Effect of organic compounds on cognac sensory profile

Introduction. The present research featured the effect of carbonyls, phenols, furans, fatty alcohols, ethers, and other chemical compounds on the sensory properties of cognac distillates of different ages. The research objective was to identify additional criteria of sensory evaluation by measuring the effect of various compounds on perception intensity. Study objects and methods. The study featured cognac samples of different ages. The experiment involved standard methods, including high-performance liquid and gas chromatography and a mathematical analysis based on Microsoft software. Results and discussion. The content of fatty alcohols, ethers, and carbonyl compounds that formed as a result of fermentation demonstrated little change during the aging period in oak casks. A longer extraction increased the content of phenolic and furan compounds and sugars. The content of terpene compounds decreased due to their high lability. The study revealed the effect of organic compounds on taste descriptors. The article introduces multivariate equations that calculate the dependences of the descriptor intensity on the content of organic compounds. A correlation and regression analysis revealed that phenolic compounds had a significant effect on the taste formation of cognac samples, depending on the aging time. Conclusion. Organic compounds proved to affect the taste profiles of cognac samples of different ages, as well as sensory evaluation descriptors.


INTRODUCTION
Formation of the flavor profile of cognac and brandy is a complex multistage process. Their aroma, taste, and color depend on too many factors, including the quality of raw materials, the technology of fermentation and distillation, etc. One of the most important factors is the aging in oak casks: its time and conditions are responsible for the numerous transformations of organic compounds, such as extraction, synthesis, biosynthesis, oxidation, etc. [1].
The gustatory sensation formation is a complex process, where a single shade of flavor may result from a whole complex of compounds [16]. People are able to perceive five basic tastes: sweet, sour, bitter, salty, and "umami", which was discovered in the early XX century.
In fact, the taste sensation forms in the brain as protein structures trigger its response to a combination of external stimuli. Several sensory stimuli shape perceptions from several descriptors. For instance, spicy tones are formed by compounds of mustard and pepper because carbon dioxide is responsible for this taste. Fresh tones depend on several compounds of plant raw materials, e.g. mint, or on individual substances, e.g. xylitol. A sense of astringency appears when saliva proteins interact with food polyphenols. How panelists evaluate one particular descriptor depends on a complex of organic compounds that enhance or minimize their effect on taste receptors due to spatial stereoisomerism, etc. [17]. Therefore, the present research objective was to study the effect that compounds in cognacs of different ages produce on the intensity of perception of individual descriptors in order to reveal extra quality assessment criteria.

STUDY OBJECTS AND METHODS
The present research featured cognac samples of various ages purchased in a network supermarket. Cognacs were stored in a dark room at 20 ± 1°С.
The reduced extract was analyzed by distillation followed by a pycnometric analysis of solids [18].
The pH of the samples was measured in sevenplicates using a pH meter (METTLER TOLEDO, USA).
All measurements were conducted in sevenplicates, standard deviation ≤ 5%. Each sample in the volume of 5 cm 3 (40% vol.) was added to 0.25 cm 3 of internal standard solution and placed in 2 cm 3 vials. Each component was introduced at a concentration of 2 g/dm 3 in absolute alcohol. The vials were hermetically sealed. A sample of 0.002 cm 3 was introduced into the chromatograph inlet. The column thermostat   3 4.0 ± 0.1 4.0 ± 0.1 4.0 ± 0.1 Reduced extract, g/dm 3 2.80 ± 0.03 4.00 ± 0.04 4.70 ± 0.50 Active acidity (рН) 3.70 ± 0.04 3.50 ± 0. 35 3.60 ± 0.36 temperature was 220°C, and the carrier gas velocity was 1.3 cm 3 /min. The sensory evaluation of the cognac samples involved seven panelists with an extensive experience in cognac industry and sensory tests. The panelists worked in separate booths, isolated from external factors. The cognac samples were served chilled to 18 ± 1°C in testing glasses at room temperature 20 ± 1°C under white diffused light. The samples were evaluated according to set of descriptors in comparison with the reference sample. The result was expressed in points from 0 to 10 (0 -impossible to evaluate; 1-2 -unsatisfactory (demonstrates a severe flaw); 3-4 -satisfactory (demonstrates an obvious flaw); 5-6 -satisfactory (violates the quality standard); 7-8very satisfactory (slightly violates the quality standard); 9-10 -excellent (corresponds with the quality standard).
The statistical analysis was performed in sevenplicates. The descriptive statistics and values were expressed as mean ± standard deviation (SD). The Student-Fisher method provided multivariate models of the correlation and regression dependence of the parameters. The reliability limit of the obtained data (P ≥ 0.95) was used to assess various factors that affected the content of polyphenols in all the experiments. The obtained statistical data were processed using the Statistics program (Microsoft Corporation, Redmond, WA, USA, 2006). Tables 3-5 show the content of ethanol, reduced extract, carbohydrates, volatiles, furans, and phenols in the cognac samples of different ages.

