Investigation of the congeners responsible for nutty/cereal aroma character in new make malt whisky

The nutty and cereal aromas of new make malt whisky are important sensory characteristics of certain distillery malts of commercial value for blending; however there is a lack of understanding regarding the volatile congeners which contribute to these complex sensory characters. The work described in this thesis aimed to improve knowledge of the chemical origins of nutty and cereal aromas in immature spirit in order to aid process control of these characters during manufacturing. Two aroma extraction methods were compared; liquid-liquid extraction (LLE) using dichloromethane and solid-phase extraction (SPE) with LiChrolut EN sorbent. New make spirit samples from industry (n=S) were evaluated by a trained whisky sensory panel using Quantitative Descriptive Analysis (QDA). Four were noted for their nutty/cereal character, the other served as a non-nutty control. Gas Chromatography-Olfactometry/ Mass Spectrometry (GC-O/MS) was used to try and identify compounds in chromatogram regions coincident with nutty/cereal descriptors. Using LLE extracts, 14 such regions were identified. LiChrolut EN SPE proved to be more selective (19 nutty/cereal odour active regions). 2,S-dimethylpyrazine (known to impart a nutty/cereal character in other food systems) was one noted congener, which was only detected using the more selective SPE method. The gross volatile compositions of the 5 spirit samples were remarkably similar, suggesting that congeners present at low concentration but with low odour thresholds are likely responsible for nutty/ cereal characters. One analytical difference of note was that the nuttier samples contained higher concentrations of long-chain esters. Thus, ethanolic Atmospheric Pressure Chemical Ionisation Mass Spectrometry (APCI-MS) was used to analyse the headspace concentrations of a test set of 14 whisky aroma volatiles above a series of aqueous ethanolic solutions differing in concentrations of alcohol (5 -40% ABV) and ethyl hexadecanoate (O-SOO mg/L). Ethanol had a significant solubilising effect (p 2.S). We propose that nutty and cereal characters are imparted by relatively polar aroma compounds, whose characters are emphasised by the selective incorporation of hydrophobic aroma compounds into the interior of micelle-like structures formed by long chain esters (typified here by ethyl hexadecanoate). Some distillers have reported that manipulation of the lipid concentrations in wash offers a method of controlling the nutty/oily character of new make spirit. A batch of fermented wash sourced from industry was spiked with varying concentrations of oleic (18:1) and linoleic (18:2) acids and (laboratory) distilled at two different temperatures, using a D-optimal experimental design to evaluate the impacts of each factor. Nutty (p = 0.0203) and oily (p = 0.0034) aroma characteristics were scored as significantly stronger in distillates of wash spiked with 100 Ilg/mL each of oleic and linoleic acids, as compared to the control. GC-O/MS of distillate extracts once again determined several odour active regions relevant to the nutty/cereal characters and concentrations of some compounds could be correlated with nutty/cereal QDA scores. New make spirit samples from 35 individual malt whisky distilleries were extracted using the LiChrolut SPE method and analysed by GC-MS. Analytical concentrations of 'candidate' nutty-cereal compounds (n = 20) were used to model sensory QDA data for the 35 spirit samples (nutty, oily, cereal and feinty characters) using Principal Components Analysis (PCA). Significant positive correlations with nutty were seen for 7 compounds (using ANOVA). These included the Maillard products 2-furanmethanol (p < 0.0001), 2-methylpyrazine (p < 0.0013) and 2,5-dimethylpyrazine (p < 0.0004). The PCA overlay bi -plot showed clustering of certain higher alcohols near to the nutty aroma descriptor (methionol, pentan-1-0I, 2-phenyethyl alcohol). Nutty and cereal characters of whisky are of complex origin and likely originate from multiple congeners in a synergistic mixture. This work has shown that processes of particular importance to the expression of this character in new make spirit are lipid oxidation and Maillard chemistry. The conditions for these reactions are to be found during malt kilning and distillation. Whilst these processes are where nutty/cereal compounds are likely to be formed, other distillery parameters such as the mashing protocol, length of fermentation (both determine the supply of key precursors such as fatty and amino acids) and the spirit cut of the distillation govern the chemical composition of the final spirit