Mechanism Of Solubilization, Degradation Of Arabinoxylans And Cloning As Well As Expression Of Acidic Xylanase Gene

The nonstarch polysaccharides of barley grain are primarily composed of arabinoxylans andβ-glucans. While the role of theβ-glucans in malting and brewing has been the subject of continuous and extensive research during the past 40 years, the arabinoxylans have received less attention. However, recently several researchers have suggested that arabinoxylans from barley grains are responsible for problems such as low extract yields, high wort viscosity, decreased rate of filtration, shortened filter life and haze formation in beer.Based on the development of the method for determining arabinoxylans, the contents of arabinoxylans from commercial beers and brewing barley samples were investigated in this paper. The effects of mashing parameters on arabinoxylans solubilization were studied systematically. A mathematical model was employed to predict the concentration of arabinoxylans in wort during mashing process. The effect of arabinoxylans molecular weight on membrane plugging was investigated by scanning electron microscopy as well. In order to realize enzymatic degradation of arabinoxylans in brewing, Aspergillus sp. ZH-26, an acidic xylanase producer, was isolated and identified. The cultivation condition and medium compositions were optimized for improving the xylanase yield. The acidic xylanase gene from Aspergillus sp. ZH-26 was cloned and expressed in E. coli.The gas chromatography method for determining arabinoxylans in beer or wort was developed after converting the hydrolyzed polysaccharides into acetylated aldononitriles. Arabinoxylans and polymeric arabinoxylans contents of thirty-six commercial beers ranged from 479-1239 mg/L and 283-849 mg/L, respectively. The contents of total arabinoxylans and water-extractable arabinoxylans from thirty-six domestic barley ranged from 3.50-5.38% and 0.31-0.67%, respectively. The correlations among the contents of arabinoxylans, polymeric arabinoxylans andβ-glucans in beer, wort gravity, beer viscosity and haze were analyzed by chemometric studies. Arabinoxylans and polymeric arabinoxylans contents in beer gave higher correlation value (0.766 and 0.736, respectively) with beer viscosity than that ofβ-glucans (0.673). The content of arabinoxylans in beer was significantly affected by original wort gravity.The solubilization of arabinoxylans during brewing and the effects of mashing parameters such as mashing temperature, ratio of grist to liquor, grist size and stirring on arabinoxylans solubilization were systematically studied. With the need of decreasing product price, more and more breweries came to use wheat or wheat malt as brewing adjuncts. So the effects of arabinoxylans solubilization on wort extract yield, wort viscosity and wort filtration were also investigated at different adjunct proportion, ratio of grist to liquor, grist size and mashing...