Effects Of Polyphenol Oxidase On "Guankou Grape" Wine Oxidative Browning

�Guankou grape� is one of Vitis vinifera�Vitis labrusca.The rapid oxidation during processing restrict their deep processing.The rapid oxidation might be caused by Polyphenol Oxidase(Polyphenol Oxidase,PPO).Basing on the studies of oxidation mechanism during the processing of grapes and wine,Riesling as a control grape variety.The experiment studied the reasons of rapid oxidation after draining,and effects of PPO on its oxidative browning,the enzymatic characteristics of PPO of two grapes at the same time.The PPOs were also tried to be purified from the two grapes.The main results were as follows:(1)The must of �Guankou grape� and Riesling were less oxidated after heating.So it is the enzymatic browning that occurred after draining,resulting in the rapid oxidation and browning.PPO promote the browning.So enzyme deactivation and inhibitor could control the oxidation browning to a extent.(2)In the untreated must,the enzyme activity of polyphenol oxidase was relatively high as soon as draing,it is the enzymatic browning that occurred.And the polyphenol was oxidated,so theie contents were relatively few.During the fermentation,the enzyme activity of polyphenol oxidase decreased.The contents of total phenols,total flavanols,flavonol and tartaric acid ester increased slowly overall,the browning degree reduced.While in the heated must,the browning degree remains low.The contents of those phenolic substance had smaller growth in the the second day,and less decreased at fifth day,reaching maximum at the end of fermentation.(3)The PPOs from Riesling and �Guankou grape� displaied only diphenolase activity,and no activity was found when monophenols and trihydroxyphenols as substrates.For�Guankou grape�,the catalytic affinity was in the order: catechinic acid > caffeic acid >4-methylcatechol>catechol,the catalytic efficiency order is: caffeic acid>catechinic acid>4-methylcatechol > catechol.For Riesling grape PPO,the catalytic affinity and efficiency order both is: caffeic acid > 4-methylcatechol > catechinic acid > catechol.The maximum activity of �Guankou grape� PPO was reached at pH 4.5 and 35?,Riesling grape PPO was at pH 4.5 and 40?.Moreover,their enzyme stability were higher than that of other grapes,sincethe minimum time needed to achieve drastically inactivation was 50 min at 75? for Riesling.And �Guankou grape� PPO was about 40 min at 75?.(4)There is an important problems during purification of grapes PPO for the richness in phenolics pose.The PPOs of two grapes were partial purified.�Guankou grape� PPO was2.06-fold purified.And Riesling PPO was purified 8.86-fold.