The impacts of frozen material-other-than-grapes (MOG) on aroma compounds of red wine varieties

Abstract

Context and purpose of the study – An undesirable note called “floral taint” has been observed in red wines by winemakers in the Niagara region caused by large volumes of frozen leaves and petioles [materials-other-than-grapes (MOG)] introduced during mechanical harvest and subsequent winemaking late in the season. The volatiles, which we hypothesized are responsible, are primarily terpenes, norisoprenoids, and specific esters in frozen leaves and petioles. The purpose of this study was to investigate the volatile compounds which may cause the floral taint problem and explore how much of them (thresholds) may lead to the problem. Also, the glycosidic precursors of some of these compounds were analyzed to see the changes happening during frost events.
Materials and methods – Research winemaking was conducted in 2016, 2017 and 2018. All fermentations were based on 40-kg replicated ferments of Cabernet Franc (CF) and Cabernet Sauvignon (CS). MOG Treatments were (by weight): 0, 0.5%, 1%, 2% and 5% petioles, and 0, 0.25%, 0.5%, 1%, and 2% leaf blades. In 2017 and 2018, different yeast strains and harvest strategies were also included in the CF treatments. Yeast treatments included CSM, EC1118 and FX10. Harvest strategies involved conventional machine harvesting (MH), Braud-New Holland Opti MH, Gregoire 8 MH, MH + optical sorting, and MH with pre-harvest leaf removal. Concentrations of key odor-active compounds were quantified by gas chromatography-mass spectrometry with stir bar sorptive extraction.
Results – Several compounds including cis- and trans-rose oxides, β-ionone, citronellol, linalool, eugenol, methyl and ethyl salicylate were higher in MOG treatments for both CF and CS and their concentrations increased linearly with the accumulative levels of petioles or leaves. Principal components analysis showed petiole and leaf treatments were separated apart from the control sample with the 5% petioles and 2% leaves as the extremes. Petiole and leaf treatments were spread out on different axes, which indicated their large differences in volatile compositions. Interestingly, eugenol and rose oxides and many other compounds followed linear curves with the addition of petioles and leaves in the 2016 vintage, which could be potentially used as a tool to communicate with winemakers on potential floral taint risk based on their sensory thresholds. Preliminary results from 2017 showed that more terpene compounds were found in the standard MH treatment than the hand-harvested control, and the yeast EC1118 produced the least terpenes out of three different yeasts among all leaf and petiole addition treatments in most cases, while yeast strain FX10 produced the highest terpene concentrations. In general, petiole additions contributed more to the floral taint problem than leaf additions. Specifically, petioles contributed terpenes and salicylates (floral notes) to the wines, and leaves contributed norisoprenoids and C6 alcohols (green notes).