Impact of Dilution on Whisky Aroma: A Sensory and Volatile Composition Analysis

Abstract

An "omics"-style approach was used to evaluate the complex relationship between whisky aroma and dilution with water, typically suggested as a way to better appreciate whisky. A set of 25 samples, including Bourbons, ryes, single-malt and blended Scotches, and Irish whiskies were chemically profiled at six dilution levels (100, 90, 80, 70, 60, and 50% whisky/water), while a subset of six whiskies (three Bourbons, three Scotches) at four dilution levels (100, 80, 60, and 40% whisky/water) were chemically profiled and subjected to sensory analysis by a trained panel (n = 20). Untargeted volatile analysis was performed using headspace solid-phase microextraction gas chromatography coupled with mass spectrometry (HS-SPME-GC-MS) and sensory analysis was performed using descriptive analysis (DA). Results were evaluated using multivariate statistical techniques, including multifactor analysis (MFA) and partial least squares discriminant analysis (PLS-DA). Dilution decreased headspace concentration of hydrophilic aroma compounds and increased concentration of more hydrophobic compounds, which agreed with DA results. Dilution above 80% whisky/20% water reduced differences within whisky styles, though differences between American (Bourbon, rye) and Scotch styles (single malt, blended) continued to increase with further dilution. This provides important insight into how dilution of whisky during consumption changes consumer perception, as well as the usefulness of HS-SPME-GC-MS as a proxy for human olfaction.

Keywords: HS-SPME-GC-MS; aroma; descriptive analysis; volatile composition; whisky.

Figure 1

Multi factor analysis (MFA) of larger dataset of headspace volatile content of 25 whiskies consisting of five blended Scotch whiskies (BS), five Bourbons (BU), four Irish whiskeys (IR), four rye whiskeys (RW), six single malt Scotch whiskies (SM), one barrel strength Bourbon whiskey (BB), measured at six different dilution levels (100, 90, 80, 70, 60, and 50% whisky in purified water) in triplicate. Analysis was performed using nine groupings consisting of three categorical variables (percent whisky, style, and sample name) and 91 volatile compounds separated in to eight compound classes, tentatively identified by GC-MS using the NIST 2020 library. Ellipses represent a 95% confidence interval about the centroid.

Figure 2

Multi factor analysis (MFA) of headspace volatile content of a subset of six whiskies used for descriptive sensory analysis (DA) consisting of three Bourbons (BU), two single malt Scotch whiskies (SM), and one blended Scotch whisky (BS). Headspace content was measured at four different dilution levels (100, 80, 60, and 40% whisky in purified water) in triplicate. Analysis was performed using 12 groupings, including three categorical variables and 131 volatile compounds tentatively identified by GC-MS using the NIST 2020 library, grouped in to 11 separate compound classes. Ellipses represent 95% confidence intervals about the centroid.

Figure 3

Partial least squares (PLS) variables plot of sensory attributes and their associated chemical compounds. A total of seven sensory attributes were found to be statistically significant (“Solvent/Chemical”, “Vanilla”, “Oak”, “Pome Fruit”, “Peat Smoke”, “Bacon”, and “Rubber”) while three were not (“Cedar”, “Malt”, and “Cornmeal/Polenta”).

Figure 4

Correlograms of “acetate ester” (A) and “phenol” (B) compound classes with sensory attributes. Irregularity of ellipsis shape corresponds to the correlation coefficient while color represents positive/negative correlation (red = negatively correlated, blue = positively correlated). Volatile compound data were measured in triplicate by HS-SPME-GC-MS and tentatively identified using the NIST 2020 library, triplicate measures were averaged for comparison. Sensory attributes are averaged duplicate measures recorded by trained descriptive analysis (DA) sensory panel (n = 20).

Figure 5

Partial least squares discriminant analysis (PLS-DA) biplot of sensory attributes and whisky styles (A) and dilution factor (B). In general, smokey terms (“Rubber”, “Bacon”, and “Peat Smoke”) were associated with peated single malt Scotch whisky (SM), “Vanilla” and “Oak” were associated with Bourbons (BU), and “Pome fruit” was associated with unpeated, blended Scotch whisky (BS). Undiluted whisky more directly correlated with “Solvent/Chemical”, “Vanilla”, “Bacon”, and “Oak” aromas while heavier dilution favored “Pome fruit”, and “Cedar” attributes.

Figure 5

Partial least squares discriminant analysis (PLS-DA) biplot of sensory attributes and whisky styles (A) and dilution factor (B). In general, smokey terms (“Rubber”, “Bacon”, and “Peat Smoke”) were associated with peated single malt Scotch whisky (SM), “Vanilla” and “Oak” were associated with Bourbons (BU), and “Pome fruit” was associated with unpeated, blended Scotch whisky (BS). Undiluted whisky more directly correlated with “Solvent/Chemical”, “Vanilla”, “Bacon”, and “Oak” aromas while heavier dilution favored “Pome fruit”, and “Cedar” attributes.