Effect of ethanol on flavor perception of Rum

Abstract This is the first sensory study to evaluate the effects of ethanol concentration on flavor perception of distilled spirits. Dilution series of two rums (R1 and R2) were evaluated to gain insight into the effects of ethanol concentration on the flavor perception of distilled spirits. Rums were diluted 1:2 (v/v) either with pure water to a final alcohol by volume (ABV) of 20% (R1‐W and R2‐W) or with an aqueous 40% ABV solution (R1‐E and R2‐E). The later dilution accounted for the flavor dilution effect while keeping the ethanol concentration the same as the original liquors. Descriptive sensory analysis was conducted on both dilution series and the original rums. Twenty‐three attributes were evaluated consisting of eight aroma, four aroma‐by‐mouth, four mouthfeel, two taste, and five aftertaste terms. Results revealed 18 significant attributes for the R1 series. With the exception of silky mouthfeel, all attributes were rated highest in R1 and lowest in R1‐E. The R2 series contained sixteen significant attributes, all of which were rated higher in R2 compared with R2‐E. The flavor profiles of the original rums and those diluted with water were very similar, with the diluted rums generally having slightly lower attribute intensities. In contrast, the rums diluted with 40% ABV had significantly different flavor profiles than the original rums. Results indicate that diluting spirits with water may reduce the odor suppression effects of ethanol or enhance flavor release which appears to counteract the flavor dilution effect.


| INTRODUC TI ON
Ethanol is arguably the most important component of alcoholic beverages, particularly distilled spirits. Alcoholic beverages span a wide range of alcohol concentrations from beers (3%-10% ABV) to distilled spirits (usually 40% ABV). Furthermore, ethanol concentration plays an important role in the flavor perception of alcoholic beverages. Molecular interactions, flavor partitioning, and sensory perceptions have all been shown to be affected by ethanol concentration.

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ICKES AND CADWALLADER 1994a,b; Onori & Santucci, 1996). Once the solution reaches an ethanol concentration of 57% ABV, a final structural shift in the water/ ethanol matrix occurs where the water molecules become monodispersed within the ethanolic matrix (D'Angelo et al., 1994a,b).
Previous research on how ethanol concentration affects aroma and sensory perception in alcoholic beverages has mainly approached the problem from an analytical perspective. Additionally, studies have mainly focused on how ethanol affects the headspace concentration of volatiles in static systems. As the ethanol concentration of solutions increases, the headspace concentration has been shown to decrease (Athès, Pena-Lillo, Bernard, Perez-Correa, & Souchon, 2004;Aznar, Tsachaki, Linforth, Ferreira, & Taylor, 2004;Boothroyd, Linforth, & Cook, 2012;Conner et al., 1998;Escalona-Buendia, Piggott, Conner, & Paterson, 1998;Tsachaki, Aznar, Linforth, & Taylor, 2006). The decrease in headspace concentration is typically attributed to an increase in the solubility of aroma compounds as the ethanol concentration increases. Ickes and Cadwallader (2017b) reviewed the current research on ethanol's effects on flavor perception, and their paper provides a more detailed evaluation of the analytical research that has been performed to date. TA B L E 1 List of final attributes, definitions, references, reference scores, and reference preparations determined for the descriptive analysis panel on ethanol's effect on the flavor perception of rums Overall, sensory research exploring the effects of ethanol concentration on the perception of alcoholic beverages is lacking. The majority of studies have evaluated ethanol's effect on wine or wine model systems (Escudero, Campo, Fariña, Cacho, & Ferreira, 2007;Goldner, Zamora, Di Leo, Gianninoto, & Bandoni, 2009;Guth, 1998;Jones, Gawel, Francis, & Waters, 2008;King, Dunn, & Heymann, 2013;Le Berre, Atanasova, Langlois, Etiévant, & Thomas-Danguin, 2007). While these studies demonstrate that ethanol concentration can impact flavor perception, the alcohol content of wine is very different from that of distilled spirits.
To date, the only sensory study to include distilled spirits was carried out in the 1970s by Williams (1972 spirits need to be diluted to ~23% ABV to get the best flavor perception of the beverage. This has been the traditional practice in the whiskey industry for years, and a common reason given for this practice is to reduce the pungency of the alcohol (Smith & Roskrow, 2012).
Both rums had a reported ethanol concentration of 40% alcohol by volume (ABV). Mention of the brand name of these rums does not imply any research contact or sponsorship and is not for advertisement or endorsement purposes. *, **, *** stand for significance at p < .05, p < .01, and p < .001, respectively. a F-ratios are shown as a source of variation. b D × P, R × P, and D × R represent the interaction between dilution samples and panelists, replications, and panelists, and dilution samples and replications, respectively.

