John M. Conner

Headspace concentrations of ethyl esters at different alcoholic strengths

Ester concentrations in the headspace influence the aroma character of alcoholic drinks. Activity coefficients for esters showed log-linear decreases as ethanol concentration was increased from 17% (v/v), with rates inversely related to ester acid chain length. At concentrations below 17% (v/v) the activity coefficient remained constant. This could be related to structural changes in ethanol/water mixtures. Below 17% (v/v), ethanol forms a monodispersed aqueous solution. Above 17% (v/v), ethanol molecules cluster to reduce hydrophobic hydration and esters partition into these ethanol-rich clusters, where the lower ester interfacial tension reduces the free energy of mixing and hence the activity coefficient. The increased solubility of the ester reduced the headspace concentration of the esters, and hence total ester content may not be a good indicator of their flavour impact.

Release of distillate flavour compounds in Scotch malt whisky

Equilibration of a flavour volatile between a distilled spirit and the headspace is a two-stage process. The first equilibrium occurs between the bulk solution and the headspace spirit interface, and the second between the surface layer and the headspace. The first stage is represented by the activity coefficient of the flavour volatile, which, for hydrophobic compounds, is greatly reduced by the aggregation of ethanol molecules in aqueous solution. The second equilibrium is governed by the vapour pressure of the solute and the ambient temperature and pressure. In mixed saturated solutions the composition of the surface layer and consequently the headspace is determined by the concentration and activity coefficients of the mixture components. Components of wood extract were found to act principally on the first equilibrium. Ethanol lignin acted in the same manner as high molecular weight esters and alcohols of the distillate, displacing volatile components from the surface layer. The suppressant effect of ethanol lignin was lost at 37 °C and consequently would only be important in nosing of spirit samples. Wood extract was found to decrease the critical point for the aggregation of ethanol, resulting in reduced activity coefficients for ethyl decanoate from 5 to 30% ethanol at both 25 and 37 °C. This effect would reduce the spirit–mouthspace partition coefficient with the resulting decreased release of flavour volatile in the mouth. This mechanism would explain the decreased impact of undesirable, immature aromas when wood matured spirits are consumed.

Anthracene and chromone derivatives in the exudate of Aloe rabaiensis

Abstract From the methanol-soluble exudate of the leaves of Aloe rabaiensis five compounds have been isolated and identified, by spectral analysis, as aloe-emodin-11- O -rhamnoside, the C-10 isomers of barbaloin (aloe-emodin anthrone-10- C -glucoside), aloe-emodin anthrone-10- C -rhamnoside, aloeresin D and 8- C -[2′- O -( E )-caffeoyl] -β-glucopyranosyl-2- [2-hydroxy] propyl-7-methoxy-5-methylchromone (trivial name rabaichromone). Rabaichromone and aloe-emodin anthrone-10- C -rhamnoside appear to be new while aloeresin D and aloe-emodin-11- O -rhamnoside are reported for the first time from a named species rather than a commercial source of Aloe resin.

Anthraquinone, anthrone and phenylpyrone components of Aloe nyeriensis var. kedongensis leaf exudate

Abstract The leaf exudate of Aloe nyeriensis var. kedongensis yielded six compounds which were identified on the basis of spectral data and inter-conversions as two groups of three allied compounds. These were (a) 4-methoxy-6(2′,4′-dihydroxy-6′- methylphenyl)-pyran-2-one, its 2′- O -β- D -glucopyranoside (aloenin) and the 2″- O - p -coumaroyl ester of aloenin, (b) the anthracene derivatives l,2,8-trihydroxy- 6-methylanthraquinone (nataloe-emodin), its 2- O -β- D -glucopyranosyl ester and the corresponding 10- C -β- D -glucopyranoside nataloin.

