Glycosidic aroma precursors of Syrah and Chardonnay grapes after an oak extract application to the grapevines

Abstract

Syrah and Chardonnay grapevines were treated with an oak extract in order to determine the effect on glycosidic aroma precursors. Grapevines were treated at three different timings of the veraison (treatment 1, 2 and 3). Aglycons were obtained by enzymatic hydrolysis, and these were identified and quantified by means of gas chromatography–mass spectrometry (GC–MS). Results suggest that after the applications the majority of compounds from the oak extract were assimilated and stored as glycosidic forms in both cultivars. Also, other compounds not present in the extract were affected, with a different behaviour observed depending on the timing of application and the variety. In general, C6 compounds, alcohols, terpenes, phenols and C13-norisoprenoids in Syrah showed a decrease and in Chardonnay an increase. Thus, this study proved a change in the glycosidic aroma profile in grapes after the oak application, so these treated grapes could produce wines with different aromatic quality.

Introduction

Glycosidic precursors are a diverse group of odourless compounds initially identified in grapes by Cordonnier and Bayonove (1974), and reputed to be the most important group responsible for some of the varietal attributes of wines. This is especially true for non-floral grapes, since frequently these forms are more common than free aromas (Bureau et al., 2000, Francis et al., 1996, López et al., 2004, Noguerol-Pato et al., 2012). Grape glycoconjugates are composed of a sugar moiety which always includes glucose bound to a volatile aglycon by a β-glucosidic bond. The nature of the aglycon and the glycosidic fraction proportion depends on grape variety, but may be influenced by other factors such as soil, climatic conditions, viticultural practises and environment where the plant grows (Bureau et al., 2000, Koundouras et al., 2006, Zoecklein et al., 2008). Some studies also suggest that glycosylation is a storage medium for volatile compounds which limits the toxicity of some of them since glycosides are much more water soluble than aglycons and are thus considered vectors for the transport and accumulation of such compounds in plants (Stahl-Biskup et al., 1993, Winterhalter and Skouroumounis, 1997). The hydrolysis of these glycoconjugates by acids or enzymes can yield odour-active aglycons such as monoterpenes, C13 norisoprenoids, phenols, alcohols and C6 compounds (Günata et al., 1985, Sefton et al., 1993), being the source of varietal aromas in wines (D’Incecco et al., 2004, Francis et al., 1996, Sánchez Palomo et al., 2006). Enzymatic hydrolysis is the most adequate technique to generate aglycons without structural changes, with the enzymatic preparation AR2000 used the most (Gómez García-Carpintero et al., 2012, Sefton et al., 1993, Sánchez Palomo et al., 2006).

On the other hand, it has been proven that factors outside the vineyards, as well as various treatments applied to these, are able to modify the aroma composition of grapes and their respective wines. Clear examples are the studies which show that chemical pesticides not only affect the fermentative aromatic compounds but also the varietal aroma (Darriet et al., 2001, Oliva et al., 2008). Moreover, some research studies have revealed that grape and grapevine exposure to smoke influences the chemical composition and gives the wine smoky sensory characteristics (Kennison et al., 2009, Kennison et al., 2011, Wilkinson et al., 2011). Recently, Martínez-Gil, Garde-Cerdán, Martínez, Alonso, and Salinas (2011) and Martínez-Gil, Garde-Cerdán, Zalacain, Pardo-García, and Salinas (2012) have also shown how the aromatic composition of wines is modified when oak extract applications are carried out on Verdejo and Petit Verdot grapevines. These studies prove that a change occurs in the free aroma profile of white and red wines when oak extract is applied at the onset of veraison, suggesting that grapes can probably store volatile compounds, mainly as non-volatile precursors. It is therefore important to complete further studies on the glycosidic compounds. Also, the studies on smoke application to grapevines prove the influence of the variety, timing and duration of smoke exposure on the aromatic composition (Kennison et al., 2009, Singh et al., 2011). Glycosylation of some volatile phenol compounds in grapes as a consequence of grapevine exposure to bushfire smoke has been demostrated (Dungey et al., 2011, Hayasaka et al., 2010, Wilkinson et al., 2011).

In consequence, the aim of this study was to determine the impact on the aroma glycosidic precursors of Syrah and Chardonnay grapes, once an oak extract was applied to grapevines via foliar at three different times. The study emphasised the aglycons that could come directly from the extract, as these are found in its composition, and the aglycons that could be modified indirectly after application, as these are not found in its composition.