Abstract
In order to solve the abnormal fermentation problems caused by the slow and difficult temperature rise in the saccharification piling during the fermentation of sauce-flavor baijiu under low temperatures in winter. A special oxygen dissolving device made of stainless steel was applied in the second sorghum input of sauce-flavor baijiu, as well as the 1th and 2th rounds of saccharification piling fermentation in this study. In addition, its effects on the saccharification piling temperature, the 1th and 2th rounds of physical and chemical indicators, microbial population, and flavor components are analyzed. The results indicate that the use of a stainless steel oxygen dissolver significantly increased the “piling core” temperature of a saccharification piling and that this improvement was highly correlated with the oxygen dissolver. Additionally, it decreased the water content, sugar concentration, and acidity of the “piling core” of the saccharification piling, all of which were negatively correlated with the height of the dissolved oxygen reactor. Furthermore, the stainless steel special dissolved oxygen device also facilitated the growth and reproduction of saccharification piling microorganisms, particularly yeast. The use of the stainless steel special dissolved oxygen device, therefore, resulted in an increase in the concentration of acetaldehyde, ethyl acetate, propanol, isoamyl alcohol, β-phenylethanol, ethyl palmitate, and other flavoring compounds. Accordingly, the stainless steel special oxygen dissolving device demonstrates a significant application potential in the 1th and 2th rounds of saccharification piling fermentation of sauce-flavor baijiu.
Introduction
With a history dating back more than 2 000 years, Chinese traditional fermented baijiu (also named as Chinese liquor or spirit) is one of the six major distilled spirits in the world. As one of the three fundamental flavor types of baijiu (sauce-flavor, light-flavor, and strong-flavor), sauce-flavor baijiu, which provides a flavor resembling soy sauce, full-body and a long-lasting aroma, plays a significant role in the baijiu industry (Zhang, et al., 2022). Sauce-flavor baijiu, exemplified by the moutai-jiu, is one of China's four most renowned baijiu. It possesses the technological features of “high-temperature distiller's yeast production, high-temperature heaping, high-temperature fermentation, and high-temperature distillation”. Among them, high-temperature piling fermentation is one of the distinctive methods for brewing sauce-flavor baijiu. By enriching microorganisms in the surrounding environment and then fermenting to produce a variety of flavor substances (also known as “secondary distiller's yeast making”), sauce-flavor baijiu is imbued with its distinctive sauce flavor (Han, et al., 2018; Wang, et al., 2019; Liu, et al., 2019).
The process of sauce-flavor Baijiu contains Jiuqu preparation, grains steaming, stacking fermentation, pit fermentation and distillation (Fig. 1). The production process of sauce-flavor baijiu is primarily characterized by “making qu (qu is the fermentation starter) at the Dragon Boat Festival (5th may) and “making sha” (sha is the sorghum) at at the Double Ninth Festival (9th September), one production cycle per year, two times of grain addition (xiasha and zaosha), nine steaming cycles, eight fermentation cycles, and seven liquor extraction cycles” (Tang, et al., 2022; Qiu, et al., 2021). Half of the raw material (sorghum) is used in xiasha batch, and the second half of sorghum is employed in zaosha batch, wherein the fermented grain obtained from the xiasha batch is mixed, and all the raw materials continue to ferment in a pit.
The first and second production cycles of fermentation occur between November and February of the following year. At this time of year, the Chishui River Valley, the principal region for the production of sauce-flavor baijiu, has the lowest wintertime ambient temperature; consequently, it is simple to induce fermentation because of the following reasons: (1) The slow rise in temperature of the saccharification piling prolongs the fermentation period, which consumes more air drying room space in the production workshop and slows down production; (2) It is difficult to raise the temperature, and even the extreme situation of “cold heart” of saccharification piling occurs. This inhibits the enrichment and propagation of saccharification piling brewing microorganisms, which in turn affects the saccharification and fermentation of fermented grains, the generation of sauce-flavor baijiu substances, and sauce-flavor precursors, ultimately affecting the yield and quality of sauce-flavor baijiu (Yuan, 2019).
