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Abstract In order to extract the scarce natural liquor flavoring which is the dream of many liquor factories from yellow water， this research first added a proper amount of foodgrade entrainer ethanol into yellow water， to form yellow water containing high content of ethanol. Then， the yellow water was subjected to azeotropic distillation and catalytic esterification， obtaining natural acetaldehyde， ethyl formate， ethyl acetate and esterification liquid containing various compounds that can influence the flavor of liquor. In the esterification liquid， the contents of ethyl propionate， ethyl butyrate， ethyl valerate， ethyl lactate and ethyl hexanoate are as higher as 19.0， 46.5， 1.5， 39.8 and 137.1 g/L， respectively. The esterification liquid prepared by one fraction of yellow water can blend 9.14 fractions of common liquor into topgrade Luzhouflavor liquor. Therefore， yellow water has great recycling value.
Key words Yellow water; Azeotropic distillation; Reaction distillation; Ethyl caproate; Esterification liquid; Liquor flavoring
Luzhouflavor liquor is the white liquor with the largest production and consumption in China， and its major flavor component is ethyl hexanoate， the content of which directly influences the quality of Luzhouflavor liquor[1]. Generally， the ethyl caproate content in new wine brewed by solid fermentation is lower than 1 000 mg/L， and the content of ethyl lactate is higher than that of ethyl caproate. However， the topgrade Luzhouflavor liquor requires an ethyl caproate content of 1 200-2 800 mg/L， and a ratio of ethyl lactate to ethyl caproate in the range of （0.6-0.8）∶1[2]. In addition， the white liquor produced by liquid fermentation， also known as the newprocess white liquor， selects edible alcohol as basic liquor and prepared by distillation through the flavorenhanced fermented grains， and adjusting flavor or blending solid with liquid[3-4]. On one hand， due to edible alcohol hardly contains the various trace components presenting flavor in the white liquor produced by solid fermentation， various spices or flavoring liquids should be added into edible alcohol generally to endow the liquid fermentation liquor with gentle flavor， natural soft sweetness， harmony acid and ester and clear taste[5-7]. However， a large portion of the spices and flavoring liquids are artificially synthesized， for instance， the caproic acid and ethyl caproate playing an important role of adjusting the flavor of liquor are both not fermentation products[8-11]. The application of synthetic spices greatly reduces the approbation of alcohol drinker to liquid fermentation liquor， thereby greatly affecting its market share， especially high end market[12]. On the other hand， the byproduct， yellow water produced during the solid fermentation of fermented grains in pits[13-14] has components similar to the base liquor produced by solid fermentation except some components. Yellow water contains about 38 of the 47 organic compounds affecting liquor flavor. In this study， the contents of caproic acid， acetic acid and butyric acid in yellowed water reached 8 604， 6 534 and 2 836 mg/L， which are more than ten times of ordinary Luzhouflavor liquor. However， these three acids and their esterification products are important flavoring components for white liquor produced by liquid fermentation， as well as the material basis for the aging of white liquor. At present， there are no methods for extracting beneficial substances from yellow water with high efficiency and low cost， which greatly limits the application of yellow water， and much yellow water is abandoned in vain and becomes one of the important water pollutants in liquor industry[15]. Therefore， how to effectively utilize the beneficial components in yellow water has become a hot research subject for white liquor scientists and manufacturers. Therefore， how to effectively utilize the beneficial components in yellow water and end liquor has become a hot research subject for white liquor scientists and manufacturers. Preparation method and process of dense end liquor and dense yellow water
Distillation and concentration of end liquor
At first， 100 fractions （referring to volume fraction unless defined otherwise） of end liquor with an ethanol content of 42% was filled into a distillation tower， and 44.7 fractions of dense end liquor with a high ethanol content was obtained through distillation. The ethanol degree and various organic compounds in the dense end liquor were analyzed by GC， as shown in Table 1. It could be seen from Table 1 that among the 42 organic compounds influencing liquor flavor in the end liquor， 21 kinds were detected in the dense liquor （in addition to ethanol and methanol）. Among them， 19 compounds had a distillate efficiency over 50%， 15 compounds showed a distillate efficiency over 80%， and especially， acetaldehyde， acetal， ethyl caproate and organic acids for synthesizing the synthetic esters including acetic acid， propionic acid， butyric acid， valeric acid and caproic acid exhibited a distillate efficiency over 90%.
Distillation and concentration of important components in yellow water influencing liquor flavor
At first， 97 fractions of yellow water and 3 fractions of dense end liquor were filled into a distillation tower and distillated （the dense end liquor was added to improve ethanol content in yellow water， so as to provide enough ethanol which formed azeotrope with organic matter in yellow water and was evaporated together with the azeotrope）. When 9.5 fractions of distillate was evaporated by elevating the temperature， distillation was stopped. Then an aerator was added into the distillate to introduce ozone， and the aeration was stopped when the yellow color of the distillate faded away， giving the discolored distillate， hereinafter referred to as dense yellow water. Various organic compounds contained in the dense yellow water were analyzed by GC， as shown in Table 1.
