, generally accounting for 62–92% of total organic acids (Kliewer 1966, Kupe et al. 2021). Studies have shown that the concentrations of tartaric and malic acids in grapes vary considerably between years, typically ranging from about 1 to 10 g l-1 across different seasons (Kunter et al. 2024). The malic acid content tends to be higher in cool-climate regions, and lower in warmer regions (Volschenk et al. 2006, Conde et al. 2007). In cooler regions, cultivar selection and viticultural practices have been focused on achieving high sugar concentrations, which are considered beneficial for wine quality. How- ever, the quality of some vintages in cool climates has not always been satisfactory due to low sugar content and high acidity (Gustafsson and Mårtensson 2005, Gąstoł 2015). Organic acids exhibit a strong vintage effect, with cool vintages associated with higher organic acid concentrations (Wang et al. 2025). The minimum maturation level achieved by most V. labrusca and hybrid grapes has been relatively low, with 44% of samples showing soluble solids content below 16 °Brix (Spinelli et al. 2024). Hybrid cultivars between V. labrusca and V. vinifera have been found to contain significantly higher total soluble sugars and fructose, as well as lower total acids and malic acid, compared to V. vinifera cultivars (Liu et al. 2006). In Estonia, previous studies have shown that must soluble solids of grapes tend to be relatively low, while total acid content tend to be high. These characteristics have been significantly influenced by cultivar properties, different growing techniques, and weather conditions (Maante-Kuljus et al. 2015, 2019a, 2019b). In previous studies, the focus has been on total acidity, but for winemaking, the content of individual acids is also important. Under cool climate conditions, a common issue has been the high content of malic acid. The malic acid level can be adjusted through winemaking techniques, such as malolactic fermentation. Therefore, it is essential to understand the full acid profile. The concentrations of glucose and fructose in grapes are the primary determinants of the final alco- hol content in wine. If the sugar content is too low for achieving proper wine alcohol content, it can be increased through the addition of sugars, which is permitted under EU legislation for wine-growing zone A. Based on the above, it can be assumed that due to general climate warming, the climatic conditions in Estonia are becoming more suitable for viticulture. However, there is limited experience with different hybrid cultivars. The primary objective of this study was to assess Estonia’s potential for viticulture through the calculation of the Heliothermal Index and analysis of its 20-year dynamics. The secondary objective was to assess the variability in sugar (total sugars, fructose, and glucose) and acid (total acids, tartaric acid, and malic acid) composition depending on vintage and cultivar characteristics in widely cultivated grape cultivars such as ‘Solaris’, ‘Regent’, ‘Leon Millot’, ‘Cabernet Cortis’, ‘Marquette’, ‘Hasansky Sladky’, ‘Zilga’, and ‘Rondo’ grown in production vineyards in Estonia. Material and methods Experimental sites and plant material The grapes for the experiment were harvested from various vineyards in South Estonia (from open field 58° 21′ 27″ N, 26° 31′ 16″ E, and tunnel 57° 55’ 24” N, 26° 43’ 9” E; 58°17’00.6”N 26°33’47.2”E; 57°58’58.4”N 26°34’03.8”E) during 2023 and 2024. Most of the cultivars were grafted onto SO4 rootstock, except for ‘Zilga’ and ‘Hasansky Sladky’, which were grown as own-rooted. In each vineyard, the vines were trained using a low double-trunk trellis system with spur pruning with 12 buds left per plant. Additionally, at the beginning of grape coloration, leaves were removed from the cluster zone. The vine rows were oriented from north to south, the ground was covered with woven ground cover fabric, and no additional irrigation system was used. The vineyards soil was high- productivity sandy loam Haplic luvisol, with a soil fertility rating of 45–50 on a 100-point scale. Agricultural and Food Science (2025) 34: 202–212 204 The harvest time for grapes was