‘Fortified’ wines volatile composition: Effect of different postharvest dehydration conditions of wine grapes cv. Malvasia moscata (Vitis vinifera L.)

Highlights

• The effect of dehydration rate was evaluated on Malvasia moscata aromatic cultivar.

• Fresh grape ripeness was related to dehydrated grape volatiles for the first time.

• The dehydration of the riper berries resulted in terpene-richer dehydrated grapes.

• Fast grape dehydration rate led to an increase in the content of free terpenes.

• Fortified wines made from fast dehydrated grapes were richer in volatile compounds.

Abstract

The impact of postharvest dehydration on the volatile composition of Malvasia moscata grapes and fortified wines produced from them was assessed. The ripeness effect of fresh grapes on volatile compounds of dehydrated grapes was evaluated for the first time in this study. Fresh grape berries were densimetrically sorted, and more represented density classes were selected. Dehydration of riper berries (20.5 °Brix) led to volatile profiles richer in terpenes, particularly linalool and geraniol. The effect of dehydration rate on the volatile composition of dehydrated grapes and fortified wines was also evaluated. Fast dehydration grapes were richer in total free terpenes, and the resulting wines contained greater amounts of volatile compounds. The predominant compounds were free esters, but linalool, rose oxide, citronellol and geraniol can also contribute to wine aroma, particularly for fast dehydration. β-Damascenone can be an active odorant, although its contribution was greater in wines made from slow dehydrated grapes.

Introduction

In recent years, the growing market demand of diversifying enological products has promoted the use of partially dehydrated grapes for the production of fortified and reinforced wines. Fortified wines are sweet wines produced from fresh or dehydrated grapes by adding alcohol or spirits, whereas reinforced wines are dry wines made with partially dehydrated grapes (weight loss less than 25% of initial fresh weight; Mencarelli & Tonutti, 2013). Wines famous for their particular aroma are produced in many viticultural areas of the world from postharvest dehydrated grapes.

Postharvest dehydration is a dynamic process of water loss from the berries occurring under more or less controlled environmental conditions. Off-vine grape dehydration can be performed by direct exposure of grapes to sun in regions with favorable climatic conditions (Ruiz, Zea, Moyano, & Medina, 2010), and indoors in naturally ventilated rooms (Rolle, Giordano, et al., 2012) or in chambers with thermohygrometric control (Bellincontro et al., 2004, Chkaiban et al., 2007, Torchio et al., 2016). This process induces changes in the chemical composition and in the physical properties of grape berries. In addition to increased sugar content, postharvest dehydration plays an important role in the concentration, synthesis and oxidation of volatile and phenolic compounds (Bellincontro et al., 2004, Costantini et al., 2006, Mencarelli et al., 2010, Noguerol-Pato et al., 2013). Moreover, changes are produced in the composition and structure of fruit surface tissues, which affect the color and texture of the berry skin (Rolle et al., 2012, Rolle et al., 2013).

During grape dehydration, water stress can alter the cellular structure of the berry, and therefore affects cell metabolism (Ramos, Silva, Sereno, & Aguilera, 2004). Generally, a first metabolic stress response occurs, involving changes in membrane permeability by activation of lipoxygenase (LOX), followed by a drastic change in basal cell metabolism from aerobic to anaerobic, related to alcohol dehydrogenase (ADH) activity (Costantini et al., 2006). The increased activity of LOX and ADH during the dehydration process promotes the formation of different volatile compounds (Chkaiban et al., 2007, Costantini et al., 2006). Dehydration time is an important commercial parameter, but weight losses by water evaporation and dehydration rate are key factors in the changes occurring in the metabolism of wine grapes (Cirilli et al., 2012). The metabolic response to water stress is faster in grapes dehydrated under uncontrolled environmental conditions, whereas the accurate control of thermohygrometric conditions delays water stress even at higher weight loss (Chkaiban et al., 2007). Dehydration temperature affects the critical weight loss; the higher the temperature, the faster the water stress (Nicoletti et al., 2013). Nevertheless, the critical water loss for grape metabolism depends on the variety (Chkaiban et al., 2007, Costantini et al., 2006). Therefore, a careful control of the environmental conditions is of great importance for the evolution of volatile compounds during the grape dehydration process.

The volatile composition of grapes contributes greatly to the varietal aroma and quality of wines. In white aromatic cultivars, such as Muscat, Riesling and Gewürztraminer, monoterpenes are the main compounds responsible for the typical floral aroma (Strauss, Wilson, Gooley, & Williams, 1986). Few studies have evaluated the changes in volatile compounds of grapes throughout the dehydration process and of the resulting wines. In both aromatic and non-aromatic wine grape cultivars, alcohols, esters and terpenes with positive odor descriptors are synthesized during the postharvest dehydration process (Serratosa, Marquez, Moyano, Zea, & Merida, 2014). Therefore, the wines made from postharvest dehydrated grapes are richer in terpenes than those made from the fresh fruit (Moreno et al., 2008). C6 volatile compounds providing herbaceous notes are also formed (Costantini et al., 2006). Although postharvest dehydration influences the volatile composition of grapes, the significance of the changes depends on weight loss (Moreno et al., 2008, Santonico et al., 2010), dehydration rate (Bellincontro et al., 2004) and also temperature (Santonico et al., 2010). Most of these studies were performed on non-aromatic wine grapes.

Vitis vinifera L. cv. Malvasia moscata is an aromatic white wine grape variety. In Italy, Malvasia grapes are used for the production of fortified wines, but no work has been published to date on the aromatic potential of Malvasia moscata grapes to produce this type of wine. Malvasia moscata is a local cultivar probably originating in Piedmont (North-west Italy). Several vines of this cultivar were identified and recovered in different (often distant) areas of this region, namely in the surroundings of Alessandria, Asti, Chieri, Pinerolo and even in the northern part of the region. While declining in Piedmont, this variety (likely introduced by immigrants from Piedmont) has developed in California, where there are more than 500 ha under the name of Malvasia bianca (Robinson, Harding, & Vouillamoz, 2012). The evaluation of the ampelographic features and of the agronomic and productive behavior, carried out in the Piedmontese grape collection of Grinzane Cavour (Cuneo), led to the enrolling of Malvasia moscata in the Italian National Register of grape varieties (in 2012). An ampelographic and agronomic presentation of this grape variety is available at http://www.vitisdb.it/varieties/show/1013 (Raimondi, Ruffa, & Schneider, 2014).

The increasing interest of grape producers and winemakers in improving the aromatic quality of fortified wines requires further effort in understanding the effect of the dehydration process on the aromatic composition of Malvasia moscata wine grapes and the resulting wines. Therefore, the main purpose of this work was to investigate the influence of maturity on free and glycosylated volatile compounds of fresh and partially dehydrated grapes. To our knowledge, this is the first time that the effect of maturity on the aroma profile of dehydrated grapes was studied. Furthermore, in a second year, the impact of dehydration conditions (fast and slow processes) was evaluated on the volatile composition of dehydrated grapes and the fortified wines made from them.