Anchoring of quality variance factors and uncovering its influence on the flavor quality of BCSGB during aging process of sun-exposure

ABSTRACT

The aging process is crucial to improve the harmony and fullness of the flavor of baijiu. Nevertheless, the traditional aging process consumes space and times and significantly increased the loss of baijiu. Hence, how to shorten the aging time through manual intervention becomes the research hotspot. Thus, a new aging method of exposure to sunlight was established to accelerate the balance of flavor compounds in baijiu. It was still unclear about the quality variance factors and its influence on the flavor quality of BCSGB (banchengshaoguo baijiu) during the sun-exposure process. Hence, adsorption pen combined with GC–MS were utilized to identify trace components, and pH liquid–liquid extraction and GC–O–MS were employed to screen and assess flavor compounds. Afterward, based on the sensory evaluation of BCSGB, three assessment systems (sensory evaluation, mental test, and random forest) were created to screen the quality variance factors. Thus, the quality variance factors were responsive to changes in the flavor quality of BCSGB during the sun-exposure process, which support process quality monitoring and aging process optimization.

GRAPHICAL ABSTRACT

KEYWORDS:

• Aging process
• dynamic change
• adsorption pen
• Mental test
• random forest

Introduction

Strong flavor baijiu is the mainstream of baijiu production and consumption in China, occupying more than 70% of baijiu market in China^[1]. Due to the different regions, raw materials and production technology, different styles of products have been sprung up, such as Chuan style (strong flavor baijiu produced in and around Sichuan province), Jianghuai style (strong flavor baijiu produced in the Yangtze River and Huaihe River basin), and Bei style (strong flavor baijiu produced in northern China) strong flavor baijiu.^[1] Of note, as the representatives of Bei style strong flavor baijiu (BSFB), Banchengshaoguo baijiu (BCSGB) is made of sorghum and wheat and is produced by the unique Laowuzeng production technique (Laowuzeng means that the Jiupei are removed from the Jiaochi, mixed with raw materials, then distilled and gelatinized at the same time in Zeng. Briefly, the Jiupei in Jiaochi are divided into four layers, namely covering layer, upper layer, middle layer, and base layer. After a round of fermentation, the covering layer is distilled and thrown away, and the upper layer is mixed with new raw materials and husks. Then the mixture is divided into a new upper layer and a middle layer in the next round of fermentation. The middle layer becomes the new base layer, and the base layer will become the new covering layer, which is abandoned after fermentation. This cyclical distillation technology is called Laowuzeng).^[2] Consequently, the flavor of BCSGB is characterized by elegant and clean. Whilst the freshly steamed baijiu (namely raw baijiu) is unfit for consumption. Because its flavor is full of spicy, stimulating, and incongruous. In practice, it is found that the flavor of the raw baijiu will be improved after a period of storage.^[3] Thus, the storage of raw baijiu is preserved and evolved into the baijiu aging process.^[4] During the aging process, physical and chemical changes such as oxidation, esterification, hydrolysis, and rearrangement continue to occur and make the flavor of raw baijiu become harmonious and mellow.^[5] Given the availability of materials and technology, pottery jars were widely applied to store the raw baijiu in ancient times (Sun). In contrast, with the improvement of production capacity and the guarantee of technique, the stainless-steel tank is more and more used to store raw baijiu.^[6] However, traditional aging process takes several years, occupies a large area, and results in significant loss of baijiu.^[7] Hence, how to shorten the aging time through manual intervention becomes the research hotspot.^[8,9]

As is well known, 98% of baijiu is ethanol and water, with the remaining 2% consisting of trace components.^[1] The flavor quality of baijiu is mainly determined by the kind, proportion, and concentration of trace components.^[10] Up to now, there have been numerous studies focused on the trace components in strong flavor baijiu.^[11] Specifically, direct injection (DI),^[12] liquid–liquid extraction (LLE),^[13] liquid–liquid microextraction (LLME),^[14] and headspace solid-phase microextraction (HS-SPME),^[14] in combination with GC–MS,^[15] and GC–O–MS,^[16] were the common methods to analyze the trace components of strong flavor baijiu. Interestingly, the above-mentioned studies were focused on a few representative brands of strong flavor baijiu from Chuan style and Jianghuai style, such as Gujinggong (a brand of strong flavor baijiu),^[17] Wuliangye (a brand of strong flavor baijiu),^[18] Luzhou laojiao (a brand of strong flavor baijiu), and Yanghe (a brand of strong flavor baijiu).^[19] The trace components in Bei style strong flavor baijiu have received limited attention. Meanwhile, there have been many studies on the flavor profile of baijiu,^[16] but the study on the change of baijiu flavor during aging has not been systematic enough.^[20] Relevant research will serve as a foundation for the regulation of the baijiu aging process. Besides, the adsorption pens have also been gradually introduced into the analysis and research of flavor compounds, which offers several advantages, including high automation, a larger adsorption capacity, and mild treatment conditions. In addition, it is highly beneficial for the extraction, separation, and identification analysis of volatile components,^[21] thus, effectively improving the stability and sensitivity of the method. Hence, adsorption pen method has become popular in flavor analysis in recent years.^[22] It is also believed that this method can be utilized to verify the trace components in baijiu.

During the production process of baijiu, several aging methods such as ultrasound, microwave, traditional aging, and ultraviolet are widely mentioned.^[23] However, due to limitations in materials, technology, and energy consumption, these methods mostly remain at the laboratory stage and have not been widely applied in practical production. While sun-exposure aging was a highly feasible method. It utilized sunlight exposure to accelerate the aging process of specific grades baijiu, aiming to improve quality and shorten aging time. Through sensory evaluation of the sun-exposed baijiu samples, it was observed that the exposed samples had a more advantageous taste compared to the non-aged samples. It was speculated that exposure to sunlight changes the composition of trace components in baijiu, thereby influencing on the sensory of raw baijiu.^[24] However, the correlation between this novel aging method and the sensory of baijiu is not yet clear. In detail, the dynamic changes in trace components in baijiu during exposure to sunlight need to be covered. Furthermore, the relationship and pattern between aroma compounds and flavor quality for raw baijiu needs to be confirmed.

In this study, different degrees of strong flavor baijiu (i.e., different sun-exposure times) were selected as object, the volatile trace components were confirmed by adsorption pen combined with GC–MS, afterward, molecular sensory omics were introduced to verify the potential aroma compounds and their dynamic changes during sun exposure process. Then, flavor matrix was established and combined with multivariate statistical analysis to investigate the correlation between aroma compounds and flavor quality for raw baijiu, and to screen the key aroma compounds for the sun exposure process. Finally, a random forest model was established to verify the relationship between flavor compounds and the sense of baijiu. The research will provide a theoretical foundation and scientific guidance for optimizing the sun exposure and aging processes.

Experimental methods

Chemicals and solvents

Analytical samples were obtained from the Chengde Qianlongzui Distillery Co., LTD (Hebei, China). Methylene chloride (analytical grade), sodium chloride (NaCl), ethanol (analytical grade), and sodium sulfate anhydrous (Na₂SO₄) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China) in analytical grade, n-alkanes (C₅−C₂₉) were purchased from Beijing Chemical Reagent Co. Ltd (Beijing, China).

Sample information

Six raw BCSGB samples (labeled as BCSGB-1, BCSGB-2, BCSGB-3, BCSGB-4B, CSGB-5, and BCSGB-6) were obtained from Chengde Qianlongzui Distillery Co., Ltd. (Hebei, China). Samples were stored at 4°C until analysis and are detailed in Table 1 and Table S1.

Table 1. Sample information for six raw BCSGB samples.

Label	Sun-exposure time (days)	pH	Alcohol (%vol)
BCSGB-1^a	0	4.16	62
BCSGB-2	6	4.16	60
BCSGB-3	12	4.16	62
BCSGB-4	20	4.16	60
BCSGB-5	30	4.16	60
BCSGB-6	50	4.16	60

^aAll samples were sourced from the same batch, ensuring that only the exposure time varies while other conditions remain constant. Specifically, BCSGB-1 represented a baijiu sample that underwent a brief aging process without exposure to direct sunlight, while other samples were the baijiu sample that had been exposed to the sunlight for different time on this basis.

Identification of trace components in raw baijiu samples

Each sample of baijiu (2 mL) was diluted to 15% (v/v) ethanol with saturated salt solution and was added to a 20 mL glass headspace vial, which was sealed with a PTFE/silicone septum and a screw cap. The sample was extracted at 240 r/min for 20 min at 50°C. Each sample was analyzed in triplicate for GC–ΜS analysis.

After extraction, an Agilent 8890 gas chromatograph equipped with an Agilent 5977B mass spectrometer (Agilent Technologies, Palo Alto, CA, USA) was used to analyze the trace components in raw baijiu. Helium (≥99.999%) at a constant flow rate of 1 mL/min was used as carrier gas. The samples are analyzed using the DB-FFAP column (60 m × 0.25 mm i.d. and 0.25 μm) in splitless mode. The oven temperature program was held at 40°C, ramped at 10°C/min to 50°C, held for 10 min, ramped at 3°C/min to 80°C for 10 min, ramped at 5°C/min to 230°C for 10 min, and then held for 10 min.