determined according to the regular soluble solids (°Brix) measurement, and the grapes were harvested when the soluble solids content of each tested cultivar reached a plateau and did not rise any longer. For the analysis, the first cluster from the lower part of the shoots was collected on the day of harvest in mid-September. A total of 12 clusters were collected from 15 plants of each cultivar. Experimental wine culti- vars and species in pedigree: ‘Solaris’ (V. vinifera, V. amurensis), ‘Regent’ (V. vinifera, V. amurensis), ‘Leon Millot’ (V. vinifera, V. riparia), ‘Cabernet Cortis’ (V. vinifera, V. amurensis), ‘Marquette’ (V. vinifera, V. riparia), ‘Hasansky Sladky’ (V. vinifera, V. amurensis), ‘Zilga’ (V. vinifera, V. labrusca), and ‘Rondo’ (V. vinifera, V. amurensis). Measurements and analysis The Heliothermal Index was calculated using the following expression (Huglin 1978): where “T” and “Tmax” are, the average mean and maximum monthly temperature (°C), respectively; “Mi” and “Mf” are the initial and the final month of the period, respectively; “d” is the length of day coefficient, with value of 1.09 for latitudes 58°. For the measurement of total sugars, fructose and glucose, and total acids, malic and tartaric acids, the frozen (–20 ℃) grape samples were thawed at room temperature until manual juice pressing was possible. The obtained juice was carried over to the 5 ml syringe. Juice samples were injected into the flow system, and scanned using a horizontal platinum diamond attenuated total reflectance (ATR) single reflection sampling module cell mounted in a Bruker Alpha FT-IR Wine analyzer (Bruker Optics GmbH, Ettlingen, Germany). Before scanning, juice samples were automatically stabilized in the cell at 40 °C as recommended by the manufacturing company with reference background spectrum recorded between different samples using deionised water. The ATR-NIR spectra were recorded by OPUS software version 7.5 provided by Bruker Optics (Bruker Optics GmbH, Ettlingen, Germany). The spectrum of each sample was obtained by taking the average of 120 scans. The accuracy of prediction for malic acid was ±0.6 (g l-1) and tartaric acid ±0.3 (g l-1). The accuracy of prediction for glucose was ±0.9 (g l-1), for fructose ±0.8 (g l-1), and for total sugars ±1.7 (g l-1). Weather conditions In both experimental years, the coolest months were May and September, with night frosts occurring in May (Fig. 1A). However, September was warmer than the average for many years. In 2023, the warmest month was August, with an average air temperature of 18 °C, whereas in 2024, the warmest month was July, with an average air temperature of 19 °C. However, the maximum recorded temperature in July reached 34 °C. July was the rainiest month, while September 2024 experienced drought conditions (Fig. 1B). According to Heliothermal Index, Estonia is located in a very cool, and in some years, it is considered a cool region (Maante-Kuljus et al. 2019a). 𝐻𝐻𝐻𝐻𝐻𝐻𝐻𝐻 = �� (𝑇𝑇𝑇𝑇 − 10) + (𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 − 10) 2 � ∗ 𝑑𝑑𝑑𝑑 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 11 17 17 18 15 15 18 19 17 14 11 15 18 17 12 0 4 8 12 16 20 Ai r t em pe ra tu re , ° C 2023 2024 AverageA 8 46 119 86 4021 59 202 109 2 54 88 67 79 55 0 50 100 150 200 250 Pr ec ip ita tio n, m m 2023 2024 AverageB Fig. 1. A Monthly average air temperature (°C); B Monthly total precipitation (mm) and average of many years (1991–2020) from May to September in 2023 and 2024 R. Rätsep et al. 205 Statistical analysis In the experiment, the influencing factors were cultivar and vintage; therefore, a two-factor analysis of variance (ANOVA) was used for data analysis. The coefficient of determination was used to explain how much the variability of one factor was caused by its relationship to another factor. Mean comparisons were conducted using Tukey honest significance difference test (p< 0.05) to confirm statistically significant differences between vintages or cultivars. The results in the figures are presented as means of three replicates, and standard deviation are added. Different letters and asterisks in the figures indicate statistically