Rice wine is rice and millet as the main raw material, through cooking, koji, saccharification fermentation, filtration, decoction, storage, blending of brewed liquor, and beer and wine alongside "the world's three ancient liquor", it has the characteristics of soft liquor, full body, mellow taste, rich nutrition, suitable liquor, etc., is an important development direction of China's brewing wine, the process of rice wine enables it to retain all the beneficial components produced in the fermentation process, and is easy to be absorbed by the human body, so the high nutritional value of rice wine is difficult to achieve by other liquors, figuratively called "liquid cake".
After the end of the decoction process, the rice wine should be stored for a period of time, so as to increase the aroma substances and improve the mellowness and coordination of the wine, and enhance the stability of the wine. The storage containers of rice wine usually have pottery jars and stainless steel tanks, etc., they all have their own advantages, pottery jar storage as a traditional storage process, the aging effect of the wine body is better, but the pottery jar is small, easy to damage, large loss of wine storage and makes the production of rice wine can not be mechanized in the whole process. The use of stainless steel large tank can solve the above problems, but because the sterilization process of supplementary air and equipment pipelines may be affected by miscellaneous bacteria, and at the same time, the heat capacity of the large tank is large and the heat dissipation is slow, so there is a certain growth space for the infected miscellaneous bacteria in the process, so there is a risk of rancidity and abnormal taste during the storage of the large tank.
In order to further grasp the flavor and quality changes of rice wine under different storage conditions, this experiment was mainly to further grasp the flavor and quality changes of rice wine under different storage conditions, and to study the aged wine with different storage methods, and quantitatively analyzed the effects of stainless steel tank and pottery jar storage on the physical and chemical indexes, organic acid composition and volatile flavor substances of rice wine, in order to optimize the storage process and link control of rice wine, improve the safety of rice wine storage, and reduce its loss and rancidity risk in the storage process. Enhance the quality of aged wines.
1 Materials and methods
1.1 Materials, Reagents and Instruments
Stainless steel jar and pottery jar for 3-year storage: provided by Shanghai Shikumen Liquor***.
Reagents: sodium chloride, sodium hydroxide, hydrochloric acid, formaldehyde, absolute ethanol, methanol, potassium dihydrogen phosphate, etc. are all analytically pure, and the rest of the gas phase and liquid phase standards are chromatographically pure.
Equipment: Near-infrared rapid alcohol analyzer, Anton Paar DMA4100; Titrator automatically, Metrohm 916DMP Ti-Touch; Gas chromatograph, Agilent Agilent 6890N; HPLC, Waters HPLC 2695, Diode Array Detector 2996.
1.2 Test Methods
1.2.1. Determination of physical and chemical indexes of rice wine.
For the determination of alcohol content, total sugar, total acid, and amino acid nitrogen content, refer to GB T 13662—2018 [4].
1.2.2 Determination of volatile aroma components in rice wine.
Refer to the determination method of Hu Jian et al.
1.2.3 Methods for the determination of organic acids in rice wine.
1) Preparation of standard series solutions of organic acids: citric acid, tartaric acid, acetic acid, malic acid, pyruvate, succinic acid, and lactic acid are taken respectively05 g, volume up to 50 ml with ultrapure water to obtain 1 g l solution. It is then diluted with ultrapure water to a new brewing platform, with an original concentration of 005 g/l、0.1 g/l、0.2 g/l、0.4 g/l、0.8 g l of mixed standard series solution.
2) Pretreatment of wine samples: wine samples are diluted 4 times, after 045 m membrane filtration.
3) Chromatographic analysis conditions: Column ATTLANTIS DC18 46×250 mm 5 μm；Column temperature 30; Injection volume: 10 l; Mobile phase 001 mol l potassium phosphate monobasic (adjust pH2 with phosphoric acid.)45）；Flow rate: 1 ml min; Detection wavelength 210 nm; The elution method is gradient elution, and the elution procedure is shown in Table 1. Qualitatively based on retention time, peak area external standard method quantification.
2 Results and Analysis
2.1 Comparison of physical and chemical indexes of rice wine with different storage methods
The most critical physical and chemical indicators in rice wine include alcohol content, total acid, total sugar and amino acid nitrogen, which reflect the quality of rice wine to a certain extent.