To solve the aforementioned problems in the heap fermentation of sauce-flavor baijiu caused by cold weather, Yuan, et al. (2019) attempted to use a piling-breaking and shifting technology to deal with the saccharification piling. In addition, Yang, et al. (2022) applied the bamboo basket dissolved oxygen device to the saccharification piling fermentation. Although these two attempts can alleviate the problems of a slow and difficult temperature rise in the saccharification piling fermentation during the period between the handling of zaosha and the 2th round of production to some extent, they are not entirely effective. Due to the high ethyl acetate content in the 1th and 2th rounds of liquor, however, the style of sauce-flavor baijiu is neither typical nor prominent, but rather resembles that of light-flavor baijiu (Yang, et al., 2004; Ma, 2019; Fan, et al., 2012). Therefore, in the practice of handling zaosha and a single round of piling fermentation, the saccharification piling is generally based on the principle of no displacement in order to reduce the consumption of raw starch and the proportion of the 1th and 2th rounds of liquor produced annually. Thus, the “breaking piling and displacing” approach is not conducive to production regulation. In addition, the bamboo basket oxygen dissolver can easily cause the fermented grains to become moldy, and it is easily damaged by mechanical equipment such as buckets when going down into the cellar. Consequently, the bamboo basket oxygen dissolver is unsuitable for commercialization and use.
The current study attempts to utilize a special oxygen-dissolving device made of stainless steel for conducting the piling fermentation of sauce-flavor baijiu from zaosha for up to two production cycles. In addition, its impact on the saccharification heap temperature, physical and chemical indicators, microorganisms, and flavor components was investigated, and its practical application value in production was evaluated.
Materials and Methods
Sorghum, reagents and equipments Organic glutinous sorghum was purchased from the local market at Zunyi City, Guizhou Province, China. Nutrient agar and Bengal red culture medium were purchased from Aoboxing Biological Co., Ltd. (Beijing, China).
The custom oxygen dissolver was constructed from food-grade 304 stainless steel in the shape of a round table with a built-in hollow chamber. The upper and lower bottom radii were 15 cm and 40 cm, respectively, and the height was 1.8 m or 1.5 m (Fig. 2). It was manufactured by the mechanical maintenance division of Guizhou Xijiu Co., Ltd (Zunyi, China).
Taishi K-contact thermometer (TES-1310) used in this study was obtained from Taishi Electronic Industry Co., Ltd (Taiwan, China). Digital display thermometer was got from Shenzhen Jinruisi Instrument Co., Ltd. (Shenzhen, China). The manufacturer of vertical pressure sterilization pot (YXQ-LS-100S II) was Shanghai Boxun Industrial Co., Ltd., (Shanghai, China). The manufacturer of the LRH-250 biochemical incubator was Shanghai Yiheng Technology Co., Ltd. (Shanghai, China). Gas chromatograph (6890N) used in this study was made by Agilent Technology Co., Ltd (Santa Clara, USA).
Application of stainless steel oxygen dissolver and saccharification piling sampling The experimental operation was shown in Fig. 3. The experiments were set four groups: group I, group I control, group II, and group II control. For the group I, the 1.8 m-tall stainless steel oxygen dissolver was positioned in the “piling core” of the saccharification reactor, gradually covering the fermented grains until the end of the piling. In addition, it was ensured that the oxygen dissolver was positioned higher than the saccharification piling when the piling was completed. And for the group II, the 1.5 m-tall stainless steel oxygen dissolver was positioned at the “piling core” of the saccharification piling, gradually covering the fermented grains until the piling was completely buried. Group I control and group II control: No oxygen dissolver was placed at the “piling core” of the saccharification reactor during piling. Each trial group was conducted three times in parallel. In addition, the samples were extracted at point O before the saccharification piling was put into the cellar, and the physicochemical indexes, microbial population, and flavor substances of the saccharification piling were analyzed.
Temperature measurement of saccharification piling and production environment Temperature measurement of saccharification piling: As depicted in Fig. 3, the temperature of the fermented grains at each sampling point was measured using a hand-held probe thermometer at three distinct positions A, B, and O, 100 cm above the ground of the saccharification piling, and the temperature value was recorded once the thermometer's reading was stable.
Temperature measurement of production environment: We used a handheld probe thermometer to measure the temperature of the production facility during the morning and afternoon production shifts, and then took the average of the two readings as the production environment temperature for the day.
Detection of the physical and chemical indices of the saccharification piling The drying method was used to determine the moisture of fermented grains (Zeng et al., 2021). In addition, acid-base titration was used to determine the acidity, and the ferlin reagent method was used to determine the reducing sugar content (Shen, 1998). The specific detection method referred to includes the DB 34/t 2264-2014 analysis method of fermented grains by solid-state fermentation (Anhui Provincial Bureau of Quality and Technical Supervision, 2014).