significant differences between means. Results Heliothermal Index ranged from 872 to 1622 in the years from 2004 to 2024, and in experimental years from 1400 to 1622 (Fig. 2). The trend line shows warming, but Heliothermal Index had large variability between test years. The fructose content was consistent across the trial years only in the ‘Regent’ (Fig. 3A). For the other cultivars, the vintage effect was significant. The ‘Leon Millot’ and ‘Rondo’ had higher fructose content in 2024, while the other cultivars had higher levels in 2023. The average in fructose content across the two vintages for the trial cultivars ranged from 72 to 98 g l-1, with the cultivar having a significant effect (Fig. 3B). The cultivar ‘Marquette’ had the highest fructose content, and ‘Zilga’ had the lowest. The vintage effect was also significant, with the 2023 vintage having higher fructose content. 1071 1164 1319 1189 1005 1112 1380 1374 1039 1300 1240 1067 1286 872 1559 1209 1108 1225 1164 1400 1622 500 700 900 1100 1300 1500 1700 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19 20 20 20 21 20 22 20 23 20 24 H el io th er m al In de x ( 01 /0 4 -3 0/ 09 ) Fig. 2. Heliothermal Index and trend line for the years from 2004 to 2024. Data is according to the Estonian Environment Agency’s Tõravere weather station. 110a 77b 79b 87a 75b 86a 110a 86b 84a 84a 98a 86b 95a 86b 74a 69b 0 20 40 60 80 100 120 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga Fr uc to se , g l-1 A 72f 81e 83d 84d 91c 92c 94b 98a 0 20 40 60 80 100 120 Zilga Rondo Leon Millot Regent Hasansky Sladky Solaris Cabernet Cortis Marquette Fr uc to se , g l-1 B Agricultural and Food Science (2025) 34: 202–212 206 The vintage affected the glucose content (Fig. 4A). The ‘Cabernet Cortis’ and ‘Marquette’ had higher glucose content in 2023, while the ‘Leon Millot’, ‘Rondo’, and ‘Hasansky Sladky’ showed lower levels. There was no vintage effect on the ‘Regent’, ‘Solaris’, and ‘Zilga’. As two-year average significant differences were observed between the cultivars (Fig. 4B). The ‘Zilga’ had the lowest glucose content (64 g l-1), while ‘Marquette’ had the highest (98 g l-1). The ‘Rondo’, ‘Leon Millot’, and ‘Hasansky Sladky’ had similar glucose content. The vintage significantly affected the total sugars content in the cultivars of ‘Cabernet Cortis’, ‘Leon Millot’, ‘Ron- do’, ‘Marquette’, ‘Hasansky Sladky’, and ‘Zilga’, although the direction of the effect varied (Fig. 5A). In 2023, the highest sugar content was observed in the ‘Cabernet Cortis’, ‘Marquette’, and ‘Zilga’, while in 2024, the highest sugar content was estimated in the ‘Leon Millot’, ‘Rondo’, and ‘Hasansky Sladky’. Significant differences between vintages were not found for the ‘Regent’ and ‘Solaris’. 110a 77b 79b 87a 75b 86a 110a 86b 84a 84a 98a 86b 95a 86b 74a 69b 0 20 40 60 80 100 120 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga Fr uc to se , g l-1 A 72f 81e 83d 84d 91c 92c 94b 98a 0 20 40 60 80 100 120 Zilga Rondo Leon Millot Regent Hasansky Sladky Solaris Cabernet Cortis Marquette Fr uc to se , g l-1 B Fig. 3. The fructose content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two- year average (B). Different letters denote significant differences. 103a 78b 70b 95a 74b 91a 100a 96b 75a 74a 77a 80a 80b 86a 64a 63a 0 20 40 60 80 100 120 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga Gl uc os e, g l-1 A 64f 74e 79d 82dc 83c 83c 90b 98a 0 20 40 60 80 100 120 Zilga Regent Solaris Leon Millot Rondo Hasansky Sladky Cabernet Cortis Marquette Gl uc os e, g l-1 B Fig. 4. The glucose content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two- year average (B). Different letters denote significant differences. R. Rätsep et al. 207 The average sugar content in the berries over the two years showed considerable variability between cultivars, ranging from 138 to 206 g l-1 (Fig. 5B). Significant differences were found between cultivars, with the ‘Marquette’ having the highest sugar content, and ‘Zilga’ the lowest. 