Effect of Sunlight Exposure on Norisoprenoid Formation in White Riesling Grapes

Abstract

Norisoprenoids are volatile secondary metabolites which are important aroma constituents of a number of grape and wine varieties. The norisoprenoids are thought to arise from carotenoid breakdown and occur in grapes as glycosidically bound precursors. However, the factors which influence carotenoid breakdown and norisoprenoid formation in grapes are not well understood. In this study we evaluated the effect of seven levels of sunlight exposure (4%, 18%, 31%, 37%, 50%, 71%, and 97% expressed as percent of full sun exposure) on norisoprenoid concentrations in White Riesling. Our results suggest that viticultural practices which affect fruit exposure to sunlight may influence norisoprenoid composition of grapes and wines. In White Riesling, TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) and Riesling acetal (2,2,6,8-tetramethyl-7,11-dioxatricycloundec-4-ene) levels increased at sunlight exposures above 20% of full sun exposure. Changes in grape berry temperature with increasing sunlight exposure may also have an influence on norisoprenoid levels.