Research On Beer Foam Proteins

Foam is one of important traits of beer. It also plays an significant role in evaluating beer's quality. Foam protein is the backbone for beer foam and one of crucial factors in determining beer foam production. Meanwhile, beer foam quality was influenced greatly by the foam protein content and quality.Protein Z and LTP are two key components of beer foam protein. Three different methods of protein separation were compared, and SDS-PAGE was chosen for the further study. The method of quantitative determination of beer foam protein by SDS-PAGE was established to detect the protein change in beer brewing. The difference of protein patterns and quality of foam protein was compared among different beers preliminarily.Proteins were isolated according to various molecular weight by SDS-PAGE, protein Z (40KD) and LTP (LTP1,9KD and LTP2,7KD) were obtained. Based on difference of isoelectric points, AKTA with DEAE anion column were used to isolate foam protein, and protein Z and LTP were better isolated. By two dimensional electrophoresis and above two methods, better isolation results were achieved, while the process was troublesome and time consuming. SDS-PAGE was chosen for further study considering all factors.Gel imaging system and Gel-Pro Analyzer software were combined to measuring the foam protein content. Operation conditions for SDS-PAGE were as follows:Separating gel concentration of 18%, Stacking gel Voltage of 75V, Separating gel Voltage of 220V. The protein detection limit and quantitation limit were 0.093mg/L and 0.31mg/L, while LTP detection limit and quantitation limit were 0.077mg/L was and 0.29mg/L. Relative standard deviation of detectable concentration of protein Z and LTP were all less than 10%, demonstrating that the method was accuracy and repeatability.Protein change during beer brewing was closely observed. During mashing, protein with molecular weight of 53KD and of 20KD-40KD was degraded. Meanwhile, protein Z and LTP content were not greatly changed. The protein content was decreased gradually during wort boiling. After that, the content of protein Z, LTP1 and LTP2 was decreased by 11%,32% and 26%, respectively. During main fermentation, content of protein Z, LTP1 and LTP2 was decreased by 12%,59% and 31%, respectively. Protein Z and LTP1 were remained stable in post fermentation, while LTP2 content was decreased gradually and reached 22% at the end.Protein was enriched during foam formation. In the raw beer, liquid from beer foam and raffinate of beer, the content of Glu (Glu and Gln) and Pro were the most, the content of Arg, Cys-s, Val,Ile and Leu was increased, while Met content was decreased in the liquid from beer foam.