Microbial detection of saccharification piling We weighed 10 g of fermented grains sample in 90 mL of sterile water, shook it at 30 °C and 180 rpm for 30 min, and then diluted 1 mL of the suspension for reserve. Subsequently, we used the gradient dilution technique and spread it on a solid plate for counting and culture (Tu, et al., 2022). The medium for detecting bacteria was nutrient agar, and the medium for detecting yeast and mold was Bengal red medium as description by Xie, et al. (2023).
Analysis of flavor components in fermented grains of saccharification piling We utilized the gas chromatography technique developed by Wu, et al. (2023). to identify the flavor components in the fermented grains. We weighed 10 g of fermented grains, extracted them in a 53 % vol. ethanol solution for 4 h, and used 0.22 μm membrane-free filtration as a backup. The GC conditions were as follows: CP97723CP-Wax57CB capillary column (50 m × 0.25 mm × 0.25 μm, Agilent, USA); the temperature of the injection port was 230 °C; the injection volume was 10 μL with a split ratio of 50:1. In addition, the temperature rise procedure was as follows: a temperature of 40 ° maintained for 3 min, temperature ramped up at a rate of 1.5 °/min up to 70 ° (not maintained); temperature ramped up at a rate of 2 °/min up to 80 ° (not maintained); temperature ramped up at a rate of 7.5 ° /min up to 215 ° (maintained it for 20 min). In addition, a carrier gas flow rate of 0.3 mL/min was utilized. By using 2-methoxy-3-methylpyrazine and 2-octanol as internal standards, the aromatic compositions were quantitatively analyzed by applying the internal standard method.
Data analysis The data were expressed as the mean ± standard deviation. SPSS 21.0 was used to conduct a one-way ANOVA test; the difference was statistically significant when p < 0.05; R was used for principal component analysis; and Microsoft Office Excel 2022 (Redmond, WA, USA) was used for routine data analysis.
Results
Changes in environmental temperature in different production rounds As depicted in Fig. 4, in the nine production links, the average ambient temperature of the final 6th rounds of sauce-flavor baijiu production was the highest, while the average ambient temperature of the 1th round of sauce-flavor baijiu production was the lowest. The average ambient temperature was: 6th rounds > 5th rounds > 7th rounds > xiasha > 4th rounds > 3th rounds > zaosha >2th round >1th round. The environmental temperature in the entire production process was lower than that in the 6th round, while the environmental temperature range was highest in the 2th round. In addition, the environmental temperature of the 1th round was the lowest, with an average value of about 10 °C, which was during the lowest annual production environmental temperature. Furthermore, the viscosity of fermented grains gradually increased after glutinous sorghum adding and handling, with the 1th round having the highest viscosity. The water content of fermented grains gradually increased as production rounds progressed. Consequently, during the initial brewing and production of sauce-flavor baijiu in Chishui River Basin, the saccharification piling fermentation conditions were the worst and the temperature rise was the slowest and most challenging.
Changes in the temperature of the saccharification piling Fig. 5 depicts the change in temperature from the zaosha to the 2th saccharification piling before entering the cellar. At sampling point A, group I and group II had temperatures that were comparable to those of the control group. At sampling point B, the temperature of group I was significantly higher than that of the control group. However, the temperature of glutinous sorghum handling rounds in group II was higher than that of the experimental group, and there was no significant difference between the temperatures of the 1th and 2th rounds. At sampling point O, the temperature of groups I and group II was significantly higher than that of the control group, and the maximum temperature was positively correlated with the height of the oxygen dissolver. Thus, the use of the stainless steel oxygen dissolver made effectively increased the temperature of the “piling core” of the saccharification piling, thereby improving the fermentation conditions. This may be attributed to the fact that the stainless steel oxygen dissolver may have increased the oxygen concentration in the “piling core” of the saccharification piling, thereby promoting the growth and reproduction of yeast and other aerobic microorganisms. In addition, it would also generate more biological heat, thereby increasing piling core temperature.
Changes in physical and chemical indicators of the saccharification piling Based on previous studies, the 1th fermentation round for the saccharification piling was conducted at the lowest ambient temperature and with the most challenging level of fermentation. Simultaneously, the transition of temperature at sampling point O from the zaosha to the 2th round of saccharification piling was observed to be optimal. With reference to the findings of Yang, et al. (2022), the sensory quality of the saccharification piling was significantly different between the test group and the control group in the 2th production round. Thus, in conjunction with the production method, the fermented grains at sampling point O of the 1th and 2th rounds of the saccharification piling were selected for subsequent detection and analysis. However, a negative correlation was observed between the height of the oxygen dissolver and the water content, acidity, and reducing sugar of the test group of the saccharification piling (as shown in Table 1). This may be attributed to the application of the dissolver, which may have increased the contact area between the “piling core” of the saccharification piling and the air, resulting in greater water loss. In addition, the application of the oxygen dissolver may have increased the oxygen concentration in the “ piling core” of the saccharification piling, thus promoting the rapid growth and nutrient metabolism of microorganisms such as yeast. Correspondingly, the activity and metabolism of anaerobic bacteria such as lactic acid bacteria were inhibited, resulting in the reduction of the acid-producing capacity. These outcomes were consistent with that proposed by Lu, et al. (2021).