It could be seen from Table 1 that there are 38 organic compounds influencing liquor flavor in yellow water， and 21 kinds were detected in the dense yellow water. Among them， 19 compounds had a distillate efficiency over 50%， 15 compounds showed a distillate efficiency over 80%， and especially， acetaldehyde， acetal， ethyl caproate and organic acids for synthesizing the synthetic esters including acetic acid， propionic acid， butyric acid， valeric acid and caproic acid exhibited a distillate efficiency over 90%. More precisely， the concentration of ethyl caproate and ethyl acetate reached 1 966.2 and 3 251.8 mg/L， respectively. Furthermore， the concentrations of the three major acids （hereinafter referred to hexanoic acid， acetic acid and butyric acid） reached 84 904.3， 67 039.3 and 29 056.6 mg/L， respectively. In the dense yellow water， not only the contents of the three major esters satisfied the requirements for topgrade Luzhouflavor liquor， the contents of the three major acids were also 119-175 times of those in ordinary Luzhouflavor liquor （the base liquor in Table 1 served as the representative of ordinary Luzhouflavor liquor， the same as below）. In addition， the total contents of ethanol， water and various organic matters （besides ethanol） in the dense yellow water were 549.1 （69.5°）， 45.7 and 249.7 g/L， respectively. The contents and mole numbers of ethanol and various shortchain organic acids and the molar ratios of ethanol to these acids in dense yellow water are shown in Table 2. Usages of Dense Yellow Water
The dense yellow water acquired by above method has following usages：
Blending of white liquor produced by liquid fermentation
The dense yellow water could be directly used as liquor flavoring， i.e.， 1% of dense yellow water is added into the white liquor produced by liquid fermentation. By such， the concentrations of the three major acids in white liquor produced by liquid fermentation could reach the concentrations in ordinary Luzhouflavor liquor produced by solid fermentation， and 21 organic compounds influencing the flavor of liquor contained in white liquor produced by solid fermentation are also added into white liquor produced by liquid fermentation， thereby making the flavor white liquor produced by liquid fermentation similar to that of white liquor produced by solid fermentation.
Improvement of white liquor produced by solid fermentation
Adding about 1% of dense yellow water into white liquor produced by solid fermentation not only could improve the concentration of the three major acids， but also could provide a material basis for the aging of white liquor. Furthermore， the concentrations of 21 organic compounds influencing the flavor of liquor contained in white liquor produced by solid fermentation are also improved， and the purposes of improving liquor quality and influencing liquor flavor are thereby achieved.
Preparation and concentration of esters and other substances required by liquor flavoring
The dense yellow water also could serve as the raw material for the preparation of the three major esters and other esters， and other liquor flavorings also could be concentrated simultaneously.
Extraction and Preparation of Liquor Flavoring with Dense Yellow Water as a Raw Material
Acquisition of lowboiling point cut fractions including acetaldehyde， ethyl formate and ethyl acetate
One hundred fractions of dense yellow water was added into an azeotropic rectification tower. At the beginning of rectification， the heating switch was turned on. The current should not be too high， so as to avoid equipment damage due to abrupt fierce heating. When the tower pot temperature reached 40 ℃， the tower insulation circuit was turned on. When the tower top temperature reached 20.8 ℃ （the boiling point of acetaldehyde）， the vapor at the tower top passed through the overhead condenser and flew to the acetaldehyde receiving tank， and when the cut fraction began to decrease and the tower top temperature increased gradually， all the natural acetaldehyde was basically vaporized. At this time， the fraction outflow pipe was switched to the position of the ethyl formate receiving tank， and the tower pot temperature was adjusted to around 65 ℃， followed by turning on the tower insulation circuit again. When the tower top temperature reached 51 ℃ （the azeotropic point of methanol and ethyl formate： 51 ℃; the azeotropic point of methanol and ethyl acetate： 62.3 ℃）， the cut fraction flowing through the overhead condenser was mainly the azeotrope of methanol and natural ethyl formate， and when the fraction began to decrease and the tower top temperature increased gradually， all the natural ethyl formate was basically vaporized. At this time， the fraction outflow pipe was switched to the position of the methanol and ethyl acetate receiving tank， and the tower pot temperature was adjusted to 70 ℃， followed by turning on the tower insulation circuit again. When the tower top temperature increased to 62.3 ℃， the cut fraction flowing through the overhead condenser was mainly the azeotrope of methanol and natural ethyl acetate， and when the fraction began to decrease again and the tower top temperature increased gradually， all the natural ethyl acetate was basically vaporized. Then， the fraction outflow pipe was switched to the position of the methanol receiving tank， and when the tower top temperature increased to 64.7 ℃ （the boiling point of methanol）， the cut fraction flowing through the overhead condenser was mainly methanol. When the fraction began to decrease again and the tower top temperature increased gradually， all the methanol was basically vaporized. Finally， distillation was stopped， and liquid in the tower was let out and transferred to the rectification device with magnetic stirring. Above tower top distillates were detected by GC， and subjected to volume metering， obtaining the results as follows：
Volume and component of the tower top distillate at 20.8 ℃
When the tower top temperature rose to about 20.8 ℃， the tower top distillate was acetaldehyde with a volume fraction of 0.288 5 fraction， and according to calculation， the acetaldehyde in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 51 ℃
When the tower top temperature rose to about 51 ℃， the tower top distillate was the binary azeotrope of methanol and ethyl formate， with a volume fraction of 0.146 0 fraction. The mass contents of methanol and ethyl formate were 15.99% and 84.01%， respectively. The contents were very close to the composition of azeotrope recorded in Data of Some Common Industrial Solvent Azeotrope[16]. According to calculation， the ethyl formate contained in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 62.3 ℃
When the tower top temperature rose to about 62.3 ℃， the tower top distillate was the binary azeotrope of methanol and ethyl acetate， with a volume fraction of 0.683 4 fraction. The mass concentrations of methanol and ethyl acetate were 44.02% and 55.98%， respectively. The contents were very close to the composition of azeotrope recorded in "Data of some common industrial solvent azeotrope". According to calculation， the ethyl acetate contained in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 64.7 ℃
When the tower top temperature rose to about 64.7 ℃， the tower top distillate was methanol， with a volume fraction of 0.449 6 fraction. According to calculation， the methanol in the dense yellow water was basically all evaporated.
