Final report for GNE22-280
Project Information
Hybrid grapes are valuable agricultural products in the Northeast region, where European grape varieties are difficult to cultivate. They are more resistant to adverse environmental conditions, pests, and diseases. It is likely that hybrid wine grapes will become more widely adopted because their resilience makes them potent tools for addressing environmental and economic sustainability concerns in the wine industry. Proanthocyanidins, also known as condensed tannins, are a key determinant of the quality and profitability of red wines. Low tannin extractability from hybrid grapes has been reported by repeated studies, and it represents a significant barrier to their wider adoption. Little is known about how environmental factors may influence extractability. Hybrid grape farmers are left with little guidance on how to improve the quality of their product, as existing research has focused on tannin accumulation in vinifera grapes. This study examined tannin extractability and wine tannin content in hybrid and Vitis vinifera wine grapes across 2 vintages and 4 locations. Total grape tannins, tannin extractability, and wine tannin content were highly variable, suggesting a complex interaction between grape genetics and grapevine environment. Notably, not all hybrid grapes produced low tannin wines. In one location, hybrid grapes were able to achieve wine tannin content comparable to Vitis vinifera. Environmental data was collected from each location in an attempt to associate environmental factors with changes in tannin extraction dynamics. A negative relationship was observed between total growing-season precipitation and wine tannin content.
- To characterize environmental drivers of phenolic extractability in vinifera and hybrid grapes. Tannin extractability will be measured in different grape varieties using established precipitation methods. We will compare grapes from the same cultivar across different locations and vintages to assess the impact of growing region and climate on tannin extractability.
- To examine variation in tannin extractability between vinifera and hybrid cultivars. Vinifera and hybrid grapes will be compared to determine whether there are significant differences in their response to environmental factors. Tannin extractability will be measured in hybrid and vinifera varieties using established methods. The impact of growing region and climate will be assessed, as in objective 1.
The purpose of this project is to investigate key factors impacting phenolic extractability in red wine grapes grown in the Northeast region. Hybrid grapes are of particular importance in this region because they are well acclimated to adverse conditions that may prevent cultivation of other grape varieties. They are different from Vitis vinifera, the classic European wine grape, because they have both vinifera and native grape heritage.
Grape phenolics are a broad class of health and sensory relevant compounds characterized by phenol-substituted benzene rings. The structure and function of phenolic compounds varies widely, including stilbenes, phenolic acids, flavanols, and proanthocyanidins, otherwise known as condensed tannins(1).
Phenolics in grapes and red wine are associated with many positive health effects. Moderate red wine consumption has been linked to a lower risk of dementia and cardiovascular disease (2,3). Grape consumption has also shown cardioprotective effects like lowered blood pressure and improved blood cholesterol composition (4).
Grape phenolics contribute to the sensory quality of wine. The color of red wine is caused by pigmenting phenolic compounds known as anthocyanins. Condensed tannins contribute to bitterness and astringency, depending on their size and structure. Astringency, the feeling of drying or puckering on the tongue is an important, desirable quality in red wines (5).
The content of phenolics in grapes and wine vary due to differences in grape genetics, environment, and winemaking practices (6,7). For example, condensed tannins in dry red wine can be found in a wide range of concentrations, from less than 100 – 1500 mg/L CE. Within Cabernet Sauvignon alone, tannin levels have been shown to vary up to 32-fold (8). Hybrid wine grapes show unique phenolic characteristics as a result of their genetic background. While the tannin concentrations found in hybrid grapes can be comparable to vinifera varieties, wine made from hybrid grapes exhibits very low concentrations of condensed tannins (9). Extractability is, therefore, a significant determining factor in the final wine tannin concentration. Limited tannin extractability from hybrid grapes can be detrimental to the quality, health attributes, and profitability of their wine products
The proportion of tannins extracted into wine can be adjusted through different winemaking parameters. Increased fermentation temperature, thermovinification, cell wall degrading enzymes, and extended maceration have been reported to increase tannin extraction from vinifera grapes (10,11). By contrast, many of these winemaking techniques result in little to no difference in the tannin extractability from hybrid grapes(12).