Anthrone and chromone components of Aloe cremnophila and a. jacksonii leaf exudates

Abstract -The leaf exudates of Aloe cremnophila and A. jacksonii have yielded four chromone and two anthrone derivatives which were identified on the basis of spectral evidence. (−)-Homonataloin, aloesin, aloeresin B and the 2′- O -tigloyl ester of aloesin were obtained from the leaf exudates of both species, whereas (+)-homonataloin and 2- carboxyethenyl-5,7-dihydroxychromone were isolated only from A. cremnophila .

Flavour development in whisky maturation

Abstract Whisky and other distilled beverages undergo major changes in flavour during maturation. These include direct flavour effects of materials extracted from the cask-wood, and of products of reactions within and between wood and distillate components. Wood-derived compounds also modify the headspace concentrations of volatile congeners. Dynamic light scattering indicated the presence of structures (agglomerates or micelles), with a mean diameter of 550 to 600 nm in unaged distillates, and 330 to 400 nm in matured distillates. The agglomerates were formed by medium- and long-chain ethyl esters and alcohols, which have limited solubilities in aqueous ethanol and are present at concentrations exceeding this limit in many malt whiskies. Agglomerates reduced the volatility of other congeners, with a proportion of these compounds being partitioned into the agglomerate. The reduction in agglomerate size during maturation, caused by the dissolution of wood components, was accompanied by changes in the solubilisation of distillate components which altered their partitioning between spirit and headspace.

The contribution of oak lactone to the aroma of wood-aged wine

Abstract Lactones are important flavour compounds in many foods and beverages because of their potency and varied sensory properties. However, while their importance in the flavours of fruits and dairy products is moderately well understood, the flavour effects of lactones in alcoholic beverages are not clear. Oak lactone has been identified in many wood-aged wines and distilled alcoholic beverages at around its threshold concentration, but it is uncertain whether this has a significant effect on the flavour. Australian white wines were found to contain between 57 and 142 mg 1−1, and reds between 13 and 433 μg l−1. Spanish white wines contained from 42 to 538 μg l−1, and reds from 208 to 479 μg l−1. The odour threshold in 12% ethanol was estimated to be 75 μg l−1. Oak lactone therefore probably contributes significantly to the aromas of some barrel-aged red and white wines.

Sensory characterization of Halloumi cheese and relationship with headspace composition

Abstract Halloumi is a traditional cheese from Cyprus, which is attracting increasing consumer interest. For successful production the factors that determine its properties must be understood, so Halloumi cheeses were analysed for chemical composition and sensory properties. Sensory properties were found to vary between cheeses, and some texture and flavor characteristics were greatly affected by cooking. Volatile compounds were contributed by mint and by milk breakdown products, and it was possible to predict panel mean scores for some flavor notes by partial least squares regression on the headspace volatiles.

A simulated mouth to study flavor release from alcoholic beverages

Abstract Static headspace techniques have contributed much to our understanding of the interactions of Scotch malt whisky solutions. Such methods, however, do not account for the numerous and changing conditions of the mouth. The Buccal Headspace Technique addresses such effects by sampling air directly from the mouth as the whisky is warmed and mixed with saliva. The complexities of Scotch malt whisky can be more fully understood by creating simple model systems in the form of whisky analogues. These analogues, which may not be suitable for human consumption, are analysed using a Simulated Mouth, the conditions of which were set using data from the Buccal Headspace technique. The Simulated Mouth may provide a useful tool in the understanding of Scotch malt whisky flavor.

Effect of ethanol strength on the release of higher alcohols and aldehydes in model solutions

Abstract Headspace concentrations of homologous series of higher alcohols and aldehydes dissolved in aqueous solutions at different ethanol concentrations were analysed by gas chromatography-flame ionisation detection. For each volatile, activity coefficients at all ethanol strengths were estimated and statistically compared to evaluate the effect of ethanol strength. There was a significant reduction of the activity coefficients between 10% and 20% v/v ethanol for all the volatiles studied. The reduction of the activity coefficients between 10% and 15% v/v was significant only for decanol, dodecanol, octanal and dodecanal. This confirms that there is a change in the effect of ethanol concentration on volatile flavour compounds in aqueous solutions at 15–20% v/v ethanol.