Variation of oak wood properties influencing the maturation of whisky
Oak casks are used for the maturation of a wide variety of alcoholic beverages including Scotch whisky. The process of maturation has a profound but variable effect on the colour and flavour of the whisky, with cask wood playing an important role, particularly through the release of extractives to the distillate. This thesis examines variation in European oak wood (Quercus petraea Matt. Liebl. and Q. robur L.) of ellagitannins, oak lactones and other extractives, and physical wood properties. Investigations particularly sought to establish whether the properties and their effects on flavour can be predicted from either the species, the various forest origins, or identifiable wood or tree characters. The treatment of wood after felling, through seasoning and particularly the toasting or charring of casks, has a major effect on the levels of many extractives. Heating reduces the concentrations of ellagitannins and increases levels of lignin-derived extractives. However, the effects are not such as to render variation in untreated wood inconsequential. Within trees, the concentration of soluble ellagitannins declines and the composition changes with heartwood age. When heartwood of a similar age is compared concentrations vary by up to ten times between different trees, making up to 14% of the heartwood dry weight. Concentrations of oak lactones appear to increase with heartwood age and are also very variable between trees. Wood samples were taken from two different forests, corresponding to opposing types of French oak used for cooperage. Over 70% of the total variation of soluble tannins in the wood occurred between the forests. A difference between the two forests in the rate of tree growth and the heartwood age of samples could not explain all of the difference in the amounts of soluble tannins. After heating the wood, the concentrations of other extractives and the flavour and colour imparted to solutions, also varied significantly between the two forests and between trees within each site. Studies on clonal, progeny and provenance material concluded that the concentration of ellagitannins, oak lactones and many physical properties of heartwood are under strong genetic control. However, a large proportion of this variation is attributable to variation between the two species Q. robur and Q. petraea. Q. robur is characterised by high concentrations of tannins and, after heating, of lignin-derived products, but low or negligible levels of oak lactones. Q. petraea has opposing extractive properties and after heating, imparts a more pleasant and complex flavour. Although there is large variation between trees within each species, this difference between the species is proposed as the main factor explaining the different flavour and extractive properties found in European oak wood from different origins.