Grapevine environment may impact the extractability of phenolics from red wine grapes. Previous studies have examined the role of environmental factors on grape phenolics, but these studies were generally concerned with accumulation of phenolics rather than extractability, and rarely included hybrid grape varieties. In this study, we will assess the impact of several environmental characteristics on basic grape characteristics in addition to phenolic extractability. This information will help farmers in the Northeast region to produce a more consistently healthy, profitable product.
Citations:
- Kennedy J, Saucier C, Glories Y. Grape and Wine Phenolics: History and Perspective. Am J Enol Vitic. 2006 Sep 1;57:239–48.
- Zorraquín-Peña I, Esteban-Fernández A, González de Llano D, Bartolomé B, Moreno-Arribas M. Wine-Derived Phenolic Metabolites in the Digestive and Brain Function. Beverages. 2019 Jan 11;5(1):7.
- Castaldo L, Narváez A, Izzo L, Graziani G, Gaspari A, Di Minno G, et al. Red Wine Consumption and Cardiovascular Health. Molecules. 2019 Jan;24(19):3626.
- Wightman JD, Heuberger RA. Effect of grape and other berries on cardiovascular health. J Sci Food Agric. 2015;95(8):1584–97.
- Ma W, Guo A, Zhang Y, Wang H, Liu Y, Li H. A review on astringency and bitterness perception of tannins in wine. Trends Food Sci Technol. 2014 Nov 1;40(1):6–19.
- Liang Z, Yang Y, Cheng L, Zhong G-Y. Characterization of polyphenolic metabolites in grape hybrids. Vitis J Grapevine Res. 2015;52:51–9.
- Nile SH, Kim SH, Ko EY, Park SW. Polyphenolic Contents and Antioxidant Properties of Different Grape (V. vinifera, V. labrusca, and V. hybrid) Cultivars. BioMed Res Int. 2013 Aug 21;2013:e718065.
- Harbertson J, Hodgins R, Thurston L, Schaffer L, Reid M, Landon J, et al. Variability of Tannin Concentration in Red Wines. Am J Enol Vitic. 2008 Jan 1;59:210–4.
- Springer LF, Sacks GL. Protein-Precipitable Tannin in Wines from Vitis vinifera and Interspecific Hybrid Grapes (Vitis ssp.): Differences in Concentration, Extractability, and Cell Wall Binding. J Agric Food Chem. 2014 Jul 30;62(30):7515–23.
- Sacchi KL, Bisson LF, Adams DO. A Review of the Effect of Winemaking Techniques on Phenolic Extraction in Red Wines. Am J Enol Vitic. 2005 Sep 1;56(3):197–206.
- Unterkofler J, Muhlack RA, Jeffery DW. Processes and purposes of extraction of grape components during winemaking: current state and perspectives. Appl Microbiol Biotechnol. 2020 Jun 1;104(11):4737–55.
- Manns DC, Coquard Lenerz CTM, Mansfield AK. Impact of Processing Parameters on the Phenolic Profile of Wines Produced from Hybrid Red Grapes Maréchal Foch, Corot noir, and Marquette. J Food Sci. 2013;78(5):C696–702.
- Medina-Plaza C, Dokoozlian N, Ponangi R, Blair T, Block DE, Oberholster A. Correlation between Skin Cell Wall Composition and Polyphenol Extractability of Pinot noir and Cabernet Sauvignon Grapes. Am J Enol Vitic. 2021 Oct 1;72(4):328–37.
- Mercurio MD, Dambergs RG, Herderich MJ, Smith PA. High Throughput Analysis of Red Wine and Grape PhenolicsAdaptation and Validation of Methyl Cellulose Precipitable Tannin Assay and Modified Somers Color Assay to a Rapid 96 Well Plate Format. J Agric Food Chem. 2007 Jun 1;55(12):4651–7.
- R Core Team. R: A language and environment for statistical computing [Internet]. Vienna, Austria: R Foundation for Statistical Computing; 2021. Available from: https://www.R-project.org/
- Amerine MA, Winkler AJ. Composition and quality of musts and wines of California grapes.
- Bindon KA, Li S, Kassara S, Smith PA. Retention of Proanthocyanidin in Wine-like Solution Is Conferred by a Dynamic Interaction between Soluble and Insoluble Grape Cell Wall Components. J Agric Food Chem. 2016 Nov 9;64(44):8406–19.