For the GC–MS system, the temperature of the ionization source was 230°C. Electron impact mass spectra were recorded at 70 eV, and the acquisitions were performed over an m/z scan range of 50 − 450 amu. Spectral Library Retrieval, Standard Comparison Method and Retention Index Comparison were used to identify the trace components in raw baijiu samples. The retention index (RI) of each compound was calculated from the retention times of n-alkanes (C₅–C2₉) according to Equation (1):

(1) $RI=100(n+\frac{\mathrm{t}\mathrm{i}-\mathrm{t}\mathrm{n}}{\mathrm{t}\mathrm{n}+1-\mathrm{t}\mathrm{n}})$(1)

In Equation (1), t_n and t_{n +1} are the retention times of n-alkanes with carbon number n and n + 1, respectively; t_i is the retention time of the untargeted compound with the peak between n and n + 1.

Quantitative analysis of the trace components in raw baijiu samples

The internal standard method was used for quantitative analysis. A series of standard solutions were prepared by dissolving authentic standard compounds in 15% aqueous ethanol solution, which were added internal standard (4-octanol, 20 mg/L). The analyzed conditions for standard solutions were the same as 2.3. Selective ion monitoring (SIM) mass spectrometry was used to quantify the trace components. All the trace components detected were quantified by constructing standard curves, where y was the peak area ratio (peak area of volatile standard/peak area of internal standard) and x was the concentration ratio (concentration of volatile standard/concentration of internal standard). All the calibrations were performed in triplicate. The concentrations of the trace components were calculated based on the standard curves.

Verification for aroma compounds in raw baijiu samples

Each sample of Baijiu was diluted to 14% (v/v) ethanol with saturated salt solution and then extracted three times with 50 mL CH₂Cl₂, and the organic-phase extracts were merged to get the organic phase O₁. Next, the organic phase O₁ was washed three times with an alkaline solution (pH = 10.0, 50 mL each time), and separated to get the organic phase O₂ and the merged water-phase extracts W₂. Then the W₂ was adjusted to pH = 2.0 with hydrochloric acid solution, the solution was saturated with NaCl. Moreover, the solution was extracted three times with CH₂Cl₂ (50 mL each time), and the combined organic phase named O₃. Anhydrous Na₂SO₄ was added to O₂ and O₃ for dry (−20°C). Then filtered, after that the mixture was concentrated to about 1.5 mL by distillation on a rotary evaporation apparatus. The neutral/basic fraction (NBF) and acidic fraction (AF) of components in BCSGB were obtained. Finally, about 1.0 ml of each sample was injected for GC–O–MS analysis.

After extraction, an Agilent 7890B GC coupled with a 5977 mass spectrometer and an olfactometer system was used (Agilent Technologies, Santa Clara, CA) in this study. Moreover, baijiu samples were analyzed with a DB-Wax capillary column (60 m × 0.25 mm × 0.25 μm). The column temperature was initially 40°C, raised to 50°C at 10°C/min for 20 min, and raised to 70°C at 1°C/min for 10 min, and then raised to 250°C at 3°C/min for 15 min. Helium (>99.999%) at a constant flow rate of 1 mL/min was used as carrier gas. The temperature of the ionization source was 230°C, respectively. A 1 μL aliquot of the concentrated baijiu sample was injected into the splitless mode.

Three trained panelists from the Key Laboratory of Brewing Molecular Engineering of China Light Industry were chosen to perform the GC–O analysis, and each panelist should record the aroma description, expression intensity, and retention time for aroma compounds. Then the C₈–C₄₀ n-alkane mixture was injected under the same GC–MS with Baijiu samples to calculate RI. All analyses were repeated in triplicate by each panelist.

Odor active value calculation and taste active value calculation

All such experiments were performed in triplicate. The contribution of each aroma compound to the overall aroma profile of BCSGB was assessed by the OAV and TAV,^[25] which was calculated based on the ratio of the concentration of each compound to its odor and taste threshold value in ethanol solution.

Sensory evaluation

Sensory panel: An 18-member sensory evaluation panel was composed of well-trained graduate students (the students are at Beijing Technology and Business University). All of them had been previously trained by describing and recognizing the aroma descriptors and the aroma characteristics of baijiu through an oronasal route.

Flavor compounds sensory evaluation: The 18-member sensory evaluation panel was trained for an additional 3 h to identify and define the descriptive terms for BCSGB using the related aroma compounds or materials reported in the literature.^[26] Then they were requested to write down the aroma descriptors of BCSGB samples. The descriptors with higher frequencies were selected. The 18-member sensory evaluation panel was called together to discuss them until an agreement was reached. Ethyl hexanoate and ethyl butanoate (fruity), 2-phenylethanol (floral), ethanol (alcoholic), hexanoic acid (cellar), steamed sorghum (grain aroma), and 2,5-dimethyl pyrazine (roasted aroma) and were selected to evaluate the aroma profiles of the BCSGB samples (as shown in Table 2). Finally, the panelists were asked to evaluate the odor intensities of the eight aforementioned aroma attributes represented by the above chemicals using a 6-point scale from 0 to 5 (0 = none, 1 = very weak, 2 = weak, 3 = moderate, 4 = strong, and 5 = very strong). All of the tests were performed in a sensory evaluation room at (25 ± 1) °C in three different sessions.

Table 2. The main sensory characteristic definitions.

Sensory characteristics	Sensory description
Fruity	ethyl hexanoate and ethyl butanoate
Floral	2-phenylethanol
Alcoholic	ethanol
Cellar	hexanoic acid
Grain aroma	steamed sorghum
Roasted aroma	2,5-dimethyl pyrazine
Zao-like aroma	The aroma of jiuzao
Fullness
Harmony

Taste compounds sensory evaluation: The sensory evaluation of taste intensity of trace components in samples was based on the scoring test method of GB/T 33,404–2016 (Institute & Association, 2016). The 5-point scoring method was selected, and each taste attribute was provided to the panels, respectively (as shown in Table 3). Then the panel tasted the sample. At the end of each taste, rinse the mouth with pure water before tasting the next sample. According to the difference between the taste intensities of the samples, score (5-point scale: no sensation − 0 points, slightly feel − 1 points, slightly − 2 points, − 3 points, strong − 4 points, very strong − 5 points).

Table 3. The main taste attributes.

Attribute	Sensory description
Acid	Lactic acid
Sweet	Sucrose
Bitter	Berberine hydrochloride
Spicy	Capsaicin
Astringent	Alum
Softness
Durability

Overall sensory attribute evaluation: The sensory evaluation of sensory attributes of BCSGB was based on the scoring test method of GB/T 33,404–2016 (Institute & Association, 2016). Each attribute (as shown in Table 4) was a rating of the aforementioned attributes.

Table 4. Baijiu overall sensory attributes.

Attribute	Definition
Color	Colorless or yellowish, clear and transparent, no suspended matter, no precipitation
Aroma	Score according to fullness and harmony
Taste	Score according to softness, purity, and durability.
Style	scored according to global characteristics

Statistical analysis

All independent experiments were performed at least in triplicate. The study was expressed as mean ±standard deviation (SD). One-way analysis of variance (ANOVA) and Turkey test at the α ≤ 0.05 level of significance were carried out to determine the significant differences between the concentration of trace components and sensory evaluation in samples with different sun exposure times. Flavor matrix was established and combined with multivariate statistical analysis to investigate the correlation between flavor compounds and flavor quality for raw baijiu using the R software package to screen the quality variance factors for the sun exposure process. The random forest analysis was performed using the R software package.

Results and discussion

Identification of trace components by the adsorption pen coupled with GC–MS

Qualitative analysis of trace components in BCSGB with six different sun-exposure times: The adsorption pen coupled with GC-MS was used for qualitative analysis of trace components in the baijiu samples to characterize the type of potential aroma compounds in six types of baijiu samples (BCSGB-1, BCSGB-2, BCSGB-3, BCSGB-4, BCSGB-5, and BCSGB-6). Consequently, 111 kinds of trace components were detected (as shown in Table 5), including 43 esters, 9 acids, 20 alcohols, 7 aldehydes, 5 ketones, 6 acetals, 4 furans, 13 aromatics, 3 hydrocarbon alkyl, and 1 sulfur compound. A group of pictures were used to demonstrate the distribution and the dynamic change for the trace components during the sun-exposure process (Figure 1). In general, esters, alcohols, and aromatics were the most abundant trace components and constituted above 50% in BCSGB samples, which was in congruence with the earlier reports on fermented beverages.^[27]

Figure 1. (a), the amount of various trace components in BCSGB. (b), the proportion of trace components in BCSGB. (c) ~ (d), the dynamic change for the trace components during sun-exposure process.

Table 5. Concentrations of 111 trace components in six BCSGB samples.