RESULTS AND DISCUSSION
The data are representatives of seven independent experiments, and values are expressed in mean (± SD). Table 3 shows that the content of ethanol stayed within the permissible values for cognac products specified in State Standard 31732-2014 "Brandy. General specifications" and did not fall below 40.0 ± 0.3% or 4.00 ± 0.03 g/dm 3 . The content of volatile compounds in the samples increased together with the aging time, which correlates with the previously published scientific data [19,20]. Table 3 clearly demonstrates that the active acidity decreased insignificantly as the aging time increased. The total acidity index depended on the origin of the wood. However, it increased as a result of long-term aging in oak casks due to the oxidation of ethanol as compounds passed from the wood to the cognac [21,22].
The pH value is known to depend on the amount of acids and the strength of the distillate. As the content of alcohol in the distillate increases, the dissociation of carboxyl groups decreases, and acidity drops. As tannins dissolve, volatile acids appear, and the strength decreases during aging, the pH decreases [23]. The pH value also depends on the amount of dissolved tannins with an acidic pH, which increases the acidity of the distillates [23]. The experimental data in Table 5 confirmed these trends.
The data are representatives of seven independent experiments, and values are expressed in mean (± SD).  Table 4 shows that the content of volatile fractions in the cognac samples increased together with the aging period, as reported in [19]. The total of higher alcohols was 1.774-2.092 mg/dm 3 . A longer aging period triggered the process of oxidation in higher alcohols (Table 4). Since the content of these alcohols in the cognac distillate was low, the oxidation of each alcohol was insignificant, in comparison with the oxidative processes of ethyl alcohol. The amounts of aldehydes, acids, and ethers formed by higher alcohols were also insignificant. Nevertheless, even in such small quantities that are elusive for conventional analysis methods, these substances still affect the taste of cognac due to the sheer fact of their existence [24][25][26]. If the cognac composition is well-balanced, higher alcohols form the basis of its sensory profile [27].
Undesirable tones may result from excessive acetaldehyde that form during oxidation, especially in the samples with a longer aging period, depending on the characteristics of oak wood [22,28]. However, if other volatile compounds are present, the excessive acetaldehyde in these samples does not disrupt the taste balance.
Ethers also affect the flavor profile of cognacs. Their content depends on the aging time [29]. If ethyl acetate exceeds the sensitivity threshold (180 mg/dm 3 ), it affects the sensory profile of the distillate, giving it undesirable tones [30].
The data are representatives of seven independent experiments, and values are expressed in mean (± SD). Table 5 shows that the cognac samples contained typical phenolic acids and aldehydes in quantities that did not exceed those featured in research publications for cognacs of 2.5-15 years of aging [31][32][33]. The content of syringaldehyde is a marker of aging time. It was in the range of 2.5-7.7 mg/dm 3 and increased with aging time, which corresponded with scientific publications on this chemical substance and other simple phenolic acids and aldehydes [31]. Table 5 illustrates the ratio of syringaldehyde and vanillin, which is also a marker of aging time. This ratio stayed within the range of 2-4, established for collection samples, and was 2.4-2.5 [31,34].
Phenolic acids are involved in the complex biochemical processes of aging and affect the sensory profile of cognacs [35]. For instance, gallic acid, a product of hydrolysis of soluble gallotannins and eluggotannins of oak wood, affects the aging processes, acts as an oxidation catalyst, and removes sulfides [36,37]. As alcohol comes in contact with oak bark during aging, it triggers solubilization with the subsequent cleavage of the covalent alkylaryl ether. This reaction leads to the cleavage of lignins and produces vanillin, syringaldehyde, and their acids, which affect the taste profile of cognac distillates [38]. Table 5 shows that phenolic acids and aldehydes increased with aging, which is consistent with the previously published research data [38].
Furan compounds appear as the temperature increases during the decomposition of non-starch polysaccharides of oak bark or during distillation from five-membered sugars [39]. The amount of furan compounds is known to affect the number of distillations [26]. The content of furan compounds increased after a prolonged contact of oak bark and cognac distillate.
