Study On The Quality Of Canned Litchi By Ultra-high Pressure Treatment

Litchi is a kind of tropical fruits with high nutritional value and delicious taste.However,it has a very short shelf life because they are easily affected by the environment.It is generally processed into litchi cans to extend the shelf life.Conventional heat treatment is generally used in the processing,resulting in a large loss of quality?The ultra-high pressure treatment can solve the above problems,but the mechanism of browning of litchi after ultra-high pressure treatment,the change of texture and organization during storage,as well as the bactericidal effect and kinetic model of soft-packaged canned litchi with other treatments aren't yet systematic.This paper,therefore,is intended to deal with the above issues,and the main research contents and results are as follows:(1)Kinetic analysis of the treatment of the UHP combined with lemon juice fence on the litchi and their bactericidal effect.Bactericidal effect of the group that treated by 100%lemon juice under 380 MPa was almost equivalent to the group of 500Pa without lemon juice,indicating that the the barrier effect of lemon juice combined with ultra-high pressure sterilization can achieve strong bactericidal effect under low ultra-high pressure.The treatments group using lemon juice(100%)under 380 MPa or 500 MPa,as well as using lemon juice(50%,100%)under 500 MPa for 15 min,presented no expanding bags stored at 37? for 7 d,which met the requirements of commercial sterility.The Weibull model was used to fit the bactericidal kinetic curve,it turned out that the fitting coefficient R~2was greater than 0.900.The combination of pressure and lemon juice affected the shape of the bactericidal curve,where the kinetic curves of 380 MPa synergistic lemon juice and 500MPa were almost coincided.(2)Effect of UHP or heat treatment on the quality of soft-packed litchi.The activity of PPO was not detected by heat treatment.UHP treatment could activate POD.Although heat treatment can inhibit the activity of POD,it cannot be completely inhibited.The UHP treatment group can maintain the content of Vc and polyphenol better than the heat-treated group,which caused the loss rate of Vc were about 92.5%and the total phenol loss rates were about 56.3%.There was no significant difference in TSS content and p H between treatment groups(P>0.05).(3)Effect of UHP on the texture,color and tissue morphology of litchi.As the pressure and pressurization time increased,the hardness of litchi decreased first and then gradually increased.Pressurized at 380 MPa and 500 MPa for 10?15 min,the elasticity of the flesh could be recovered,but chewiness with increasing pressure presented a clear downward trend.After being treated with 380 MPa and 500 MPa for 15 min,compared with the group treated by UHP when stored at 5? for 3 months,the color was still acceptable.For tissue morphology,the greater the pressure was,the thinner the cell wall of the pulp was.After litchi flesh was stored at 25? for 12 d,the cells of pulp of the control group had no partition,while the cells by the UHP treatment still showed visible partition.(4)Effect of UHP on the the browning mechanism of litchi.The results showed that polyphenol oxidase(PPO)activity declined significantly with the increase of pressure,while the peroxidase(POD)activity was activated.Compared with the control,there was no obvious difference in the content of vitamin C(Vc),5-hydroxymethylfurfural(5-HMF)and polyphenol(P>0.05)after storage at 35? for 7 days.UHP accelerated the browning of litchi,that is,the greater the pressure was,the more serious the browning of was.It is concluded that browning caused by UHP may be related to the activation of POD and the oxidative polymerization of phenolics.