No.	trace components	CAS	RI		Identification	Concentration^c（μg/L）
			DB-WAX^a	HP-5 MS^b		BCSGB-1	BCSGB-2	BCSGB-3	BCSGB-4	BCSGB-5	BCSGB-6
1	2-methylpropanal	78-84-2	812	553	MS,RI,S	1103.12 ± 180.91b			1525.84 ± 85.83a		1104.02 ± 28.9b
2	aldehyde acetal	105-57-7	866	729	MS,RI,S	36960.21 ± 1264.23b	40986.7 ± 1389.98a	42449.58 ± 2734.36a	32639.68 ± 1130.13c	20238.92 ± 474.81d
3	ethyl acetate	141-78-6	900	613	MS,RI,S	63332.36 ± 11044.26b	52090.53 ± 360.78c	58558.64 ± 2942.68c	73241.2 ± 1984.22b	63294.3 ± 139.59b	96633.77 ± 4542.83a
4	3-methylbutanal	590-86-3	916	650	MS,RI,S	7568.03 ± 704.09a	5975.53 ± 287.24a	4742.27 ± 1590.57b	6714.48 ± 476.9a	5308.06 ± 151.24b	6626.47 ± 174.51a
5	ethyl propanoate	105-37-3	953	708	MS,RI,S	3291.16 ± 680.28b	3961.79 ± 585.83b	8120.96 ± 2602.41a	4279.53 ± 861.54b	3777.78 ± 212.73b	1991.52 ± 87.52b
6	ethyl 2-methylpropanoate	97-62-1	960	751	MS,RI,S	6769.3 ± 1046a	7670.58 ± 273.89a	7061.44 ± 389.59a	5710.27 ± 122.35b	5103.07 ± 154.37b	5481.06 ± 242.1b
7	propyl acetate	109-60-4	970	728	MS,RI,S	4601.26 ± 643.41a		5226.25 ± 458.88a	4064.28 ± 103.64b	3578.02 ± 111.72b	3835.77 ± 617.48b
8	1,1-diethoxy-2-methylpropane	1741-41-9	976	858	MS,RI,S		13758.07 ± 128.55
9	2-methylpropyl acetate	110-19-0	1011	783	MS,RI,S	5654.64 ± 126.49b	6196.27 ± 111.66a	4412.24 ± 213.96c	1355.09 ± 59.4e	2838.2 ± 137.95d	2840.83 ± 52.18d
10	2-butanol	78-92-2	1016	603	MS,RI,S	3582.66 ± 36.64a		3349.65 ± 295.53a	820.77 ± 9.93c		1413.16 ± 12.79b
11	ethyl butanoate	105-54-4	1033	804	MS,RI,S	88313.84 ± 3515.71a	86149.85 ± 1085a	89563.85 ± 9.79a	73103.74 ± 854.99c	66632.13 ± 200.54d	78252.24 ± 331.26b
12	ethyl 2-methylbutanoate	7452-79-1	1047	893	MS,RI,S	6262.27 ± 148.73a	2767.41 ± 50.71c	2127.82 ± 24.45d	1433.66 ± 22.2e	3097.42 ± 112.11b	2808.56 ± 190.4c
13	ethyl 3-methylbutanoate	108-64-5	1065	852	MS,RI,S	3546.54 ± 288.08b	4442.97 ± 163.82a	3167.88 ± 60.17c	1138.1 ± 51.9f	2439.18 ± 110.48d	1805.77 ± 96.52e
14	butyl acetate	123-86-4	1069	812	MS,RI,S	10032.3 ± 94.05b	12378.45 ± 239.66a	9288.57 ± 69.22c	6540.45 ± 205.01d	5820.58 ± 33.84e	6212.67 ± 51.76d
15	1,1-diethoxy-3-methylbutane	3842-03-3	1078	954	MS,RI,S	21772.44 ± 3205.41b	38254.92 ± 1293.42a	19356.56 ± 184.47b	11141.42 ± 514.11d	8252.01 ± 211.82d	15200.41 ± 55.05c
16	2-methyl-1-propanol	78-83-1	1084	629	MS,RI,S	3407.13 ± 199.14b	4607.09 ± 57.28a	2850.82 ± 86.14c	2158.65 ± 175.22d	1690.54 ± 59.28e	2732.66 ± 110.57c
17	2,4,6-trimethyl-1,3,5-trioxane	123-63-7	1092		MS,RI,S	3495.88 ± 209.77a	3155.86 ± 18.96b	3216.82 ± 156.79a	641.89 ± 49.37d	596.29 ± 18.9d	1773.24 ± 88.74c
18	1-ethoxy-1-pentoxyethane	13442-89-2	1093		MS,RI,S						1335.56 ± 80.71
19	2-pentanol	6032-29-7	1115	706	MS,RI,S	1480.16 ± 14.55b	1691.98 ± 57.68a	567.22 ± 21.83d	574.91 ± 2.84d	441.76 ± 8.94e	961.11 ± 29.76c
20	3-methylbutyl acetate	123-92-2	1122	883	MS,RI,S	15284.37 ± 737.5b	20560.07 ± 506.39a	14009.01 ± 603.09b	9931.13 ± 293.1c	9189.74 ± 329.67c	10444.29 ± 551.9c
21	ethyl pentanoate	539-82-2	1136	902	MS,RI,S	58925.99 ± 597.44b	61777.12 ± 683.97a	60925.55 ± 205.65a	40076.75 ± 277.93c	38928.35 ± 78.68d	36689.75 ± 170.29e
22	1-butanol	71-36-3	1140	662	MS,RI,S	8655.52 ± 241a	7539.27 ± 396.84b	8859.05 ± 201.09a	8634.77 ± 504.27a	6392.59 ± 212.13c	8100.78 ± 33.01a
23	2-heptanone	110-43-0	1182	889	MS,RI,S	1868.61 ± 46.55b	2112.56 ± 42.82a	1720.37 ± 20.23c	1183.9 ± 17.23d	1034.42 ± 27.1e	1180.86 ± 95.31d
24	methyl hexanoate	106-70-7	1186	925	MS,RI,S	1191.77 ± 7.18b	1337.22 ± 13.01a	1033.86 ± 56.25c	552.29 ± 1.29e	818.19 ± 6.56d
25	ethyl 4-methylpentanoate	25415-67-2	1190	969	MS,RI,S	2421.93 ± 100.36b	3473.52 ± 17.22a	1541.82 ± 10.85c	809.01 ± 24.93e	1347.7 ± 5.11d	805.79 ± 60.74e
26	3-methyl-1-butanol	123-51-3	1204	760	MS,RI,S	31309.89 ± 1099.92b	29666.76 ± 1107.1c	32094.51 ± 1064.26a	33547.37 ± 527.92a	24901.6 ± 92.46d	33923.58 ± 921.77a
27	butyl butanoate	109-21-7	1214	996	MS,RI,S	3398.82 ± 287.5a	1236.87 ± 45.12d	2780.25 ± 49.28b	1526.79 ± 11.84d	1511.34 ± 68.1d	1918.48 ± 94.37c
28	4-octanone	589-63-9	1220	974	MS,RI,S	4125.29 ± 4.51b	2432.82 ± 83.69f	3487.98 ± 10.55d	3278.43 ± 10.26e	4722.72 ± 162.95a	3800.8 ± 23.32c
29	ethyl hexanoate	123-66-0	1231	998	MS,RI,S	338792.37 ± 2245.79a	331632.51 ± 3109.76b	323531.58 ± 1282.58c	190219.78 ± 1003.34d	184647.69 ± 852.59d	147255.38 ± 4537.04e
30	1-pentanol	71-41-0	1244	764	MS,RI,S	2169.24 ± 14.99c	2275.52 ± 101.32b	2475.77 ± 112.25a	1934.43 ± 15.89d	2616.2 ± 118.24a	1810.1 ± 55.84d
31	1-ethoxy-1-hexoxyethane	54484-73-0	1258		MS,RI,S						143.86 ± 9.69
32	3-methylbutyl butanoate	106-27-4	1260	1054	MS,RI,S	1303.8 ± 69.88a	1215.6 ± 40.14b	813.9 ± 13.65c	288.95 ± 6.04d	371.35 ± 4.21d	210 ± 5.01e
33	hexyl acetate	142-92-7	1268	1025	MS,RI,S	4074.72 ± 101.71a	4516.47 ± 228.44a	3801.6 ± 109.56a	1477.86 ± 73.3b	4636.27 ± 981.93a	768.81 ± 1.48b
34	2-octanone	111-13-7	1280	990	MS,RI,S	348.54 ± 23.72a	362.56 ± 4.37a	162.01 ± 1.14c	212.76 ± 3.15b	174.01 ± 1.14c	125.91 ± 7.56d
35	ethyl 4-hexenoic	1000302-90-0	1286		MS,RI,S						140.4 ± 2.36
36	ethyl 3-hexenoate	64187-83-3	1289	993	MS,RI,S	388.51 ± 6.11a	312.24 ± 13.22b	212.63 ± 7.3d	136.45 ± 4.2e	257.94 ± 4.7c
37	2-ethoxymethylfuran	6270-56-0	1297	876.1	MS,RI,S						10160.19 ± 230.95
38	2-heptanol	543-49-7	1297	905	MS,RI,S						525.29 ± 3.04
39	1,1,3-triethoxypropane	7789-92-6	1303	1075	MS,RI,S	1846.58 ± 138.12a	804.62 ± 19.04c		999.06 ± 12.44b	707.71 ± 15.36c	434.2 ± 15.56d
40	butyl pentanoate	591-68-4	1312	1098	MS,RI,S	414.57 ± 9.87a	349.7 ± 10.3b	285.5 ± 2.61c		89.23 ± 3.42d
41	propyl hexanoate	626-77-7	1315	1091	MS,RI,S	2264.5 ± 75.36b	2542.22 ± 3a	2336.53 ± 3.1b	897.83 ± 4.3c	804.99 ± 11.29d
42	propyl 2-ethylbutanoate	5129-46-4	1320	1005	MS,RI,S	971.11 ± 36.7a			812.76 ± 9.14b
43	pentyl hexanoate	540-07-8	1320	1280	MS,RI,S	181.8 ± 1.09d	231.99 ± 1.13c	180.24 ± 5.27d	692.59 ± 0.27b	793.43 ± 7.03a
44	2-methyl-3-heptanol	18720-62-2	1330	970	MS,RI,S				116.18 ± 2.85c	138 ± 0.09b	207.09 ± 5.52a
45	ethyl heptanoate	106-30-9	1332	1098	MS,RI,S	20136.78 ± 431.84b	22897.89 ± 271.14a	19356.09 ± 238.24c	5208.66 ± 23.22d	5685.44 ± 134.57d	2377.51 ± 0.19e
46	6-methylhept-5-en-2-one	110-93-0	1335	986.4	MS,RI,S	235.11 ± 3.68b	239.05 ± 2.56b	286.43 ± 6.85a	161.9 ± 5.55c	130.13 ± 3.11d
47	ethyl 2-hydroxypropanoate	97-64-3	1342	813	MS,RI,S	38953.47 ± 4560.65b	37952.53 ± 1672.53b	48734.71 ± 3610.92a	40177.58 ± 1993.01b	23016.2 ± 752.97c	35775.47 ± 1240.71b
48	1-hexanol	111-27-3	1351	850	MS,RI,S	13909.11 ± 867.14b	15282.57 ± 775.52b	18294.04 ± 749.73a	15105.21 ± 280.98b	11721.3 ± 483.56c	14798.32 ± 1086.99b
49	2-methyl-1-propyl hexanoate	105-79-3	1353	1144	MS,RI,S	673.08 ± 42.28a	689.62 ± 14.89a
50	3-methylbutyl pentanoate	2050-09-1	1362	1108	MS,RI,S	178.12 ± 12.96b	174.91 ± 8.29b	195.75 ± 5a
51	dimethyl trisulfide	3658-80-8	1375	977.2	MS,RI,S	298.28 ± 16.52a	286.64 ± 8.29a	265.15 ± 0.37c	0.27 ± 0.13d
52	2-methyl-4-heptanol	21570-35-4	1390		MS,RI,S	223.45 ± 7.73c	232.65 ± 2.72b	262.28 ± 2.67a