60% of the variability in sugar content in the grapes can be explained by the differences between cultivars (determination coefficient 0.6). The interaction between vintage and cultivar (determination coefficient 0.39) showed that 39% of the variability in sugar content is explained by the interaction of cultivar and vintage. This indicates that the sugar content of some grape cultivars was influenced by the specific vintage, while others remained stable across years. The tartaric acid content in 2023 was higher compared to 2024 in most cultivars, with only ‘Regent’ showing a higher tartaric acid content in 2024 (Fig. 6A). ‘Zilga’ (5.9 g l-1) stood out with significantly higher tartaric acid content, while ‘Leon Millot’ (3.5 g l-1) and ‘Regent’ (3.7 g l-1) had the lowest value (Fig.6B). 216a 157b 151b 199a 153b 184a 213a 199b 160a 161a 176a 178a 177b 187a 141a 135b 0 50 100 150 200 250 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga To ta l s ug ar , g l-1 A 138g 160f 169e 175d 177d 182c 187b 206a 0 50 100 150 200 250 Zilga Regent Rondo Leon Millot Solaris Hasansky Sladky Cabernet Cortis Marquette To ta l s ug ar , g l-1 B Fig. 5. The total sugars content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two-year average (B). Different letters denote significant differences. 5.6a 4.1b 4.6a 2.4b 4.6a 3.6b 4.9a 4.5b 3.2b 4.3a 5.6a 4.5b 5.6a 4.5b 5.4a 3.8b 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga Ta rt ar ic a ci d, g l-1 A 3.5e 3.7e 4.1d 4.6c 4.7c 4.9bc 5.1b 5.9a 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Leon Millot Regent Rondo Hasansky Sladky Marquette Cabernet Cortis Solaris Zilga Ta rt ar ic a ci d, g l- 1 B Agricultural and Food Science (2025) 34: 202–212 208 There were noticeable differences in malic acid content between the vintages for the cultivars (Fig. 7A). In 2023, the ‘Leon Millot’, ‘Rondo’, ‘Regent’, and ‘Hasansky Sladky’ showed higher malic acid content, while ‘Cabernet Cor- tis’, ‘Solaris’, and ‘Zilga’ had higher values in 2024 compared to 2023. ‘Hasansky Sladky’ (5.5 g l-1) stood out with the highest malic acid content, while ‘Regent’ (2.1 g l-1) and ‘Solaris’ (2.5 g l-1) had the lowest. On average, the malic acid content across cultivars was similar in 2024 compared to 2023 (Fig. 7B). The total acid content was higher in the ‘Solaris’ and ‘Zilga’ in 2024, and the most noticeable annual change was observed in the ‘Cabernet Cortis’, where the total acid content increased from 8.5 g l-1 (2023) to 11.1 g l-1 (2024) (Fig. 8A). Across the vintage’s, the ‘Rondo’ and ‘Marquette’ had similar acid content. Fig. 6. The tartaric acid content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two-year average (B). Different letters denote significant differences. 5.6a 4.1b 4.6a 2.4b 4.6a 3.6b 4.9a 4.5b 3.2b 4.3a 5.6a 4.5b 5.6a 4.5b 5.4a 3.8b 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga Ta rt ar ic a ci d, g l-1 A 3.5e 3.7e 4.1d 4.6c 4.7c 4.9bc 5.1b 5.9a 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Leon Millot Regent Rondo Hasansky Sladky Marquette Cabernet Cortis Solaris Zilga Ta rt ar ic a ci d, g l- 1 B 1.2b 5.9c 4.6a 3.0b 4.6a 3.8b 4.8a 3.9b 2.8a 1.4b 1.3b 3.7a 5.8a 5.2b 3.1b 4.6a 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga M al ic a ci d, g l-1 A 2.1d 2.5d 3.5c 3.8c 3.9bc 4.2b 4.3b 5.5a 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Regent Solaris Cabernet Cortis Leon Millot Zilga Rondo Marquette Hasansky Sladky M al ic a ci d, g l-1 B Fig. 7. The malic acid content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two-year average (B). Different letters denote significant differences. R. Rätsep et al. 209 The total acid content varied between different grape cultivars (Fig.8B). The highest acid content was in ‘Cabernet Cortis’ (9.8 g l-1), followed by ‘Zilga’ (9.4 g l-1), and ‘Hasansky Sladky’ (9.1 g l-1). The cultivars with the lowest acid content were ‘Rondo’ (6.5 g l-1), ‘Solaris’ (6.5 g l-1), and ‘Regent’ (6.8 g l-1). The cultivar effect on acid content was highly significant: 71% of the variation in total acids content could be explained by cultivar characteristics (deter- mination coefficient 0.71). The interaction between cultivar and vintage explained 27% of the variation (determi- nation coefficient 0.27). Discussion Over the past 20 years, the HI has been ≤1500 in most years. However, for the first time, in