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.
Sensory profile, cognac, organic compounds, fatty alcohols, ethers, volatile compounds, polyphenolic compounds, 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].
Different classes of compounds contribute to
the formation of the sensory profile of cognacs with
different aging periods (Tables 1 and 2) [2–9].
Figure 1 shows descriptors that make up the sensory
profile of cognac [15].
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].
Table 1 Compounds that affect the sensory profile of cognacs
Compounds Source Effect on cognac quality
Fatty alcohols Amino acids of raw materials
during fermentation
Resinous, honey, floral, and ripe fruity tones
Ethers Raw materials; fermentation
and aging in oak casks
Fruity tones; ethyl acetate is responsible for floral
and anis aroma
Aldehydes and ketones Raw materials; fermentation
and aging in oak casks
Unpleasant unripe tones; nutty and floral tones
Norizoprenoids and terpenes Fermentation of plant
raw materials
Resinous tones, e.g. myrcene; fruity and floral tones;
caryophylenes are responsible for the tone of cedar pine nuts
Polyphenols and phenols Oak wood during aging A wide range of flavors and colors 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).
The list of phenolic and furan compounds included
gallic, syringic, vanilla and sinapic acids, vanillin,
syringaldehyde, coniferaldehyde, sinapaldehyde,
5-hydroxymetifurfural, furfural, and 5-methylfurfural.
Their content was analyzed by high performance liquid
chromatography (HPLC) using a diode array detector
Agilent Technologies 1200 (Agilent, USA). We also used
a Hypersil 5 um C18 250×4.6 mm column (Thermo,
USA) with wavelengths of 270 and 310 nm. The test samples and standards (0.02 cm3) were introduced in a
reversed-phase column at 40°C. The mobile phase was
represented by a 0.025 mol/dm3 solution of potassium
dihydrogen phosphate (A) with pH = 2.5, and a solution
of acetonitrile (B) in the ratio of A:B = 87:13. The elution
rate was 1.3 cm3/min.
The mass concentration of sugars, i.e. fructose,
glucose, and sucrose, was analyzed by HPLC using an
Agilent Technologies 1200 diode array detector (Agilent,
USA). A Hypersil 5 um C18 250×4.6 mm column
(Thermo, USA) had wavelengths of 440 and 540 nm.
The test samples and standard solutions were injected
in a volume of 0.02 cm3 of a reversed-phase column at
40°C. The mobile phase was represented by distilled
water (A) and acetonitrile solution (B) in the ratio of
A:B = 87:13. The elution rate was 600 cm3/min.
The mass concentration of higher alcohols,
ethers, and hydrocarbons was assessed using gas
chromatography (HPHC). A flame ionization
detector (GC-FID) was used to detect various
volatile components, including methanol, ethanol, 1-,
2-propanol, 1-, 2-butanol, isobutanol, isoamilol, hexanol,
phenylethyl alcohol, acetaldehyde, isobutyl aldehyde,
acetone, ethyl formate, diethyl formate, ethyl acetate,
isoamyl acetate, ethyl caproate, ethyl lactate, ethyl
caprylate, ethyl caprate, guaiacol, and eugenol. The
analysis also involved such non-volatile components as
o-cresol, tyrosol, myrcene, and β-terpineol.
All measurements were conducted in sevenplicates,
standard deviation ≤ 5%. Each sample in the volume
of 5 cm3 (40% vol.) was added to 0.25 cm3 of internal
standard solution and placed in 2 cm3 vials. Each
component was introduced at a concentration of
2 g/dm3 in absolute alcohol. The vials were hermetically
sealed. A sample of 0.002 cm3 was introduced into
the chromatograph inlet. The column thermostat temperature was 220°C, and the carrier gas velocity was
1.3 cm3/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); 7–8 –
very 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).
RESULTS AND DISCUSSION
Tables 3–5 show the content of ethanol, reduced
extract, carbohydrates, volatiles, furans, and phenols in
the cognac samples of different ages.
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/dm3. 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/dm3. 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 [24–26]. 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/dm3), 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 [31–33]. The content of
syringaldehyde is a marker of aging time. It was in the
range of 2.5–7.7 mg/dm3 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 (Y1, Y2,
Y3, Y4, Y5, Y6 and Y7) and the variables. In group Y1, the
variables at X3 had a larger coefficient because o-cresol
had a greater effect on descriptor Y1; in groups Y2 and Y3,
phenolic alcohols contributed; in Y4 – volatile phenolic
compounds and aldehydes; in Y5 – oxymethylfurfural;
inY6 – terpene compounds, and in Y7 – vanillin.
Table 8 demonstrates equations for the dependence
of the compounds (X) on the aging period (Yx) obtained
by the method of pair linear correlation.
The greatest value belonged to variable X3.
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 Y4 (resinousness) and
Y7 (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 v Figure 2 Descriptors for sensory evaluation of cognac samples arious organic compounds
o-cresol
[X3]
Ethers
[X3]
Ethers
[X2]
Fruity tone [Y6]
Chocolate tone [Y7]
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.
CONTRIBUTION
M.N. Eliseev supervised the research project.
O.A. Kosareva developed the research plan,
I.N. Gribkova and O.M. Alexeyeva performed the
experimental research, obtained the data, and analyzed
them.
CONFLICT OF INTEREST
The authors declare that there is not conflict of
interests regarding the publication of this article.
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