| Panelists
All materials related to panelist recruitment and test design were approved by the Institutional Review Board (IRB) at the University

| Test design
A hybrid of Qualitative Descriptive Analysis ® (Stone, 1992) and the Spectrum™ method (Meilgaard et al., 2007;Muñoz & Civille, 1992) was used (Ickes & Cadwallader, 2017a). Two rums were evaluated as a dilution series. At each session, panelists received one dilution series consisting of three samples: straight rum, a 1:2 dilution with 40% ABV ethanol, and a 1:2 dilution with water (e.g., R1, R1-E and R1-W, respectively). On the first day of the panel, the panelists were refreshed about the DA method to be used in the study. The first four sessions (1 hr each) consisted of term and reference generation, followed by reference refinement. Panelists were presented with a dilution series of samples labeled with random three-digit codes and asked to generate the attributes they perceived in the rum samples for aroma, aroma-by-mouth, mouthfeel, taste, and aftertaste modalities. Panelists were provided with a rum flavor wheel (Ickes, Lee, & Cadwallader, 2017)    Superscripts of the same letter within an attribute indicate no significant difference by Fisher's least significant difference (LSD) test at α = .05. "R1" is rum 1, "R1-W" is 1:2 dilution of rum 1 with water to achieve 20% ABV, "R1-E" is 1:2 dilution of rum 1 with 40% ethanol to achieve 40% ABV.
(p > .05) for either the R1 dilution series (except silky mouthfeel and sweet taste) or the R2 dilution series (except toasted aroma, woody aroma, sweet taste, and plastic aftertaste). This lack of variation shows the panelists were able to rate the sample attributes across replications consistently.
Significant panelist variation did exist (p < .05) for all attributes for the R1 dilution series (except alcohol aroma) and R2 dilution series. This type of variation is typical of descriptive analysis panels and is most likely a result of panelists not using the entire scale or using different parts of the scale to rate the samples (Lawless & Heymann, 1999b;Stone, Sidel, Oliver, Woolsey, & Singleton, 1974