53	nonyl aldehyde	124-19-6	1393	1081	MS,RI,S	1073.33 ± 32.2b	1557.73 ± 29.47a	1006.93 ± 10.66c
54	butyl hexanoate	626-82-4	1411	1190	MS,RI,S	2214.84 ± 81.54b	2333.66 ± 16.64a	1790.74 ± 17.37c	368.06 ± 14.48d	423.77 ± 12.95d	151.18 ± 0.7e
55	hexyl butanoate	2639-63-6	1414	1191	MS,RI,S	415.6 ± 14.63a	371.03 ± 0.31b	280.57 ± 3.81c	78.54 ± 3.8e	97.59 ± 0.71d
56	5-methyl-2-heptanol	54630-50-1	1417		MS,RI,S				82.44 ± 3.25	114.14 ± 10.18
57	3-methyl-2-butanol	598-75-4	1420	856	MS,RI,S						130.3 ± 7.47
58	2-octenal	2548-87-0	1429	1058	MS,RI,S	153.45 ± 7.28c	180.42 ± 4.97b	195.97 ± 3.44a	95.53 ± 4.08d	102.83 ± 4.21d	71.37 ± 3.65e
59	1,1-diethoxyoctane	54889-48-4	1432	1270	MS,RI,S	50.34 ± 0.8c	87.83 ± 2.36a	66.71 ± 1.73b
60	ethyl octanoate	106-32-1	1437	1193.8	MS,RI,S	16687.12 ± 598.03a	14780.48 ± 154.42b	10644.26 ± 151.9c	2205.72 ± 19.13d	2514.91 ± 43.52d	710.92 ± 13.01e
61	1-octen-3-ol	3391-86-4	1451	979.2	MS,RI,S	200.38 ± 14.01a	204.89 ± 96.59a	260.11 ± 9.27a			140.32 ± 3.91b
62	acetic acid	64-19-7	1453	602	MS,RI,S	96543.85 ± 11590.85b	79519.19 ± 3479.77c	74667.28 ± 2946.96c	148509.9 ± 2794.03b	330368.98 ± 61926.87a	42400.43 ± 2806.17c
63	3-methylbutyl hexanoate	2198-61-0	1460	1250.9	MS,RI,S	1446.18 ± 59.63a	1445.89 ± 18.12a	1230.38 ± 78.51b
64	furan-2-carbaldehyde	98-01-1	1469	835	MS,RI,S	5443.73 ± 509.32b	6054.58 ± 335.22b	8150.87 ± 666.24a	4842.18 ± 290.48c	4381.53 ± 190.64d	5532.58 ± 432b
65	2-ethyl hexanol	104-76-7	1486	1029	MS,RI,S	87.24 ± 33.17b	156.65 ± 38.06a	149.21 ± 4.4a	77.74 ± 0.08b	92.17 ± 0.25b	63.89 ± 2.41b
66	decanal	112-31-2	1495	1209.1	MS,RI,S	210.15 ± 1.98b	291.51 ± 7.02a	192.28 ± 3.19c
67	2,3-dimethylcyclohexanol	1502-24-5	1506		MS,RI,S	46.75 ± 1.57c	59.02 ± 0.72b	64.43 ± 3.72a	61.52 ± 0.39a
68	1-methyl-1-octanol	628-99-9	1515	1098	MS,RI,S	61.07 ± 1.74b	69.96 ± 2.15a	64.44 ± 2.36b	48.13 ± 0.71c
69	butyl 2-hydroxypropanoate	138-22-7	1518		MS,RI,S	195.05 ± 5.79b	208.46 ± 4.64b	257.93 ± 18.85a	187.83 ± 0.83b	77.31 ± 0.68c	133.06 ± 3.53c
70	Benzaldehyde	100-52-7	1524	921	MS,RI,S	538.3 ± 2.51c	678.97 ± 24.32b	799.59 ± 63.15a	514.64 ± 22.71c	514.37 ± 21.73c	663.41 ± 38.24b
71	1,1-diethoxynonane	54815-13-3	1528	1381	MS,RI,S	115.62 ± 4.12c	232.86 ± 20.14a	204.28 ± 5.85b
72	ethyl nonanoate	123-29-5	1535	1295	MS,RI,S	366.05 ± 14.77a	330.36 ± 5.12b	273.21 ± 8.24c
73	furan-2-ylmethyl acetate	623-17-6	1540	991	MS,RI,S	597.78 ± 43.07c	712.73 ± 28.34b	860.4 ± 63.77a	574.29 ± 7.86c	442.4 ± 16.75d	464.25 ± 34.73d
74	2,6-dimethyl-4-heptanol	108-82-7	1541		MS,RI,S	3101.82 ± 130.75a	3115.53 ± 93.7a	3193.36 ± 89.83a	3181.52 ± 43.97a	1182.79 ± 95.48b
75	1-cyclopropylpentane	2511-91-3	1550	812	MS,RI,S						789.2 ± 188.27
76	1-octanol	111-87-5	1555	1071	MS,RI,S	847.83 ± 0.63b	939.75 ± 10.71a	978.52 ± 39.13a	752.52 ± 9.36c	599.01 ± 11.94d
77	3-methylbutyl 2-hydroxypropanoate	19329-89-6	1567	1047	MS,RI,S	580.76 ± 19.65c	913.2 ± 36.59a	787.27 ± 69.34b	749.38 ± 0.13b	233.45 ± 4d	750.85 ± 39.46b
78	2-undecanone	112-12-9	1595	1292	MS,RI,S	187.72 ± 1.24a
79	hexyl hexanoate	6378-65-0	1607	1385	MS,RI,S	222.3 ± 10.38b	363.19 ± 8.63a	230.14 ± 5.02b	78.91 ± 3.58c	343.36 ± 19.01a
80	butanoic acid	107-92-6	1625	789	MS,RI,S	10589.02 ± 73.25b	11414.31 ± 517.09b	10969.36 ± 68.02b	11593.51 ± 548.51b	20108.4 ± 806.62a	11471.6 ± 643.55b
81	ethyl decanoate	110-38-3	1637	1392.2	MS,RI,S	275.14 ± 11.36a	283.13 ± 6.65a	171.08 ± 4.12b
82	2-phenylacetaldehyde	122-78-1	1648	1045.6	MS,RI,S	557.87 ± 24.26b	840.85 ± 14.24a	877.17 ± 80.92a	613.04 ± 4.88b	449.85 ± 16.84c	546.04 ± 27.2b
83	1-nonanol	143-08-8	1656	1171.6	MS,RI,S	306.37 ± 12.2c	402.67 ± 2.92a	337.57 ± 17.56b			190.77 ± 10.9d
84	furan-2-ylmethanol	98-00-0	1662	875	MS,RI,S	249.5 ± 15.38c	267.2 ± 10.16c	317.29 ± 31.11b	314.74 ± 8.83b	437.56 ± 1.97a	214.82 ± 13.9d
85	3-methylbutanoic acid	503-74-2	1666	839	MS,RI,S		4444.69 ± 122.73	3627.12 ± 197.14	3501.01 ± 144.37	4476.96 ± 189.88	4505.88 ± 292.28
86	ethyl benzoate	93-89-0	1670	1172	MS,RI,S	268.87 ± 12.2d	298.01 ± 7.05c	335.94 ± 10.53b	212.59 ± 10.06e	226.91 ± 8.68e	369.41 ± 9.67a
87	diethyl butanedioate	123-25-1	1677	1191	MS,RI,S	373.81 ± 5.48c	451.51 ± 0.59a	509.92 ± 60.94a	439.49 ± 5.9b	219.9 ± 3.52d	492.55 ± 4.75a
88	2,5-dimethylbenzaldehyde	5779-94-2	1705	1208	MS,RI,S		311.6 ± 13.83		773.89 ± 14
89	2,2-diethoxyethylbenzene	6314-97-2	1719	1328	MS,RI,S	3010.15 ± 176.46a	1339.53 ± 16.19c	819.73 ± 61.57d	1694.68 ± 53.71b	988.45 ± 24.88d	571.52 ± 59.93e
90	pentanoic acid	109-52-4	1735	933.3	MS,RI,S	2999.19 ± 169.1c	3174.09 ± 16.8b	2833.09 ± 43.05d	3175.61 ± 31.15b	5093.4 ± 199.23a	3353.33 ± 50.85b
91	methyl benzofuranone	32267-71-3	1777		MS,RI,S	213.16 ± 14.8a
92	3,4-dimethylbenzaldehyde	5973-71-7	1790	1165	MS,RI,S						254.17 ± 9.61
93	ethyl 2-phenylacetate	101-97-3	1791	1248.3	MS,RI,S	1191.46 ± 15.17a	1187.65 ± 39.47a	1300.38 ± 84.69a	898.06 ± 30.59b	722.29 ± 28.94c	750.32 ± 55.37c
94	2-phenylethyl acetate	103-45-7	1822	1255	MS,RI,S	521.64 ± 22.82b	651.9 ± 25.02a	678.2 ± 54.07a	400.94 ± 14.64c	253.85 ± 9.6d
95	hexanoic acid	142-62-1	1842	978	MS,RI,S	47337.16 ± 128.59d	55532.2 ± 841.15b	51065.7 ± 541.51c	60794.08 ± 686.43a	55113.01 ± 1075.09b	56767.35 ± 2069.94b
96	ethyl 3-phenylpropionate	2021-28-5	1892	1370	MS,RI,S	820.37 ± 36.51a	593.3 ± 13.25c	642.7 ± 6.24b	503.09 ± 17.5d	577 ± 7.73c	275.87 ± 11.66e
97	2-phenylethanol	60-12-8	1918	1113	MS,RI,S	400.95 ± 0.77a	395.21 ± 18.51a	343.97 ± 13.46b	327.82 ± 4.34b	314.25 ± 4.56c	292.11 ± 13.19d
98	heptanoic acid	111-14-8	1949	1071	MS,RI,S	3019.88 ± 417.74a	2909.71 ± 138.94a	1989.15 ± 0.93c	2511.19 ± 33.29b		2640.15 ± 42.08a
99	2-methyl-1-hexadecanol	2490-48-4	1964		MS,RI,S	140.15 ± 3.51b	119.63 ± 5.27c		256.1 ± 6.08a	145.07 ± 7.47b
100	phenol	108-95-2	2008	992	MS,RI,S						247.88 ± 17.29
101	1,1’−biphenyl	92-52-4	2009	1368.5	MS,RI,S	183.62 ± 2.21a		169.12 ± 3.86a			88.02 ± 19.64b
102	propan-2-yl tetradecanoate 1	110-27-0	2038	1824.1	MS,RI,S	133.05 ± 8.46c	106.88 ± 4.3d		331.91 ± 4.93a	283.37 ± 7.97b
103	ethyl tetradecanoate	124-06-1	2050	1799.4	MS,RI,S	308.29 ± 1.36c	671.07 ± 23.29a	505.13 ± 22.88b	747 ± 84.15a	208.71 ± 2.5d	53.01 ± 4.69e
104	octanoic acid	124-07-2	2057		MS,RI,S	4986.95 ± 58.55a	5163.45 ± 176.92a	4064.13 ± 268.8b	5498.34 ± 437.53a		2393.46 ± 654.16c
105	ethyl pentadecanoate	41114-00-5	2153	1897	MS,RI,S				64.74 ± 2.63a		60.74 ± 17.99a
106	nonanoic acid	112-05-0	2174	1267	MS,RI,S						1390.61 ± 297.76
107	methyl hexadecanoate	112-39-0	2219	1925.9	MS,RI,S				147.03 ± 20.09a		94.58 ± 44.04b
108	ethyl tetradecanoate	628-97-7	2256	1799.4	MS,RI,S		4357.18 ± 290.68c	5379.52 ± 605.89b	8768.38 ± 107.36a		3213.14 ± 2204.32c
109	ethyl 9-hexadecenoate	54546-22-4	2284	1978.3	MS,RI,S		1104.33 ± 97.35a	1116.71 ± 166.48a
110	2,4-bis(1,1-dimethylethyl)-phenol	96-76-4	2311	1513	MS,RI,S	586.56 ± 72.06c	568.78 ± 68.17c	992.02 ± 45.85b		337.88 ± 1.15d	1358.84 ± 29.91a
111	benzoic acid	65-85-0	2454	1170	MS,RI,S	15877.33 ± 535.16a		7303.43 ± 184.63c			10582.7 ± 1460.29b

