Decoding the unique peaty aroma of Islay scotch single malt whisky by means of the sensomics concept

After application of an Aroma Extract Dilution Analysis and Stable Isotope Dilution Assays, 39 odorants with Odour Activity Values ≥ 1 were mixed in their natural concentrations in 40 % ABV (alcohol by volume) ethanol for a recombination experiment to verify the correct characterisation of all aroma impact compounds. The smoky, clovelike and phenolic character of the peaty whisky was caused by a set of 14 phenol derivatives, such as 3-ethylphenol with an Odour Activity Value up to 940. Comparing the concentrations of phenol derivatives in the raw whisky with the matured ready to drink product, it seems that the maturation process also contributes to the smoky aroma by increasing the concentrations of 4-allyl-2-methoxyphenol and 2-ethylphenol, while the other phenol derivatives mainly originated from the special kilning process with peat reek. Introduction Whisky making has a long tradition in Scotland and its islands. After mashing barley malt with yeast, a double-batch distillation yields the raw spirit, which is then aged for at least 3 years in second hand oak casks before bottling as single malt whisky. Especially, whiskies from the island Islay are particularly known to elicit a peaty odour. The malting process on Islay contains the traditional step of kilning with so-called peat reek (peat smoke) which is responsible for the typical smoky and phenolic aroma of the spirit. It is already suggested that this “peatiness” is caused by a spectrum of phenolic compounds including phenol, methylphenol and dimethylphenol derivatives and 2-methoxyphenol with a total amount up to 80 ppm [1-5]. Early studies could correlate the cumulated concentrations of all phenolic compounds to the degree of peatiness [1,2] or identified some phenol derivatives, such as 4-ethyl-2-methoxyphenol and 2-, 3-, and 4methylphenol in Scotch and Japanese whiskies as aroma impact compounds based odour activity values (OAV) calculated, however, using threshold data in 10 to 20 % ABV ethanol [3-5]. Poisson and Schieberle were the first to fully characterise an American Bourbon whiskey by means of the Sensomics concept. Their investigations resulted in a set of 26 impact aroma compounds, including ethyl (S)-2-methylbutanoate, 3methylbutanal, 4-hydroxy-3-methoxybenzaldehyde and (E)-β-damascenone with highest OAV [6,7]. They also investigated a peaty whisky from Islay and could trace back the distinctive smoky aroma to the high OAVs of several phenol derivatives, such as 2methoxyphenol, 4-allyl-2-methoxyphenol and 5-methyl-2-methoxyphenol. However, their recombination experiments did not lead to a satisfying outcome [8] as not all phenol derivatives could be identified. In order to decode the unique aroma with focus on the peatiness of Scotch Single Malt whiskies from Islay on a molecular basis, a whisky from the Ardbeg distillery was investigated by means of the Sensomics concept [9]. Additionally, selected aroma compounds were quantitated in a sample of the corresponding raw whisky to investigate the impact of the maturation process to the smoky aroma of the whisky. Veronika Mall et al. 84 Experimental Samples. The whisky “Uigeadail” from the distillery Ardbeg was purchased at a local spirit shop. The shop owner also kindly provided a sample of the raw whisky, intended to be matured into an “Uigeadail” whisky. Workup. After solvent extraction and SAFE (solvent assisted flavour evaporation) distillation and Vigreux column distillation, the concentrated distillate was subjected to aroma extract dilution analysis (AEDA), which was carried out by two panellists to assure the detection of the whole set of important odorants. Impact aroma compounds with high FD factors were quantitated by means of stable isotope dilution assays (SIDA), using C or H-labelled analogues. OAVs were then calculated by using the respective odour threshold concentration in 40 % ABV ethanol from the literature [7,8,10]. Sensory trials. Unavailable odour threshold concentrations were newly determined in 40 % ABV ethanol by a sensory trained panel according to the method