| Summary: | Estery and woody flavour notes are important characteristics of distilled spirit flavour. It has been reported for malt whisky that the estery character of mature whiskies typically declines relative to that of the new make spirit, even though the analytical concentrations of esters remain broadly constant. One potential explanation for this observation would be a sensory interaction between mature and estery characters. The work described in this thesis was designed to test this hypothesis and to further explore the nature of the congeners responsible for eliciting these characteristics across different spirit types, as influenced by their maturation conditions (time, temperature, cask provenance etc.). In the research described in Chapter 2, four pairs of non-mature and mature spirits (tequila, bourbon and 2 malt whiskies) were characterized by instrumental analysis with the aim of defining the key aroma compounds that determined the mature character in each spirit. According to PLS analysis of the full data set, concentrations of 17 congeners were positively correlated with ageing time and might thus influence the mature character of the aged spirits. In Chapter 3, the same eight spirit samples were analysed by GC-Olfactometry using the AEDA (aroma extraction dilution analysis) approach. Aged spirits presented a more complex aroma than new make spirits, and contained more compounds with the highest FD-factors. Whilst a full GC-O characterisation was completed, the main focus was on identifying compounds which contributed to the estery and woody/mature characters of each spirit. In Chapter 4 we attempted to reproduce these characters for each spirit through aroma recombination, based on blends of the odiferous compounds identified at high FD factors and their analytical concentrations in the actual samples as reported in Chapter 3. It soon became apparent that relatively simple mixtures of esters on the one hand and maturation-linked compounds on the other did not adequately reproduce the nature of these characteristics in the spirits themselves. This implied either that our analysis had missed some significant compounds contributing to these characteristics, or that the complexity of the full spirit matrix is required to give the groups of compounds the nuanced flavour that they lacked in isolation. The latter hypothesis was tested by adding in additional blends of compounds to increase the complexity of the recombinant aroma mixtures. It was concluded that the authenticity of the aroma blends overall was improved by both the addition of a cocktail of ‘low boiling compounds’ (those analysed by a separate direct injection GC technique) and the introduction of a ‘structuring’ compound (ethyl hexadecanoate) at a concentration that would cause agglomeration within the whisky (micellar structures) thus influencing aroma partitioning and release. It was concluded that these modifications produced recombinant aromas which were close enough to the authentic spirit characters to use them in sensory interaction studies (Chapter 5). As opposed to interaction effects there was simply a tendency for the woody/mature characters to suppress the corresponding estery character of mature spirits, particularly at the higher concentrations of added wood extractives. Because the woody/mature compounds which characterised maturation were broadly similar across the spirit types, but differed in concentration according to the maturation conditions, we decided finally to investigate the extraction kinetics of wood-derived compounds from oak sticks as a function of ageing time, temperature, spirit type and alcohol content (Chapter 6). Temperature and alcohol content were the most significant factors that determined the extraction rate and final concentrations of all 18 wood-extractive compounds (P < 0.05) analysed. Not surprisingly, extraction rates increased with increasing temperature, but the trend in terms of alcoholic strength depended on the particular compound. Overall this thesis has improved knoEstery and woody flavour notes are important characteristics of distilled spirit flavour. It has been reported for malt whisky that the estery character of mature whiskies typically declines relative to that of the new make spirit, even though the analytical concentrations of esters remain broadly constant. One potential explanation for this observation would be a sensory interaction between mature and estery characters. The work described in this thesis was designed to test this hypothesis and to further explore the nature of the congeners responsible for eliciting these characteristics across different spirit types, as influenced by their maturation conditions (time, temperature, cask provenance etc.). In the research described in Chapter 2, four pairs of non-mature and mature spirits (tequila, bourbon and 2 malt whiskies) were characterized by instrumental analysis with the aim of defining the key aroma compounds that determined the mature character in each spirit. According to PLS analysis of the full data set, concentrations of 17 congeners were positively correlated with ageing time and might thus influence the mature character of the aged spirits. In Chapter 3, the same eight spirit samples were analysed by GC-Olfactometry using the AEDA (aroma extraction dilution analysis) approach. Aged spirits presented a more complex aroma than new make spirits, and contained more compounds with the highest FD-factors. Whilst a full GC-O characterisation was completed, the main focus was on identifying compounds which contributed to the estery and woody/mature characters of each spirit. In Chapter 4 we attempted to reproduce these characters for each spirit through aroma recombination, based on blends of the odiferous compounds identified at high FD factors and their analytical concentrations in the actual samples as reported in Chapter 3. It soon became apparent that relatively simple mixtures of esters on the one hand and maturation-linked compounds on the other did not adequately reproduce the nature of these characteristics in the spirits themselves. This implied either that our analysis had missed some significant compounds contributing to these characteristics, or that the complexity of the full spirit matrix is required to give the groups of compounds the nuanced flavour that they lacked in isolation. The latter hypothesis was tested by adding in additional blends of compounds to increase the complexity of the recombinant aroma mixtures. It was concluded that the authenticity of the aroma blends overall was improved by both the addition of a cocktail of ‘low boiling compounds’ (those analysed by a separate direct injection GC technique) and the introduction of a ‘structuring’ compound (ethyl hexadecanoate) at a concentration that would cause agglomeration within the whisky (micellar structures) thus influencing aroma partitioning and release. It was concluded that these modifications produced recombinant aromas which were close enough to the authentic spirit characters to use them in sensory interaction studies (Chapter 5). As opposed to interaction effects there was simply a tendency for the woody/mature characters to suppress the corresponding estery character of mature spirits, particularly at the higher concentrations of added wood extractives. Because the woody/mature compounds which characterised maturation were broadly similar across the spirit types, but differed in concentration according to the maturation conditions, we decided finally to investigate the extraction kinetics of wood-derived compounds from oak sticks as a function of ageing time, temperature, spirit type and alcohol content (Chapter 6). Temperature and alcohol content were the most significant factors that determined the extraction rate and final concentrations of all 18 wood-extractive compounds (P < 0.05) analysed. Not surprisingly, extraction rates increased with increasing temperature, but the trend in terms of alcoholic strength depended on the particular compound. Overall this thesis has improved knowledge of the chemical and sensory changes that accompany spirit maturation and has highlighted some of the factors that cause differences in mature character across spirit types. === Moreover, it concludes that the sensory perception of woody/mature generally suppresses the fresh estery characteristic of new make spirits, even though analytically the esters are still there in similar concentrations. wledge of the chemical and sensory changes that accompany spirit maturation and has highlighted some of the factors that cause differences in mature character across spirit types. Moreover, it concludes that the sensory perception of woody/mature generally suppresses the fresh estery characteristic of new make spirits, even though analytically the esters are still there in similar concentrations.
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