From the above， following components were distillated from the 100 fractions of dense yellow water： 0.288 5 fraction of pure acetaldehyde， 0.146 0 fraction of azeotropic ethyl formate， 0.683 4 fraction of azeotropic ethyl acetate and 0.449 6 fraction of pure methanol， and totally， 1.567 6 fractions of distillate were evaporated.
Purification method of ethyl formate and ethyl acetate
The azeotropic ethyl formate and ethyl acetate obtained in "Acquisition of lowboiling point cut fractions including acetaldehyde， ethyl formate and ethyl acetate" were added with proper amounts of foodgrade CaCl2， which formed with methanol in the azeotrope： CaCl2·4CH3OH， which was then removed by distillation. The cut fractions were collected， i.e.， pure ethyl formate and ethyl acetate. The addition amounts of CaCl2 were calculated to be 124.61 g per liter of azeotropic ethyl formate and 325.80 g per liter of azeotropic ethyl acetate. In a word， 0.119 6 fraction of pure natural ethyl formate and 0.360 5 pure natural ethyl acetate could be obtained from 100 fractions of dense yellow water through azeotropic distillation and removal of methanol. Preparation of ester substances including ethyl caproate
It could be seen from Table 2 that the molar ratios of ethanol to acetic acid， propanoic acid， butyric acid， valeric acid and hexanoic acid were 10.7∶1， 68.4∶1， 36.1∶1， 304.1∶1 and 16.3∶1， respectively. The molar ratio of ethanol to total acid was about 5∶1. That is to say that the esterification reaction with the tower liquid after rectification as a raw material is the esterification under excessive ethanol.
The tower liquid （98.432 4 fractions） was input to the rectification device with magnetic stirring together with 6 fractions of ultrastrong solid acid. The magnetic stirrer and heating switch were turned on. The electric current should not be too high， so as to avoid equipment damage due to abrupt fierce heating. Under total reflux， the tower pot temperature was improved to 80 ℃， and the tower insulation circuit was turned on while keeping the total reflux stage， so as to ensure that the esterification reactions were completely finished. Meanwhile， attention should be paid to the rotation speed of the magnet stirrer， which exactly kept the solid acid suspending in the dense yellow water. During the period， the esterification reactions of several organic acids and ethanol in the reactor were as follows：
CH3COOH+C2H5OHSolid acidCH3COOC2H5+H2O（1）
C2H5COOH+C2H5OHSolid acidC2H5COOC2H5+H2O（2）
C3H7COOH+C2H5OHSolid acidC3H7COOC2H5+H2O（3）
C4H9COOH+C2H5OHSolid acidC4H9COOC2H5+H2O（4）
C5H11COOH+C2H5OHSolid acidC5H11COOC2H5+H2O（5）
During above reaction， acetic acid content in the tower liquid was monitored once every 2 h， and the monitoring times and total reaction time were recorded [in future， under the same condition， when repeating this reaction， this reaction time could serve as the time for etherification reaction （1） to reach equilibrium]. When the adjacent two detection results hardly changed， acetic acid content in the tower tended to be stable， and etherification reaction （1） basically reached equilibrium. At this time， the reflux ratio was changed from total reflux to 2， and the tower top temperature was controlled smaller or equal to 70.4 ℃ （azeotropic points of ethanol， water and ethyl acetate： 70.23 ℃; azeotropic points of water and ethyl acetate： 70.4 ℃）， which created the condition for the formation of binary or ternary azeotrope from ethyl acetate， water and a small amount of reactant ethanol. At this tower top temperature， partial water and all ethyl acetate produced in the esterification reaction of acetic acid and ethanol and partial ethanol could form azeotrope which was evaporated from the tower top timely， driving the reactions （1）-（5） towards the right. The distillate evaporated from the tower top flowed to the natural ethyl acetate receiving tank after being condensed by the condenser， and when no distillate flowed out from the tower top at 70.4 ℃， ethyl acetate was no longer produced in the tower. The distillate was detected by GC to contain， respectively， 7.56%、8.71% and 83.73% of ethanol， water and ethyl acetate， which were 0.941 1， 0.855 4 and 9.116 7 fractions， respectively， with a total volume fraction of 10.913 2 fractions. The composition of above distillate is different from "the ternary azeotrope composition of ethanol， water and ethyl acetate and the binary azeotrope composition of water and ethyl acetate" recorded in Data of Some Common Industrial Solvent Azeotrope[15]. It could thus been deemed that this distillate is not a simple ternary azeotrope or simple binary azeotrope， but should be the mixture of binary azeotrope and ternary azeotrope. It was referred to as synthesized multicomponent azeotropic mixture of natural ethyl acetate hereinafter. Above synthesized multicomponent azeotropic mixture of natural ethyl acetate was added with a proper amount of foodgrade CaO， which formed Ca（OH）2 with H2O in the multicomponent azeotrope， and then， the precipitated multicomponent azeotropic mixture was distillated， obtaining the distillate which was the mixture of ethanol and ethyl acetate. The mixture of ethanol and ethyl acetate was called binary mixture of natural ethyl acetate hereinafter. The addition amount of CaO was calculated to be 244.882 7 g per litre of synthesized multicomponent azeotropic mixture of natural ethyl acetate， which could form the precipitate Ca（OH）2 with all water in the multicomponent azeotropic mixture. Secondly， 10.913 2 fractions of synthesized multicomponent azeotropic mixture of natural ethyl acetate could produce 9.116 7 fractions of binary mixture of natural ethyl acetate after removal of water， i.e.， the acetic acid and ethanol in 100 fractions of dense yellow water could produce 9.116 7 fractions of ethyl acetate under heating and catalysis by solid acid （according to check computation， 83.6% of acetic acid contained in the dense yellow water was converted to ethyl acetate）. The ethyl acetate formed with water and ethanol， a multicomponent azeotropic mixture， which could give 10.057 8 fractions of synthesized binary mixture of natural ethyl acetate.