Reducing sugars, i.e. glucose, arabinose, and fructose, were also registered in the distillate samples. During aging, the contact of alcohol and oak wood led to the hydrolysis of hemicelluloses and hydrolyzable tannins [40]. Sugars affected the sensory profile of cognacs, and their quantity increased with aging (Table 5).
Volatile phenolic compounds, phenols, and terpene compounds are responsible for some characteristic tones in the cognac bouquet. The content of phenolic compounds increases with aging, while the concentration of terpene compounds decreases as a result of their lability ( Table 5).
The cognac samples underwent a sensory evaluation (Table 6) using the descriptors presented in Fig. 2, which demonstrates how certain organic compounds compose particular descriptors.
In low alcohol drinks, bitterness is known to depend on alcohol content [41]. This study proved that bitterness depends not only on aliphatic alcohols, but also on phenolic compounds.
Aldehydes are responsible for mildness [42]. However, aliphatic alcohols with their different tones also might help make the taste of cognac milder, and the content of o-cresol might also produce a certain effect on the mildness [2]. Astringency appears when phenolic compounds are released during aging as a result of contact with oak wood, depending on the aging time and pH [5,6].
The resinous tones result from the combined action of organic compounds in the distillate; it defines the quality of the finished product [33]. This descriptor is formed during fermentation, distillation, and aging [1,33]. As a result, resinousness may depend on the content of aliphatic alcohols, phenolic compounds, and terpenoids [5,7,33].
Oiliness, another cognac descriptor, appears mainly due to secondary fermentation products that remain after distillation, and partly due to the contact of alcohol with oak [33]. Fruity tones depend on such secondary fermentation products as aldehydes and alcohols, as well as on terpene compounds, which is associated with the fermentation of fruit raw materials [9].
Chocolate tones are more difficult to form than the rest of the descriptors. Chocolate tones are known to depend on secondary fermentation products, volatile phenolic compounds, vanillin, and methylfurfural, the latter also being responsible for sweet-nutty tones [26].
The intensity indicators for each descriptor were quantitatively correlated with the results of the sensory evaluation (Tables 3-5). They were processed in order to obtain correlation and regression equations that made it possible to calculate the dependence of the tones on particular compounds ( Table 7).
The values of the coefficients were analyzed in modulus in each group of the dependencies (Y 1 , Y 2 , Y 3 , Y 4 , Y 5 , Y 6 and Y 7 ) and the variables. In group Y 1 , the variables at X 3 had a larger coefficient because o-cresol had a greater effect on descriptor Y 1 ; in groups Y 2 and Y 3, phenolic alcohols contributed; in Y 4 -volatile phenolic compounds and aldehydes; in Y 5 -oxymethylfurfural; inY 6 -terpene compounds, and in Y 7 -vanillin. Table 8 demonstrates equations for the dependence of the compounds (X) on the aging period (Y x ) obtained by the method of pair linear correlation.
The greatest value belonged to variable X 3 . Therefore, the change in the content of volatile phenolic compounds affected the sensory profile of the cognac samples more than other compounds. Probably, descriptor groups Y 4 (resinousness) and Y 7 (chocolate tone) had a greater affect on the taste perception in comparison with other descriptor groups. Phenolic compounds, i.e. acids, aldehydes, alcohols, and volatile compounds, were especially important for the development of the sensory profile of the cognac samples.

CONCLUSION
The correlation and regression analysis made it possible to assess the role of various organic compounds  in the development of taste profiles for cognac samples of different ages. The paper introduced equations of multivariate models that describe the effect of organic compounds on the descriptors of cognac products. Linear regression equations revealed that phenolic compounds of various classes played a major role in the taste profile formation. The obtained data will make it possible to form a list of additional criteria for sensory evaluation of cognac products.

CONFLICT OF INTEREST
The authors declare that there is not conflict of interests regarding the publication of this article.