Table 2 shows the changes and comparison of the physical and chemical indexes of aged wine with different storage methods, combined with the national standard GB T 13662-2018, it can be seen that the aged wine of the two storage methods first meets the index standards of traditional high-grade rice wine, and secondly, the alcohol content of stainless steel tank storage is 11% higher than that of pottery jar, and the total sugar is 19% lower, which may be due to the fact that pottery jar has a certain air permeability and is not as sealing as stainless steel tank, resulting in the loss of part of the liquor body during storage and the alcohol content is lower due to volatilization. At the same time, it also makes the wine have a concentration effect, so that the total sugar content is relatively high; In terms of total acid content, the storage of stainless steel tanks may be due to the larger total amount of substrate, and it is more difficult to dissipate heat in the original load of the new brewing platform under high temperature conditions such as summer, and the biochemical reaction speed of various microorganisms becomes faster under relatively high temperature conditions, resulting in more acid production, and the total acid content is 12% higher than that of pottery jar storage; For the amino acid nitrogen content, the two storage methods are basically at the same level.
2.2 Comparison of organic acid components of rice wine with different storage methods
Acid is an important part of the body of rice wine, an appropriate amount of acid can play a role in regulating the flavor in the wine, and gradually form aromatic esters with alcohols during storage, acid can not only make the body of rice wine coordinate, but also enhance the mellow feeling of rice wine. Most of the acids in rice wine are produced by microbial metabolism during fermentation. Among them, the volatile acids dominated by acetic acid are the main substances that make rice wine irritate, and the non-volatile acids dominated by lactic acid, citric acid, and succinic acid are the main substances that make the wine have a mellow aftertaste. The acidity of rice wine is greatly affected by the aging and storage technology, and excessive acidity should be avoided during storage, otherwise it will damage the flavor of rice wine, reduce the quality, and eventually lead to rancidity.
In this experiment, high performance liquid chromatography was used to determine organic acids, first of all, under the defined chromatographic conditions, the spectra of the organic acid mixture were determined (Figure 1), and the retention times of organic acids such as tartaric acid, pyruvate, malic acid, lactic acid, acetic acid, citric acid, and succinic acid were obtained (Table 3).
The organic acid mixed standard series solution was diluted according to a certain gradient, and the obtained gradient solution of each concentration was 0After the 45 m mixed fiber resin membrane was filtered, the analysis was carried out under the determined chromatographic conditions in the order from low to high, and the peak area-concentration curves of each organic acid were plotted by peak area external standard method, and the standard equations and correlation coefficients of seven organic acids were obtained, as shown in Table 4.
As can be seen from Table 4, the solution content and the response value of these 7 organic acid standards showed a good linear relationship, and the correlation coefficient was 09984～0.9999, which means that this method has a high sensitivity and can be used for the determination of organic acid content.
The two groups of wine samples with different storage methods were compared, and the results are shown in Table 5. It can be seen from Table 5 that the total acid content of stainless steel tank storage is higher, and among the seven organic acids, the contents of pyruvate, malic acid, acetic acid, citric acid and succinic acid are slightly higher than those stored in pottery jars, and tartaric acid and lactic acid are slightly lower. At the same time, focusing on the two main organic acids in rice wine, lactic acid and acetic acid, it can be found that compared with stainless steel tanks, the content of non-volatile acids represented by lactic acid in pottery jar storage samples with greater contribution to mellow feeling is higher, which is relatively higher by 9%, while the irritating acetic acid content is lower, which is 14% lower than that of stainless steel tank, which may be one of the reasons why the wine stored in pottery jars is more harmonious and has a stronger taste than that stored in stainless steel tanks.
Table 5 Organic acid content (mg l) of rice wine with different storage methods
2.3 Comparison of volatile flavor compounds in rice wine with different storage methods
The unique aroma of rice wine does not refer to a single aroma, but a series of compound aromas, which are composed of esters, alcohols, aldehydes, acids and phenols. Although the content of flavor compounds is low, its aromatic effect is significant and varied, which determines the style characteristics and quality differences of rice wine. In addition to the products in the fermentation process, the modification of the flavor of the wine during the storage stage is also very important, and the influence of different aging and storage techniques on the interaction of aroma substances is also different, so the aroma of different rice wines is ever-changing.
