How to increase the tannin content of cold-hardy interspecific hybrid grape wines cultivated in cold climate?

By Pamela Nicolle and Karine Pedneault

Cold-hardy interspecific hybrid grape (CIHG) cultivars are crosses between the European Vitis specie V. vinifera and North American Vitis species, such as V. labrusca and V. riparia (Pedneault and Provost, 2016). CIHG cultivars have contributed extensively to the expansion of northern wine production areas, such as the Province of Quebec (Canada) that typically have harsh winters and short growing seasons (Pedneault et al., 2013). Despite the real advantage they present from a viticulture standpoint, including high tolerance to fungal diseases and frost, CIHG are challenging to work with in the winery, due to their unique biochemical composition (Pedneault and Provost, 2016). For instance, CIHG cultivars produce wines with high anthocyanin but low tannin concentrations (Manns et al., 2013), making their tannin/anthocyanin ratio unsuitable for tannin polymerisation during winemaking (Ribéreau-Gayon et al., 2006). Thus, young red CIHG wines typically exhibit a low mean degree of polymerisation (≤ 4) (Manns et al., 2013) that lead to low mouthfeel wines. However, consumers associate higher mouthfeel to higher quality in wine (Kassara and Kennedy, 2011).
Red wine astringency positively relates to the concentration of tannins and to their degree of polymerisation, as well as their degree of galloylation. Appropriate winemaking processes, mainly based on duration and temperature of maceration (e.g. pre-fermentative heat treatment, must freezing, extended maceration), contribute to optimising the extraction of phenolic compounds in wine, including tannins (Sacchi et al., 2005). The use of additives, either in the form of natural grape additives (e.g. pomace) (Nicolle et al., 2018) or as commercial enological tannins (Kyraleou and al., 2016), is another common practice to compensate for tannin deficiency and improve wine structure. But grape composition may also significantly impact tannin extractability and retention in wine, eventually leading to conflicting results. For example, practices such as tannin addition, cold pre-fermentative maceration and thermovinification, showed little to no effect on both tannin concentration and mouthfeel of red wines made from the CIHG varieties Marechal Foch, Corot noir, and Marquette (Manns et al., 2013).
High molecular tannins are known to interact with proteins and polysaccharides from the grape cell wall, which eventually removes them from must (Bindon, Kassara, and Smith, 2017). The pulp of CIHG varieties contains high concentration of pathogenesis-related proteins compared to typical wine grape varieties (V. vinifera) (Springer and Sacks, 2014). Pathogenesis-related proteins contribute to limit the extraction and retention of high molecular weight tannins in CHIG red wines (Springer et al., 2016).
Recently, our research group explored the impact of different treatments aiming at reducing the concentration of proteins in CHIG must on tannin retention in CHIG wine. Thus the impact of (1) must protein treatment, bentonite and heat; (2) pomace, fermented with and without; (3) tannin addition, 0-9 g/L; (4) and time of maceration, 0-11 days on tannin and protein extraction/retention in red CHIG wine were tested using a factorial experimental design (Figure 1). The CHIG variety used for this experiment was Frontenac, a cold-hardy variety developed at the University of Minnesota. Results were published in the journal Food chemistry (Nicolle et al., 2019)

Figure 1: Factorial experimental design used to produce experimental Frontenac wines, including the following factors: must protein treatment (untreated, bentonite-treated, and heat-treated must), pomace (must fermented with and without pomace), tannin addition (0, 1, 3, and 9 g/L), and time of maceration (0, 4, and 11 days after the end of alcoholic fermentation).

One of the most relevant finding was that protein removal from must prior to alcoholic fermentation, using bentonite addition or heat treatment, did not significantly improve tannin retention in CHIG Frontenac wines. On the contrary, conducting fermentation without pomace significantly increased tannin retention in wine, when exogenous tannins were used. In this case, concentrations of exogeneous tannins comprised between 5-15 times the recommended dose were necessary to obtain a significant increase in wine tannin concentration.

Figure 2: Tannin content (m/L epicatechin equivalent) in experimental Frontenac wines made from untreated, bentonite-treated and heat-treated must, related to the dose of tannin addition (0, 1, 3, and 9 g/L) at the end of alcoholic fermentation, in wines fermented with and without pomace. Different letter (lower case: oligomeric tannins; capital: polymeric tannin) indicate significant differences between wines at the 0.05 probability level.

In the light of these results, thermovinication has been proposed to the winemakers of Quebec wine industry as a suitable method to increase tannin retention in CHIG wines. Heating must and pomace prior to alcoholic fermentation (60-80°C) remove proteins from CHIG wines but, most importantly, limit any interaction between the cell wall components of the pomace and therefore maximises the retention of the exogeneous tannins required. Moreover, this winemaking approach has proved to be efficient for maximising the colour and improving its stabilisation. Work on sensory acceptability of this practise coupled with high exogeneous tannins addition need to be evaluated in terms of wine colour and stability as well as wine astringency and aroma profile.

Pamela Nicolle is a Ph.D. student at Laval University. She has been studying the compounds involved in tannin retention of red interspecific hybrid grape wines cultivated in cold climate. She has over 10 years of experience in fermented beverage in France and Canada. She has an expertise in winemaking as well as in chemical and sensory analysis.

Karine Pedneault is an assistant professor at the Department of Sciences of Sainte-Anne University (Nova Scotia, Canada) and an associated research scientist in Institut de Recherche en Biologie Végétale (IRBV; Montreal, Canada). Karine Pedneault's research focuses on factors affecting the quality of ciders and wines produced in cold climate. She is interested in the relations between the growing conditions of horticultural fruit plants (vine, apple), the quality of fruits (sugars, acidity, aromas, tannins) and the quality of wines.

References

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