Grape skins as supplements for color development in Pinot noir wine

Highlights

• Demonstrates the impact of tannin supplements on wine color stability.

• In six months, 20% more grape skins increased stable pigment concentration by 40%.

• Up to 85% of anthocyanin from recycled grape skins retained as stable wine pigments.

• Color benefit of grape skin supplement compromised when seed equivalent included.

Abstract

A particular challenge to making wine from Pinot noir grapes is the delicate flavor, light color and poor ageing potential of the wine. Conventional Pinot noir must preparations were compared with those made using a skin-based supplement to assess the impact on non-bleachable (sulfur resistant) pigments in the wine. When supplemented with either fresh grape pomace of Pinot noir, Pinot gris or Chardonnay grapes; Pinot noir grape marc or a commercial liquid grape skin extract, the additional seeds and pulp from the supplements were shown to compromise the development of stable pigments in the wine. To compare the relative merits of tannin derived from grape skins and seeds, the supplements used in a parallel experiment were the skins alone of the same three grape varieties and at six months bottle age, the stable pigment concentration was found to exceed the amount attributable to the supplement. A third experiment used fermented grape skins as the supplement, with 85% of the supplementary anthocyanin recovered as stable pigment complexes in the wine. Notably, this series of experiments showed that supplements containing grape seeds appeared to compromise non-bleachable pigment formation in the wine while skin only supplements stimulated their development.

Introduction

Anthocyanin pigments and tannins are major components of red wine and their relative concentrations have been correlated with measures of wine quality (Cozzolino et al., 2008, Holt et al., 2008, Kassara and Kennedy, 2011). The different properties, localizations and extractability of skin and seed tannins influence the contribution of each tissue source to phenolic attributes of the wine (Cortell et al., 2005, Koyama et al., 2007, Sparrow et al., 2015, Vidal et al., 2003). Consequently wine quality is governed by the extraction of skin and seed tannins in optimal proportions (Bautista-Ortin et al., 2007, Sparrow and Smart, 2017, Sparrow et al., 2015, Sparrow et al., 2016). All phenolic compounds are unstable and undergo numerous enzymatic and chemical reactions during winemaking and aging, making the effect of a supplement difficult to predict. Many of the changes in appearance and sensory properties during aging of red wine have been ascribed to anthocyanin-tannin reactions (Cheynier et al., 2006). Flavanols are the building blocks of both wine pigments and tannin, therefore the quantity and source of the flavanols influence the degree of polymerization of proanthocyanidins (condensed tannins) and the astringency and bitterness of red wine (McRae et al., 2013, Singleton, 1992). Both the grape variety and the tissue source (skin or seed) determine the amount and structure of the flavanols that occur in the wine matrix (Mattivi et al., 2009, Soares et al., 2017, Sparrow et al., 2015).

The concentration of stable, non-acylated forms of anthocyanin in Pinot noir grapes is low when compared to other red wine grape varieties, which generally have a greater range and concentration of anthocyanins (Mazza, Fukumoto, Delaquis, Girard, & Ewert, 1999). Pinot noir grapes also have an unusual distribution of tannins within the berry, with seed tannins being considerably higher in concentration than skin tannins (Downey et al., 2003, Kennedy, 2008, Mattivi et al., 2009, Sparrow et al., 2015). In addition, the procyanidins and proanthocyanidins found in grape seeds have a different subunit composition from those occuring in grape skins, which in turn impacts the structural configuration of the proanthocyanidins and consequently wine quality (Rousserie, Rabot, & Geny-Denis, 2019).

To balance these characteristics of Pinot noir grapes, supplementary sources of grape pigments or tannins are sometimes included with the fermenting must to improve the wine quality parameters and color stability of the wine. Food processing legislation however, places constraints on the addition of supplementary compounds to wines, a condition that is supported by the market demand for products containing only locally produced, naturally occurring raw materials.

The use of enotannins provides one option (Soares et al., 2017), these being commercially available products that consist of tannins extracted from oak wood or other suitable plant products such as grape seeds or skins and are sometimes used in red winemaking to improve aging characteristics such as texture, depth of color and wine color stabilization, attributes that are particularly vulnerable in Pinot noir wines (Crespy, 2003). However, the effectiveness of enotannins is not guaranteed; in a recent report on the influence of winemaking supplements on tannin composition and sensory properties of Shiraz wines (Li, Bindon, Bastian, Jiranek, & Wilkinson, 2017), the effect of a commercial, grape derived enotannin was found to be negligible.

The main difference between white and red grapes is the color of their skins. The red skin pigment is due to the presence anthocyanins (flavan-3-ols) found in the skin cells (Nel, 2018). Many commercial wineries produce a number of wine styles from different grape varieties, such that a range of grape solids; stalks, seeds and skins are produced as by-products. In the case of white or sparkling wines, fresh juice is pressed off the grapes prior to fermentation leaving the fresh grape solids (the pomace: composed of skins, pulp and seeds), whereas grape marc (fermented skins and seeds) is a common waste product of red wine production. While these by-products are normally either composted, sent off-site for tannin extraction or used as stock-feed, both are readily accessible sources of grape phenolics that might be used to supplement new wine (Muhlack et al., 2018, Somers, 1971).

Our previous investigations showed that source of grape tannin (skin or seed) has a significant impact on the non-bleachable pigment concentration of Pinot noir wine (Sparrow and Smart, 2017, Sparrow et al., 2015, Sparrow et al., 2016). Furthermore, in a recent review on the practical interventions influencing the sensory attributes of red wines and their relationship to the phenolic composition of grapes, Harrison (2018) points out that the ability to influence the relative extraction and subsequent reaction of phenolic components from the skin and seed of grapes is important in the production of full color development, optimal aromatic quality and sustained aging potential of the wine.

Consequently, the purpose of this investigation was to compare the effect of stable color development in Pinot noir wines made with a conventional Pinot noir must and those made using skin-based tannin supplements, sourced from either fresh red or white grape pomace, Pinot noir marc, or a commercially available grape skin extract. This latter treatment allowed the comparison of a commercially available grape skin tannin supplement with those that are readily available at low cost.

To further examine the effect of fresh grape skins as a supplement in the absence of seeds and of pulp, a second experiment was conducted using only the skins of the grape varieties that had been selected for the pomace supplement investigation. Acknowledging that it is much simpler to separate skins from grape marc, than from fresh grapes, a third experiment was conducted to compare the impact of fresh grape skin and fermented skin supplements on Pinot noir color development. The detailed comparisons described in these experiments were made possible using micro-vinification techniques (Sparrow & Smart, 2015) and provided valuable insights on aspects of color development in Pinot noir wine during the first six months of wine maturation.