Effect of resin extraction on toasted wood flavours in wine
Woods other than the traditional oak wood have been studied for their applicability to winemaking, aiming to produce wider ranges of wine flavours with a distinctiveness associated with New Zealand. Prior studies with woods other than oak have shown that unpleasant flavours from infusion of some woods could be linked to the presence of resin. The main thrust of this research was to test the hypothesis that any wood free of resin would be suitable for flavouring wine since all trees are composed of the same major constituents. The study involved various organic solvent extractions to remove resins from manuka, macrocarpa, totara, kahikatea, radiata pine, gorse, and American oak, prior to toasting and wine infusion. The woods were cut to a defined chip dimension, and Soxhlet-extracted with dichloromethane before toasting to 200 and 210°C for two and three hours, respectively. These were the light and heavy toasts. In discriminative triangle trials comparing unextracted with resin-extracted infusion treatments in unwooded chardonnay at the two toasting levels, the 50 panellists could distinguish a difference in only three of the 14 trials. The exceptions were manuka heavy toast (P < 0.01), and both macrocarpa toasts (P < 0.05). The remaining 11 trials did not elicit significant levels of correct judgements from the panellists. The mass of resin recovered ranged from 1% (gorse) to 11 % (manuka), but there was no relationship between the quantity of resin and the discrimination results. Other parameters relating to the wood chips were measured in parallel to the discrimination trials. Colour changes in untoasted woods due to resin extraction were usually statistically significant but minor. Colour changes were unrelated to weight losses due to extraction. Light and heavy toasting resulted in significant and often major changes in colour parameters due to resin extraction. However there was no clear pattern of change and thus had no meaningful outcomes. In the discrimination trials, subtleties of the responses to macrocarpa and manuka, suggested that not all resin was extracted by the single extraction with dichloromethane. A further extraction of toasted chips not used for infusion showed that resinous matter was still present in all woods, although pyrolytic generation of dichloromethane-soluble matter could not be excluded. These collective results prompted an exhaustive extraction of wood chips prior to a hedonic trial with manuka, macrocarpa, and American oak. The solvents in sequence were dichloromethane, hexane and diethyl ether. Each extracted some resinous matter, clearly showing that the single dichloromethane extraction prior to the discrimination trials left some resin in the chips, potentially affecting wine flavour. The triple-extracted woods were light toasted and infused in wine destined for a hedonic trial in six retail wine shops, for which the overall statistical significance was P < 0.001. The 121 consumers found that the unwooded chardonnay (control) was most favoured whereas the wine infused with macrocarpa was very significantly the least favoured. Its dislike was clearly caused by presence of its resin in the wine and/or because of a very low flavour threshold for that resin. The manuka treatment was numerically the most favoured ahead of American oak, but not significantly so. The resins extracted at various points of this study were also evaluated by panellists focusing on descriptive qualities. These descriptions and the results of the discrimination and hedonic trials led to the conclusion that variation in wine flavour when infused with toasted wood was in most cases not related to the occurrence of resin in woods. Finally, future research possibilities have been described, with an emphasis of the most potentially useful wood, manuka.