Study On The Effect Of Malt Protein Modification On Wort Foam Protein

The foam proteins in beer are mainly from malt.The abundance and molecular weight of foam proteins play important roles in the quality of beer foam.In this paper,the relationships among Kolbach index,protein content,protease and high molecular protein content in wort were studied and established,which could be used to predict the protein content related to foam protein in wort.By preparing malts with different Kolbach index,the relationships among protease and high molecular weight protein content in wort and fermentation broth,,as well as the dynamics of protein ratios in malt,wort and fermentation liquid were explored.The results provided basis in malting process for regulating protein solubility and improving beer foam.It was found that the high molecular weight protein content was significantly positively correlated with protein content in wort when suing malt from different habitats and barley varieties but similar saccharification procedure.There was a good linear equation among high molecular weight protein content and Kolbach index and malt protein content in malt.Therefore,these two indexes can be used to predict the content of high molecular weight protein in wort,using the variety corresponded relative linear equation.The protease activity,protein molecular consistitution and foam stability in wort and fermentation liquid and foam stability of fermented liquid using of Copeland and Gairdner malt with different Kolbach index were analyzed.It was found that when use malt with low Kolbach index,the wort and fermentation liquid has a relatively lowprotease activity and high concent of high molecular weight protein.As malt Kolbach index increased,the protease activity was higher,with less amount of high molecular weight protein.When the soluble protein of malt were properly or excessively solubilized,the protease activity increased significantly,while the high molecular weight protein content decreased significantly.When the Kolbach index varied in the range of 32.8%~50.3%,there exhibited a linear relationship between protease and high molecular weight protein content when.In worts made from Copeland and Gairdner malts,the proportion of protein with a molecular weight of ≤10 kDa increased gradually as Kolbach index and protease activity increased,which were mainly derived from the degradation of 10~20 kDa proteins.Comparing of the beer fermentation liquid with a proper-and over-dissolved malt,the latter one had a relatively weaker foam stability,and less amount of proteins of 10~20 kDa proteins.And the decrease of the 10~20 kDa protein may result in a deficiency of cytoskeletal proteins in the foam,and less interaction between proteins with other moleculars in wort,which will lead to a low foam stability.Hence,how to retain a molecular weight of 10~20 kDa in 16.9%~25.0% is the key factor to have a good beer foam.During he 24 hours of the steeping and sprouting process,a variety of enzymes were formed and activated.It was showed that the contents of protein with 10~20 kDa and 35~100 kDa was closely related with this activation process.After 24 h of germination,it can be found that the change of protein proportion in each molecular segment was directly related to protease activity.At the same time,by adjusting the kolbach index in the malting process to keep 10~20 kDa proteins accounts for 18%~21%,the foam stability of fermentation liquid of beer could meet the requirements of its national standard sensory evaluation.