a DB-WAX, linear retention index on DB-WAX.

b HP-5 MS, linear retention index on HP-5 MS.

c The concentrations of trace components were represented as the mean value of triplicate samples ± standard deviation (mean ± SD).

Specifically, the total type of trace components decreased during the sun-exposure time. Ninety-two kinds of trace components were identified in the sample BCSGB-1, including 38 esters, 7 acids, 16 alcohols, 11 aromatics, 6 aldehydes 5 ketones, 5 acetals, 3 furans, and 1 sulfur compound. Among them, the types of trace components showed a trend of decreasing first and then increasing. The most trace components were identified in the sample BCSGB-1, and the fewest were found in the sample BCSGB-5. Among them, a phenomenon was found that the types of esters, ketones, and sulfur compounds decreased and the types of furans increased. For other types of trace components, no significant changes were found during the sun-exposure process. This corresponds to the variation in the concentration of esters in light, sauce and feng flavor of baijiu [28, 29]. In detail, four types of esters (3-methylbutyl hexanoate, ethyl nonanoate, ethyl decanoate, and 3-methylbutyl pentanoate) were not found after 12 days of solarization. Two types of alcohols (2,6-dimethyl-4-heptanol, 1-methyl-1-octanol) were hard to catch after 12 days solarization. A sulfur compound (dimethyl trisulfide) and three types of alkanes (1-ethoxy-1-pentoxyethane, 1-ethoxy-1-hexoxyethane, and 1-cyclopropylpentane) emerged gradually during the sun-exposure process. It was worth noting that 52 types of trace components were always present in the sun-exposure process; thus, these trace components were thought to be the main trace components of BCSGB.

Qualitative analysis of trace components in BCSGB with six different sun-exposure times: In order to further clarify the concentration distribution of trace components in the process of sun-exposure, six samples of BCSGB were subjected to the adsorbent pen combined with GC–MS to determine the concentrations of trace components. Roughly, it was consistent with the trend of type changes, the total concentration of trace components showed a general decreasing tendency with the length of sun-exposure time increased, as indicated in Table 1 and Figure 2. Obviously, the sample BCSGB-1 (1051456.38 μg/L) obtained the greatest total concentration of trace components, followed by the samples BCSGB-2 (1047420.27 μg/L), BCSGB-3 (1014404.49 μg/L), BCSGB-4 (851042.76 μg/L), BCSGB-5 (949190.27 μg/L), and BCSGB-6 (696786.31 μg/L). In detail, ethyl hexanoate (147255.38–338792.37 μg/L), ethyl butanoate (78252.24–88313.84 μg/L), ethyl acetate (52090.53–96633.77 μg/L), ethyl pentanoate (36689.75–61777.12 μg/L), hexanoic acid (47337.16–60794.08 μg/kg), and ethyl 2-hydroxypropanoate (23016.2–40177.58 μg/L) had relatively high concentrations in six BCSGB samples. Additionally, butyl 2-hydroxypropanoate (77.31–257.93 μg/L), 2-octenal (71.37–195.97 μg/L), and 2-ethyl hexanol (63.89–156.65 μg/L) presented low concentrations in such samples. In particular, esters have the highest concentration in BCSGB, followed by acids and alcohols. To further clarify the variation tendency in the concentrations for trace components during the sun-exposure process, the 111 trace components were standardized and exhibited in Figure 2a.

Figure 2. (a), The post-standardized concentration of various trace components in six BCSGB samples. (b) ~ (d), standardized results of esters in six BCSGB samples. (e), standardized results of alcohols in six BCSGB samples. (f), standardized results of aldehydes in six BCSGB samples. (g), standardized results of acids in six BCSGB samples. (h), standardized results of aromatics in six BCSGB samples. (i), standardized results of ketone in six BCSGB samples. (j), standardized results of acetals in six BCSGB samples. (k), standardized results of furans in six BCSGB samples. (l), standardized results of sulfur compounds in six BCSGB samples.

