| High ethanol beer, which has a mellow taste and rich aroma, is characterized as highcalories, high fermentation degree and lowered residual sugars, and thus, is very suitable fordrinking in winter. Moreover, high ethanol beer would be a good solution, to so me extent, tothe seasonal variation of production and consumption in beer industry. In order to producehigh ethanol beer, screening of yeast strains was conducted firstly. And then, a novelprocedure for high ethanol beer brewing was established, which could be described as glucosesyrups feeding during the primary fermentation. Glucose syrups were fed successively toreveal the effects of ethanol on sugars uptake and growth of yeast cells. Concernedfermentation parameters were investigated during the process of8%vol ethanol beer brewing,as well as flavor compounds in final beer. Lastly, fermentation conditions were optimized byfactorial design and desirability function to find out the optimal condition and control themain flavor compounds at appropriate levels. Details and results are listed below:(1) Three yeast strains were obtained at the first step. They were domesticated purposely inthe wort containing13%vol ethanol and isolated consecutively. Then, fermentation trials werecarried out in worts with8%vol and13%vol ethanol, respectively. The results show that theselected strain SC-9exhibited good fermentation performance with the highest sugars uptakerate,1.01oP/d. Although interrupted fermentations were observed in worts with13%volethanol, the strain SC-9was still superior to the other two strains. When fermentation stopped,the ethanol concentration of wort fermented by SC-9was15.1%vol which was16.2%higherthan the initial yeast strain.(2) Fed-batch fermentation with glucose syrups as adjuncts for high ethanol beer brewingduring the primary fermentation was established. Glucose syrups feeding were repeated7times during the fermentation process and found that yeast cells proliferated each time afterglucose feeding. The ethanol concentration was up to16.35%vol, but the cell’s viability, sugars uptake rate and yield of ethanol decreased significantly when the ethanol content washigher than14%vol.(3) As glucose feeding proceeded, yeast cells proliferated. The maximum concentration ofyeast cells and proliferation multiple were about2-fold higher compared with traditional beerbrewing. Assimilation of free amino nitrogen was enhanced and assimilation rate was about83.3%. Ethanol content was8.6%vol while the yield decreased due to the extension growth ofyeast. Two additional peaks of diacetyl were observed exactly after each feeding. The highestpeak value of diacetyl concentration was1.90mg/L. Concerning with flavor compounds infinal beer, over-productions of acetaldehyde, isoamyl alcohol and ethyl hexanoate were obtained.The concentrations of them were2-fold or more compared with their threshold values.(4)23full factorial design and regression analysis were employed to explore the effect of wortgravity, fermentation temperature and yeast pitching rate on fermentation parameters and flavorcompounds. Results demonstrate that wort gravity and fermentation temperature gave rise tosignificant influences on fermentation process and flavor compounds. Synthesis of higher alcoholswas strongly enhanced by the glucose uptake and extension growth of yeast cells. Concentration ofesters, generally, fell nearby their threshold values. However, acetaldehyde, isoamyl alcohol and ethylhexanoate were still over-produced. Then, optimization of fermentation conditions was executed, andthe optimal conditions were found to be15oP for initial wort gravity,10oC for fermentationtemperature and20×106cells/ml for yeast pitching rate. A desirability of0.7198was achieved,meaning that71.98%of the goals were achieved. The predicted values against the experiment resultsindicated a good fit. Concentration of acetaldehyde, isoamyl alcohol and ethyl hexanoate, under theoptimal fermentation conditions, resulted64.2%,17.9%and54.4%decrease, respectively. |
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