Effects Of Zinc Ion On Yeast Autolysis Of Cloudy Wheat Beer

Cloudy wheat beer contains active beer yeast.When yeast is influenced by all kinds of physical and chemistry factors,protease begins to degrade intracellular matericals which lead to the breakdown of yeast cell wall,and yeast autolysis subsequently begins.Polysaccharide,nitrogen compounds,nucleotides,lipids and other materials are released in the process of yeast autolysis,making the stability of flavor,colloid,foam,biological of beer body worse and effecting the appearance and quality of cloudy wheat beer.In beer brewing,zinc is the one essential trace element for yeast growth and metabolism.Zinc deficienty can lead to the stagenation of fermention,but excess zinc is toxic to yeast cells.The paper explored preliminarily the relationship of the acid protease activity and yeast properties in the production of cloudy wheat beer,and adding yeast cultured with different concentrations of zinc ion to beer,then confirmed appropriate zinc ion addition amount by accelerated autolysis experiment,which was verified by fementation and accelerated autolysis tests finally.The main results were as follows:(1)Changes of acid protease acitivity of cloudy wheat beer and yeast properties in the process of brewingThe acid protease activity of cloudy wheat beer fermentation liquor rase obviously after tank sealed and during eaelier stage of post-fermentation,the increment of which was about0.015 u.Yeast mortality of tankA and tankB increased to 2.99% and 2.27% respectively on the 4th day after cooling.Yeast budding ratio increased significantly when tank fulled,and decreased to 8.45% and 6.54% at the end of cooling.The rate of yeast glycogen dyed was 0when tank was fulled and increased to 100.00% during the sealing,then decreased rapidly.(2)Effects of zinc-added yeast on beer during the accelerated autolysis processS.cerevisiae sp cultivated by different zinc ions was added to ordinary bottled beer,and stored at 40 ? to accelarate autolysis.According to the change of beer index,the speed and degree of yeast autolysis could be judged.Acid protease activity of beer increased at first and then decreased along with accelerated autolysis time,and acid protease activity with adding0.4 mg/L zinc ion culturing yeast had been low,which had the maximum of 0.060 u on the 9th day.After 24 h of accelerated autolysis,beer with adding 0.4 mg/L zinc ion culturing yeast had the maximum number of live yeast cell,which was 5.26×105 cfu/mL.The content of protein in beer showed a trend of descending after ascending with accelerated autolysis time,while the content of ?-AN,total nitrogen,FAAs and purines increased at first and then kept stable or decreased lightly.The ?-AN content in beer with adding 0.2 mg/L and 0.4 mg/L zinc ion cultivating yeast changed obviously between the 5th and 7th day,and the increment was 4.84 mg/L and 5.44 mg/L respectively.The content of total nitrogen in beer with adding 0.4 mg/L zinc ion cultivating yeast had the maximum of306.33 mg/L on the 9th day.On the 1st day of the accelerated autolysis period,the FAAs content in beer with adding 0.2 mg/L and 0.4mg/L zinc cultivating yeast was apparently lower than that with adding 0.8 mg/L and 1.6 mg/L zinc ion cultivating yeast.During accelerated autolysis period,the purine content in beer with adding 0.4 mg/L zinc ion cultivating yeast changed more slowly.Therefore,adding 0.4 mg/L zinc ion to wort could delay beer yeast autolysis based on the extent and the velocity of yeast autolysis.(3)Effects of zinc ion addtion on yeast autolysis in cloudy wheat beerProtease activity of cloudy wheat beer with adding no zinc ion was higher than that with adding 0.4 mg/L zinc ion during fermentation and the accelerated autolysis period,and the difference of which was 0.012 ~ 0.024 u from the 1st to the 11 th day of the accelerated storage.Acid protease activities of both were up to maximum on the 3rd day of the accelerated autolysis period,which were 0.055 u and 0.043 u respectively.Protein content with adding 0.4 mg/L zinc ion and adding no zinc ion were up to maximum on the 3rd day of the accelerated autolysis period,which were 157.03 mg/L and 150.03 mg/L,respectively.During the 1st to the 3rd day of the accelerated storage,the ?-AN content of cloudy wheat beer with adding no and 0.4 mg/L zinc ion increased by 94.72 mg/L and 90.27 mg/L,respectively.During the 3rd to the 5th day of the accelerated storage,the increment of ?-AN content with adding no and 0.4 mg/L zinc were 2.27 mg/L and 3.15 mg/L respectively,and?-AN content of both were up to maximum on the 7th day of the accelerated autolysis period.