As a result, esters were the major trace components identified, with ethyl hexanoate, ethyl butanoate, ethyl acetate, and ethyl 2-hydroxypropanoate comprising more than 53% of the total esters collected from the BCSGB samples. Among them, ethyl hexanoate showed a significant downward trend, while others had no significant change (as shown in Figure 2(b-d)). As reported, these trace components were predominant in many types of baijiu. Of note, their ratio of esters to corresponding acids could significantly affect the flavor of baijiu, which was found in literature reports and actual production. In detail, the ratio of ethyl acetate to acetic acid increased during the sun-exposure process, while the ratio of ethyl hexanoate to hexanoic acid decreased. A decreasing tendency for esters and an increasing tendency for acids were obviously observed, which was speculated to have an important effect on the flavor of baijiu. For acetic acid, its concentration was significantly higher than other kinds of acids, comprising at least 40% of the total acids, while the concentration of 3-methylbutanoic acid was observed to significantly change during the sun-exposure process (as shown in Figure 3g). For alcohols, 3-methyl-1-butanol and 1-hexanol showed a higher concentration than others and exhibited a fluctuation decline during the sun-exposure process. Of note, only butyl pentanoate, 2-methyl-4-heptanol, and 2,3-dimethylcyclohexanol were found to be significantly different between these BCSGB samples. As a result, 12 kinds of aromatics were identified and a remarkable change was found in the concentration of ethyl benzoate, ethyl 3-phenylpropionate, and 2-phenylethanol (as shown in Figure 3h). For sulfur compounds, only dimethyl trisulfide had a gradual reduction during the sun-exposure time (as shown in Figure 3l). For other types of trace components, no significant variance was obtained during the sun-exposure process (as shown in Figure 3e-k).

Figure 3. (a) and (d), OAVs and TAVs are shown, with a color gradient denoting Spearman’s correlation coefficient. Aroma compounds and taste compounds was related to sensory attributes by partial (geographic distance-corrected) Mantel tests. Edge width corresponds to the Mantel’s r statistic for the corresponding distance correlations, and edge color denotes the statistical significance based on 9,999 permutations. (b) and (c), Aroma and taste descriptive profiles of BCSGB samples. The scores of selected descriptors were evaluated by 18 panelists on average. Significance was indicated at * p < .05 (significance), ** p<.01 (very significant), *** p < .001 (very highly significant).

Based on this, the change in type and concentration was deemed to be the result that affected the flavor of baijiu. However, the contribution of trace components has yet to be discovered. Hence, it was necessary to screen its flavor substances (aroma compounds and taste compounds) and evaluate their contribution to the flavor of baijiu.

Screening of flavor compounds contributing to BCSGB by pH liquid–liquid extraction/GC–O–MS

The study showed that not all the trace components had a direct contribution to the flavor of baijiu, and it was important to screen and evaluate the flavor compounds that made an important contribution to the flavor of Baijiu. Thus, pH liquid–liquid extraction/GC–O–MS was used to screen the dynamic change for aroma compounds during the sun-exposure process, and time intensity method was used to evaluate the aroma expression of the aroma compounds. As exhibited in Table 6, 35 kinds of flavor compounds were screened, including esters (15 types), acids (5 types), alcohols (4 types), aromatics (6 types), aldehyde (1 types), acetals (1 type), furans (2 types), fruity, floral, honey, and cheese-aroma were found to be the main aroma characteristic for flavor compounds. Thus, these aroma characteristics were considered as the basis of sensory evaluation in subsequent experiments. Of note, the aroma expression for hexanoic acid, butanoic acid, 2-phenylacetaldehyde, and ethyl hexanoate were higher than other aroma compounds.

Table 6. Osme for 34 flavor compounds in six BCSGB samples.

No^.a	flavor compounds	Descriptor^b	Osme						Basis of ID
3	ethyl acetate	fruity, apple	2	1	1	2	2		MS, RI, odor
11	ethyl butanoate	ripe apple	3	3	3	1	2	2	MS, RI, odor
12	ethyl 2-methylbutanoate	Fruity, sweet,			2	1	1	1	MS, RI, odor
13	ethyl 3-methylbutanoate	Apple, pear						1	MS, RI, odor
14	butyl acetate	banana	2				2	2	MS, RI, odor
16	2-methyl-1-propanol		1	2			2	2	MS, RI, odor
20	3-methylbutyl acetate	banana	3	1	3	1	2	3	MS, RI, odor
21	ethyl pentanoate	apple	3	2	3	2	2	1	MS, RI, odor
22	1-butanol	fermentation-like						2	MS, RI, odor
26	3-methyl-1-butanol	ripe apple	2	2	2	1	2	3	MS, RI, odor
29	ethyl hexanoate	Fruity, sweet	3	3	2	2	3	3	MS, RI, odor
33	hexyl acetate	fruity	2	3	4	1	1		MS, RI, odor
38	2-heptanol	vegetal	1						MS, RI, odor
45	ethyl heptanoate	grape	3	3	3		1		MS, RI, odor
47	ethyl 2-hydroxypropanoate	sweet	1	1	1	1	1	2	MS, RI, odor
48	1-hexanol	vegetal		3	2		2	2	MS, RI, odor
50	3-methylbutyl pentanoate							3	MS, RI, odor
54	butyl hexanoate	fruity	3	1	2	2	2	1	MS, RI, odor
60	ethyl octanoate	ripe apple	3	3	2			2	MS, RI, odor
62	acetic acid	sour	3	2	2	1	1	1	MS, RI, odor
63	3-methylbutyl hexanoate							3	MS, RI, odor
64	furan-2-carbaldehyde	roasted	2	3	1	1	3	1	MS, RI, odor
66	decanal							1	MS, RI, odor
70	Benzaldehyde			2	1	1	2		MS, RI, odor
73	furan-2-ylmethyl acetate	spicy	1	3	1	2	1	2	MS, RI, odor
80	butanoic acid	cheesy	4	2	2	2	3	4	MS, RI, odor
82	2-phenylacetaldehyde	floral	3	3	3	1	3	4	MS, RI, odor
86	ethyl benzoate	fermentation-like						1	MS, RI, odor
89	2,2-diethoxyethylbenzene	cheesy	2		2	2	2	2	MS, RI, odor
91	methyl benzofuranone	honey	1	1				1	MS, RI, odor
95	hexanoic acid	cheesy	4	1	3	3	4	4	MS, RI, odor
96	ethyl 3-phenylpropionate	honey						3	MS, RI, odor
97	2-phenylethanol	floral	2	3	2	1	2	2	MS, RI, odor
98	heptanoic acid	cheesy					2		MS, RI, odor

^aThe numbers were the same as the numbers listed in Table 5.

Esters, the most significant type of aroma compounds in strong flavor baijiu, were what gave baijiu its intense fruity fragrance.^[28] Based on this, higher aroma expression intensity for ethyl butanoate, 3-methylbutyl acetate, ethyl pentanoate, ethyl hexanoate, ethyl 2-hydroxypropanoate, and butyl hexanoate was observed by GC–O during the sun-exposure process. Ethyl hexanoate was a distinctive aroma compound of strong flavor baijiu with fruity aroma. Of note, due to hydrolysis and volatilization during the sun-exposure process, ethyl acetate, hexyl acetate, and ethyl heptanoate were hard to catch after 30 days of solarization. Acids appeared to be the most important contributors to the desirable aroma of vinegar and cheese-aroma. Moreover, alcoholics and furans contributed to alcoholic and malty, floral and honey, sweet and roasted aroma, respectively. Notably, the aroma expression of acetic acid, benzaldehyde, furan-2-carbaldehyde, 3-methyl-1-butanol, and furan-2-ylmethyl acetate changed greatly during the sun-exposure process. Thus, it was hypothesized that the concentration and aroma expression of aroma compounds changed during the sun-exposure process, resulting in different sensory characteristics of baijiu.

OAVs and TAVs of flavor compounds.

As described above, the contributions of aroma compounds in baijiu were not just determined by their concentration, the matrix effect should also be considered. Hence, in order to reveal the contributions of the flavor compounds in BCSGB, the odor and taste thresholds of flavor compounds from the literature were used to estimate the OAVs and TAVs. The results (Table 7) showed that there were 35 (BCSGB-1), 34 (BCSGB-2), 35 (BCSGB-3), 26 (BCSGB-4), 26 (BCSGB-5), and 25 (BCSGB-6) aroma compounds with OAVs ≥ 1, respectively. Among them, ethyl hexanoate had the highest average OAVs (4566), followed by ethyl pentanoate (1850), ethyl octanoate (615), ethyl butanoate (985), dimethyl trisulfide (786), ethyl 3-methylbutanoate (399), 3-methylbutanal (372), 3-methylbutyl acetate (140), and ethyl 4-methylpentanoate (63). The result showed that these aroma compounds were thought to have substantial contributions to the aroma profiles of BCSGB. However, a significant difference for OAVs was found in 13 aroma compounds during the sun-exposure time (p < .05). Notably, a decreased trend was found in the OAVs of ethyl hexanoate, ethyl 3-phenylpropionate, dimethyl trisulfide and an increased trend was found in hexanoic acid during the sun-exposure process, which was responsible for the differences in the flavor characteristic of six BCSGB samples.