Cooperators
- (Educator and Researcher)
- (Educator and Researcher)
Research
Objectives 1 and 2 – 1. Grape Sample collection and preparation
Each year for two years, grape samples from V. vinifera cultivar Cabernet Franc and hybrid cultivar Chambourcin were collected from 4 sites in Pennsylvania, labelled BG, HV, SM, and WW. While hybrid grapes are the focus of this work, comparing them to a common vinifera cultivar adds important context to our results and increases the impact of our conclusions. At commercial maturity, 5 lbs of each grape sample was collected and frozen at -20C prior to analysis. Grapes were destemmed and mixed to ensure random sampling. Basic grape characteristics such as Brix, pH, and grape size were measured.
Objectives 1 and 2 – 2. Tannin extraction
Model wine and exhaustive tannin extractions were performed in triplicate using a procedure modified from Medina-Plaza et al (13). Grape tissue was extracted in either model wine (14% ethanol, 5g/L tartaric acid, pH = 3.5) or an 80% acetone solution. Grape tissue was homogenized in the extraction solution using 0.5mm silica/zirconia beads and a benchtop bead homogenizer. After 1 hour, samples were centrifuged, and the grape extract was decanted. Homogenization and extraction steps were then repeated. Pooled grape extract was concentrated using a rotary evaporator and freeze-dryer, then resuspended in 40% methanol. Extracts were stored at -20C until analysis.
Objectives 1 and 2 – 3. Microvinification
Winemaking was performed in triplicate for each grape sample at a 50mL scale. Grapes were destemmed and crushed by hand. Fresh juice was reserved for pH and Brix analysis. SO2 was added prior to fermentation at a concentration of 25ppm in the form of potassium metabisulfite (KMBS). Grape musts were inoculated using Go-Ferm Protect and Lalvin ICV-GRE yeast. Fermentation took place over 8 days in a 26C water bath with de-gassing and punch-downs performed twice daily. Complete fermentation was confirmed using AimTab Reducing Substances tablets. Wines were pressed after fermentation and racked 48 hours later. Tannin quantification was performed after racking.
Objectives 1 and 2 – 4. Tannin quantification
Tannin content of grape extracts and wine samples were quantified using methyl-cellulose precipitation, adapted from Mercurio et al(14). Briefly, 25-100 uL of sample was mixed with a 0.04% methylcellulose solution, incubated for 3 minutes, and centrifuged at 10,000 rpm to precipitate tannin-methylcellulose complexes. After discarding supernatant, the tannin pellet was resuspended in a 40% acetonitrile solution. Absorbance of the resulting solution was measured at 280nm. Tannin content was calculated using a standard curve of epicatechin ranging from 0 to 300 mg/L. Tannin extractability was calculated as the model wine tannin content divided by the exhaustive extraction tannin content.
Objectives 1 and 2 – 5. Environmental Data
Environmental data was collected for each vineyard using either in-vineyard weather stations or the nearest available NOAA weather station. Relevant weather conditions included growing-degree-days(GDDs), precipitation, and winter lows. GDDs were measured using a baseline of 50F. Cumulative GDDs and precipitation were recorded for the growing season, approximately April to October. To calculate winter lows, the average minimum temperature was recorded for each month in the winter prior to the growing season. The coldest mean monthly low was recorded for each site.
Objectives 1 and 2 – 6. Statistical Analysis
Tannin extraction experiments were conducted with 5 biological replicates for each site using randomly sampled grapes to reduce the effect of individual grape/cluster differences. Tannin quantification experiments were conducted with three technical replicates for each extract or wine sample. ANOVA and Tukeys HSD were used to evaluate significant differences between grape extract and wine tannin content. Regression analysis was conducted using R (15) with location, cultivar, and vintage as independent factors to determine their effect on tannin extractability.
Grape and Environmental Descriptors
Grapevine environment was variable between vineyard locations and vintages. Growing-degree-days were similar for most locations except for BG, which was ~500 units higher than the other locations and the climate normal for that area. A GDD of 3000-3500 is generally favorable for high quality wine grape production(16), and most sites were able to reach that threshold. Higher precipitation(~35in. compared to ~25in.) was observed at BG, HV Year 1, and SM Year 2, with the lowest precipitation found at WW. Winter lows were more moderate in Year 2, around 25F compared to around 15F observed in Year 1.
