The effects of oak cask previous use, origin and filling strength in the maturation of Scotch Whisky

While the use of oakwood casks in the maturation of Scotch Whisky is a legal requirement for the definition and production of Scotch Whisky, these casks play a crucial role in the final organoleptic properties of the matured spirit. During the ageing process in maturation warehouses, maturing Scotch Whisky gradually loses strength both as a function of maturation time and as a function of the prevailing environmental conditions. In Scotland, there are generally no great extremes of temperature and/or humidity throughout the calendar year. Empirically, some maturing Scotch single malt whiskies have been characterised by relatively high wood sugar concentrations and while this feature is clearly related to the origin of cooperage oak wood and barrel construction, it may also be related to the changing ethanol to water ratio in the cask over the maturation period. Ethanol is a better extractor of the more hydrophobic extractives while water, in addition to extracting the more hydrophilic congeners, may facilitate changes in esterification equilibria of some perceived organoleptic qualities of maturing Scotch whisky. In this work, new make Scotch malt spirit was filled at specific filling strengths of alcohol into a range of different types of oak casks constructed of both American and French oak. In addition to newly constructed casks, some of these casks had different previous uses while some have had rejuvenation treatments. All the casks were palletised, warehoused and sampled over a 22-month period. Samples of fresh, air-dried and kilned staves supplied by American and French cooperages, were randomly core sampled in triplicate from each stave. The cores were subjected to various controlled toasting temperatures and subsequent laboratory extraction procedures while the particulate matter derived from the coring process underwent elemental analysis. During the 22-month period the casks were sampled at 3, 9 and 22 months. These samples were analysed by high performance liquid chromatography (HPLC) and gas chromatography with coupled mass spectrometry (GC-MS) for a range of extractive congeners which included lignin breakdown compounds, lactones and wood sugars. The 22-month warehouse maturing samples were also assessed for organoleptic properties using a trained sensory panel. The core samples were incubated at a controlled temperature (20°C) in NMS at different strengths and the liquid extracts were then also determined for the same analytes as for the maturing samples. The particulate matter deriving from the core drillings was microwave digested and then analysed for elemental content by inductively coupled plasma mass spectrometry (ICP-MS). Maximum levels of wood-derived congeners were often found in those casks filled at 63.5% ABV whilst French oak casks that had been charred reached congener levels equivalent to that of American charred oak casks. This demonstrated the significance of the cask construction process, and this was confirmed by the laboratory toasting experiments. The organoleptic analyses highlighted that casks previously used for wine may be susceptible to stale and sulphury aromas and may benefit from being filled at lower strengths to prevent formation of this aroma. These off-notes were also linked to the presence of eugenol which may be an indicator of a defective cask. Elemental analysis of oak core particulates using ICP-MS found significant differences in elemental content comparing particulate from different oak origins. This included manganese, for which French oak yielded significantly higher levels than oak of American origin. Manganese may be important in promoting fungal growth during air seasoning and is also implicated in the formation of diethyl acetal during maturation. The research reported here, highlights novel information on the extractive potential of various types and origins of cooperage oakwood including previous use and fill strength effects. This information thereby provides a practical rationale for reasoned warehouse inventory decisions, delivering the potential for improved oak cask utilisation in Scotch whisky production.