Accelerated seasoning of Manuka and Oak Wood chips destined for wine and spirit flavour

Sun, Zhongyi
Young, Owen
Robertson, John
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Master of Applied Science
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Auckland University of Technology

Based on the concept of geographic exclusivity, a number of research projects have been conducted at AUT as an effort to explore the applicability of winemaking by using a number of unique New Zealand wood species other than traditional oak to age wines or whiskies. One of the researches has shown that manuka (Leptospermum scoparium) previously dry toasted to about 200°C, can substitute for oak in wine. However, two New Zealand wine experts have remarked that the desirable flavours of toasted manuka are overlaid with a flavour note reminiscent of when oak wood is unseasoned prior to barrel construction and toasting. Desirable as some flavours may be, if manuka is to be used as an alternative to oak, this species may also have to be subject to a seasoning procedure.

Designed to simulate natural seasoning but accelerated by extreme conditions, leaching due to water (but not fungal colonisation) was used to treat the wood while parallel treatments were also applied to American oak (Quercus alba) as a control. The objectives of the study are to compare the different seasoning methods for manuka which may affect the flavour of either wine or spirit, and to build knowledge about manuka seasoning.

Both manuka and American oak woods were cut into chips 20 x 10 x 4 mm and received various leaching treatments at four different conditions: none; boiling under reflux for 1 h; reflux for 8 h; soaking at ambient temperature for 3 wk. A toasting with 200 °C for 2 h (light toasting) or 210 °C for 3 h (dark toasting) was also applied. With a range of combination from these leaching and toasting methods, a total of 16 treatments were thus established. Weight losses and changes in CIE colour space were monitored at both stages of leaching and toasting, and ultraviolet absorbances due to leaching were also recorded. Subsequent infusions for the toasted chips after the 16 treatments were done using 60% v/v of ethanol. Ultraviolet absorbances and gas chromatographic analyses were both conducted for the 16 infusions using authentic whisky as reference. Meanwhile, semi-formal sensory assessment for these infusions was conducted, and weight changes for the infused chips were also recorded.

Leaching always resulted in weight loss, except in the situations where toasting preceded reflux. Fewer changes in CIE colour were observed in manuka chips, and ultraviolet absorbances due to leaching were found much lower on manuka curves, both indicating manuka was more stable to leaching. Manuka had greater weight losses than oak on toasting. The absorbance due to whisky in the UV range is much lower than for the experimental infusions, suggesting that the quantity of woody matter in authentic whisky is lower than in the current infusions, in turn suggesting less exposure of commercial whisky spirit to wood. GC results showed that oak infused more strongly, that parallels the results from UV absorbance analyses. The semi-formal assessment confirmed only treatment for oak with dark toasting to be more mellow and lower in a woody note.

The aim of the research is not to emulate existing whisky styles, but rather to produce a whisky-coloured spirit that is closely identified with New Zealand. Current research has discovered the limitation of manuka due to its strong resistance to the treatments. It is recommended that more wood species are to be explored for this purpose where totara has already shown a lot of potential (Young et al. 2010). Therefore, The key words for future research might include “totara”, “dark toasting”, limited exposure”, “esters”, and “caramelised sugar” etc.

Manuka , Leaching , Reflux
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