Grape characteristics were highly variable, but showed some consistent cultivar, vintage, and location effects. Brix was location and vintage dependent, with most samples falling in an appropriate range for red wine production between 22-25 brix. HV samples were the exception, having brix below 20 in both vintages. Grape juice pH was mainly cultivar dependent, however some vintage variation was noted. Chambourcin exhibited lower pH around 3.2-3.4, while Cabernet Franc had pH around 3.5-3.8. Grape size was highly variable, with cultivar, vintage, and location playing a role. Notably, Chambourcin grapes were consistently larger than Cabernet Franc grapes grown at the same location, and Year 2 grapes were consistently larger than Year 1 grapes. Wine yield was very similar between grape samples, however some cultivar and location effects were observed. Chambourcin grapes had consistently higher yield than Cabernet Franc. WW showed consistently lower yield, while HV grapes had the highest yields.
Model Tannin Extractions:
Total grape tannins and model wine extractable tannins were highly variable. Total grape tannins estimated by exhaustive extraction were found between 2.4-4 mg epicatechin eq. / g grapes. Location was the most important factor for determining total grape tannins. Cultivar and vintage effects did not lead to significant differences in this quantity. Model wine extractions contained significantly lower tannin content in all cases, between 0.5-2.1 mg epicatechin eq. / g grapes. Again, model wine tannin content was comparable between cultivars and vintages, but highly variable between different locations. In both extracts, grapes from WW had the highest tannin content and BG the lowest. The only significant vintage effect was observed in WW Cabernet Franc samples, where model wine extractable tannins were significantly higher in Year 2.
Figure 1: Tannin Content of Model wine and Exhaustive Extracts.
Mean tannin content of exhaustive grape extracts (light blue bars) and model wine extracts (dark blue bars) are shown for each grape sample. Error bars represent the 95% confidence interval of the mean. ANOVA and Tukey’s HSD were performed within each extraction and cultivar group, with letters above each bar representing significant differences. The percent tannin extractability, calculated as the ratio of ethanol extract tannin content to exhaustive extract tannin content, is shown at the bottom of each bar.
Tannin extractability, measured as the percentage of total grape tannins that are extracted in an ethanol solution, was highly variable. Values ranged from 24.8-63.7%, with location and cultivar playing a major role. Chambourcin grapes had a higher tannin extractability overall, contrary to expectations. High tannin extractability did not always predict model wine tannins. HV Chambourcin samples had the highest extractability, but the model wine tannins were more moderate. Vintage did not impact tannin extractability except in the case of WW Cabernet Franc.
Figure 2: Final Tannin Content of Wine Samples
Mean wine tannin content is shown for each grape sample, with light grey bars representing the Year 1 vintage, and dark grey bars representing the Year 2 vintage. Error bars represent the 95% confidence interval of the mean. ANOVA and Tukey’s HSD were performed with letters above each bar representing significant differences.
Microvinification:
Wine tannin content was highly variable across location, cultivar, and vintage, ranging from 250-1600mg/L. Wine tannin showed similar trends to model tannin extractability, with BG having the lowest tannin content and WW having the highest. There were, however, numerous exceptions. Significant vintage effects were found in 3 locations, with SM Cabernet Franc having almost 900mg/L lower tannin content in Year 2. The magnitude of these effects was larger in the wine tannin measurements. The highest tannin content in model wine extracts was 2.3x the lowest, while the highest wine tannin content was 5.4x the lowest. Finally, Chambourcin wine samples exhibited significantly lower tannin content compared to cabernet franc.
Discussion:
Existing literature on hybrid wine grapes emphasizes the low tannin content observed in these varieties. This has been attributed to lower tannin extractability(Springer and Sacks 2014). Our research noted, contrarily, that comparable tannin content was extracted from hybrid and vinifera grapes. This was only in the model extracts, with wine tannins being characteristically lower in the hybrid cultivar. We believe that this observation points to the importance of tannin stability in determining final wine tannin content. In our experiments, model tannin extracts were processed immediately to dissolve the tannins in a 40% methanol solution in which tannins are highly soluble. In winemaking, extracted tannins remain in solution with suspended cell-wall-material and grape proteins which can bind and precipitate tannins (17).