Table 1. Physical and chemical indices of different groups during the 1th and 2th saccharification piling round.
| Groups |
1th round |
2th round |
| Water content (%) |
Acidity
(mmol/10 g) |
Reducing sugar
(g/100 g) |
Water content
(%) |
Acidity
(mmol/10 g) |
Reducing sugar
(g/100 g) |
| Group I |
40.82 ± 0.18b |
1.60 ± 0.03b |
1.06 ± 0.06c |
44.20 ± 0.02c |
1.99 ± 0.07c |
1.14 ± 0.02b |
| Control of group I |
42.50 ± 0.08a |
1.91 ± 0.08a |
1.88 ± 0.03a |
45.53 ± 0.11b |
2.26 ± 0.08a |
1.73 ± 0.05a |
| Group II |
42.11 ± 0.10a |
1.80 ± 0.06a |
1.60 ± 0.10b |
45.16 ± 0.03b |
2.10 ± 0.03b |
1.65 ± 0.04a |
| Control of group II |
42.35 ± 0.04a |
1.85 ± 0.01a |
1.80 ± 0.05a |
46.03 ± 0.07a |
2.30 ± 0.05a |
1.68 ± 0.01a |
Values with different lowercase letters are significantly different (p < 0.05).
Table 2. Microbial population of different groups during the 1th and 2th saccharification piling round.
| Groups |
1th round |
2th round |
| Bacteria |
Yeast |
Mould |
Bacteria |
Yeast |
Mould |
| Group I |
6.90 × 106a |
3.70 × 106 a |
ND |
6.05 × 106 a |
4.54 × 107b |
1.00 × 103 |
| Control of group I |
2.30 × 106c |
1.10 × 105b |
ND |
1.50 × 106d |
7.60 × 105a |
ND |
| Group II |
6.35 × 106a |
1.60 × 105b |
ND |
4.50 × 106 b |
3.00 × 106c |
ND |
| Control of group II |
3.35 × 106b |
1.52 × 105b |
ND |
2.55 × 106c |
1.60 × 106d |
ND |
Values with different lowercase letters are significantly different (p < 0.05).
Changes in the microbial population of the saccharification piling The populations of bacteria and Saccharomycetes in the 1th and 2th rounds of saccharification pilinging in groups I and II were significantly higher than those in the control group, as shown in Table 2. This could be attributed to the application of the stainless steel dissolver to the saccharification piling, which had a supporting effect on its “core” and increased the “core” oxygen concentration. In addition, it may have assisted the saccharification piling in enriching the environment with microorganisms and facilitated the growth and reproduction of bacteria and Saccharomycetes. This is consistent with the conclusion reached by Tang, et al. (2007), wherein, at the end of fermentation, the number of Saccharomycetes and bacteria in the fermented grains in the location with more contact with the environment in the saccharification piling was greater than those in the location with less contact with the environment. Moreover, in the 1th and 2th rounds, few or no molds were detected in group I, group II, and their control groups. Consequently, from the standpoint of microbial growth and distribution, the application of a special stainless steel oxygen dissolver aided the heap fermentation in the saccharification piling, promoted the growth and reproduction of Saccharomycetes and bacterial microorganisms, and increased the population of these species.
Table 3. Results of alcohols of different groups in the 1th and 2th saccharification piling rounds.