Table 7. OAVs of 35 flavor compounds in six BCSGB samples.

No.	Compounds	Order threshold (μg/L)	OAVs
			BCSGB-1	BCSGB-2	BCSGB-3	BCSGB-4	BCSGB-5	BCSGB-6
29	ethyl hexanoate	55.33	6123.12 ± 40.59a	5993.72 ± 56.2b	5847.31 ± 23.18c	3437.91 ± 18.13d	3337.21 ± 15.41d	2661.4 ± 82e
21	ethyl pentanoate	26.78	2200.37 ± 22b.31	2306.84 ± 25.5a4	2275.04 ± 7.68a	1496.52 ± 10.38c	1453.64 ± 2.94d	1370.04 ± 6.36e
60	ethyl octanoate	12.87	1296.59 ± 46.47a	1148.44 ± 12b	827.06 ± 11.8c	171.38 ± 1.49d	195.41 ± 3.38d	55.24 ± 1.01e
11	ethyl butanoate	81.5	1083.61 ± 43.14a	1057.05 ± 13.31a	1098.94 ± 0.12a	896.98 ± 10.49c	817.57 ± 2.46d	960.15 ± 4.06b
51	dimethyl trisulfide	0.36	828.56 ± 45.89a	796.23 ± 23.03a	736.52 ± 1.04b	0.75 ± 0.35c	0 ± 0c	0 ± 0c
13	ethyl 3-methylbutanoate	6.89	514.74 ± 41.81b	644.84 ± 23.78a	459.78 ± 8.73c	165.18 ± 7.53f	354.02 ± 16.04d	262.09 ± 14.01e
4	3-methylbutanal	16.51	458.39 ± 42.65a	361.93 ± 17.4b	287.24 ± 96.34b	406.69 ± 28.89b	321.51 ± 9.16b	401.36 ± 10.57b
20	3-methylbutyl acetate	93.93	162.72 ± 7.8b5	218.89 ± 5.39a	149.14 ± 6.4b2	105.73c ± 3.12	97.84 ± 3.51c	111.19 ± 5.88c
25	ethyl 4-methylpentanoate	27	89.7 ± 3.72b	128.65 ± 0.64a	57.1 ± 0.4c	29.96 ± 0.92e	49.91 ± 0.19d	29.84 ± 2.25e
61	1-octen-3-ol	6	33.4 ± 2.34b	34.15 ± 16.1b	43.35 ± 1.55a	0 ± 0c	0 ± 0c	23.39 ± 0.65b
27	butyl butanoate	110	30.9 ± 2.61a	11.24 ± 0.41d	25.27 ± 0.45b	13.88 ± 0.11d	13.74 ± 0.62d	17.44 ± 0.86c
95	hexanoic acid	2517	18.81 ± 0.05d	22.06 ± 0.33b	20.29 ± 0.22c	24.15 ± 0.27a	21.9 ± 0.43b	22.55 ± 0.82b
2	aldehyde acetal	2090	17.68 ± 0.6b	19.61 ± 0.67a	20.31 ± 1.31a	15.62 ± 0.54c	9.68 ± 0.23d	0 ± 0e
24	ethyl hexanoate	87	13.7 ± 0.08b	15.37 ± 0.15a	11.88 ± 0.65c	6.35 ± 0.01e	9.4 ± 0.08d	0 ± 0f
23	2-heptanone	140	13.35 ± 0.33b	15.09 ± 0.31a	12.29 ± 0.14c	8.46 ± 0.12d	7.39 ± 0.19e	8.43 ± 0.68d
80	butanoic acid	964.64	10.98 ± 0.08b	11.83 ± 0.54b	11.37 ± 0.07b	12.02 ± 0.57b	20.85 ± 0.84a	11.89 ± 0.67b
53	nonyl aldehyde	122.45	8.77 ± 0.26b	12.72 ± 0.24a	8.22 ± 0.09c	0 ± 0d	0 ± 0d	0 ± 0d
89	2,2-diethoxyethylbenzene	389.11	7.71 ± 0.43c	8.16 ± 0.04b	7.28 ± 0.11d	8.16 ± 0.08b	13.09 ± 0.51a	8.62 ± 0.13b
34	2-octanone	50	6.97 ± 0.47a	7.25 ± 0.09a	3.24 ± 0.02c	4.26 ± 0.06b	3.48 ± 0.02c	2.52 ± 0.15d
96	Ethyl 3-phenylpropionate	125	6.56 ± 0.29a	4.75 ± 0.11c	5.14 ± 0.05b	4.02 ± 0.14d	4.62 ± 0.06c	2.21 ± 0.09e
9	2-methylpropyl acetate	922	6.13 ± 0.14b	6.72 ± 0.12a	4.79 ± 0.23c	1.47 ± 0.06e	3.08 ± 0.15d	3.08 ± 0.06d
83	1-nonanol	50	6.13 ± 0.24c	8.05 ± 0.06a	6.75 ± 0.35b	0 ± 0e	0 ± 0e	3.82 ± 0.22d
14	butyl acetate	1830	5.48 ± 0.05b	6.76 ± 0.13a	5.08 ± 0.04c	3.57 ± 0.11d	3.18 ± 0.02e	3.39 ± 0.03d
22	1-butanol	2733.35	3.17 ± 0.09a	2.76 ± 0.15b	3.24 ± 0.07a	3.16 ± 0.18a	2.34 ± 0.08c	2.96 ± 0.01a
91	methyl benzofuranone	407	2.93 ± 0.04a	2.92 ± 0.1a	3.2 ± 0.21a	2.21 ± 0.08b	1.77 ± 0.07c	1.84 ± 0.14c
33	hexyl acetate	1500	2.72 ± 0.07a	3.01 ± 0.15a	2.53 ± 0.07a	0.99 ± 0.05b	3.09 ± 0.65a	0.51 ± 0b
48	1-hexanol	5370	2.59 ± 0.16b	2.85 ± 0.14b	3.41 ± 0.14a	2.81 ± 0.05b	2.18 ± 0.09c	2.76 ± 0.2b
82	2-phenylacetaldehyde	262	2.13 ± 0.09b	3.21 ± 0.05a	3.35 ± 0.31a	2.34 ± 0.02b	1.72 ± 0.06c	2.08 ± 0.1b
3	ethyl acetate	32551.6	1.95 ± 0.34b	1.6 ± 0.01c	1.8 ± 0.09c	2.25 ± 0.06b	1.94 ± 0b	2.97 ± 0.14a
104	octanoic acid	2701	1.85 ± 0.02a	1.91 ± 0.07a	1.5 ± 0.1b	2.04 ± 0.16a	0 ± 0d	0.89 ± 0.24c
55	hexyl butanoate	250	1.66 ± 0.06a	1.48 ± 0b	1.12 ± 0.02c	0.31 ± 0.02e	0.39 ± 0d	0 ± 0f
45	ethyl heptanoate	13153.17	1.53 ± 0.03b	1.74 ± 0.02a	1.47 ± 0.02c	0.4 ± 0d	0.43 ± 0.01d	0.18 ± 0e
10	2-butanol	2730	1.31 ± 0.01a	0 ± 0d	1.23 ± 0.11a	0.3 ± 0c	0 ± 0d	0.52 ± 0b
92	3,4-dimethylbenzaldehyde	406.83	1.28 ± 0.06b	1.6 ± 0.06a	1.67 ± 0.13a	0.99 ± 0.04c	0.62 ± 0.02d	0 ± 0e

a The numbers were the same as the numbers listed in Table 5.

b OAVs were calculated by dividing the concentration by the respective odor threshold.

c Odor thresholds were taken from ref^[29] and.^[30]

Considering the importance of baijiu’s taste quality and the fact that individuals predominantly choose baijiu based on their taste preferences, it is imperative to further investigate the contribution of flavor compounds to the taste of baijiu.^[25]aa Hence, the TAV value of flavor compounds was calculated as the ratio of the concentration determined in the BCSGB samples to its threshold value. As exhibited in Table 8, the TAVs for ethyl butanoate, 3-methylbutanal, and 3-methylbutyl acetate were higher than other taste compounds. Thus, these taste compounds were considered to have an important contribution in subsequent experiments. Based on this, a decreased trend was found in the TAVs of 2-phenylethanol, ethyl 3-phenylpropionate, and 3,4-dimethyl benzaldehyde and a fluctuate was found in hexyl hexanoate and 1-butanol.

Table 8. TAVs of 35 flavor compounds in six BCSGB samples.

