Research On Beer Foam Proteins-Structural Characteristics And Changes During Brewing

Beer foam is one of the most appealing beer characteristics that a consumer to evaluate the beer quality. Rich and stable beer foam could provide drinkers the visual feel about freshness and refreshingness, the tactile to the lips and mouthful, and the aromas of beer. Proteins play an important role in the beer foam quality, contributing to the foam promotion and stability. In this paper, beer and barley malt were taken as research materials and the protein composition and structure as the research point. The structure characteristic of beer foam-proteins and the changes in composition and secondary structure of proteins during brewing were researched, so as to improve the theory basis of beer foam-proteins, and supply research theories and ideas for improving beer foam quality and the anticipation of foam performance.Composition and structure characteristics of foam-proteins from different type beers and four cultivars barley malt water soluble proteins were researched and analyzed. SDS-PAGE showed that beer foam-proteins mainly composed by 40kDa and 5-17kDa, and there were some differences in diverse beer samples. And there was a good correlation between beer foam-proteins content and beer foam stability. Beer foam-protein bands were positively identified and categorized into 11 protein species, which included protein Z4, LTP1, BDAI-1, and some proteinase inhibitors and trace amount hordein fragments. Among these Proteins, 40kDa was protein Z, and 9kDa was LTP1. In the MS, the incomplete fragments of protein Z and LTP1 were also detected, which could be related to the strenuous brewing conditions and some enzyme degradation. The research on beer foam-proteins'physicochemical and structure properties indicated that foam-proteins were characteristic of more hydrophobic amino acid, higher surface hydrophobicity, more random coil andβ-sheet structure and a higher stability for heat, pH and ethanol. And there were obvious differences between beer foam-proteins and barley malt water soluble proteins in the relative molecular weight, surface hydrophobicity and secondary structure, which indicated that beer foam-proteins were in an unfolding form, so as to gathering when bubble formatting and beneficial to the beer foam stability. Barley malt is the main source of beer foam-proteins, and the significant differences between the two suggest that brewing process must have an important impact in proteins.Take Subei malt and Australian malt as raw materials, the changes in composition and structure of proteins during the brewing process were studied, and the important factor was preliminary discussed. The two cultivars had similar changing regularities during brewing. Throughout the brewing process, most proteins were removed, forming two predominant proteins from barley grains survive: 40kDa and 5-17kDa, which were similar in wort, fermented liquor and beer. This composition was formed at the end of mashing basically, and thoughout boiling and fermentation, protein band around 40kDa and 9kDa were intensive, becoming the major components. Changes on properties of protein surface hydrophobicity and secondary structure indicated that, structure change of proteins began in mashing, boiling was decisive, and the change was unobvious during fermentation though the protein contents decreased. All in all, after brewing process proteins mainly show random coil andβ-sheet form, being characteristic of unfolding form.Though heat-stable components of malt water soluble proteins had higher structure stability and similar protein molecular weight in wort, fermented liquor and beer, protein structure had occurred significantly change during boiling. After preliminary research on influencing factors, the significantly changes were closely related to the mode of heating and reducing substance, and the complexity of the composition of the wort was the important factor of the protein structural changes.