both 2018 and 2024, the HI exceeded 1500, which would shift Estonia from the very-cold to the cold viticultural region category according to the classification by Tonietto and Carbonneau (2004). These trends indicate significant changes in Estonia’s climate, with important implications for the region’s viticultural potential. In the cold viticultural cli- mate class, the Heliothermal potential allows for a much wider range of grape cultivars, both white and red, to reach full ripeness. If this warming trend continues, it may become possible to grow a greater diversity of grape cultivars in open-field conditions in the future. In general, summers are becoming warmer, but a significant limiting factor – spring night frosts – remains, which can have a severe negative impact on grape production. In cooler climates, an early spring warming that is followed by late spring frosts can highly likely damage the fruit-bearing shoots, and later also affect the yield and berry ripening (Schrader et al. 2020). A similar effect was observed during the experimental years: late spring frosts in May damaged the shoots, leading to the growth of new shoots from dormant buds. The extent of damage from spring frosts depends on the growth vigor and recovery capacity of the vines. Due to the reasons above, ‘Caber- net Cortis’ and ‘Solaris’ were particularly affected, suffering significant damage from spring frost, which resulted in a considerably lower grape sugar content and a higher acid content in 2024. The vintage affected the sugar profile of the experimental cultivars differently, as also presented in other exper- iments. For example, the composition of sugars in grapes remained more stable across years, while acids were sensitive to climate changes (Liu et al. 2006, Preiner et al. 2013). However, at the same time, no significant differ- ences were observed in the malic, citric, and titratable acid, and glucose contents nor changes in the grape pH of Fig. 8. The total acids content (g l-1) in grape cultivars for the years 2023 and 2024 (A), and as the two- year average (B). Different letters denote significant differences. 8.5b 11.1a 9.4a 6.3b 6.3a 6.7a 8.7a 8.3a 7.2a 6.4b 6.2b 6.8a 9.4a 8.8b 9.0b 9.8a 0.0 2.0 4.0 6.0 8.0 10.0 12.0 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 2023 2024 Cabernet Cortis Leon Millot Rondo Marquette Regent Solaris Hasansky Sladky Zilga To ta l a ci d, g l- 1 A 6.5f 6.5f 6.8f 7.9e 8.5c 9.1b 9.4b 9.8a 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Rondo Solaris Regent Leon Millot Marquette Hasansky Sladky Zilga Cabernet Cortis To ta l a ci d, g l -1 B Agricultural and Food Science (2025) 34: 202–212 210 ‘Vidal’ among the three production areas (Yan et al. 2022). Long-term experiments in Estonia have demonstrated a significant effect of the vintage on the grape’s maturity, which was associated to the variable weather conditions across the ten years of observations (Maante-Kuljus et al. 2019a). This finding is consistent with other studies showing that a year with increased precipitation and cooler temperatures is characterized by higher organic acid levels, whereas berries from a warmer and drier vintage contain relatively less organic acid (Pereira et al. 2006), due to the increased accumulation of sugars. The test years were distinguished by a warmer and drier September than average for many years, which positively affected grape ripening. The sugar content was significantly influenced by the cultivar and vintage. The most stable cultivars in terms of sugar content were ‘Regent’ and ‘Solaris’. It has been shown that in general, the sugar content of ripe grapes varies between 150 and 250 g l⁻¹ (Dharmadhikari 2006). Most of the cultivars studied (except ‘Zilga’) remained within this range in both years (151–216 g l⁻¹). ‘Zilga’ stood out for its low sugar content, having both the lowest fructose and glucose levels. However, it has also been noted that labrusca-type cultivars should be harvested before full matu- rity, as wine made from fully ripe berries with high sugar content tends to have a stronger foxy taste (Plocher and Parke 2008). On average, the tested berries contained more fructose than