Chapter 11 Production of Scotch and Irish whiskies: their history and ...

Production of Scotch and Irish whiskies: their history and evolution 165US proof (54.5 o GL) bourbon, on a 100 o proof(50 o GL) basis, are shown in Figure 14. Thenature and origin of some of these compoundshave been examined in some detail. Among thealdehydes, scopoletin and the aromaticaldehydes syringaldehyde, sinapaldehyde,coniferaldehyde and vanillin are important.According to Baldwin et al. (1967), thesecompounds could be formed by ethanol reactingwith lignin in the oak wood to produce coniferylalcohol and sinapic alcohol. Under mildlyoxidizing conditions, these alcohols could beconverted into coniferaldehyde andsinapaldehyde, respectively. Vanillin could thenarise from coniferaldehyde, and syringaldehydefrom sinapaldehyde. The increase in aldehydecontent during maturation is also attributable inpart to formation of acetaldehyde by oxidationof ethanol. Formation of ethyl acetate probablyaccounts for the steady rise in the ester contentof whisky during maturation.Several other groups of compounds notdescribed in Figure 14 are also important in thematuration process. Monosaccharide sugars arefound in mature whisky, and probably arise fromthe pentosans and other polysaccharides in theoak wood. Otsuka et al. (1963) reported that amature Japanese whisky contained xylose,arabinose, glucose and fructose, while Black andAndreasen (1974) added rhamnose to this listwhen they analyzed a mature bourbon. The latterworkers found that the concentrations ofarabinose and glucose increased at a faster ratethan those of xylose and rhamnose over a 12year maturation period. Salo et al. (1976) alsodetected low concentrations of mannose andgalactose in a matured Scotch malt whisky inaddition to the sugars already noted. Theconcentrations of sugars in mature whiskies (ofthe order of 100 mg/L) are too low to suggestany sweetening effect on the beverage. Phenolsare also detectable in mature whisky, althoughsome of these probably arise during mashing(Steinke and Paulson, 1964) or from maltproduced using peat-fired kilns (MacFarlane,1968). However, Salo et al. (1976) reported anincrease during a one year maturation of aScotch malt whisky in the concentration ofeugenol, which is a major phenol extracted fromoak chips by ethanol (Suomalainen andLehtonen, 1976). Also present in mature whiskiesare sterols, which may precipitate in bottledwhisky stored at room temperature. Black andAndreasen (1973) found campesterol,stigmasterol and sitosterol in mature bourbon,in addition to sitosterol-D-glucoside, although thepossibility that some of these were formedduring mashing cannot be excluded. Finally,reference has already been made to the whiskylactone, ß-methyl-octalactone, and its origin.Not surprisingly, the nature and amounts ofcompounds extracted from charred oak wooddepend on the ethanol concentration of thewhisky. It is to some extent an advantage tomature whisky at a high proof, since this requiresfewer barrels and saves on storage space. Until1962 the US Treasury Department limited thebarrelling proof of whisky to a maximum of 110 oUS proof (55 o GL). In anticipation of this limitbeing raised to 125 o US proof (62.5 o GL),Baldwin and Andreasen initiated a series ofexperiments in 1962 to establish the importanceof barrelling proof on changes in color andconcentrations of organoleptically-importantcompounds during maturation of bourbonwhiskies. Their report in 1973 indicated that colorintensity and congener concentration of whiskiesmatured for 12 years decreased as the barrellingproof was raised from 109 o US proof (54.5 o GL)to 155 o US proof (77.5 o GL). The one exceptionwas the higher alcohol content, which remainedapproximately constant.AcknowledgmentsThe author wishes to thank his friends andcolleagues in whisky production in manycountries of the world for their cooperation andwillingness to supply information on theprocesses of whisky production. Particular thanksare due to Dr. Gareth Bryan-Jones of UnitedDistillers, Menstrie, Scotland, for his assistancein providing information on recent researchreports.