M. Blank1, V. Giebe1, C. Stein1 und J. Sturm2
1 Departement of Enology, LVWO Weinsberg, Traubenplatz 5, 74189 Weinsberg
2 Departement for Grape Breeding, LVWO Weinsberg, Traubenplatz 5, 74189 Weinsberg
Work presented at the International Scientific Congress GreenWINE, held on 19–20 May 2025 in Verona.
Context and Purpose of the Study
PIWI grape varieties (Pilzwiderstandsfähig, fungus-resistant) present innovative solutions for sustainable viticulture by addressing environmental challenges faced by traditional Vitis vinifera. PIWIs demonstrate resistance or tolerance to fungal diseases such as downy and powdery mildew, growing PIWIs enable the reduction of chemical treatments by ca. 60-80%. This reduction in fungicide use promotes environmentally friendly vineyard management and economic sustainability (Merdinoglu et al. 2018).
Despite these advantages, PIWIs are relatively new, and comprehensive studies on their oenological quality, especially regarding phenolic composition, remain limited (Watrelot et al. 2023). Previous studies indicate that hybrid grape varieties possess distinct phenolic profiles compared to traditional Vitis vinifera, notably in anthocyanin composition and tannin content (Pedneault et al., 2016, Watrelot and Norton 2020).
Currently, few studies have evaluated the phenolic profiles of hybrids and species other than Vitis vinifera (Merkytė et al., 2020). Phenolic composition, especially anthocyanins, is genetically controlled and varies significantly among grape varieties (Revilla et al., 2001), with reported anthocyanin concentrations ranging from 186 to 561 mg malvidin/L (González-Centeno et al., 2019).
Anthocyanin fingerprints have been characterized for various varietal wines (Ehrhardt et al., 2014; Tampaktsi et al., 2023). Specifically, anthocyanin diglucosides serve as markers for non-vinifera grapes and wines (Cheng et al., 2022a), as demonstrated by Regent grapes, which accumulate at least twice as much 3,5-anthocyanidin diglucoside derivatives compared to the PIWI Cabernet Cortis (Ehrhardt et al., 2014).
According to Watrelot and Norton (2020), studies on tannin extractability and retention generally report lower condensed tannin contents in Vitis vinifera grapes and wines compared to hybrid varieties (Springer and Sacks, 2014; Norton et al., 2020; Nicolle et al., 2019; Cheng et al., 2022b, Gapinsky et al. 2024).
However, most available data focus on cold-hardy hybrids from the U.S. Midwest (Rice et al. 2017). Springer and Sacks (2014) noted tannin concentrations were approximately 5.5 times lower in French hybrids (e.g., Leon Millot, Maréchal Foch, DeChaunac) than in classic Vitis vinifera varieties (Pinot Noir, Limberger, Sangiovese), particularly in skin tannins (Manns et al. 2013). Overall, interspecific hybrids showed about 1.8-fold lower tannin levels compared to Vitis vinifera (Springer and Sacks, 2014). In contrast, other studies indicate that certain PIWI varieties can exhibit higher tannin concentrations in red wines (Ehrhardt et al., 2014; Socha et al., 2015; Tampaktsi et al., 2023).
Therefore, it is essential to characterize the phenolic profiles of PIWI varieties to effectively tailor winemaking protocols to their specific characteristics. This study aimed to characterize the phenolic profiles of German-bred PIWI varieties intended for red wine production, emphasizing their suitability for sustainable winemaking practices.
Materials and Methods
Experimental Design: A vineyard managed by the grape breeding department compared 30 different PIWI varieties in three replicates over three vintages (2022–2024). Twelve PIWI red varieties (Accent, Baron, Cabertin, Cabernet Cortis, Cabernet Cantor, Levitage, Monarch, Pinotin, Prior, Regent, Rondo, Satin Noir) (Figure 1) and four traditional Vitis vinifera grape varieties (Bl. Trollinger, Bl. Limberger, Pinot Noir, Cabernet Sauvignon) were analyzed using a randomized block design with six vines per variety.
Berry Sampling: At harvest, ten grape clusters per replicate were selected to assess cluster weight, berry count per cluster, and average berry weight. Total soluble solids (TSS, °Brix) were measured using FTIR spectroscopy (FT2 Winescan Flex -Foss, Hilleroed, Denmark). From each replicate, 100 berry samples were frozen at -20°C for further analysis.
Micro-scale Winemaking: Small-scale fermentations were conducted in 250 mL fermentors (Figure 2). After thawing and manual crushing, musts were inoculated with Saccharomyces cerevisiae (Uvaferm BDX™, 250 mg/kg). Fermentation proceeded at approximately 20°C for six days, with cap manually plunged down three times daily. Following a two-week post-fermentation period, pomace was pressed at 1 bar. Phenolic compounds in resulting wines were analyzed using the Harbertson-Adams assay (Harbertson et al., 2002; Heredia et al. 2006).