glucose in 2023, but in 2024, glucose and fructose were present at equal levels, which is consistent with findings reported in the literature (Dharma- dhikari 2006). At the ripening stage, glucose and fructose are usually present in equal amounts (1:1 ratio), though some variation in the glucose-to-fructose ratio may occur among cultivars. In wine production, analyzing the sugar profile is important, as yeasts generally exhibit a slightly higher preference for glucose than fructose, confirming the glucophilic nature of Saccharomyces wine yeasts (Tronchoni et al. 2009). Grape sugar concentration is the primary determinant of the final alcohol content of wine, and is therefore an important factor in choosing winemaking techniques. The expected ethanol production can be estimated based on the measured sugar concentration, applying the official European conversion ratio of 16.83 g l⁻¹ of sugar to pro- duce 1% (v/v) ethanol. The average sugar content of the tested grapes ranged from 138 to 206 g l⁻¹, from which it can be concluded that the potential alcohol content would be between 8.2% and 12.2%. Sugar supplementation is necessary for ‘Zilga’, because the alcohol content of the wine should be above 8.5%. For other tested cultivars, this may not be necessary in every year. According to the legislation, sugar may only be added to increase alco- hol by up to 3%, so based on the data provided, mainly dry wine can be produced in Estonia. In traditional wine regions, the production of dry wines has become increasingly challenging. For example, Duchêne and Schneider (2005) showed that over the last 30 years, the estimated alcohol level of ‘Riesling’ grapes in Alsace has increased by 2.5% (v/v) due to warmer ripening periods and earlier phenology. However, the results of this experiment indicate that it is possible to produce dry wine with a lower alcohol content in Estonia, which could be more appealing to consumers. It is important to note that the composition of organic acids was significantly influenced by vintage, though not for all tested cultivars. The most stable cultivars in terms of acids were ‘Rondo’ and ‘Marquette’. It also became evident that the vintage affected the ripening of most of the tested cultivars, but the direction of this effect varied for each cultivar. Malic acid is the most problematic among the acids, as wines with excessive malic acid can taste aggressively sour and tart. In the tested years, malic acid was lower in 2023, which was affected by significantly warmer weather conditions in August and September. Too high malic acid content can be regulated in winemaking through malolactic fermentation, a process in which malic acid in wine is converted into softer, cream- ier lactic acid. The experimental results showed that ‘Cabernet Cortis’ has a low malic acid content in a favorable year, but after spring damage and therefore, late ripening, the content may become higher than expected. In the case of ‘Hasansky Sladky’, malolactic fermentation would be recommended in both years. Based on the two years average total acids content, the cultivars can be divided into three categories. Cultivars with high acidity (>9 g l-1): ‘Cabernet Cortis’, ‘Zilga’, ‘Hasansky Sladky’; with medium acidity (7–9 g l-1): ‘Leon Millot’, ‘Marquette’; with low acidity (<7 g l-1): ‘Rondo’, ‘Solaris’, ‘Regent’. Previous wine trials showed that the average total acid content in young wines over a three-year period was 7.7±0.6 g l⁻¹ for ‘Solaris‘, and 8.0±0.4 g l⁻¹ for ‘Zilga‘ (Maante-Kuljus et al. 2024). These results indicate that Estonian-grown grapes contain high levels of organic acids, making them suitable for wine styles that emphasize freshness and aging potential. Conclusions Over the last twenty years, the trendline shows warming with important implications for the region’s viticultural potential. The composition of grapes organic acids and sugars was influenced by vintage, though not for all tested cultivars. The most stable cultivars in terms of sugars content were ‘Regent’ and ‘Solaris’, and in terms of acids R. Rätsep et al. 211 content ‘Rondo’ and ‘Marquette’. It was also found that some grape cultivars grown in Estonia