Progress 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 will characterize environmental drivers of tannin extractability in hybrid wine grapes. Grape samples of 2 cultivars will be collected from 5 locations in the Northeast region across 2 growing seasons. Environmental data will be provided by weather monitoring stations located within the vineyard. Tannin extractability will be determined using high performance liquid chromatography. Outreach materials will be developed with the help of the Penn State Grape and Wine Team, which hosts regular industry meetings and workshops. Knowledge on environmental drivers of tannin extractability will help farmers to develop approaches to maximize the quality and profitability of their crops.
- To characterize environmental drivers of phenolic extractability in hybrid grapes. Tannin extractability will be measured in hybrid grape varieties using established HPLC 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 HPLC 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.
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 will be collected from 5 sites in the Northeast region including central New York, Erie, PA, and southeastern PA. While hybrid grapes are the focus of this work, comparing them to a common vinifera cultivar will add important context to our results and increase the impact of our conclusions. At commercial maturity, 5 lbs of each grape variety will be collected from each vineyard and stored at -20C until all samples are ready for analysis. Basic grape chemistry will be measured using established methods for pH, titratable acidity, and Brix (24).
Objectives 1 and 2 – 2. Tannin extraction
Model wine and exhaustive tannin extractions will be performed in triplicate as shown in Medina-Plaza et al. 2021 (25). For the model wine extraction, crushed berries will be added to a 14% ethanol model wine solution at a ratio of 1:10 w/v. Tannin extraction will take place over 72 hours at 22℃ with agitation. This solution will then be filtered and concentrated under vacuum. For the exhaustive extraction, berries will be processed in a homogenizer and added to methanol containing 0.1% HCl at a ratio of 1:10 w/v. Methanol extraction will take place at 4℃ until the grape material is colorless. A final extraction with 70% acidified(0.1%HCl) acetone will take place overnight at 22℃. Methanol and acetone extracts will be pooled and concentrated under vacuum. Model wine and exhaustive extraction products will be stored at -20℃ until tannin quantification
Objectives 1 and 2 – 3. Tannin quantification
Tannin content of model wine and exhaustive extraction products will be quantified using reverse phase high-performance liquid chromatography (HPLC). The method is based on Watrelot 2021, which found that HPLC may be a more relevant method for measuring hybrid wine tannins than either protein precipitation or methylcellulose precipitation(26). Briefly, compounds will be separated using an Agilent HPLC unit with a polystyrene divinylbenzene reverse-phase column and diode array detector. A gradient of two mobile phases will be used for separation Mobile phase A will consist of water containing 1.275% orthophosphoric acid. Mobile phase B will consist of 80% aqueous acetonitrile containing 0.255% orthophosphoric acid. Tannin content will be calculated using a standard curve of (-) epicatechin and absorbance at 280nm. Tannin extractability will be calculated as the model wine tannin content divided by the exhaustive extraction tannin content.
Objectives 1 and 2 – 4. Environmental Data
Established weather stations in commercial and research vineyards will be used to collect temperature, humidity, precipitation, and solar radiation measurements across two growing seasons. Approximate topographical information including elevation, slope, and aspect will be collected from vineyard managers during grape collection visits. Vineyards will be evaluated based on topographical characteristics and weather summaries for each growing year to determine how each of these factors may impact tannin extractability.
Objectives 1 and 2 – 5. Statistical Analysis
Tannin extraction experiments will be conducted with three biological replicates for each site using randomly sampled grapes to reduce the effect of individual grape/cluster differences. Tannin quantification experiments will be conducted with three technical replicates for each extract to ensure that our method is sufficiently precise. Mean and standard deviations for tannin extractability and environmental characteristics will be collected for each site, variety, and vintage. Regression analysis will be conducted using R (27) with location, environmental factors, and vintage as independent factors to determine the effect on tannin extractability. Regression will be conducted separately for each variety, as it is expected that this will a significant effect. Results for vinifera and hybrid varieties will be compared to assess how tannin extractability may respond differently to environmental factors in these groups.
Sample Collection:
Ten commercial vineyards in the Northeast region who grew both Cabernet Franc and Chambourcin grape varieties were contacted requesting a sample of both grape cultivars and access to weather data from the previous year. Though most responded positively, only 5 vineyards were able to freeze both sets of grape samples at harvest, with an additional 2 vineyards that provided only Chambourcin or Cabernet Franc grapes.
Grape and Environmental Descriptors
Grapevine environment was variable between vineyard locations. Growing-degree-days were similar for most locations except for BG, which was ~600 units higher than the other locations. Cumulative precipitation was higher for BG and HV locations (around ~35 in.) and lower for SM and WW locations (around ~25 in.). Winter lows were found around 15F for all locations except for HV, which was milder at 20.7F. All grapes achieved appropriate sugar development for red winemaking (Brix = 22-24) except for the HV location. Grape pH and berry mass were mostly discriminated by cultivar, with Chambourcin having consistently lower pH and higher grape mass.
Table 1: Grape Sample and Environmental Descriptors
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 mg epicatechin eq. / g grapes. Cultivar status did not significantly affect total grape tannins; however a significant location effect was observed in the Chambourcin grape samples. Model wine extractions contained significantly lower tannin content in all cases, between 0.5-1.8 mg epicatechin eq. / g grapes. Again, model wine tannin content was comparable between cultivars, but highly variable between Chambourcin grapes from different locations.
Figure 1: Tannin Content of Model Wine and Exhaustive Grape Extracts
We expected to observe a consistent location effect between cultivars, with the hybrid grapes having overall lower extractable tannins. Instead, extractable tannins were only significantly different between locations in the Chambourcin grapes, with some reaching higher tannin content than the V. Vinifera cultivar. This seems to contradict an array of hybrid winemaking literature, which repeatedly finds lower tannin content in hybrid wines. However, it’s possible that the tannin losses observed in hybrid grapes occur during fermentation, racking, or aging, which we did not perform in our model extractions. Cabernet Franc’s relative indifference to environment was also unexpected. This may be an issue with sample size, in which case we may observe significant differences after studying the second vintage from these locations. Alternatively, Cabernet Franc grapes may be less tolerant to Pennsylvania’s climate, resulting in a “bottoming-out” of tannin values, which appears as low variability.
While location effects were observed, we did not observe any clear correlations between environmental measures and tannin extractability in this vintage. This is likely to be remedied when the year 2 samples are added, as they will double the sample size and allow us to discriminate between geography-based location effects and climate-related location effects.
Ongoing Studies:
Tannin extractability and basic grape chemistry measurements are in progress for year 2 samples. HPLC tannin analysis is in progress for year 1 and year 2 samples.
Education & Outreach Activities and Participation Summary
Participation Summary:
The results of this research will help grape farmers to develop viticultural methods to improve tannin extractability in hybrid wine grapes. Penn State University has a strong agricultural extension program with agricultural extension educators located across the state of Pennsylvania. The Penn State Grape and Wine Team regularly publishes outreach material relevant to grape growers and winemakers in the region. The team produces articles, videos, fact sheets, workshops, and webinars available on the extension website ( https://extension.psu.edu/food-safety-and-quality/grape-and-wine-production) and Facebook page (https://www.facebook.com/PennStateExtensionVitEnol). Dr. Cain Hickey is an extension educator on the Grape and Wine Team and will help us to develop effective outreach materials. It is expected that we will publish one article on the Penn State Extension website and host one webinar for grape and wine producers. Through these materials, we 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.
Results from this research project will be presented at the 2023 National Conference for the American Society of Enology and Viticulture.