| Numbers |
Coumpounds |
Odor description |
Group I (μg/g) |
Control of group I (μg/g) |
Group II (μg/g) |
Control of group II (μg/g) |
| A1 |
Propanol |
Wine aroma,
Fermented aroma |
35.12 ± 0.12a |
20.85 ± 0.33b |
33.28 ± 0.65a |
18.01 ± 0.16c |
| B1 |
108.30 ± 2.88a |
26.16 ± 1.98c |
32.31 ± 2.00c |
52.36 ± 1.91b |
| A2 |
Isobutanol |
Malty |
156.19 ± 3.66b |
184.91 ± 5.04a |
99.54 ± 2.43c |
100.47 ± 2.52c |
| B2 |
26.08 ± 2.23a |
28.20 ± 1.65a |
29.36 ± 3.83a |
28.50 ± 0.52a |
| A3 |
2-Methyl-1-butanol |
- |
164.51 ± 1.41a |
101.06 ± 1.33d |
110.04 ± 3.22c |
126.86 ± 1.23b |
| B3 |
24.18 ± 2.86c |
22.30 ± 1.96c |
79.94 ± 1.57a |
63.12 ± 0.55b |
| A4 |
Isoamyl alcohol |
Fruity aroma |
17.24 ± 2.14a |
11.78 ± 2.01b |
8.44 ± 1.68c |
4.74 ± 2.52c |
| B4 |
Flower fragrance |
422.81 ± 6.92b |
125.89 ± 3.10d |
356.09 ± 1.69c |
448.77 ± 2.08a |
| A5 |
Strong fusel |
1 419.07 ± 5.34c |
1671.51 ± 4.67b |
2 107.46 ± 2.83a |
1 354.64 ± 3.89d |
| B5 |
Amyl alcohol |
Smell, fruity aroma |
179.72 ± 1.08c |
164.93 ± 3.65d |
181.40 ± 2.53b |
213.49 ± 2.65a |
| A6 |
β-Phenylethanol |
Flower fragrance |
30.80 ± 1.55a |
29.01 ± 1.09a |
18.98 ± 0.34b |
15.87 ± 1.45b |
| B6 |
44.26 ± 1.22a |
41.44 ± 1.01a |
31.43 ± 1.36b |
29.79 ± 0.82b |
| A7 |
1,2-Propanediol |
|
50.08 ± 1.98a |
47.25 ± 1.53a |
50.87 ± 2.08a |
55.47 ± 3.73a |
| B7 |
17.69 ± 0.10b |
16.25 ± 0.07b |
27.39 ± 0.43a |
30.07 ± 0.89a |
| A8 |
2,3-Butanediol |
Buttery aroma, |
52.22 ± 2.01a |
44.34 ± 1.29b |
27.64 ± 2.00d |
36.26 ± 0.98c |
| B8 |
Creamy aroma |
27.70 ± 0.66b |
27.27 ± 0.78b |
42.39 ± 0.68a |
44.94 ± 1.02a |
| ΣA |
|
1 925.23 ± 18.21c |
2 110.71 ± 17.29b |
2 456.25 ± 15.23a |
1 712.32 ± 16.48d |
| ΣB |
|
850.74 ± 19.75b |
452.44 ± 14.20d |
780.31 ± 14.09c |
911.04 ± 10.44a |
Values with different lowercase letters are significantly different (p < 0.05).
Changes in flavor substances of the saccharification piling
Alcohols High levels of alcohol are present in baijiu, which may be roughly categorized as monohydric alcohols, polyols, and aromatic alcohols. These comprise essential substances that give baijiu its mellow and sweet flavor, as well as act as the precursors to esters (Fan, et al., 2012), which affect the flavor of sauce-flavor baijiu. As demonstrated in Table 3, eight alcohols were detected in each of the four groups. The alcohol content of group II in the 1th round and group I in the 2th round increased significantly compared to the control group. It was worth mentioning that the content of propanol in the 1th and 2th rounds of group I, isoamyl alcohol in the 2th round, and amyl alcohol in the 1th round of group II was significantly higher than those in the control group. These compounds may impart fruit flavor, fermentation flavor, and other flavors to sauce-flavor baijiu. In addition, in the 1th round of saccharification, pentanol was the most abundant alcohol compound, comprising 73.71 %, 85.80 %, 79.19 %, and 79.11 % of group I, group II, the control group of group I, and the control group of group II, respectively.
Acids The acid flavor substances in sauce-flavor baijiu consist primarily of organic acids, which are a type of essential trace component in baijiu. They have an evident taste-forming effect, contribute significantly to the aftertaste of sauce-flavor baijiu, and are precursor components of ester flavor-forming substances. There is a proverb that states, “Where there is acid, there is ester” (Li, et al., 2019). As shown in Table 4, five types of acids were found in the saccharification piling. These acids were acetic acid, propionic acid, isobutyric acid, isovaleric acid, and hexanoic acid. In the 1th and 2th rounds, the concentration of various acids exhibited an upward trend; however, the concentration of acetic acid in groups I and II was lower than that of the control group. Additionally, in the 1th round, the concentrations of propionic acid and isovaleric acid in group II were less than those in the control group, and the concentrations of isobutyric acid in group II in the 1th and 2th rounds were also less than those in the control group.