Then， in order to discharge the water produced in esterification reactions （1）（5） and the water inherent in original dense yellow water， above synthesized binary mixture of natural ethyl acetate obtained after water removal， was dropped into the tower at constant speed. With the proceeding of the reaction， on the one hand， the synthesized binary mixture of natural ethyl acetate was dropped into the tower continuously， and on the other hand， the synthesized multicomponent azeotropic mixture of natural ethyl acetate was also evaporated continuously， while the ethyl acetate carried all the water produced in the esterification reactions and the water inherent in the reaction system out of the reaction system （the reevaporated synthesized multicomponent azeotropic mixture of natural ethyl acetate was subjected to water removal according to above method， for latter continuous dropping）. Specifically， at the beginning， the synthesized binary mixture of natural ethyl acetate was dropped faster， so as to carry the water produced in the esterification reactions and the water inherent in the reaction system out of the system， as well the water which was produced but had not carried out of the system. Because the water produced in esterification reactions （1）（5） and the water inherent in the system were carried out of the reaction system timely， esterification reactions （1）（5） were driven towards the right to the largest extent. When the tower top temperature was kept equal to or lower than 70.4 ℃， and no distillate continued to flow out， esterification reactions （1）-（5） basically reached the equilibrium state. Then， the remaining synthesized binary mixture of natural ethyl acetate was all input into the tower， and the tower top temperature increased gradually at this time. When the tower top temperature rose to 71.8 ℃， the cut fraction flowing out from the tower top was the binary azeotrope of ethyl acetate and ethanol. After a certain time， when no distillate continued to flow out and the tower top temperature rose again， stirring and heating were stopped， and the liquid in the tower was filtered， obtaining the filtrate （i.e.， the esterification liquid） and the solid acid catalyst. On the one hand， dropping the synthesized binary mixture of natural ethyl acetate into the reaction tower not only could bring water in the reaction system out to drive the esterification reaction （2）（5） towards the right， but also would not bring other impurities into the reaction system. On the other hand， the ethanol carrying out by the evaporated ethyl acetate was resupplied to the reaction system， thereby ensuring the large molar ratio of ethanol to acid in the reaction system.
Volume fraction of the tower top distillate at 71.8 ℃ and contents of various components in it
It could be known from volume metering and GC analysis that when the tower top temperature was 71.8 ℃， the tower top distillate was the binary azeotrope of ethyl acetate and ethanol， with a volume fraction of 15.887 0 fractions， which included 10.512 6 fractions of ethyl acetate and 5.374 3 fractions of ethanol （according to check computation， the conversion rate of acetic acid reached 96.4%）. The mass contents of ethanol and ethyl acetate were 30.9% and 69.1%， respectively， and the proportional relation between ethanol and ethyl acetate is closer to the composition of azeotrope recorded in Data of Some Common Industrial Solvent Azeotrope[16]. That is to say that 100 fractions of dense yellow water could finally produce 15.887 0 fractions of binary azeotrope of ethyl acetate and ethanol with an ethyl acetate content of 69.1% through catalytic esterification. The binary azeotrope of ethyl acetate and ethanol was called synthesized binary azeotropic natural ethyl acetate.
Volume fractions and contents of various compounds in esterification liquid
The volume of the esterification liquid （tower liquid） was metered to be 74.699 7 fractions. The esterification liquid was analyzed by GC， and the contents of various components are listed in Table 1. The mass content of ethanol， ethyl propionate， ethyl butyrate， ethyl valerate， ethyl lactate and ethyl caproate were 65.7%， 2.3%， 5.7%， 0.2%， 4.8% and 16.7%， respectively. The amounts of ethyl propionate， ethyl butyrate， ethyl valerate， ethyl lactate and ethyl caproate in the esterification liquid were calculated to be 2.138 9， 5.324 7， 0.167 2， 3.861 1 and 15.759 5 fractions， respectively. The total volume fraction of the five esters was 27.251 4 fractions （plus above obtained 0.119 6 fraction of natural ethyl formate， 0.360 5 fraction of natural ethyl acetate and 10.512 6 fractions of synthesized natural ethyl acetate， the total volume fraction of obtained esters was 38.244 1 fractions）. The conversion rates of acetic acid， propionic acid， butyric acid， valeric acid and caproic acid were 96.4 % （further converted）， 79.9%， 89.9%， 21.4% and 95.3%， respectively. It could be seen from the conversion rates of acids that not only the conversion rate of valeric acid was lower than 22% due to its toolow concentration in the substrate， but also the conversion rate of acetic acid was only of 83.6% at the beginning of the esterification reactions， which might be due to the existence of water in the reaction system. However， with the proceeding of the reaction， especially the removal of water in the reaction system， its conversion rate was further improved to 96.4%. Comprehensively from above esterification reactions， it could be seen that the set temperature was proper， the selected process route is rational， and the used solid acid catalyst had a good catalytic effect. [10] ZENG JY， ZHOU XH， ZHANG J， et al. Study on enzymatic synthesis of ethyl caproate in organic medium[J]. Food Science， 2009， 30（6）： 123-127. （in Chinese）
[11] WEI LJ， XU ZG， LIU RR， et al. Enzymatic synthesis of ethyl hexanoate in nonaqueous media[J]. China Brewing， 2001， 20（6）： 20-22. （in Chinese）
[12] GONG WC. A few ideas for liquor production reformation[J]. China wine， 1996（1）： 6-7. （in Chinese）
[13] WANG M， FU XH， FENG XY， et al. Study of different pretreatment methods on gas chromatographic detection of yellow water volatile components influence[J]. Sichuan Food and Fermentation， 2015， 51（1）： 101-106. （in Chinese）
[14] YANG R， ZHOU J. Byproduct in liquor productionyellow water & its development and utilization status[J]. LiquorMaking Science & Technology， 2008（3）： 90-92. （in Chinese）
[15] YAO Q， TU XY， YAO JC. A new approach to byproducts in liquormaking utilization[J]. Liquor Making， 2009， 36（6）： 69-72. （in Chinese）
[16] Shanghai Chemical Industry Designing Institute. Data of some common industrial solvent azeotrope[M]. Shanghai： Shanghai Peoples Publishing House， 1979： 173-180.
