Flavour components of whiskey

MacNamara, Kevin (2002-03)

Thesis (PhD)--Stellenbosch University, 2002.

Thesis

ENGLISH ABSTRACT: Aged distilled spirits such as whiskey are complex mixtures of flavour compounds in an ethanol-water matrix. The flavour compounds involved can have widely different volatility and relative amounts. Many of the organoleptic properties that make whiskey suitable for commercial sale have their origin in reactions occurring during the ageing process in oak wood barrels. To investigate the complex changes that take place during spirit ageing a preparative fractional vacuum distillation process was developed. Both high and low volatility compounds could be individually isolated as fractions and free from both the ethanol matrix and the fermentation fusel alcohols. This allowed a range of sensory and analytical procedures to be conducted on these fractions, in particular to investigate changes occurring during ageing. Gas chromatographic (GC) analysis of the low volatility fraction is complicated by the fact that both the compounds and their ethanol matrix have very similar chromatographic behaviour when separated simultaneously on standard chromatographic phases. Compound and matrix co-elution becomes a major problem and conditions for mass spectrometric (MS) investigation are disadvantageous. A two-dimensional GC configuration using dissimilar chromatographic phases was configured to overcome these limitations. Using this approach 27 compounds were separated and identified. Headspace injection was used to increase detection sensitivity. Changes with ageing for seven compounds present at very low levels were quantified. In addition changes in the most abundant compounds were quantified by standard split injection, and changes in trace level sulfur compounds by headspace injection with sulfur chemiluminescent detection (SCD). Increases of the concentrations of pleasant fruity ethyl esters and acetates were established. Volatile sulfides with generally objectionable aroma showed concomitant major decreases. Appropriate techniques could also be applied to the low volatility compounds recovered from the whiskey water fraction. High temperature GC-MS analysis of an extract of the water fraction allowed the identification of 30 compounds. Three phenolic esters were identified in whiskey for the first time. These compounds were synthesised and shown to be contributory to desirable ageing flavour. Increases in concentrations of 16 oak derived compounds during alO year ageing period were established. Several compounds increased significantly over this time period. Ratios of aromatic phenolic aldehydes, and changes in these ratios during ageing, were unique to the type of barrel used in these experiments. This suggests that the final sensory properties of aged whiskey may be more dependent on wood parameters than previously thought. Preparative reverse phase High Pressure Liquid Chromatography (HPLC) with an ethanol water gradient was used to further fractionate an extract of the low volatility compounds. Subsequent analysis and sensory testing allowed a group separation of compounds with each group contributing characteristic attributes to the total flavour. One group contained the three new phenolic esters together with a number of other unidentified compounds. This group was found to be important for desirable ageing flavour that seems to develop slowly with time. Further studies in this area to understand the individual and synergistic contributions of the many facets of ageing chemistry will have important commercial implications.

AFRIKAANSE OPSOMMING: Verouderde spiritus soos Whiskey is 'n komplekse mengsel van geurstowwe in 'n etanolwater oplossing. Die vlugtigheid van die geurstowwe asook die konsentrasies waarin hul aanwesig is, varieer aansienlik. Verskeie van die sintuiglike eienskappe wat kommersiële waarde aan whiskey verleen, het huloorsprong in reaksies wat tydens die verouderingsproses in eikehoutvate plaasvind. Ten einde die ingewikkelde veranderinge wat tydens die veroudering van spiritus plaasvind, te ondersoek, is 'n preparatiewe fraksionele vakuumdistillasieproses ontwikkel. Hoogs vlugtige en minder vlugtige verbindings kon geskei word in afsonderlike fraksies wat vry was van etanol en fuselalkohole. Dit het die sintuiglike en fisies-chemiese analises van die fraksies moontlik gemaak, veralom die veranderings wat tydens veroudering plaasvind, te ondersoek. Gaschromatografiese (GC) analise van die fraksie met 'n lae vlugtigheid word gekompliseer deur die feit dat hierdie komponente en die etanol waarin dit opgelos is soortgelyke chromatografiese eienskappe toon wanneer hul gelyktydig op standaard gaschromatografie fases geskei word. Die gelyktydige eluering van dié komponente en etanol waarin hul opgelos is, skep 'n probleem wat nadelig vir massaspektrometriese (MS) analise is. Die beperkings is oorkom deur die gebruik van tweedimensionele GC en stasionêre fases met uiteenlopende eienskappe. Op dié wyse is 27 verbindings geskei en geïdentifiseer. Die veranderinge in konsentrasies tydens veroudering is vir sewe verbindings gekwantifiseer. Veranderinge in die konsentrasies van die verbindings teenwoordig in die hoogste konsentrasies is gekwantifiseer deur split-inspuitings, terwyl veranderinge in die spoorkonsentrasies van vlugtige swawelverbindings mbv dampfase-inspuitings en met swawel chemolumisensie deteksie (SCD) bepaal is. Toenames in die konsentrasies van die aangename vrugtige esters en asetate is bepaal. Vlugtige sulfiede met meesalonaanvaarbare aromas toon gelyktydige groot afnames. Geskikte tegnieke is ook gebruik vir die herwinning van minder vlugtige verbindings met die waterfase van whiskey. Hoë temperatuur GC-MS analises van 'n ekstrak van die waterfase het die identifikasie van 30 komponente moontlik gemaak. Drie fenoliese esters is vir die eerste keer in whiskey gevind. Hierdie verbindings is gesintetiseer en hul bydrae tot die gewenste verouderingsgeur is sintuiglik bevestig. Toenames in die konsentrasies van 16 eikehoutverwante verbindings gedurende 'n verouderingsperiode van 10 jaar is bepaal. 'n Betekenisvolle toename het voorgekom in die konsentrasies van verskeie van hierdie whiskey verbindings. Die verhoudings van aromatiese fenoliese aldehiede en die verandering in die verhoudings tydens veroudering was kenmerkend van die tipe eikehoutvat wat gebruik is. Dié bevinding dui daarop dat die fenole sintuiglike eienskappe van verouderde whiskey meer afhanklik mag wees van eikehout parameters as wat voorheen algemeen aanvaarbaar is. Preparatiewe omgekeerde fase hoëdrukvloeistofchromatografie met etanol/water as 'n gradient elueermiddel is gebruik om 'n ekstrak van die minder vlugtige verbindings verder te fraksioneer. Verdere GC-, MS- en sintuiglike analise het die skeiding van groepe van verbindings waarvan elk kenmerkende bydraes tot die totale geur lewer, moontlik gemaak. Een groep het drie nuut geïdentifiseerde fenoliese esters, tesame met 'n aantal ongeïdentifiseerde verbindings, bevat. Daar is vasgestel dat hierdie groep 'n belangrike bydrae maak tot die gewenste geur wat klaarblyklik stadig tydens veroudering ontwikkel. Verdere ondersoeke in hierdie verband om die individuele en sinergistiese bydraes van verskeie fasette van die chemie van veroudering te verstaan, kan belangrike kommersiële implikasies hê.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/52849
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