Esters Esters contribute important aroma in sauce-flavor baijiu. Compared to other distilled liquors in the world, the ester flavor of baijiu is particularly prominent (Franitza, et al., 2016). According to Wang, et al. (2021), ethyl acetate had the highest ester content in the 1th and 2th rounds of base liquor, which had a significant effect on the style typicality of the 1th and 2th rounds of liquor. As shown in Table 5, the content of esters in group I in the 1th round was significantly higher than in the control group, and the content of esters in group II in the 2th round was also significantly higher than in the control group. In addition, ethyl acetate content increased significantly in group I during the 1th and 2th rounds, which helped to improve the typical style of the 1th and 2th rounds of liquor; ethyl oleate also increased significantly in group I during the 1th and 2th rounds, and in group II during the 2th round. The total contents of ethyl acetate and ethyl oleate accounted for 70 % of ester compounds in group I, the control group of group I, group II, and the control group of group II in the 1th round, 48 %, 59.25 %, 62.06 %, and 68.19 % respectively, and the proportions of ester compounds in the 2th round were 56.27 %, 70.24 %, 78.08 %, and 65.34 % respectively.
Table 4. Results of acids of different groups in the 1th and 2th saccharification piling rounds.
| Number |
Compounds |
Odor description |
Group I (μg/g) |
Control of group I (μg/g) |
Group II (μg/g) |
Control of group II (μg/g) |
| A1 |
Acetic acid |
Vinegar fragrance |
41.82 ± 1.32a |
41.04 ± 1.87a |
29.26 ± 2.49b |
25.61 ± 2.72b |
| B1 |
17.77 ± 2.56d |
32.95 ± 1.01c |
38.85 ± 0.61b |
49.04 ± 0.49a |
| A2 |
Propionic acid |
Pungent sour fragrance |
170.34 ± 2.44ab |
141.17 ± 1.59c |
165.68 ± 1.85b |
185.91 ± 4.38a |
| B2 |
304.05 ± 3.68c |
227.96 ± 2.66d |
451.25 ± 1.84a |
377.63 ± 3.56b |
| A3 |
Isobutyric acid |
Sweaty, vinegar fragrance |
437.89 ± 3.67c |
390.72 ± 2.44d |
510.54 ± 5.03b |
832.04 ± 2.38a |
| B3 |
580.43 ± 1.43b |
517.80 ± 2.61c |
591.03 ± 1.98b |
822.82 ± 5.06a |
| A4 |
Isovaleric acid |
Sweaty, vinegar fragrance |
77.92 ± 1.55c |
72.25 ± 3.43c |
93.46 ± 2.86b |
115.07 ± 2.74a |
| B4 |
71.08 ± 0.51b |
70.74 ± 0.11bc |
74.79 ± 1.05a |
74.68 ± 0.10a |
| A5 |
|
Sweaty odor, Animal odor, sour |
20.09 ± 2.01a |
17.34 ± 2.46a |
12.90 ± 1.34b |
10.54 ± 1.83b |
| B5 |
Caproic acid |
Odor, sweet fragrance, fruity aroma |
53.88 ± 0.58a |
46.92 ± 0.62b |
44.24 ± 0.37b |
39.59 ± 0.99c |
| ΣA |
|
|
748.06 ± 10.99c |
662.52 ± 11.79d |
811.84 ± 13.57b |
1169.17 ± 14.05a |
| ΣB |
|
|
1 027.21 ± 8.76c |
896.37 ± 7.01d |
1 200.16 ± 5.85b |
1 363.76 ± 10.20a |
Values with different lowercase letters are significantly different (p < 0.05).
Table 5. Results of esters of different groups in the 1th and 2th saccharification piling rounds.