While the effect of cultivar on wine tannin content is well documented, we have observed significant location and vintage effects on total grape tannins, tannin extractability, and wine tannin content. This is an optimistic finding for hybrid winemakers, as it seems that hybrid cultivars from certain locations may be able to achieve appropriate tannin content for high-quality dry red wine styles.
One potentially important connection was noted between tannin content and precipitation. Other studies have noted that tannin content tends to be lower when the vine receives more water during the growing season. This is attributed to a biosynthesis pathway connected to tannins which is upregulated during periods of water-stress. In our study, the location that had the highest precipitation also had the lowest wine tannin content, and the location that had the lowest precipitation had the highest wine tannin content. This was observed in vintage effects as well. Year 2 precipitation was higher at SM, with a corresponding drop in wine tannin content. Year 2 precipitation was lower in HV, with a corresponding rise in wine tannin content. Given the small sample size of our study, the degree of variability in observed tannin content, and the sheer number of environmental factors that may influence wine tannin content, we cannot rule out extraneous factors. Future studies should examine this problem more specifically, potentially using viticultural interventions to decrease the water-load of grapevines in high-precipitation areas.
Our research has found significant variability in total grape tannins, tannin extractability, and wine tannin content across cultivars, locations, and vintages. These results indicate that there is a complex interaction between grape genetics and grapevine environment responsible for the tannin extraction characteristics of each grape sample.
This research found that grape tannin extractability and wine tannin content are highly variable across locations and vintages. While hybrids are assumed to have lower tannin content in every case, we found that tannin extractability was similar between the two cultivars. Wine tannin content was lower in hybrids overall, however one location produced hybrid grapes with a similar tannin content to the vinifera. Wine tannin content was generally associated with lower precipitation at the vineyard site. These results suggest first that hybrid grapes are not necessarily doomed to have low tannin content. Under the right conditions, hybrid grapes may produce wines with tannin content similar to their vinifera counterparts. Second, these results suggest that tannin stability may be particularly important in hybrid winemaking. Future research in this area should examine the role of tannin stability in hybrid winemaking as well as potential vineyard management strategies to modulate vine water status.
Education & Outreach Activities and Participation Summary
Participation Summary:
Research from this project was presented at two conferences. A poster and short talk were presented at the national conference for the American Society of Enology and Viticulture in 2023 in Napa, California. This conference is targeted at grape farmers, winemakers, wine researchers, and wine industry professionals throughout the United States with attendance around 800. A poster and short talk were also presented at the eastern conference for the American Society of Enology and Viticulture in 2024 in Cleveland, Ohio. This conference is targeted at grape farmers, winemakers, wine researchers, and wine industry professionals working outside of the west coast. This research will also be presented at the Eastern Winery Exposition in Spring 2025. These presentations allowed us to distribute our research to a variety of stakeholders including many farmers working with hybrid grapes.
Research from this project will be submitted to a peer-reviewed journal, such as the American Journal of Enology and Viticulture. This will help distribute our work to other grape and wine researchers.
Penn State University has a strong agricultural extension program with agricultural extension educators located across the state of Pennsylvania. To distribute our results to grape farmers and winemakers in Pennsylvania, we are developing a publication with the Penn State Grape and Wine Team. This publication will discuss the difficulties in managing phenolics when using hybrid grapes as well as our project’s efforts to improve our understanding of tannin extractability.
Project Outcomes
This project has improved our knowledge of tannin extraction dynamics in hybrid wine grapes. Additionally, we have connected some characteristics of the grapevine environment to the final wine tannin content. A better understanding of these interactions will allow researchers to develop better methods for managing tannins in hybrid winemaking. This will lead to economic benefits for farmers, as tannins are important for the sensory quality and profitability of grape and wine products. More effective tannin management strategies will also increase the utility of hybrid wine grapes, allowing for more sustainable grape production with lower fungicide and pesticide use.
Performing research for this grant has provided significant knowledge and experience both for me personally and for our lab as a whole. Speaking with farmers and other stakeholders when collecting samples and at various conferences gave me insights into the motivations, concerns, and needs of farmers dealing with a rapidly changing climate. As our lab previously worked mostly on cacao, I had to make connections with local grape producers and fellow wine researchers to complete this project. These connections will allow our lab to continue to work on topics relevant to Pennsylvania wine grapes even after I graduate. I plan to pursue a career in industry food science research, where I will certainly be able to utilize the skills gained during this project.