may be character- ized by low sugar and elevated acid levels. The two-year average sugar content in the grapes showed considerable variability between cultivars. The ‘Marquette’ had the highest sugar content and the potential alcohol content of the wine would be 12.2%. In the case of the ‘Zilga’, it is recommended to add sugar, because the natural alcohol content of 8.2% does not meet the requirements for wine. The malic acid content was high in the grapes of the ‘Hasansky Sladky’ in both years, therefore malolactic fermentation is recommended when producing wine from this cultivar. In conclusion, it can be stated that, according to the requirements of the legislation of viticulture zone A, dry wine can be made from the cultivars that were tested. Acknowledgments This research was funded by the Estonian Ministry of Rural Affairs. Project: “Mapping and quality analysis of grape wine cultivars grown in Estonia for domestic grape breeding”. References Colak, N. & Bengu, A. 2024. Ripening-Related Changes in the Nutritional Profile of a Little-Known Red ‘Isabel’ Grape (Vitis vinifera L. × Vitis labrusca L.). Applied Fruit Science 66: 1877–1889. https://doi.org/10.1007/s10341-024-01166-w Conde, C., Silva, P., Fontes, N., Dias, A.C.P., Tavares, R.M., Sousa, M.J., Agasse, A., Delrot, S. & Gerós, H. 2007. Biochemical changes throughout grape berry development and fruit and wine quality. Food 1: 1–22. Dharmadhikari, M. 2006. Composition of Grapes. Iowa State University: Midwest Grape and Wine Industry Institute. https://www. extension.iastate.edu/wine/publications/composition-of-grapes/ Duchêne, E. & Schneider, C. 2005. Grapevine and climatic changes: A glance at the situation in Alsace. Agronomy for Sustainable Development 24: 93–99. https://doi.org/10.1051/agro:2004057 Gąstoł, M. 2015. Vineyard performance and fruit quality of some interspecific grapevine cultivars in cool climate conditions. Folia Horticulturae 27: 21–31. https://doi.org/10.1515/fhort-2015-0011 Gustafsson, J.-G. & Mårtensson, A. 2005. Potential for extending Scandinavian wine cultivation. Acta Agriculturae Scandinavica Section B - Soil & Plant Science 55: 82–97. https://doi.org/10.1080/09064710510029097 Huglin, P. 1978. New way to evaluate the solar thermal possibilities of a wine growing environment. Proceedings of the French Academy of Agriculture 64: 1117–1126. Jones, N.K. 2018. An investigation of trends in viticultural climatic indices in Southern Quebec, a cool climate wine region. Jour- nal of Wine Research 29: 120–129. https://doi.org/10.1080/09571264.2018.1472074 Karvonen, J. 2014a. Vitis cv. Zilga is a vine for the northern temperate climate. Horticultural Science 41: 147–151. https://doi.org/10.17221/17/2014-HORTSCI Karvonen, J. 2014b. Northern European viticulture compared to Central European high altitude viticulture: Annual growth cy- cle of grapevines in the years 2012–2013. International Journal of Wine Research 6: 1–7. https://doi.org/10.2147/IJWR.S60208 Kliewer, W.M. 1966. Sugars and organic acids of Vitis vinifera. Plant Physiology 41: 923–931. https://doi.org/10.1104/pp.41.6.923 Kunter, B., Unal, O.B., Keskin, S., Hatterman-Valenti, H. & Kaya, O. 2024. Comparison of the sugar and organic acid components of seventeen table grape varieties produced in Ankara (Türkiye): a study over two consecutive seasons. Frontiers in Plant Science 15: 1321210. https://doi.org/10.3389/fpls.2024.1321210 Kupe, M., Ercisli, S., Karatas, N., Skrovankova, S., Mlcek, J., Ondrasova, M. & Snopek, L. 2021. Some Important Food Quality Traits of Autochthonous Grape Cultivars. Journal of Food Quality 2021: 1–8. https://doi.org/10.1155/2021/9918529 Liu, H.F., Wu, B.H., Fan, P.G., Li, S.H. & Li, L.S. 2006. Sugar and acid concentrations in 98 grape cultivars analyzed by principal com- ponent analysis. Journal of the Science of Food and Agriculture 86: 1526–1536. https://doi.org/10.1002/jsfa.2541 Maante-Kuljus, M., Karp, K., Rätsep, R., Mainla, L., Koort, A., Põldma, P., Kaldmäe, H. & Moor, U. 2024. Polyphenol composition of skin-contact fermented ‘Solaris’ and ‘Zilga’ wines. Beverages 10: 59. https://doi.org/10.3390/beverages10030059 