The repeatability of the method was between 4 and 12 % for tannins and 5 and 9 % for anthocyanins (Blank et al., 2021). The concentration of tannins obtained after micro-scale fermentations was linearly correlated to that obtained after higher scale fermentation (R² = 0.86), which was also the case for anthocyanins (R² = 0.89) (Blank et al., 2021).
Statistical analysis Statistical evaluations and graphs were generated using the open-source software R (version 3.3.1; R Foundation for Statistical Computing, Vienna, Austria) within the RStudio environment (version 2024.04.2). Differences between treatment means were assessed using Tukey’s HSD test (P = 0.05). Principal component analysis (PCA) was performed with the FactoMineR package after scaling variables to unit variance.
Results
Significant differences were observed among the phenolic profiles of the PIWI wines. Cabernet Cortis and Rondo showed up to three times higher tannin concentrations compared to Pinot noir and Bl. Limberger, while Monarch and Accent exhibited roughly twice the tannin levels (Figure 3a). In contrast, Pinotin, Prior, and Levitage had significantly lower tannin concentrations, similar to those of Bl. Trollinger.
Figure 3: Phenolic composition of wines from 14 hybrid grape varieties compared to the Vitis vinifera varieties (Trollinger and Limberger). Wines were produced by micro-scale fermentation of 100 berries per replicate collected at harvest. Box plots represent the distribution of means from three replicates over the vintages 2022–2024. a) Anthocyanin concentration expressed as mg Malvidin-3-O-Glucoside (M3OG)/L; b) Tannin concentration expressed as mg Catechin Equivalent (CE)/L.
Anthocyanin levels in PIWI wines were consistently higher than those in traditional varieties, ranging between 500 mg/L and 3000 mg/L, with the highest concentrations found in Rondo and Accent (Figure 3b). Differences in polymeric pigment content among PIWI varieties indicate varying potentials for color stability. Multivariate analysis of phenolic profiles grouped the studied varieties into four distinct clusters, highlighting their phenolic diversity and potential for winemaking (Figure 4).
Discussion
Significantly lower tannin concentrations, comparable to those of Vitis vinifera Bl. Trollinger, were consistently observed over three vintages for the hybrid varieties Pinotin, Prior, and Levitage. This reduced tannin content likely results from tannin binding to proteins and cell wall components, causing lower extractability. Springer and Sacks (2014) reported tannin extraction rates of only 2.2–5.7% in interspecific hybrids, notably lower than the 8–22% typical for V. vinifera. For hybrids with low tannin extractability, traditional methods such as cold soak (Springer et al., 2014), saignée (Cheng et al., 2022b), enzyme treatments (Manns et al., 2013), and enological tannin additions (Fredrickson et al., 2020) have generally been ineffective. Therefore, alternative approaches, including protein binding with bentonite and protein modification using high-power sonication, have been explored to improve tannin extraction and retention (Cheng et al., 2022). In our own trials, thermovinification and post-fermentation maceration at 35 °C were tested to enhance tannin extraction (results not shown).
Tannin concentrations in Cabernet Cortis and Rondo wines were up to three times higher than in Pinot noir and Bl. Limberger, while Monarch and Accent exhibited tannin levels approximately twice as high. Similar results for Rondo were observed elsewhere (Stój et al. 2020). Among the German-grown PIWI varieties studied, Cabernet Cortis reached high tannin levels (~3000 mg CE/L), similar to Monarch and Prior (Tampaktsi et al., 2023), though other PIWI cultivars, such as Vidoc, can exceed 5000 mg CE/L (Kapusta et al. 2018). To effectively manage these high tannin concentrations, adapted winemaking methods, including micro-oxygenation, have been suggested (Teissedre, 2018; Ferenbach et al., 2023).
The formation of anthocyanin–tannin complexes plays a key role in color stabilization in wines made from Vitis vinifera grapes. However, little is known about the interaction between tannins and anthocyanins in hybrid grape varieties. Burtch and Mansfield (2016) suggested that wines from hybrid grapes are less prone to forming polymeric pigments, despite often containing higher anthocyanin levels than V. vinifera wines. Our results, however, do not support this finding, as the levels of total polymeric pigments (TPP) in PIWI wines were comparable to those observed in V. vinifera wines within the same study. This suggests that PIWI wines may achieve similar color stability, which has positive implications for their oenological potential.
Conclusion
Evaluating the phenolic profiles of PIWI varieties is crucial for assessing their suitability in red wine production and for refining winemaking strategies based on tannin and color extraction potential. The results of this study provide a solid foundation for integrating PIWIs into sustainable winemaking, aligning both environmental and quality objectives. Duley et al. (2023) emphasized the promising potential of PIWIs for high-quality wine production, while also highlighting the need to adapt winemaking protocols to the specific traits of each cultivar. Future research should expand the number of samples and varieties analyzed, and include sensory evaluations of wines produced from hybrid grapes to better assess their organoleptic quality. Continued investigation into the phenolic and volatile composition, along with consumer acceptance, will be essential for promoting the broader adoption of hybrid grape varieties in modern winemaking.
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