No.	Compounds	Taste threshold	TAV
			BCSGB-1	BCSGB-2	BCSGB-3	BCSGB-4	BCSGB-5	BCSGB-6
11	ethyl butanoate	150	588.76 ± 23.44a	574.33 ± 7.23a	597.09 ± 0.07a	487.36 ± 5.7c	444.21 ± 1.34d	521.68 ± 2.21b
4	3-methylbutanal	120	63.07 ± 5.87a	49.8 ± 2.39a	39.52 ± 13.25b	55.95 ± 3.97a	44.23 ± 1.26b	55.22 ± 1.45a
20	3-methylbutyl acetate	850	17.98 ± 0.8b7	24.19 ± 0.6a	16.48 ± 0.71b	11.68 ± 0.34c	10.81 ± 0.39c	12.29 ± 0.65c
2	aldehyde acetal	2760	13.39 ± 0.46b	14.85 ± 0.5a	15.38 ± 0.99a	11.83 ± 0.41c	7.33 ± 0.17d	0 ± 0e
25	ethyl 4-methylpentanoate	320	7.57 ± 0.31b	10.85 ± 0.05a	4.82 ± 0.03c	2.53 ± 0.08e	4.21 ± 0.02d	2.52 ± 0.19e
85	3-methylbutanoic acid	970	0 ± 0c	4.58 ± 0.13a	3.74 ± 0.2b	3.61 ± 0.15b	4.62 ± 0.2a	4.65 ± 0.3a
13	ethyl 3-methylbutanoate	800	4.43 ± 0.36b	5.55 ± 0.2a	3.96 ± 0.08c	1.42 ± 0.06f	3.05 ± 0.14d	2.26 ± 0.12e
12	ethyl 2-methylbutanoate	1790	3.5 ± 0.08a	1.55 ± 0.03c	1.19 ± 0.01d	0.8 ± 0.01e	1.73 ± 0.06b	1.57 ± 0.11c
3	ethyl acetate	50000	1.27 ± 0.22b	1.04 ± 0.01c	1.17 ± 0.06c	1.46 ± 0.04b	1.27 ± 0b	1.93 ± 0.09a
64	furan-2-carbaldehyde	5610	0.97 ± 0.09b	1.08 ± 0.06b	1.45 ± 0.12a	0.86 ± 0.05c	0.78 ± 0.03b	0.99 ± 0.08b
91	methyl benzofuranone	1440	0.83 ± 0.01a	0.82 ± 0.03a	0.9 ± 0.06a	0.62 ± 0.02b	0.5 ± 0.02c	0.52 ± 0.04c
47	ethyl 2-hydroxypropanoate	70000	0.56 ± 0.07b	0.54 ± 0.02b	0.7 ± 0.05a	0.57 ± 0.03b	0.33 ± 0.01d	0.51 ± 0.02b
48	1-hexanol	29860	0.47 ± 0.03b	0.51 ± 0.03b	0.61 ± 0.03a	0.51 ± 0.01b	0.39 ± 0.02c	0.5 ± 0.04b
5	ethyl propanoate	10000	0.33 ± 0.07b	0.4 ± 0.06b	0.81 ± 0.26a	0.43 ± 0.09b	0.38 ± 0.02b	0.2 ± 0.01b
22	1-butanol	22500	0.38 ± 0.01a	0.34 ± 0.02b	0.39 ± 0.01a	0.38 ± 0.02a	0.28 ± 0.01c	0.36 ± 0a
97	2-phenylethanol	2290	0.18 ± 0a	0.17 ± 0.01a	0.15 ± 0.01b	0.14 ± 0b	0.14 ± 0c	0.13 ± 0.01d
96	ethyl 3-phenylpropionate	4790	0.17 ± 0.01a	0.12 ± 0c	0.13 ± 0b	0.11 ± 0d	0.12 ± 0c	0.06 ± 0e
92	3,4-dimethylbenzaldehyde	5670	0.09 ± 0b	0.11 ± 0a	0.12 ± 0.01a	0.07 ± 0c	0.04 ± 0d	0 ± 0e
79	hexyl hexanoate	6710	0.03 ± 0b	0.05 ± 0a	0.03 ± 0b	0.01 ± 0c	0.05 ± 0a	0 ± 0d

^aThe numbers were the same as the numbers listed in Table 5.

^bTAVs were calculated by dividing the concentration by the respective taste threshold.

^cTaste thresholds were taken from ref.^[25]

Above all, seven flavor compounds (ethyl butanoate, 3-methylbutanal, 3-methylbutyl acetate, aldehyde acetal, ethyl 4-methylpentanoate, ethyl 3-methylbutanoate, and ethyl acetate) were considered as the essential factors that were attributed to the fact that their OAVs and TAVs were both greater than 1. However, the correlation between flavor compounds and flavor characteristics was uncovered. Hence, it was necessary to establish the correlation between flavor compounds (aroma compounds and taste compounds) and various flavor characteristics (aroma characteristics and taste characteristics) of BCSGB and to explore the interaction between various flavor compounds.

Correlation analysis between flavor compounds and flavor characteristics for BCSGB

To evaluate the flavor profile of BCSGB, nine aroma attributes (fruity, floral, alcoholic, cellar, grain aroma, roasted aroma, zao-like aroma, fullness, and harmony) and seven taste attributes (acid, sweet, bitter, spicy, astringent, durability, and softness) were chosen. As shown in Figure 3C, sun-exposure had no significant effect on individual taste perception, but it did have a notable effect on overall taste perception (p < .001). Moreover, a remarkable difference was found in the aroma attribute of grain (p < .05) and the taste of softness (p < .01).

Although the OSME values from GC–O and calculated OAVs and TAVs could be used to evaluate the flavor (odor and taste) intensities and contributions of flavor compounds, the contribution of these flavor compounds to the corresponding flavor characteristics of BCSGB still needed further study. Hence, the relationship between flavor compounds and flavor characteristics of corresponding baijiu samples was established by mental test. As shown in Figure 3a, ethyl butanoate and 1-octen-3-ol had a strong positive correlation with grain aroma, ethyl 3-methylbutanoate exhibited a significant positive impact on roasted aroma, while zao-like aroma was found to be closely positive related to 1-octen-3-ol. As shown in Figure 3d, a positive correlation was found between 3-methylbutanoic acid and sweet taste, while furan-2-carbaldehyde and 1-hexanol had a positive impact on astringent taste. Interestingly, the aroma expression among esters had a positive correlation with each other, while there was a negative aroma expression between esters and acids. Thus, it was speculated that the perception of aroma compounds was also influenced by the presence of flavor compounds. Above all, ethyl butanoate, 1-octen-3-ol, ethyl 3-methylbutanoate, 3-methylbutanoic acid, furan-2-carbaldehyde, and 1-hexanol were thought to influence the sensory characteristics of baijiu, which were responsible for the quality change of BCSGB during the sun-exposure process.

To verify the relationship between flavor compounds and the sense profile of baijiu, a random forest model based on flavor compounds and overall sensory attribute evaluation (as shown in Figure 4) was established.^[31] Based on the importance analysis, it can be seen that the characteristics of the four trace components, ethyl hexanoate, 2,6-dimethyl-4-heptanol, butyl-2-hydroxypropanoate, and 2-phenylethanol, were of great importance for the flavor of baijiu. In consideration of three evaluation methods (OAVs and TAVs, mental test, and random forest), esters, acids and aromatics were regarded as the most important kinds of flavor compounds influencing the flavor of baijiu. It was discovered that ethyl butanoate, 3-methylbutanal, 3-methylbutyl acetate, aldehyde acetal, ethyl 4-methylpentanoate, ethyl 3-methylbutanoate, ethyl acetate, 1-octen-3-ol, 3-methylbutanoic acid, furan-2-carbaldehyde, 1-hexanol, ethyl hexanoate, and 2,6-dimethyl-4-heptanol were significant factors influencing the quality of baijiu, their modifications improve the harmonious aroma and soft taste of baijiu, as well as the aroma expression of grain note. In detail, the concentration of esters decreased, leading to a weakening of the fruit and roasted aromas, while acetal demonstrated a downward trend, resulting in the enhancement of the floral aroma. It was worth noting that lowering the esters to acids ratio results in more sensual harmony for baijiu. Furthermore, it was discovered the sweet taste faded with the concentration of aromatics and acids decreased. Esters and alcohols showed a positive relation to astringency and improved the fullness of baijiu. Above all, exposure to sunlight affected the distribution of esters, acids, and alcohols, resulting in corresponding changes in the fruity aroma, floral aroma, astringency, and harmony of the baijiu.

Figure 4. (a) Flavor profile analyses (post-standardized) of BCSGB samples. The scores of selected descriptors were evaluated by 18 panelists on average. Significance was indicated at * p < .05 (significance), ** p < .01 (very significant), *** p < .001 (very highly significant). (b), the error of random forest model. (c), Curve of significant influence factors.

Conclusion

In summary, a total of 111 trace components were identified. Further, 34 aroma compounds and 11 taste compounds were recognized as the important flavor compounds owing to their relatively high OAVs or TAVs, which were mainly esters, acids, and alcohols. Based on flavor expression, mental test, and random forest, it was indicated that 13 kinds of flavor compounds were verified as significant factors influencing the quality of baijiu, which can be used as a grapple to grade product quality and monitor the brewing process. This study provides strategies and ideas for optimizing directional processes and improving the quality of baijiu.

Acknowledgments

This paper is supported by the Young Elite Scientist Sponsorship Program by Bast (No. BYESS2023055), the National Natural Science Foundation of China (32001826), the project of the National Key R&D Program of China (No. 2022YFD2101205), 2023 Beijing Technology and Business University Graduate Student Discipline Competition Project, and 2023 postgraduate research capability improvement program funding.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

Data will be made available on request.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/10942912.2023.2281885

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [32001826]; the project of the National Key R&D Program of China [2022YFD2101205]; the 2023 Beijing Technology and Business University graduate research ability improvement project., and the 2023 postgraduate research capability improvement program Young Elite Scientist Sponsorship Program by Bast [BYESS2023055].