| Number |
Compounds |
Odor description |
Group I (μg/g) |
Control of group I (μg/g) |
Group II (μg/g) |
Control of group II (μg/g) |
| A1 |
|
Pineapple fragrance, apple fragrance, fruity aroma |
30.70 ± 0.34a |
20.89 ± 0.18d |
27.86 ± 0.42b |
25.02 ± 0.22c |
| B1 |
Ethyl acetate |
80.60 ± 1.68b |
23.95 ± 2.35c |
143.55 ± 1.98a |
142.81 ± 3.78a |
| A2 |
Ethyl palmitate |
|
112.39 ± 3.39a |
82.47 ± 1.67bc |
87.27 ± 2.76b |
56.55 ± 1.69d |
| B2 |
|
141.79 ± 2.01a |
111.13 ± 0.99b |
91.76 ± 1.75c |
95.13 ± 1.26c |
| A3 |
Fat fragrance, fragrance of flowers |
298.67 ± 1.25a |
123.86 ± 0.99d |
143.56 ± 2.15b |
130.19 ± 1.16c |
| B3 |
Ethyl oleate |
123.86 ± 1.00c |
298.67 ± 2.03a |
245.70 ± 1.36b |
66.44 ± 2.30d |
| A4 |
Fat fragrance, flower and fruit aroma |
25.55 ± 2.82a |
17.09 ± 1.67b |
17.54 ± 2.48b |
15.85 ± 1.76b |
| B4 |
Ethyl linoleate |
17.09 ± 0.64b |
25.55 ± 0.56a |
17.54 ± 0.31b |
15.85 ± 0.88b |
| ΣA |
|
|
467.31 ± 7.8a |
244.31 ± 4.51bc |
276.23 ± 7.81b |
227.61 ± 4.83d |
| ΣB |
|
|
363.34 ± 5.33c |
459.30 ± 5.93b |
498.55 ± 5.5a |
320.23 ± 8.22d |
Values with different lowercase letters are significantly different (p < 0.05).
Aldehydes and ketones Aldehydes and ketones are primarily generated by microbial metabolism during fermentation and by alcohol oxidation, ketone acid decarboxylation, and other pathways during wine aging (Cao, et al., 2021). Because its threshold is generally low, it is believed to have a significant effect on the overall flavor and aroma of sauce-flavor baijiu. Tang, et al. (2021) demonstrated that acet aldehyde is the precursor for acet al production and that acet als can impart a soft fragrance to alcoholic beverages. As shown in Table 6, the concentration of acet aldehyde was significantly elevated in group I in the 1th and 2th rounds, and in group II in the 1th.
Table 6. Results of aldehydes and ketones of different groups in the 1th and 2th saccharification piling rounds.
| Number |
Compounds |
Odor description |
Group I (μg/g) |
Control of group I (μg/g) |
Group II (μg/g) |
Control of group II (μg/g) |
| A1 |
Acet aldehyde |
Pungent aldehyde fragrance |
86.89 ± 1.21a |
41.64 ± 0.99c |
73.47 ± 0.58b |
31.10 ± 1.32d |
| B1 |
114.08 ± 3.01a |
48.24 ± 2.64d |
90.93 ± 2.42bc |
98.06 ± 4.31b |
| A2 |
3-Hydroxy-2-butanone |
50.08 ± 2.64 |
72.55 ± 3.56 |
207.36 ± 4.19 |
254.73 ± 3.75 |
| B2 |
1641.47 ± 9.53b |
1899.26 ± 12.35a |
1227.08 ± 4.93c |
553.1 1 ± 4.00d |
| ΣA |
136.97 ± 3.85a |
114.19 ± 4.55c |
280.83 ± 4.77b |
285.83 ± 5.07b |
| ΣB |
1755.55 ± 12.54b |
1947.5 ± 14.99a |
1318.01 ± 7.35c |
651.17 ± 8.31d |
Values with different lowercase letters are significantly different (p < 0.05).
Main flavor compounds As depicted in Fig. 6, the control group of group II had the highest concentration of acidic compounds, which was essentially the same in both the 1th and 2th rounds. The content of volatile acids in the saccharification piling decreased, which was consistent with the acidity change of physical and chemical indicators; compared to the control group, the content of esters in group I increased in the 1th round, and it was the highest in group I and group II in the 2th round; there was no significant difference in the content of aldehydes and ketones between group I, group II, and the control group. According to table 3– 6, the total content of alcohols, acids, esters, aldehydes, and ketones in the saccharification piling treated with a stainless steel oxygen dissolver increased (The 1th round: 3 277.57 μg/g in group I, 3 825.15 μg/g in group II; the 2th round: 3996.84 μg/g in group I, 3 797.03 μg/g in group II), with flavor components such as acet aldehyde, propanol, isoamyl alcohol β-phenylethanol, ethyl acetate, ethyl palmitate and so on increasing significantly. Similarly, the performance of the 2th round was superior to the performance of the 1th round, which was consistent with the sensory performance. The substance had a pineapple flavor, a cream flavor, a rose flavor, a fermentation flavor, and a unique aroma (Yi, et al., 2022). In conclusion, the use of stainless steel oxygen dissolver could aid in enhancing the flavor content of the saccharification piling and enhancing the quality of sauce-flavor baijiu.