The volatile flavor compounds of stainless steel jar aged wine and pottery jar aged wine were detected separately, and the results are shown in Table 6. It can be seen from Table 6 that among the alcohols, the content of -phenylethanol stored in stainless steel tanks is slightly higher, while the storage of pottery jars can reduce the content of higher alcohols by 13%. An appropriate amount of high-grade alcohol can bring the unique aroma of rice wine, and make the wine full and coordinated, and the taste is better, but if the original content of high-grade alcohol content exceeds a certain limit, it will lead to miscellaneous flavors and strong intoxication, which significantly reduces the post-drinking experience, commonly known as "Shangtou", so drinking rice wine should try to avoid too high high-grade alcohol; For the total ester, the total ester stored in stainless steel tanks was 33% higher than that stored in pottery jars, of which ethyl lactate was increased by 28%, and ethyl acetate was increased by 37%. This may be due to the fact that the tightness of the tank allows the esters to be retained more completely, and the high temperature caused by the larger total amount of substrate promotes the esterification reaction, which increases the ester content. The total aldehyde content stored in stainless steel tanks increased by 3% compared with that stored in pottery jars, and the proportion of furfural increased by 50%. In terms of volatile acid, the acetic acid content stored in the pottery jar is 88% of that stored in stainless steel tanks.
Table 6 Content of volatile flavor compounds (mg l) of rice wine with different storage methods
2.4 Comparison of sensory tasting of rice wine with different storage methods
Gas chromatography analysis was combined with artificial sensory evaluation to obtain more comprehensive flavor substance results, and according to the description standard of sensory requirements in GB T 13662-2018, the characteristics of appearance, aroma, taste and style were evaluated, and the focus was on aroma and taste.
Table 7 Sensory evaluation results of rice wine with different storage methods.
The results of expert tasting are shown in Table 7, in general, the sensory evaluation of rice wine stored in pottery jar is better, the aging effect is better than that of stainless steel tank storage, and the various flavor substances in the wine are more harmoniously combined, which may be due to the different oxidation and maturation rates of different storage methods, resulting in different changes of various aroma components in the wine. The tiny pores of the pottery jar itself can make the gas exchange between the inside and outside of the jar slowly, so that the trace amount of oxygen slowly penetrates into the wine, so as to play a catalytic role in the oxidation reaction of the compounds in the wine, such as ethanol is oxidized to acetaldehyde, and acetaldehyde will condense with ethanol to form acetal, so that the aroma of the wine becomes softer. Although the total ester content is higher than that of pottery jar storage, it does not fully represent the harmony of its aroma substances, and its high-grade alcohol content is also higher, which may also affect its post-drinking experience.
3 Conclusions and prospects
In this experiment, the alcohol content of stainless steel jar storage was 11% higher than that of pottery jar, and the total sugar content was 19% lower than that of pottery jar. In terms of total acid content, the storage of stainless steel tanks is 12% higher than that of pottery jars. There was no significant difference in amino acid nitrogen content between the two storage methods. In terms of organic acids, compared with the pottery jar, the content of pyruvate, malic acid, acetic acid, citric acid and succinic acid is higher, and the lactic acid and tartaric acid are lower, of which acetic acid is relatively increased by 16% and lactic acid is 9% lower; From the perspective of volatile flavor compounds, the content of high-grade alcohols stored in stainless steel tanks was 13% higher than that stored in pottery jars, and the total ester content was 33% higher, and the content of total aldehydes and volatile acids was higher respectively.
In general, the storage of pottery jars was better than that of stainless steel tanks in terms of reducing higher alcohols, and the aging effect and sensory evaluation were better, and the content of total acids and most organic acids was lower. The storage of stainless steel can better maintain the alcohol content and total ester content, and is better in mechanized production, which is the general trend of the industrialization of rice wine production.
This experiment provides a useful exploration for improving the storage technology of rice wine, and also provides some references for the quality study of aged wine with different storage methods, in order to finally produce higher quality rice wine. At the same time, the wine samples of different storage methods used in this experiment were all 3-year-old wines, and the effects of different storage methods on the flavor and quality of rice wine under longer storage time need to be further studied.