The Role Of Laccase In Longan Pericarp Browning And The Effects Ofthehumidity Control On Processing On Litchi Storage

Longan（Dimocarpus longan Lour.）and litchi（Litchi chinensis Sonn.）are popular and important fruits in southern C hina,with high economic value,and it is popular in consumers both at home and overseas.Longan fruit and litchi fruit turn brown rapidly after harvest,which is the main factor limiting its long-term storage and transportation,resulting in short shelf life and low price.So far the mechanism of pericarp browning is not satisfactorily explained,particularly the key gene still not confirmed,and the methods of preventing pericarp browning and maintaining freshness are not effective.Based on the results of our research,Laccase（LAC）may be a key enzyme in litchi pericarp.However,there is no lacquer enzyme in the presence of longan peel.At present in the production practice,longan and lychee after the anti-brown preservation did not get satisfactory results.Based on the above research background,this paper compared the effect of smoked sulfur on the browning and the laccase activity of the longan after the mining,and isolated and purified the longan laccase from the peel,to explore the role of laccase in browning of longan fruit.In addition,materials for litchi,from the perspective of humidity controltreatment,the effect of new polyacrylamide composite humidity regulator on the freshness of postharvest litchi fruit was discussed.The aim was to provide reference for the postharvest preservation technology oflongan and litchi.The main results are as follows:1.By Na₂S₂O₅（sodium metabisulfite）and HC l reaction to release SO₂ method,calculate the release of SO₂ content of 2%,the fresh longan for sulfur treatment.The results showed thatsulfur treatment could inhibit the activity of laccase and prolong the browning of the longan and the postharvest peel of the longan.The re sults showed that the SO₂ could inhibit the laccase activity of the longan to delay the longan postharvest pericarp browning and rotting flesh.2.Through ammonium sulfate fractionation,DEAE-Sepharose column chromatography,Sephadex G-200 gelfiltration chromatography,epicatechinenzyme was purified from longan pericarp to homogenuous,the epicatechin-degrading enzyme with a purification factor of 65.2 was obtained from fresh longan peel.SDS-PAGE gelelectrophoresis,Western blot analysis to determine the laccase,the molecular weight of about 110 kDa,similar to the size of litchi laccase,suggesting that it is longan laccase.3.The enzymatic properties of longan laccase were studied.The optimum reaction temperature is 40oC.And the temperature was kept at different temperature for 60min,The activity was stable in the temperature range of 20-40oC.After reaching or exceeding 40oC,the enzyme activity decreased sharply;the optimum reaction p H was 6,which was stable in the range of p H 5.5-8,showing strong acid and alkali resistance.According to the K m of epicatechin,catechin,chlorogenic acid and 4-methyl-catechol were measured at 40oC and pH 5.0,indicating that theenzyme has a certain affinity for these substrates.Laccase equivalent substrate has a strong affinity.Six common representative compounds of Na₂SO₃,phytic acid,ethanol,EDTA,kojic acid and L-cysteine were selected as inhibitors of enzymatic reaction of laccase,and catechins were used as substrate for enzymatic reaction.It was found that Na₂SO₃ and L-cysteine had a strong inhibitory effect on laccase.Select the common metal ions,Fe²⁺and Fe³⁺,Go²⁺,Mg²⁺,K⁺,Ba²⁺,Zn²⁺,Cu²⁺,Ca²⁺metal ions was studied on the effect of laccase,found that 0.1 mol/LBa²⁺ions have a activatio n effect on longan laccase,other metal ions have weak inhibitory effect on laccase.4.Polyacrylamide new composite humidity regulator has a higher water absorption capacity.The different content of the compound humidity regulator for the treatment of litchi,and found that the amount of compound humidity regulator added 5.0 g on lychee（20 about 450 g）the best preservation effect.At the same time,through the determination of anthocyanin content,TSS,Vc and TA,it was found that the addition of 5.0 g ofhygroscopic agent could keep the appearance and nutrient quality of litchi fruit duringstorage.Comprehensive consideration,the compound moisture absorption capacity of 5.0 g can play a better role in regulating humidity.