Influence of leaf removal and reflective mulch on phenolic composition of white wines

摘要

Aim: The aim of this research was to evaluate if leaf removal and red geotextile reflective mulch “Vitexsol” could affect phenolic composition of wines from three white cultivars, Riesling italico, Traminer, and Manzoni bianco.Methods and results: A two-year study was conducted to evaluate the effects of leaf removal and reflective mulch on the phenolic composition of white wines from Zagreb vineyards in northwestern Croatia. Leaf removal (LR) and reflective mulch (RM) “Vitexsol”, made from weave of aluminum platelets protected by a transparent film and sewn together with red polypropylene threads, were tested separately and combined (LR+RM) on vines of Traminer, Riesling italico and Manzoni bianco in 2008 and 2009. LR and RM had no consistent effect on must sugar content and titratable acidity. All treatments resulted in higher total phenol and flavan-3-ol content in wines of all cultivars, but in different years. LR+RM generally resulted in the highest phenolic acid and individual flavan-3-ol content in all wines except Traminer. RM treatment had the least effect on phenolic composition of wines.Conclusion: LR+RM generally resulted in the highest content of most phenolic compounds, especially when compared to control wines.Significance and impact of the study: This work provides some useful informations for adjusting vineyard practices and thus optimizing phenolic quality of white wines. IntroductionPhenols are present in grapes and wines in quite low concentrations, but with significant contribution to sensorial attributes of wines, such as color, bitterness, and astringency (Hufnagel and Hofmann 2008a, 2008b, Keller 2010). Phenols in wines are also important because of great health benefits as antioxidants in the human body (Kanner et al. 1994). In wines, these compounds are mostly originated from grape berries, and some are generated by yeast activity or extraction from oak wood barrels. As a result, grape variety, climatic conditions, vineyard practices and vinification techniques all affect the composition and content of phenolic compounds in wine (Jackson and Lombard 1993, Boulton et al. 1998, Downey et al. 2006, Keller 2010).The phenolic compounds commonly found in white grapes and wines are non-flavonoid hydroxycinnamic acids, especially caffeic and caftaric acids (Chamkha et al. 2003). In white wines, small amounts of phenolics are considered desirable in providing body to the finished product, while their contribution to oxidative browning represents a less desirable role (Singleton and Noble 1976).The positive aspects of leaf removal through increased fruit exposure to sunlight are well known. Leaf removal can increase soluble solids and reduce total acidity, as well as increase total phenol content in grapes (Smart et al. 1990). Besides other canopy management techniques, leaf removal is a common practice, especially in cooler regions, used to improve canopy microclimate and fruit ripening. Increased sunlight exposure causes the elevation of berry temperature, and both factors are closely related with fruit composition and quality.Reflective mulches might also increase sunlight in the fruit zone, and consequently improve berry ripening and disease suppression (Hostetler et al. 2007a, 2007b). There are only few published works lately reporting about effects of reflective mulches or geotextiles on fruit composition and yield (Pearson 2004, Vanden Heuvel and Neto 2006, Hostetler et al. 2007a, 2007b, Sandler et al. 2009). Reflective mulch studies demonstrated various effects on yield, fruit composition and wine quality.Riesling italico is the most widespread grape cultivar in Croatia, where it is also known as Gra?evina. Manzoni bianco, which is the progeny of Pinot blanc and White Riesling, is a cultivar recently introduced in Croatia that shows excellent potential to produce different types of wine, while Traminer is a very popular cultivar all around the world.The aim of this research was to evaluate if leaf removal and red geotextile reflective mulch “Vitexsol” could affect phenolic composition of wines from three white cultivars, Riesling italico, Traminer, and Manzoni bianco and to provide some useful informations for adjusting vineyard practices and thus optimizing white wine phenolic quality. For example, flavan-3-ols and resveratrol are important because of great health benefits on human body. But when present at high concentrations, catechin and epicatechin can contribute to undesirable bitterness and astringency of wine, while high concentrations of hydroxybenzoic and hydroxycinnamic acids increase the risk of oxidation process as well as browning of wines.Materials and methods1. Study area and treatmentsResearch was performed in two consecutive years (2008 and 2009). Grape harvest as well as vinification process were conducted in the experimental field Jazbina, which is part of the Department of Viticulture and Enology, Faculty of Agriculture, University of Zagreb (Croatia). The white