Evaluation of the variability of skin thickness and berry physical properties in cv. ‘Grenache’ grown in different areas across the Ebro River Valley (Spain)

Abstract

The berry skin plays a very relevant role in red wine grape varieties, as the epidermal and subepidermal cells contain anthocyanins and other phenolic compounds that remarkably affect wine quality. This may be particularly relevant in those varieties that are considered to have thinner skins, such as ‘Grenache’. However, despite it being a widely grown variety in the Mediterranean area, little information on the skin characteristics of this variety is available. The aim of this study is to evaluate the variability of the berry skin properties of ‘Grenache’ grown at different locations across the Ebro River Valley in Spain. Samples of 300 randomly picked berries were obtained at harvest from 15 different vineyards, embracing a wide range of edaphic-climatic conditions that included vineyards with altitudes ranging from more than 600 m (S. Martín de Unx, Navarra) to barely 80 m above sea level (Constantí, Tarragona). From each sample, a 25-berry subsample was used to determine skin thickness (Sp_sk) and three parameters related to skin hardness: the maximum break force (F_sk), the break energy (W_sk), and the skin Young’s modulus (E_sk), using a universal testing machine for texture measurement. The remaining berries were used to measure berry weight, total soluble solids, pH and total anthocyanin content. Skin thickness for ‘Grenache’ was 0.23 mm, ranging from 0.15 – 0.29 mm, depending on the vineyard. This indicated that there is a significant variability for this parameter, depending on the growing conditions and/or between clones of this variety. Moreover, the range of this variation is similar to the whole range reported in earlier literature for all wine grape varieties studied, and indicates that although ‘Grenache’ berries have thin skins under some circumstances, some clones and/or growing conditions can result in much thicker skins. A similar degree of variability was found for the parameters related to skin hardness: F_sk, W_sk and E_sk. Berry weight was positively related to Sp_sk, and F_sk and W_sk were positively related to total anthocyanins. Even at a very wide scale such as that considered in this study, skin mechanical properties showed to play a role in red grape quality evaluation for ‘Grenache’. Further research is necessary in order to determine how climatic conditions, agricultural practices and clonal variability condition skin properties, and to evaluate to what extent they are related to final grape and wine quality.