To further analyze the effect of an oxygen dissolver made of stainless steel on the flavor components of a saccharification piling, the principal component analysis method was used to analyze the primary flavor compounds (Ji, et al., 2016). As demonstrated in Fig. 7, the cumulative contribution rates of the first two principal components in the 1th and 2th rounds reached 79.7 % and 85.1 %, respectively, which may represent the predominant trend of aroma substances in the saccharification piling. According to Fig. 7, group I and its control group were located in the first quadrant and the fourth quadrant, whereas group II and its control group were located in the second quadrant and the negative half axis of the X axis, respectively; Group I and its control group in the 2th round of saccharification piling were located in the second quadrant and the third quadrant, whereas group II and its control group were located in the junction area of the fourth quadrant and the negative half axis of the In conclusion, group I and group II differed significantly from their respective control groups, and the dispersion of flavor substances was significant, indicating that the application of stainless steel oxygen dissolver could significantly influence the flavor components of saccharification piling.
Discussion
This study aims to apply a special oxygen dissolver made of stainless steel to the saccharification piling fermentation in order to resolve the abnormal fermentation of sauce-flavor baijiu in the wintertime production environment. The results indicate that the use of an oxygen dissolver made of stainless steel can significantly raise the temperature of the “piling core” of a saccharification piling and stimulate piling fermentation. Moreover, the detection results of physical and chemical indicators, microorganisms, and flavor compounds of the “piling core” of the saccharification piling in the 1th and 2th rounds applied with the special stainless steel oxygen dissolver indicate that this method can reduce the moisture, acidity and reducing sugar of the “ piling core” of the saccharification piling, and increase the number of microorganisms and flavor substance content in the “piling core” of the saccharification piling. Because the oxygen dissolver is made from stainless steel, it is simple to clean after use, making it resistant to bacterial staining and suitable for re-use. In addition, stainless steel is hard, resistant to damage, durable, and has application potential.
Due to the rich and complex composition of flavor compounds in baijiu, as well as the vast differences in their mass concentration range, polarity, solubility, volatility, and thermal stability, it is difficult to have a single method for detecting and analyzing all flavor compounds in baijiu, according to studies (Tang, et al., 2022). However, only gas chromatography was used in this study to detect and analyze the flavor substances of the saccharification piling. Therefore, in the follow-up study, additional flavor omics technologies, such as headspace solid-phase microextraction gas chromatography-mass spectrometry, liquid chromatography-mass spectrometry, gas chromatography ion mobility spectrometry, and electronic nose analysis technology, should be used to analyze the effect of the special stainless steel oxygen dissolver on the flavor substances of the saccharification piling.
In the heap fermentation of the saccharification piling, proteases, lipases, tannases, esterases, and other enzymes can convert biological macromolecular substances such as starch and protein into small molecular substances such as amino acids, oligosaccharides, fatty acids, which is conducive to the progress of piling fermentation and the metabolism and release of flavor substances, thereby influencing the flavor and quality of baijiu. Additionally, glucoamylase α-amylase, cellulase, and pectinase can impact the utilization rate of raw materials and wine production rate (Dai, 2019). However, the effect of the application of a special stainless steel oxygen dissolver on amylase, protease, glucoamylase, cellulase, and pectinase, as well as other enzyme systems in the saccharification piling is unknown, which must be investigated and analyzed in the future.
In the production of alcoholic beverages, food-grade stainless steel should be utilized to ensure food safety. The oxygen dissolver utilized in this study is made from 304 steel. Thus, further clarification is required regarding whether the effects of other food-grade stainless steel materials, such as 316 L, are consistent with the results of 304. In addition, how to balance the quantity of stainless steel oxygen dissolver with production costs requires additional consideration.
In conclusion, the results of this study indicate that the application of a special stainless steel oxygen dissolver can significantly increase the temperature of the “piling core” of the saccharification piling, reduce the moisture and sugar content and acidity of the “piling core” of the saccharification piling, promote the growth and reproduction of microbes in the saccharification piling, and increase the content of flavor compounds such as acet aldehyde, ethyl acetate, propanol, and isoamyl alcohol β-phenylethanol, ethyl palmitate and so on. Consequently, the special stainless steel oxygen dissolver has application potential in the 1th and 2th rounds of heap fermentation in the saccharification piling of sauce-flavor baijiu.
Author Contributions Conceptualization and data analysis, Zhao Hubing, Huo Yingyu, Yang Gangren, Liu Xiaozhu et al; methodology, Li Xu; supervision, X.L. All authors have read and agreed to the published version of the manuscript.
Acknowledgements This research was funded by Guizhou Fruit Wine Brewing Engineering Research Center [Qianjiaoji (2022)050].
Conflict of interest There are no conflicts of interest to declare.
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