Structure Characteristics Of Arabinoxylan Extracted From Cloudy Wheat Beer

Wheat beer is more and more popular for its golden color,strong fruit and malt aroma,white and delicate foam,as well as fresh and mellow taste.Arabinoxylan(AX),as one of the main non-starch polysaccharides,is the main component of cell walls in barley and wheat malt.In the beer brewing,AX can enhance the stability of wheat beer foam,promote the taste of beer,and increase the sensory characteristics of wheat beer.Therefore,this thesis was dependent on whole barely beer,mixed barley-wheat beer,and the whole wheat beer made by different ratio of barley malt,wheat malt as brewing raw materials.The characteristic of basic indexes,monosaccharide composition,ferulic acid(FA)and 4-Vinylguaiacol(4-VG)in three different beers during the process of brewing could provide theory for the brewing techniques in beer manufacture.To explore the different characteristics and distribution of AX in wheat beer,beer foam and defoamed beer were separated by separating funnel.Enzyme and gradient ethanol precipitation were used to purified AX in this thesis.The microstructure,monosaccharides composition,degree of substitution(A/X)and average degree of polymerization(av DP)of AX,molecular weight and FTIR spectrum were investigated to reveal the molecular structural characteristics of AX in wheat beer.The main research results are as follows:1.The content of FA during wort preparation with different proportion of raw materials was significantly different.With the development of saccharification process,the content of FA decreased and become stability in the wort.Ferulic acid was decarboxylated to produce 4-vinylguaiacol(4-VG)which led to the concent of 4-VG increase during beer fermentation and then stabilized during beer storage.2.The highest monosaccharide content in wort preparation and fermentation was glucose,followed by xylose,arabinose,mannose and galactose.Arabinose showed a decreasing trend in the wort preparation and then tended to be stable.The changes of xylose and arabinose were similar.During the wort preparation and fermentation,arabinoxylan decreased and then stabilized.The degree of substitution of arabinoxylan remained at about 0.50,and the average degree of polymerization increased at the saccharification stage,and stabilized at about 3.00 during fermentation and post-storage.3.The physico-chemical indexes of raw material were analyzed.The powder extraction rates of whole beer,beer foam and defoamed beer were 31.88 ± 0.37 %,32.93 ± 0.72 % and 31.83 ± 1.10 %,respectively.The analysis of monosaccharides component showed that the glucose content was the highest in raw materials.The contents of xylose and arabinose were 20.50 ～ 23.11 mg/g and 15.17 ～ 16.99 mg/g,respectively.The contents of mannose and galactose were the lowest,about 5.00 ～ 6.81 mg/g.4.The extraction rate of AX with different ethanol concentration was various.About 20% extraction rate was found in 67 % ethanol precipitation which was the highest.Besides,the extraction rate of AX from 75 % ethanol precipitate was next,about 19 %,while 12.56 ～ 14.01 % was analyzed from 50 % ethanol precipitate.Under the same ethanol concentration condition,there was no significant difference in extraction rate between whole beer,beer foam and defoamed beer.After precipitation,the protein content in AX was significantly lower than that in raw material.5.The appearance and microstructure of AX purified by enzymatic hydrolysis and precipitated by gradient ethanol showed significantly different.AX precipitated by 50 % ethanol was slightly gray-white and presented a loose state.There were many irregular flaky structures according SEM assay.However,AX precipitated by 67 % ethanol showed brown and blocky.It displayed large and small globules under microscopic observation.AX powders precipitated by 75 % and 80 % ethanol showed brown and hard powders with spherical structures.Thus,the appearance and microstructure of AX indicated that gradient ethanol could extract AX with different apparent structures.6.There were significant differences in monosaccharide composition among AX.Arabinose and xylose were the main components of AX precipitated by 50 % ethanol.The content of xylose was the highest,followed by arabinose.Mannose was first found in 50 % ethanol precipitation,and a small amount of galactose was detected,but no glucose exist.In 67 % ethanol precipitation,the contents of arabinose and xylose were decreased compared with 50% ethanol precipitates,in which xylose content decreased significantly,mannose content decreased sharply,galactose content increased,and glucose began to appear.With the increase of ethanol concentration to 75 % and 80 %,the contents of arabinose and xylose in AX were lower regardless of the whole beer,beer foam and defoamed beer,while the glucose content increased significantly.Therefore,the monosaccharide composition of AX which precipitated by gradient ethanol showed significant differences.Xylose was mainly concentrated in 50 % ethanol precipitates,while arabinose was mainly present in 50 % and 67 % ethanol precipitates,and the highest content was in the beer foam.7.The AX in ethanol precipitates was increased after enzymatic hydrolysis and ethanol precipitation in wheat beer,defoamed beer and beer foam by analyzing the content of polysaccharides.In the 50 % ethanol precipitate,AX content and the average degree of polymerization was the highest,and the mannose polymer appeared.In 67 % ethanol precipitates,AX content and av DP value decreased compared with 50 % ethanol precipitates and A/X value increased slightly.What’s more,arabinogalactan content reached the highest in 67 % ethanol precipitates.In 75 % and 80 % ethanol precipitates,AX content decreased,while the contents of dextran and starch increased sharply.Therefore,we can find that gradient ethanol precipitation can be used as an effective way to separate and extract AX which was the highest in 50 % ethanol precipitate and more accumulated in beer foam with the highest degree of polymerization and substitution.With the increase of ethanol concentration,the content of AX was decreased,while the content of dextran and starch increased rapidly.Arabinogalactan is more likely to present in 67 % ethanol precipitate.8.AX with different molecular weight can be separated and precipitated by gradient ethanol.The highest weight-average molecular weight of AX was obtained by 50 % ethanol precipitation,but its molecular weight distribution was wide.With the increase of ethanol concentration,the molecular weight decreased sharply,but the distribution was narrowed and uniform.AX with high molecular weight was mainly present in 50 % and 67 % ethanol precipitated components.9.FTIR was carried out on the purified AX and it was found that the scope of a polysaccharide absorption of intermolecular O-H stretching vibration in 3600 ～ 3000cm-1,while the scope of 3000 ～ 2800 cm-1 was C-H absorb vibration.The absorption of 800 ～ 1200 cm-1 was the fingerprint area of carbohydrate.AX precipitated by 50% and 67 % ethanol showed the different molecular structure,however,there were no significant differences in cloudy wheat beer,beer foam and defoamed beer in FTIR spectrum.