3.1. Sensory Evaluations
The aroma profiles of the two gins are plotted in Figure 1
. Both gins were dominated by an ethanolic
odor (not depicted), perceived with intensities of 7.0 and 5.3, respectively, for Gspusi and Kini; a lack of significance between these ratings do not allow conclusions to be drawn as to its higher rating in the gin with fewer botanicals, although it might be speculated that the use of many botanicals elicits a more complex aroma of the gin that acts to reduce the ethanolic impression. Since the present study focuses on key aroma (impressions) in relation to the use of botanicals, however, the ethanolic impression will not be considered in the ensuing discussion (and therefore, this impression has been omitted from the aroma profile plot of Figure 1
Kini exhibited an aroma profile that was dominantly citrus-like (5.1), orange-like (3.7) and fruity (3.6), with these aroma notes being rated with significantly higher intensities than in Gspusi (rated at 2.6, 1.1 and 1.6, respectively). The strong cola-like impression in Kini was only weakly perceived in Gspusi (respective ratings of 4.1 and 1.4), although the difference was not statistically significant due to the higher variance in the individual ratings across the panel. Based on the most intensely rated attributes, the aroma profile of Gspusi was dominantly juniper-like (4.0), which might be attributed to the use of fewer botanicals and an associated emphasis of this aroma note; juniper-like in Kini was rated with similar intensity (3.4), but this was less pronounced than some of the other aroma impressions (as discussed above). A peach-like note was rated with higher intensity for Kini than for Gspusi (1.1 and 0.3, respectively), whereas a eucalyptus-like impression was rated higher for Gspusi than for Kini (2.4 and 1.4, respectively), albeit in both cases without statistical significance. The aroma notes soapy/coriander-like, anise-like and malty were each rated with similar intensities in both gins (within the range 2.4–3.0).
In terms of the hedonic evaluations, both gins were rated positively, albeit with Kini being rated higher than Gspusi (6.5 and 5.1, respectively). Although this difference was not statistically significant, the trend nevertheless indicates a more pleasant overall aroma of Kini compared with Gspusi. This might be explained by the richer and more complex aroma of the former compared to the latter, or the less intense ethanolic note in Kini, or a combination of both. It should be noted, however, that like the attribute ratings, the hedonic evaluation was made orthonasally and not retronasally, and preferences in flavor during consumption of the gins might be different. In a study that compared orthonasal and retronasal profiles of Chilean Pisco spirit, however, no significant differences were observed between these two methods of assessment [30
3.2. Identification of Aroma Compounds
Overall, 48 aroma-active compounds were detected in the two gins, of which 38 were identified, as listed in Table 1
. In AEDA, the FD factors relate to the last dilution in which an individual odorant is still perceivable during GC-O analysis, and these thereby indicate the most potent odorants in a sample [31
]. It is worthy of note that compounds perceived at FD 2048 might still be perceivable at the next AEDA dilution, namely FD 4096 (not performed in the present study), hence the representation as ≥2048 to indicate that these compounds might be present at higher dilutions. The compounds in Table 1
are listed in order of decreasing FD factor with respect to their presence in Kini; as such, the most potent odorants (those with the highest FD factors) in Kini appear at the top of the table, whilst those in Gspusi appear throughout the table, although primarily also at or near the top.
Comparing the FD factors between the two gins, Kini contained the most compounds – eight in total – at the highest FD factor of ≥2048, compared to two in Gspusi. Common compounds present at the highest FD factor of ≥2048 in both gins were (Z)-rose oxide and linalool, with flowery, rose-like and flowery notes, respectively. The additional FD ≥2048 compounds in Kini were trans-anethole, eugenol, limonene, δ-carene, octanal and coumarin, exhibiting odor impressions ranging from anise-like, clove-like, orange/lemon peel-like, citrus-like, eucalyptus-like, soapy, coconut-like and cinnamon-like.
The second highest FD factor of 1024 was represented by three compounds in Kini, and two compounds in Gspusi. Specifically, α-pinene, γ-terpinene and an unknown compound were present at FD 1024 in Kini, with odor impressions rosiny and conifer-like, turpentine-like and soapy, and musty, respectively. By comparison, the two compounds at FD 1024 in Gspusi were myrcene and (E,E)-2,6-nonadienal, with odor qualities of earthy, metallic and geranium-like, and fatty and cucumber-like, respectively. Kini and Gspusi respectively contained nine and five compounds at FD 512.