reported previously [10]. For a descriptive analysis of the recombinate and the original whisky, the sensory panel was asked to rate the intensities of nine aroma attributes from 0 (no perception) to 10 (very strong intensity) on an unscaled line. Results and discussion AEDA and identification experiments resulted in 36 aroma active compounds with FD factors ranging from 32 to 4096. Next to (E)-β-damascenone, cis-whisky lactone and 4-hydroxy-3-methoxybenzaldehyde with high FD factors, a group of phenol derivatives with FD factors ranging from 4 to 4096 with smoky, phenolic or clove-like odour attributes were identified (data not shown). Based on these data quantitations followed by the determination of OAVs of 44 aroma compounds were carried out. Highest OAVs were found for 3-ethylphenol (940), followed by 3-methybutanal (640), (S)-ethyl 2methylbutanoate (410), ethanol (390), 2-methoxy-5-methylphenol (590) und 2methoxyphenol (280). Altogether, 39 aroma compounds with an OAV ≥ 1 (Table 1), including 14 phenol and 2-methoxyphenol derivatives, contributed to the complex aroma of the peaty single malt whisky. A recombination experiment with all 39 impact aroma compounds in their natural concentration could mimic the original whisky very well (Figure 1) confirming their correct characterisation as impact aroma compounds. The typical smoky and phenolic aroma of the whisky was generated by the set of phenol and 2-methoxyphenol derivatives with high to very high OAVs, such as 3-ethylphenol, 2methoxy-5-methylphenol, 4-ethyl-2-methoxyphenol, 4-methylphenol, 2-methoxy-4propylphenol, 2-methylphenol und more. Figure 1: Aroma profiles of the original whisky (a) and the corresponding aroma recombinate (b) Decoding the unique peaty aroma of Islay scotch single malt whisky by means of the sensomics concept 85 Table 1: Impact aroma compounds with OAV ≥ 1 of the peaty single malt whisky from Islay. odorant OAV odorant OAV 3-ethylphenol 940 2-phenylethyl acetate 29 3-methylbutanal 640 (E)-2-nonenal 29 (S)-ethyl 2-methylbutanoate 410 3-methylbutyl acetate 25 ethanol 390 4-allyl-2-methoxyphenol 20 2-methoxy-5-methylphenol 380 ethyl cinnamate 18 2-methoxyphenol 280 4-ethylphenol 16 ethyl octanoate 250 3-methyl-1-butanol 15 (E)-β-damascenone 220 2-phenylethanol 14 4-ethyl-2-methoxyphenol 200 3-methylphenol 12 ethyl methylpropanoate 160 2-ethylphenol 10 vanillin 140 decanoic acid 9 ethyl 3-methylbutanoate 120 ethyl 3-phenylpropanoate 9 4-methylphenol 97 2-methoxy-4-methylphenol 7 ethyl hexanoate 80 methyl-1-propanol 5 1,1-diethoxyethane 68 γ-nonalactone 5 ethyl butanoate 67 phenol 4 2-methoxy-4-propylphenol 52 acetaldehyde 3 2-methylphenol 46 2,3-dimethylphenol 3 2-methylbutanal 43 3,5-dimethylphenol 1 cis-whisky lactone 30 1 OAV; odour activity value using odour threshold concentrations in 40 % ABV ethanol. In order to investigate the impact of the maturation process on the smoky aroma of the whisky, a sample of the raw spirit intended for the production of “Uigeadail” with 69 % ABV ethanol was investigated focussing on known maturation derived compounds, such as cis-whisky lactone and 4-hydroxy-3-methoxybenzaldehyde, as well as on the previously identified phenol derivatives. Since the investigated whisky had cask strength (59 % ABV) meaning the spirit did not undergo dilution after the maturation process, the concentrations of the selected compounds in the raw spirit and final whisky were directly compared without conversion. Next to the typical maturation derived compounds, only two phenol derivatives, such as 2-ethylphenol and 4-allyl-2-methoxyphenol showed noteworthy concentration increases after oak cask maturation. The remaining phenol derivatives were already present in the raw whisky, thus confirming their origin from the peat smoke used for kilning the malt. Minor concentration differences could be explained by the use of different starting material and vintage. Veronika Mall et al. 86 Table 2: Concentrations of impact aroma compounds in the raw and matured whisky. odorant concentration [μg/L] raw whisky matured whisky increase/ decrease [%] maturation compounds cis-whisky lactone < 1.3 2000 +150000 vanillin 23.1 3140 + 13500