Maante-Kuljus, M., Rätsep, R., Mainla, L., Moor, U., Starast, M., Põldma, P. & Karp, K. 2019a. Technological maturity of hybrid vine (Vitis) fruits under Estonian climate conditions. Acta Agriculturae Scandinavica Section B - Soil & Plant Science 69: 706–714. https://doi.org/10.1080/09064710.2019.1641547 Maante-Kuljus, M., Vool, E., Rätsep, R. & Karp, K. 2015. The effect of genotype on table grapes soluble solids content. Agronomy Research 13: 141–147. Maante-Kuljus, M., Vool, V., Mainla, L., Starast, M. & Karp, K. 2019b. Berry quality of hybrid grapevine (Vitis) cultivars grown in the field and in a polytunnel. Agricultural and Food Science 28: 137–144. https://doi.org/10.23986/afsci.76822 Pereira, G.E., Gaudillere, J.P., Van Leeuwen, C., Hilbert, G., Maucourt, M., Deborde, C., Moing, A. & Rolin, D. 2006. HNMR metab- olite fingerprints of grape berry: Comparison of vintage and soil effects in Bordeaux grapevine growing areas. Analytica Chimica Acta 563: 346–352. https://doi.org/10.1016/j.aca.2005.11.007 Plocher, T. & Parke, B. 2008. Northern wineworks: Growing grapes and making wine in cold climates. 2nd ed. Hugo, MN: North- ern Winework. Eau Claire Printing, USA. 214 p. Agricultural and Food Science (2025) 34: 202–212 212 Preiner, D., Tupajić, P., Karoglan Kontić, J., Andabaka, Ž., Marković, Z. & Maletić, E. 2013. Organic acids profiles of the most important Dalmatian native grapevine (Vitis vinifera L.) cultivars. Journal of Food Composition and Analysis 32: 162–168. https://doi.org/10.1016/j.jfca.2013.09.005 Schrader, J.A., Cochran, D.R., Domoto, P.A. & Nonnecke, G.R. 2020. Yield and berry composition of cold-climate grape cultivars and advanced selections in Iowa climate. HortTechnology 30: 193–203. https://doi.org/10.21273/HORTTECH04557-19 Spinelli, F.R., André, J.A., Celso, P.G., Vicenzi, M.S., Kinast, E.J., Lamb, C.R.C. & Bertoldo, G. 2024. Soluble solids profile of Brazil- ian Vitis labrusca and hybrid grape musts, from the 2012–2022 harvest. Journal of Food Composition and Analysis 125: 105797. https://doi.org/10.1016/j.jfca.2023.105797 Tonietto, J. & Carbonneau, A. 2004. A multicriteria climatic classification system for grape-growing regions worldwide. Agricul- tural and Forest Meteorology 124: 81–97. https://doi:10.1016/j.agrformet.2003.06.001 Tronchoni, J., Gamero, A., Arroyo-López, F.N., Barrio, E. & Querol, A. 2009. Differences in the glucose and fructose consumption profiles in diverse Saccharomyces wine species and their hybrids during grape juice fermentation. International Journal of Food Microbiology 134: 237–243. https://doi.org/10.1016/j.ijfoodmicro.2009.07.004 Van Leeuwen, C., Destrac-Irvine, A., Gowdy, M., Farris, L., Pieri, P., Marolleau, L. & Gambetta, G.A. 2023. An operational model for capturing grape ripening dynamics to support harvest decisions: This article is published in cooperation with the 22nd GiESCO International Meeting, hosted by Cornell University in Ithaca, NY, July 17–21, 2023. OENO One 57: 505–522. https://doi.org/10.20870/oeno-one.2023.57.2.7399 Volschenk, H., van Vuuren, H.J.J. & Viljoen-Bloom, M. 2006. Malic acid in wine: Origin, function and metabolism during vinifica- tion. South African Journal of Enology and Viticulture 27: 123–136. https://doi.org/10.21548/27-2-1613 Wang, H., Yao, X., Liu, M., Xu, X., Wang, Y., Kong, J., Chen, W., Xu, Z., Kuang, Y., Fan, P., Liang, Z., Liu, H. & Dai, Z. 2025. Climate, soil, and viticultural factors differentially affect the sub-regional variations in biochemical compositions of grape berries. Scientia Horticulturae 339: 113858. https://doi.org/10.1016/j.scienta.2024.113858 Yan, H.K., Ma, S., Lu, X., Zhang, C.C., Ma, L., Li, K., Wei, Y.C., Gong, M.S. & Li, S. 2022. Response of wine grape quali- ty to rainfall, temperature, and soil properties in Hexi Corridor. American Society for Horticultural Science 57: 1593–1599. https://doi.org/10.21273/HORTSCI16845-22 Assessing Estonia’s viticultural potential based on thecompositional analysis of sugars and acids of wine grape cultivars Introduction Material and methods Experimental sites and plant material Measurements and analysis Weather conditions Statistical analysis Results Discussion Conclusions Acknowledgments References