Study On The Enzymatic Properties And Passivation Effects Of Two Forms Of Polyphenol Oxidase In Lijiang Snow Peach

Lijiang snow peach is a late-maturing variety unique to the Lijiang Plateau in Yunnan Province,China.It is prone to enzymatic browning during processing and storage.Polyphenol oxidase(PPO)is one of the key factors leading to enzymatic browning of fruits and vegetables.In this study,partially purified soluble polyphenol oxidase(sPPO)and membrane bound polyphenol oxidase(mPPO)were obtained by ammonium sulfate precipitation,and its enzymatic properties and its inhibitors have been systematically studied.The effects of heat treatment,high pressure processing(HPP)and high-pressure homogenization(HPH)on the activities of sPPO and mPPO were also studied:(1)The enzymatic properties of sPPO and mPPO were compared.After the crude extracts of sPPO and mPPO were extracted by homogenate method,sPPO and mPPO were partially purified by 40-80% ammonium sulfate precipitation.The specific enzyme activities of sPPO and mPPO were 1138 U/mg and 3954 U/mg,respectively,and the purification times were 1.3 and 2.4 times,respectively.It was found that sPPO and mPPO showed different enzymatic characteristics: sPPO had the highest activity at pH 7.0,but mPPO had two optimum pH values(pH 5.0 and pH 7.0).The optimum temperature for sPPO and mPPO were 30? and 50?,respectively.Compared with sPPO,mPPO is more resistant to high temperature.Both sPPO and mPPO belong to diphenol oxidase,and their common high specific substrate is 4-methylcatechol.In addition,caffeic acid is the most suitable substrate for sPPO.0.1 mM L-cysteine and sodium pyrosulfite could inhibit the activity of sPPO,while mPPO had strong resistance to inhibitors,and the inhibition rate of 10 mM ascorbic acid was only 59%.(2)The effects of heat treatment and non-heat treatment on the activities of sPPO and mPPO were compared.The relative activity of sPPO decreased with the increase of heat treatment temperature.When treated at 85? for 5 min,the relative activity of sPPO was about 10%,and after treated for 25 min,sPPO was completely inactivated.Compared with sPPO,mPPO showed a higher tolerance to heat treatment.With the increase of treatment temperature(45-85?)and treatment time(5-45 min),the residual activity of mPPO remained above 60%.Four kinetic models were used to fit the thermal inactivation curves of sPPO and mPPO.The relative activity of sPPO decreases logarithmically with time,and the first-order kinetic model and Weibull model can be used to describe the inactivity mechanics of sPPO.The Two-fraction model and Fractional-conversion model can be used to fit the nonlinear relationship between the relative activity of mPPO and the heating time.The effect of HPP on PPO activity under 150-550 MPa was studied.After 250-550 MPa HPP treatment,the relative activity of sPPO was reduced by 59-89%.After treatment at 550 MPa for 50 min,the relative activity of sPPO was activated to 202%.mPPO was relatively stable after 550 MPa HPP treatment,and its relative activity was between 88-106%.The activity of sPPO was inhibited after HPH treatment at 50 MPa,100 MPa,200 MPa and300 MPa.The activity of sPPO was only 1.5% when the homogenizing pressure was300 MPa.(3)The effects of HPP on the activities of sPPO and mPPO in fresh-cut peach slices were studied,and the changes of color and other parameters in fresh-cut peach slices during 12 days of storage at 4? were studied.The peach slices treated with 400 MPa and 600 MPa HPP showed better color only on the 0 th day,and the BI(Browning Index)decreased by 3.4 and 11.9,respectively,compared with the untreated group.The L*,a * and b * of peach slices in the HPP group all changed,and the L* values of 200 MPa,400MPa and 600 MPa samples decreased by 26.7%,25.4% and 15.3%,respectively.In the subsequent storage process,the browning index of the HPP group was significantly higher than that of the untreated group,BI continued to increase during the storage period,L* continued to decrease,and a * and b * values increased.In general,the period of the fastest Browning occurred in the HPP group was 2-6 d.TPA(Texture Profile Analysis)was used to test the texture properties of the untreated group and the HPP group.The hardness of the 200 MPa,400 MPa and 600 MPa groups decreased by 22.6%,38.7% and 67.1%,respectively.During the storage period,the hardness of the peach slices in the untreated group and the 200 MPa treatment showed a decrease on the whole,while the hardness changes after 400 MPa and 600 MPa treatment were relatively stable.The phenolic content of untreated group showed an increasing trend during the storage period,while the phenolic content of HPP treated peach slices increased significantly,and then decreased gradually during the storage period.The content of MDA in peach tablets after HPP treatment decreased by at least 37%,indicating that HPP can effectively reduce membrane lipid oxidation.