Key words Yellow water; Azeotropic distillation; Reaction distillation; Ethyl caproate; Esterification liquid; Liquor flavoring
Luzhouflavor liquor is the white liquor with the largest production and consumption in China， and its major flavor component is ethyl hexanoate， the content of which directly influences the quality of Luzhouflavor liquor[1]. Generally， the ethyl caproate content in new wine brewed by solid fermentation is lower than 1 000 mg/L， and the content of ethyl lactate is higher than that of ethyl caproate. However， the topgrade Luzhouflavor liquor requires an ethyl caproate content of 1 200-2 800 mg/L， and a ratio of ethyl lactate to ethyl caproate in the range of （0.6-0.8）∶1[2]. In addition， the white liquor produced by liquid fermentation， also known as the newprocess white liquor， selects edible alcohol as basic liquor and prepared by distillation through the flavorenhanced fermented grains， and adjusting flavor or blending solid with liquid[3-4]. On one hand， due to edible alcohol hardly contains the various trace components presenting flavor in the white liquor produced by solid fermentation， various spices or flavoring liquids should be added into edible alcohol generally to endow the liquid fermentation liquor with gentle flavor， natural soft sweetness， harmony acid and ester and clear taste[5-7]. However， a large portion of the spices and flavoring liquids are artificially synthesized， for instance， the caproic acid and ethyl caproate playing an important role of adjusting the flavor of liquor are both not fermentation products[8-11]. The application of synthetic spices greatly reduces the approbation of alcohol drinker to liquid fermentation liquor， thereby greatly affecting its market share， especially high end market[12]. On the other hand， the byproduct， yellow water produced during the solid fermentation of fermented grains in pits[13-14] has components similar to the base liquor produced by solid fermentation except some components. Yellow water contains about 38 of the 47 organic compounds affecting liquor flavor. In this study， the contents of caproic acid， acetic acid and butyric acid in yellowed water reached 8 604， 6 534 and 2 836 mg/L， which are more than ten times of ordinary Luzhouflavor liquor. However， these three acids and their esterification products are important flavoring components for white liquor produced by liquid fermentation， as well as the material basis for the aging of white liquor. At present， there are no methods for extracting beneficial substances from yellow water with high efficiency and low cost， which greatly limits the application of yellow water， and much yellow water is abandoned in vain and becomes one of the important water pollutants in liquor industry[15]. Therefore， how to effectively utilize the beneficial components in yellow water has become a hot research subject for white liquor scientists and manufacturers. Therefore， how to effectively utilize the beneficial components in yellow water and end liquor has become a hot research subject for white liquor scientists and manufacturers. Preparation method and process of dense end liquor and dense yellow water
Distillation and concentration of end liquor
At first， 100 fractions （referring to volume fraction unless defined otherwise） of end liquor with an ethanol content of 42% was filled into a distillation tower， and 44.7 fractions of dense end liquor with a high ethanol content was obtained through distillation. The ethanol degree and various organic compounds in the dense end liquor were analyzed by GC， as shown in Table 1. It could be seen from Table 1 that among the 42 organic compounds influencing liquor flavor in the end liquor， 21 kinds were detected in the dense liquor （in addition to ethanol and methanol）. Among them， 19 compounds had a distillate efficiency over 50%， 15 compounds showed a distillate efficiency over 80%， and especially， acetaldehyde， acetal， ethyl caproate and organic acids for synthesizing the synthetic esters including acetic acid， propionic acid， butyric acid， valeric acid and caproic acid exhibited a distillate efficiency over 90%.
Distillation and concentration of important components in yellow water influencing liquor flavor
At first， 97 fractions of yellow water and 3 fractions of dense end liquor were filled into a distillation tower and distillated （the dense end liquor was added to improve ethanol content in yellow water， so as to provide enough ethanol which formed azeotrope with organic matter in yellow water and was evaporated together with the azeotrope）. When 9.5 fractions of distillate was evaporated by elevating the temperature， distillation was stopped. Then an aerator was added into the distillate to introduce ozone， and the aeration was stopped when the yellow color of the distillate faded away， giving the discolored distillate， hereinafter referred to as dense yellow water. Various organic compounds contained in the dense yellow water were analyzed by GC， as shown in Table 1.