Previous studies have similarly identified terpenes, terpenoids and aldehydes in gin, specifically α
-pinene, camphene, sabinene, δ
-carene, myrcene, limonene, 1,8-cineole, γ
)-rose oxide, linalool, citronellal, citral, geraniol, as well as hexanal, octanal, nonanal and decanal via GC-O or GC-MS analysis [10
], and estragole and eugenol by high performance liquid chromatography (HPLC) analysis [32
]. A number of compounds detected in the present study have not been previously reported for gin and are reported here for the first time, specifically (E
)-2-hexenal, 1-octen-3-one, 2-acetyl-1-pyrroline, 1-(R
)-fenchone, trimethylpyrazine, 1-octen-3-ol, (Z
-anethole, 2-methoxyphenol, p
-anisaldehyde, thymol, myristicin, coumarin and vanillin. The number and nature of aroma compounds present in gin closely reflect the number and type of botanicals used in their production. A richer diversity of aroma compounds, therefore, would be expected to be present in gins produced with a wide range of botanicals, as is the case for the Kini gin in this study, and to a lesser extent the Gspusi gin.
Comparing the sensory data with the volatile constituents identified, the high ratings of citrus-like
impressions for Kini are in good agreement with the aroma compounds exhibiting citrusy notes detected at high FD factors, specifically δ
-carene, limonene (also described as orange peel-like), octanal, citronellal, and citral (each perceived with at least four times higher FD than in Gspusi). Further citrus-like
impressions are associated with the aldehyde nonanal (also soapy
), which was present in both gins at the same FD (512), and an unidentified odorant (also described as green
) with an RI 1390 on the DB-FFAP column that was similarly present at FD 512 in Gspusi but only at FD 8 in Kini. The strong fruity
impression of Kini reported by sensory analysis might reflect the presence of the fruity smelling odorants camphene, (E
)-2-hexenal, and an unidentified compound at RI 1142 on DB-FFAP, whereby the first compound was present at identical FD in both gins, whereas the other two were higher (at least twice as high) in Kini than in Gspusi. The soapy
impression was rated similarly in both gins, yet the soapy compounds octanal and γ
-terpinene were present at higher FD factors in Kini (FD ≥2048 and FD 1024, respectively) than in Gspusi (FD 128 and FD 4, respectively). Similarly, the intensity ratings of the anise-like
notes in the two gins were not significantly different, yet the anise-like compounds estragole and trans
-anethole were present at higher FD factors several times higher in Kini. In general, differences in the FD factors of individual aroma compounds of the two gins are likely associated with the use of diverse individual botanicals during gin production. Additionally, however, it is a well-known phenomenon that complex odorant mixtures may have suppressive, synergistic or additive interactions, leading to unpredictable effects. As such, although the presence of individual aroma compounds at high FD factors certainly elicit strong odor impressions, specific notes of complex mixtures cannot be solely represented by their single constituents [33
Overall, terpenes and terpenoids were found to be the dominant odor-active compounds in the Kini and Gspusi gins. These classes of compounds are mainly formed in plants via the mevalonate pathway [36
] and are well known aroma-active constituents in a large variety of essential oils from different botanicals used in gin production (Table S2
), including juniper, citrus fruits and ginger. Comparing the gins, approximately 40% of the substances identified had higher FD factors in Kini than in Gspusi, which can be explained by the greater number and variety of botanicals used in the production of the former. By comparison, the saturated and unsaturated short chain aldehydes, including odorants such as hexanal, (E
)-2-hexenal and (E,E
)-2,6-nonadienal, are mainly formed by the degradation of fatty acids. Oleic, linoleic and linolenic acid are especially susceptible to autoxidation, which leads to the formation of unsaturated aldehydes and ketones, and subsequently saturated aldehydes [37
]. In relation to the present study, FD factors of related compounds were either similar for both gins or higher for Kini with the exception of (E
)-2-decenal. Citrus fruits are most likely the main source of the aldehydes found in the two gins, so differences in their respective potencies might be a reflection of different amounts used in the production of Kini and Gspusi.
The compounds eugenol and estragole, detected in both gins, are phenylpropanoids that are formed in plants via the shikimate biosynthetic pathway with phenylalanine as an intermediate [38
]. Phenylpropanoids are also a common component of essential oils, besides terpenes, and represent the majority of naturally occurring phenolic phytochemicals or their precursors [38
]. Botanicals with high amounts of phenylpropanoids were used in the production of Kini (botanicals listed in Table S2 in the Supplementary Material
), which might explain the presence of these compounds in this gin, especially eugenol, which was present at the highest FD factor (≥2048).
3.3. Quantitation of Selected Aroma Compounds
Seven individual compounds, listed in Table 2
, were chosen for quantitation in the gins via SBSE and SIDA. The selection of compounds for quantitation was made based on their presence at high or differing FD factors in the two gins.