| Effect of dilution on sensory profiles
The dilution of the rum samples, either with water or ethanol (40% ABV), caused significant changes to the sensory profiles of the rums.
The results indicated that rum samples diluted with ethanol (R1-E and R2-E) had the lowest intensities for all attributes (except silky mouthfeel in the R1 series).
For the R1 dilution series, R1 was significantly different from R1-E for all attributes, having higher intensities for all attributes except silky mouthfeel (Table 4). Additionally, the two dilutions, R1-W and R1-E, significantly differed from each other for most attributes, except for caramel aroma, maple aroma, vanilla aroma, maple aromaby-mouth, and vanilla aroma-by-mouth. R1-W had a higher intensity of all attributes except silky mouthfeel. R1 and R1-W were not significantly different from each other except for caramel aroma, maple aroma, vanilla aroma, and brown spice aftertaste. Selected significant attribute correlations for the R1 dilution series samples (Table 5) include those between astringent mouthfeel and roasted aroma, warming mouthfeel, and alcohol aroma-by-mouth, and a significant negative correlation existed between silky mouthfeel and roasted aroma, astringent mouthfeel, warming mouthfeel, and alcohol aroma-by-mouth.
For the R2 dilution series, all attributes were rated higher in R2 compared with R2-W and R2-E (Table 6). In addition, all attributes were rated higher in R2-E than in R2-W, except for vanilla aroma, vanilla aftertaste, and plastic aftertaste. R2 and R2-W were significantly different from each other for alcohol aroma, dark fruit aroma, astringent mouthfeel, caramel aroma-by-mouth, maple aroma-bymouth, and vanilla aroma-by-mouth attributes. Selected significant attribute correlations for the R2 dilution series (Table 7) include those between: bite vs aftertaste, warming mouthfeel vs alcohol TA B L E 6 Mean intensity rating for significant aroma, mouthfeel, taste, aftertaste, and aroma-by-mouth attributes for R2, R2-W and R2-E Superscripts of the same letter within an attribute indicate no significant difference by Fisher's least significant difference (LSD) test at α = .05. "R2" is rum 2, "R2-W" is 1:2 dilution of rum 2 with water to achieve 20% ABV, "R2-E" is 1:2 dilution of rum 1 with 40% ethanol to achieve 40% ABV.
aroma-by-mouth, slick mouthfeel vs brown spice aftertaste, plastic aftertaste vs alcohol aroma-by-mouth, and astringent mouthfeel vs slick mouthfeel, bitter, brown spice, and plastic aftertaste, and alcohol and maple aroma-by-mouth.
Interestingly, rums diluted with water possessed nearly the same sensory profiles as the original rums, with only slightly lower intensities for most attributes as shown in the constructed spider plots ( Figure 1). In contrast, the results indicate that dilution with 40% ABV profoundly changed the sensory profiles of the rums, especially for the R2 series.
Principal component analysis was conducted to reduce the complexity of the data and gain a better visual representation of the results as shown in Figures 2 and 3 (Lawless & Heymann, 1999a,b).
and rum dilutions were scored by a trained panel (Jolliffe, 2014). For the R1 dilution series (Figure 3), the first factor (PC1) contained the majority of the variation between samples (88.4%) and the second factor (PC2) contained the remaining sample variation (11.6%). PC1 contrasts samples high in brown sugar, caramel, maple, vanilla, coconut and chocolate aroma, brown spice and caramel aftertaste, caramel, maple, vanilla and coconut aroma-by-mouth, with samples high in alcohol, citrus, and phenolic aroma, warming mouthfeel, and bitter taste. PC2 was mainly defined by slick mouthfeel.
Focusing on the R2 dilution series ( Figure 3) the first factor contained the majority of the variation as well (94.8%), with the second factor minimally loaded with the remaining variation (5.2%). PC1 contrasted samples that were high in all significant attributes with those that had lower intensities of those attributes. PC2 was defined by samples differentiated by dark fruit aroma.
These results validate the initial hypothesis that the dilution to a lower alcohol concentration would cause a decrease in attribute intensity, particularly regarding aroma. Previous analytical studies on dynamic systems showed that higher ethanol concentration had higher headspace concentrations of volatiles (Taylor et al., 2010;Tsachaki, Linforth, & Taylor, 2005;Tsachaki et al., 2008). In this study, all aroma attributes were highest in the original rums.  (Marangoni, 1865;Rayleigh, 1916) would still occur at 20% ABV, as it has been previously demonstrated in 12% ABV systems (Taylor et al., 2010), and this may explain why the aroma attribute intensities did not decrease as much as expected.
Alternatively, the similar aroma profiles observed between the straight rum and water dilution may be a result of reducing the suppressant effect of ethanol during dilution with water. Ethanol is known to have anesthetic qualities and stimulate trigeminal sensations (Taylor et al., 2010). It is possible that at high ethanol concentrations these qualities may suppress or mask the other odor-active compounds in distilled beverages. When the ethanol concentration is reduced with water, the suppressant effect of ethanol as an antagonist may diminish.
Therefore, in the water dilution, even though the concentration of the In mouth sensory perceptions, including mouthfeel and taste, also differed as a result of dilution. Regarding mouthfeel, the F I G U R E 1 Spider plot of mean significant attribute intensities for (a) R1, R1-W and R1-E and (b) R2, R2-W and R2-E rum 1. "R1" is rum 1, "R1-W" is 1:2 (v/v) dilution of rum 1 with water to achieve 20% ABV, "R1-E" is 1:2 (v/v) dilution of rum 1 with 40% ethanol to achieve 40% ABV. "R2" is rum 2, "R2-W" is 1:2 dilution (v/v) of rum 2 with water to achieve 20% ABV, "R2-E" is 1:2 (v/v) dilution of rum 1 with 40% ethanol to achieve 40% ABV. "A" is aroma, "ABM" is aroma-by-mouth, "AT" is aftertaste, "MF" is mouthfeel, "Ta" is taste warming sensation was the same between the original rums and dilutions with water but significantly decreased in the ethanol dilution. This is surprising since previous research has demonstrated that increased ethanol concentration caused a higher rating of hotness or burning mouthfeel sensation (Demiglio & Pickering, 2008;Jones et al., 2008;Nolden & Hayes, 2015). It was expected that the ethanol dilutions would have had the highest warming sensation followed by the original rums and then the dilutions with water. It may be that ethanol, while one factor contributing to the warming sensation of spirits, may not be the only chemical contributing to that perception. These results are interesting as the ethanol used as the reference for warming mouthfeel was the same ethanol used to dilute the samples.
Additionally, previous research demonstrated that increased ethanol concentration causes a decrease in astringency (Demiglio & Pickering, 2008;Fontoin, Saucier, Teissedre, & Glories, 2008). In agreement, R1-E and R2-E had the lowest perception of astringency in both series. In the R2 series, the R2-W was also significantly lower in astringency than R2. This difference could be attributed to the fact that previous studies focused on wines. The high concentration of tannins present in wines may alter the perception of astringency differently than distilled spirits, and in particular as function of ethanol concentration.
Bitter taste was also shown to be significantly lower for the ethanol dilutions in comparison with the original rums and water dilutions.
Previous studies have shown increases in ethanol concentration can cause an increase in bitterness (Fontoin et al., 2008;Jones et al., 2008;Nolden & Hayes, 2015), which is contrary to our results. It is possible that other volatile and nonvolatile components dissolved in the rum matrix could account for these differences.
Sweetness was not shown to be significantly different among the rum dilutions; however, previous research has shown that sweetness perception increases with ethanol concentration in wines (Nurgel & Pickering, 2006;Zamora, Goldner, & Galmarini, 2006). It is possible that nonvolatile composition of the two beverages may affect how ethanol concentration impacts sweetness perception.

| CON CLUS ION
This was the first study to evaluate the sensory effects of ethanol on distilled spirits. Our results showed that the original rums and dilutions with water were more similar to one another than expected.
The samples were only statistically different for several attributes in each series. These results support the age-old industry tradition of diluting distilled spirits to 20% or 23% ABV for blending and evaluation purposes, and in essence demonstrating that while the intensity of the attributes decreased slightly in the dilutions, the overall flavor profiles were very similar. The results for the ethanol dilutions were not expected, and further research is needed to better understand how ethanol interacts with sensory perceptions at high ethanol concentrations.