It could be seen from Table 1 that there are 38 organic compounds influencing liquor flavor in yellow water， and 21 kinds were detected in the dense yellow water. Among them， 19 compounds had a distillate efficiency over 50%， 15 compounds showed a distillate efficiency over 80%， and especially， acetaldehyde， acetal， ethyl caproate and organic acids for synthesizing the synthetic esters including acetic acid， propionic acid， butyric acid， valeric acid and caproic acid exhibited a distillate efficiency over 90%. More precisely， the concentration of ethyl caproate and ethyl acetate reached 1 966.2 and 3 251.8 mg/L， respectively. Furthermore， the concentrations of the three major acids （hereinafter referred to hexanoic acid， acetic acid and butyric acid） reached 84 904.3， 67 039.3 and 29 056.6 mg/L， respectively. In the dense yellow water， not only the contents of the three major esters satisfied the requirements for topgrade Luzhouflavor liquor， the contents of the three major acids were also 119-175 times of those in ordinary Luzhouflavor liquor （the base liquor in Table 1 served as the representative of ordinary Luzhouflavor liquor， the same as below）. In addition， the total contents of ethanol， water and various organic matters （besides ethanol） in the dense yellow water were 549.1 （69.5°）， 45.7 and 249.7 g/L， respectively. The contents and mole numbers of ethanol and various shortchain organic acids and the molar ratios of ethanol to these acids in dense yellow water are shown in Table 2. Usages of Dense Yellow Water
The dense yellow water acquired by above method has following usages：
Blending of white liquor produced by liquid fermentation
The dense yellow water could be directly used as liquor flavoring， i.e.， 1% of dense yellow water is added into the white liquor produced by liquid fermentation. By such， the concentrations of the three major acids in white liquor produced by liquid fermentation could reach the concentrations in ordinary Luzhouflavor liquor produced by solid fermentation， and 21 organic compounds influencing the flavor of liquor contained in white liquor produced by solid fermentation are also added into white liquor produced by liquid fermentation， thereby making the flavor white liquor produced by liquid fermentation similar to that of white liquor produced by solid fermentation.
Improvement of white liquor produced by solid fermentation
Adding about 1% of dense yellow water into white liquor produced by solid fermentation not only could improve the concentration of the three major acids， but also could provide a material basis for the aging of white liquor. Furthermore， the concentrations of 21 organic compounds influencing the flavor of liquor contained in white liquor produced by solid fermentation are also improved， and the purposes of improving liquor quality and influencing liquor flavor are thereby achieved.
Preparation and concentration of esters and other substances required by liquor flavoring
The dense yellow water also could serve as the raw material for the preparation of the three major esters and other esters， and other liquor flavorings also could be concentrated simultaneously.
Extraction and Preparation of Liquor Flavoring with Dense Yellow Water as a Raw Material
Acquisition of lowboiling point cut fractions including acetaldehyde， ethyl formate and ethyl acetate
One hundred fractions of dense yellow water was added into an azeotropic rectification tower. At the beginning of rectification， the heating switch was turned on. The current should not be too high， so as to avoid equipment damage due to abrupt fierce heating. When the tower pot temperature reached 40 ℃， the tower insulation circuit was turned on. When the tower top temperature reached 20.8 ℃ （the boiling point of acetaldehyde）， the vapor at the tower top passed through the overhead condenser and flew to the acetaldehyde receiving tank， and when the cut fraction began to decrease and the tower top temperature increased gradually， all the natural acetaldehyde was basically vaporized. At this time， the fraction outflow pipe was switched to the position of the ethyl formate receiving tank， and the tower pot temperature was adjusted to around 65 ℃， followed by turning on the tower insulation circuit again. When the tower top temperature reached 51 ℃ （the azeotropic point of methanol and ethyl formate： 51 ℃; the azeotropic point of methanol and ethyl acetate： 62.3 ℃）， the cut fraction flowing through the overhead condenser was mainly the azeotrope of methanol and natural ethyl formate， and when the fraction began to decrease and the tower top temperature increased gradually， all the natural ethyl formate was basically vaporized. At this time， the fraction outflow pipe was switched to the position of the methanol and ethyl acetate receiving tank， and the tower pot temperature was adjusted to 70 ℃， followed by turning on the tower insulation circuit again. When the tower top temperature increased to 62.3 ℃， the cut fraction flowing through the overhead condenser was mainly the azeotrope of methanol and natural ethyl acetate， and when the fraction began to decrease again and the tower top temperature increased gradually， all the natural ethyl acetate was basically vaporized. Then， the fraction outflow pipe was switched to the position of the methanol receiving tank， and when the tower top temperature increased to 64.7 ℃ （the boiling point of methanol）， the cut fraction flowing through the overhead condenser was mainly methanol. When the fraction began to decrease again and the tower top temperature increased gradually， all the methanol was basically vaporized. Finally， distillation was stopped， and liquid in the tower was let out and transferred to the rectification device with magnetic stirring. Above tower top distillates were detected by GC， and subjected to volume metering， obtaining the results as follows：
Volume and component of the tower top distillate at 20.8 ℃
When the tower top temperature rose to about 20.8 ℃， the tower top distillate was acetaldehyde with a volume fraction of 0.288 5 fraction， and according to calculation， the acetaldehyde in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 51 ℃
When the tower top temperature rose to about 51 ℃， the tower top distillate was the binary azeotrope of methanol and ethyl formate， with a volume fraction of 0.146 0 fraction. The mass contents of methanol and ethyl formate were 15.99% and 84.01%， respectively. The contents were very close to the composition of azeotrope recorded in Data of Some Common Industrial Solvent Azeotrope[16]. According to calculation， the ethyl formate contained in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 62.3 ℃
When the tower top temperature rose to about 62.3 ℃， the tower top distillate was the binary azeotrope of methanol and ethyl acetate， with a volume fraction of 0.683 4 fraction. The mass concentrations of methanol and ethyl acetate were 44.02% and 55.98%， respectively. The contents were very close to the composition of azeotrope recorded in "Data of some common industrial solvent azeotrope". According to calculation， the ethyl acetate contained in dense yellow water was basically all evaporated.
