Study On The Effect Of Proteinase A On Draft Beer Foam Stability

Draft beer refers to the one which adopts the technology of low-temperature microporous membrane filtration and sterile filling. Since entering the markets, the draft beer has been popular among consumers for its unique fresh taste. However, because of proteinase A remaining in the beer, which could degrade the foam proteins, t he draft beer has a serious defect in the aspect of stability, which has become the bottleneck restricting the healthy development of draft beer. Therefore, it is necessary to study the foam stability problems of draft beer.For the purpose of industrial application and the establishment of control standards of proteinase A, the change rules of proteinase A activity in the process of brewing and storage were studied, and the effects of yeast, alpha amino nitrogen, hops and hop products on proteinase A were systematically investigated. Furthermore, the control standards and measures for beer foam stability had been put forward, basing on the above findings. The study aims is to provide technological references which can solve the general degenerative problems of draft beer foam stability in industrial production. The main results were as follows:1. Based on the change rules of draft beer foam stability during the shelf storage period, the study found that:(1) Compared with foam proteins, proteinase A was the main reason affecting draft beer foam stability, and significant negative correlation(r =-0.837, P<0.05) between proteinase A and foam stability was observed.(2) Under ambient temperature storage conditions, the decline of foam stability in draft beer mainly occurred in the first two months, and the decline would become flat or even disappear after two months. In addition, proteinase A activity also existed degenerative trend, but did not change obviously with foam stability. There may be related to the precursors of proteinase A.(3) If the yeast used in the fermentation accorded with the following conditions that: the storage time of yeast was too long(five days), or the generation of yeast was over three generations, or the proteinase A activity recycled from the fermentation liquid of high concentration were higher, there was a linear decline trend of draft beer foam stability during storage, while the proteinase A activity presented the opposite case.2. Based on the study of factors which influence the secretion of proteinase A, the control measures were established to improve draft beer foam stability, as follows :(1) The upgrade measures from saccharification technique : the draft beer foam stability would be significantly improved when we used the particle hops and the content of alpha amino nitrogen was controlled at around 190 mg/L in industrial production.(2) The standards of inoculated yeast: the storage time of yeast should not be more than three days, the generations should be less than three, the proteinase A activity ought to less than 15×10-3 U/mL, and the viability of yeast had more than 92%(detected by flow cytometry).(3) The controlling standards of brewing process and finished products: on the one hand, the yeast should be recycled as soon as possible. On the other hand, the proteinase A activity from fermenting wort and bottled commercial beer should not be more than 20×10-5 U/mL and 15×10-5 U/mL, respectively.According to the above control targets, the foam stability of draft beer could be improved, and achieved excellent standard in six months(≥180 s).3. In order to improving the defects and deficiencies of the current detection method of proteinase A activity, this experiment shown that there was a significant negative correlation between the proteinase A activity and the viability of yeast cells measured by flow cytometry instrument(r =-0.982, P<0.05). Therefore, we can replace the fluorescent peptide using flow cytometry assay method to determine whether yeast cells were suitable for revaccination.