Most of the selected aroma compounds quantified were generally present at higher concentrations in Kini than in Gspusi. Especially noteworthy were the eucalyptus-like
smelling 1,8-cineole and the flowery
smelling linalool, which were at concentrations almost 30 times higher in Kini than in Gspusi. Similarly, the anise-like
smelling estragole was 20 times higher in Kini. These quantitative data appear to contradict the semi-quantitative results of AEDA, which showed only minor differences in intensities of 1,8-cineole and estragole between the two gins, and no difference for linalool. On the other hand, the AEDA data are in agreement with the sensory evaluations, whereby no significant differences in the attributes anise-like
between the gins were observed (flowery
was not evaluated). Contradictory results like these have been previously reported and discussed in the literature [34
]. Studies have shown that concentration does not linearly correlate with OAV of individual compounds when these are present at higher concentrations. Moreover, odor thresholds are significantly influenced by the presence of other odorants in the matrix, as previously mentioned in relation to synergistic or additive effects. Nevertheless, the OAV model is generally valid for odor-active compounds present at lower concentrations [34
] and provides an indication of the importance of an odorant to the overall aroma, as explored in the next section.
The concentrations of the selected aroma compounds determined in the two gins in the present analysis are generally in good agreement with values reported elsewhere for gin, demonstrating that the present methods yield concentrations in comparative ranges to other approaches. Myrcene, for example, exhibited slightly lower or equivalent mean concentrations to those reported in the literature, with 0.82 mg/L and 2.71 mg/L here for Kini and Gspusi, respectively, compared to values ranging between 2.28 mg/L [8
] and 11.09 mg/L [14
] elsewhere. This was similarly the case for linalool, with mean concentrations of 0.93 mg/L in Gspusi and 28.53 mg/L in Kini compared to 1.9 mg/L [8
] and 36.99 mg/L [14
] reported in other studies. By comparison, limonene was found here to be fully within the range of the literature values, with mean concentrations of 3.83 mg/L and 7.03 mg/L for Kini and Gspusi, respectively, compared to between 1.22 mg/L and 17.21 mg/L reported in the literature [14
As highlighted in the introduction, besides the common use of juniper, different gins are produced with a highly diverse number and type of botanicals to create their unique or characteristic individual flavor profiles. Accordingly, the exact composition of constituent aroma compounds vary across distilleries and for different gins depending on which botanicals are used. Further, the amount of each botanical used, as well as how each is processed and added during the production process, will affect the concentrations of individual aroma constituents. Many botanicals contain common aroma compounds, albeit in varying amounts, thus individual compounds can be rarely attributed solely to a single botanical. As such, the omission of any particular botanical during gin production does not automatically lead to the absence of a specific constituent aroma compound in the gin, which might be imparted by another botanical ingredient. Further, in the present study, although a list of botanicals used in each gin was kindly provided by the distillery, the quantities implemented are proprietary information. As such, a detailed and comprehensive comparison of the quantitative or semi-quantitative (FD factor) differences of individual aroma compounds within each gin type or between the two gins in relation to the botanical ingredients cannot be made here.
3.4. Odor Activity Values
As mentioned above, OAVs are defined as the ratio of the concentration of an odorant in a sample to the odor threshold of that odorant, and these give an indication of the relative importance of an individual odorant to the overall aroma. The OAVs were determined for the seven quantified aroma compounds (see Table 2
) using experimentally determined thresholds of the individual aroma compounds in 45%vol. ethanolic solutions (Table 3
), with the exception of linalool for which a published literature value was used [20
Among the selected aroma compounds quantified, the flowery
smelling linalool had the highest OAV and thereby contributed most to the overall aroma in both gins. Further, with an OAV of 1189 in Kini, this compound was more pronounced than in Gspusi (OAV 39), although its FD factor was identical in both gins (with the highest value of FD ≥2048; see Table 1
), indicating its overall dominant contribution. The eucalyptus-like
1,8-cineole had the second highest OAV in Kini (OAV 12), but was not prominent in Gspusi (OAV <1), followed by the earthy
smelling myrcene at OAV 8 in Kini, but this was more dominant in Gspusi, at OAV 27. This observation is reflected in the AEDA data, whereby 1,8-cineole exhibited a higher FD factor in Kini (FD 32 vs FD 4 in Gspusi), and conversely, myrcene was higher in Gspusi (FD 1024 vs FD 512 in Kini). The anise-like trans
-anethole exhibited OAVs of 7 and 2 in Kini and Gspusi, respectively, indicating its higher contribution to the overall aroma of the former compared to that of the latter; again, the AEDA data reflect this observation, with respective FD factors of ≥2048 and 512 for Kini and Gspusi. OAVs of the other compounds ranged from <1 to 3 in the two gins, reflecting varying minor contributions to the aroma. Although these OAVs provide indications of the contributions of individual odorants to the overall aroma of a sample, the significance of their impact on the odor profile cannot be inferred from these data, but rather require additional recombination and omission experiments [40
]. Using the latter approach it has been shown that odor qualities of a less intense aroma compound can be entirely suppressed in the presence of a more intense odorant [34
], thereby highlighting the complexity of odor interactions and how these affect perception.