Volume and component of the tower top distillate at 64.7 ℃
When the tower top temperature rose to about 64.7 ℃， the tower top distillate was methanol， with a volume fraction of 0.449 6 fraction. According to calculation， the methanol in the dense yellow water was basically all evaporated.
From the above， following components were distillated from the 100 fractions of dense yellow water： 0.288 5 fraction of pure acetaldehyde， 0.146 0 fraction of azeotropic ethyl formate， 0.683 4 fraction of azeotropic ethyl acetate and 0.449 6 fraction of pure methanol， and totally， 1.567 6 fractions of distillate were evaporated.
Purification method of ethyl formate and ethyl acetate
The azeotropic ethyl formate and ethyl acetate obtained in "Acquisition of lowboiling point cut fractions including acetaldehyde， ethyl formate and ethyl acetate" were added with proper amounts of foodgrade CaCl2， which formed with methanol in the azeotrope： CaCl2·4CH3OH， which was then removed by distillation. The cut fractions were collected， i.e.， pure ethyl formate and ethyl acetate. The addition amounts of CaCl2 were calculated to be 124.61 g per liter of azeotropic ethyl formate and 325.80 g per liter of azeotropic ethyl acetate. In a word， 0.119 6 fraction of pure natural ethyl formate and 0.360 5 pure natural ethyl acetate could be obtained from 100 fractions of dense yellow water through azeotropic distillation and removal of methanol. Preparation of ester substances including ethyl caproate
It could be seen from Table 2 that the molar ratios of ethanol to acetic acid， propanoic acid， butyric acid， valeric acid and hexanoic acid were 10.7∶1， 68.4∶1， 36.1∶1， 304.1∶1 and 16.3∶1， respectively. The molar ratio of ethanol to total acid was about 5∶1. That is to say that the esterification reaction with the tower liquid after rectification as a raw material is the esterification under excessive ethanol.
The tower liquid （98.432 4 fractions） was input to the rectification device with magnetic stirring together with 6 fractions of ultrastrong solid acid. The magnetic stirrer and heating switch were turned on. The electric current should not be too high， so as to avoid equipment damage due to abrupt fierce heating. Under total reflux， the tower pot temperature was improved to 80 ℃， and the tower insulation circuit was turned on while keeping the total reflux stage， so as to ensure that the esterification reactions were completely finished. Meanwhile， attention should be paid to the rotation speed of the magnet stirrer， which exactly kept the solid acid suspending in the dense yellow water. During the period， the esterification reactions of several organic acids and ethanol in the reactor were as follows：
CH3COOH+C2H5OHSolid acidCH3COOC2H5+H2O（1）
C2H5COOH+C2H5OHSolid acidC2H5COOC2H5+H2O（2）
C3H7COOH+C2H5OHSolid acidC3H7COOC2H5+H2O（3）
C4H9COOH+C2H5OHSolid acidC4H9COOC2H5+H2O（4）
C5H11COOH+C2H5OHSolid acidC5H11COOC2H5+H2O（5）
During above reaction， acetic acid content in the tower liquid was monitored once every 2 h， and the monitoring times and total reaction time were recorded [in future， under the same condition， when repeating this reaction， this reaction time could serve as the time for etherification reaction （1） to reach equilibrium]. When the adjacent two detection results hardly changed， acetic acid content in the tower tended to be stable， and etherification reaction （1） basically reached equilibrium. At this time， the reflux ratio was changed from total reflux to 2， and the tower top temperature was controlled smaller or equal to 70.4 ℃ （azeotropic points of ethanol， water and ethyl acetate： 70.23 ℃; azeotropic points of water and ethyl acetate： 70.4 ℃）， which created the condition for the formation of binary or ternary azeotrope from ethyl acetate， water and a small amount of reactant ethanol. At this tower top temperature， partial water and all ethyl acetate produced in the esterification reaction of acetic acid and ethanol and partial ethanol could form azeotrope which was evaporated from the tower top timely， driving the reactions （1）-（5） towards the right. The distillate evaporated from the tower top flowed to the natural ethyl acetate receiving tank after being condensed by the condenser， and when no distillate flowed out from the tower top at 70.4 ℃， ethyl acetate was no longer produced in the tower. The distillate was detected by GC to contain， respectively， 7.56%、8.71% and 83.73% of ethanol， water and ethyl acetate， which were 0.941 1， 0.855 4 and 9.116 7 fractions， respectively， with a total volume fraction of 10.913 2 fractions. The composition of above distillate is different from "the ternary azeotrope composition of ethanol， water and ethyl acetate and the binary azeotrope composition of water and ethyl acetate" recorded in Data of Some Common Industrial Solvent Azeotrope[15]. It could thus been deemed that this distillate is not a simple ternary azeotrope or simple binary azeotrope， but should be the mixture of binary azeotrope and ternary azeotrope. It was referred to as synthesized multicomponent azeotropic mixture of natural ethyl acetate hereinafter. Above synthesized multicomponent azeotropic mixture of natural ethyl acetate was added with a proper amount of foodgrade CaO， which formed Ca（OH）2 with H2O in the multicomponent azeotrope， and then， the precipitated multicomponent azeotropic mixture was distillated， obtaining the distillate which was the mixture of ethanol and ethyl acetate. The mixture of ethanol and ethyl acetate was called binary mixture of natural ethyl acetate hereinafter. The addition amount of CaO was calculated to be 244.882 7 g per litre of synthesized multicomponent azeotropic mixture of natural ethyl acetate， which could form the precipitate Ca（OH）2 with all water in the multicomponent azeotropic mixture. Secondly， 10.913 2 fractions of synthesized multicomponent azeotropic mixture of natural ethyl acetate could produce 9.116 7 fractions of binary mixture of natural ethyl acetate after removal of water， i.e.， the acetic acid and ethanol in 100 fractions of dense yellow water could produce 9.116 7 fractions of ethyl acetate under heating and catalysis by solid acid （according to check computation， 83.6% of acetic acid contained in the dense yellow water was converted to ethyl acetate）. The ethyl acetate formed with water and ethanol， a multicomponent azeotropic mixture， which could give 10.057 8 fractions of synthesized binary mixture of natural ethyl acetate.
Then， in order to discharge the water produced in esterification reactions （1）（5） and the water inherent in original dense yellow water， above synthesized binary mixture of natural ethyl acetate obtained after water removal， was dropped into the tower at constant speed. With the proceeding of the reaction， on the one hand， the synthesized binary mixture of natural ethyl acetate was dropped into the tower continuously， and on the other hand， the synthesized multicomponent azeotropic mixture of natural ethyl acetate was also evaporated continuously， while the ethyl acetate carried all the water produced in the esterification reactions and the water inherent in the reaction system out of the reaction system （the reevaporated synthesized multicomponent azeotropic mixture of natural ethyl acetate was subjected to water removal according to above method， for latter continuous dropping）. Specifically， at the beginning， the synthesized binary mixture of natural ethyl acetate was dropped faster， so as to carry the water produced in the esterification reactions and the water inherent in the reaction system out of the system， as well the water which was produced but had not carried out of the system. Because the water produced in esterification reactions （1）（5） and the water inherent in the system were carried out of the reaction system timely， esterification reactions （1）（5） were driven towards the right to the largest extent. When the tower top temperature was kept equal to or lower than 70.4 ℃， and no distillate continued to flow out， esterification reactions （1）-（5） basically reached the equilibrium state. Then， the remaining synthesized binary mixture of natural ethyl acetate was all input into the tower， and the tower top temperature increased gradually at this time. When the tower top temperature rose to 71.8 ℃， the cut fraction flowing out from the tower top was the binary azeotrope of ethyl acetate and ethanol. After a certain time， when no distillate continued to flow out and the tower top temperature rose again， stirring and heating were stopped， and the liquid in the tower was filtered， obtaining the filtrate （i.e.， the esterification liquid） and the solid acid catalyst. On the one hand， dropping the synthesized binary mixture of natural ethyl acetate into the reaction tower not only could bring water in the reaction system out to drive the esterification reaction （2）（5） towards the right， but also would not bring other impurities into the reaction system. On the other hand， the ethanol carrying out by the evaporated ethyl acetate was resupplied to the reaction system， thereby ensuring the large molar ratio of ethanol to acid in the reaction system.
Volume fraction of the tower top distillate at 71.8 ℃ and contents of various components in it
It could be known from volume metering and GC analysis that when the tower top temperature was 71.8 ℃， the tower top distillate was the binary azeotrope of ethyl acetate and ethanol， with a volume fraction of 15.887 0 fractions， which included 10.512 6 fractions of ethyl acetate and 5.374 3 fractions of ethanol （according to check computation， the conversion rate of acetic acid reached 96.4%）. The mass contents of ethanol and ethyl acetate were 30.9% and 69.1%， respectively， and the proportional relation between ethanol and ethyl acetate is closer to the composition of azeotrope recorded in Data of Some Common Industrial Solvent Azeotrope[16]. That is to say that 100 fractions of dense yellow water could finally produce 15.887 0 fractions of binary azeotrope of ethyl acetate and ethanol with an ethyl acetate content of 69.1% through catalytic esterification. The binary azeotrope of ethyl acetate and ethanol was called synthesized binary azeotropic natural ethyl acetate.
Volume fractions and contents of various compounds in esterification liquid
The volume of the esterification liquid （tower liquid） was metered to be 74.699 7 fractions. The esterification liquid was analyzed by GC， and the contents of various components are listed in Table 1. The mass content of ethanol， ethyl propionate， ethyl butyrate， ethyl valerate， ethyl lactate and ethyl caproate were 65.7%， 2.3%， 5.7%， 0.2%， 4.8% and 16.7%， respectively. The amounts of ethyl propionate， ethyl butyrate， ethyl valerate， ethyl lactate and ethyl caproate in the esterification liquid were calculated to be 2.138 9， 5.324 7， 0.167 2， 3.861 1 and 15.759 5 fractions， respectively. The total volume fraction of the five esters was 27.251 4 fractions （plus above obtained 0.119 6 fraction of natural ethyl formate， 0.360 5 fraction of natural ethyl acetate and 10.512 6 fractions of synthesized natural ethyl acetate， the total volume fraction of obtained esters was 38.244 1 fractions）. The conversion rates of acetic acid， propionic acid， butyric acid， valeric acid and caproic acid were 96.4 % （further converted）， 79.9%， 89.9%， 21.4% and 95.3%， respectively. It could be seen from the conversion rates of acids that not only the conversion rate of valeric acid was lower than 22% due to its toolow concentration in the substrate， but also the conversion rate of acetic acid was only of 83.6% at the beginning of the esterification reactions， which might be due to the existence of water in the reaction system. However， with the proceeding of the reaction， especially the removal of water in the reaction system， its conversion rate was further improved to 96.4%. Comprehensively from above esterification reactions， it could be seen that the set temperature was proper， the selected process route is rational， and the used solid acid catalyst had a good catalytic effect. [10] ZENG JY， ZHOU XH， ZHANG J， et al. Study on enzymatic synthesis of ethyl caproate in organic medium[J]. Food Science， 2009， 30（6）： 123-127. （in Chinese）
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