Study On The Reason Of Color Change In Clear Red Pitaya Juice During Thermosonication Processing And Of Its Color Retention

Red pitaya(Hylocereus polyrhizus)is a fruit with bright color,sweet taste and juicy quality.It is suitable for juice processing besides that fresh-eating.Thermal sterilization is usually used in juice processing,though which may easily leads to deterioration of juices,including color deterioration induced by thermal degradation and loss of natural pigments,reduced nutritional and economic values of juice.In this present study,thermosonication(TS)technology was used for clear red pitaya juice processing.The color change rule of clear red pitaya juice during treatment was explored and causes of its color change were analyzed.Causes of color change during storage were analyzed and its shelf life was predicted.Based on the main causes of discoloration during processing and storage conducted color retention in order to obtain high quality juice with stable color.The main results were as follows:Firstly,the change rule of red pitaya juice color with TS treatment parameters were investigated.Results showed that both a*(redness or greenness)and b*(yellowness or blueness)values decreased significantly(p<0.05),there was no significant change in L*(lightness)value(p>0.05).Temperature was the most important parameter.For example,theΔE value increased twice when temperature increased from 20℃to 60℃at 380 W for 40min.Secondly,the causes of color change of juice treated by TS were analyzed from pigment retention,non-enzymatic and enzymatic browning.Results showed that the content of betacyanins reduced,its degradation rate accelerated when≥50℃,and a linear decline trend of its was observed.According to UPLC-MS analysis,there were 12monomers of betacyanins in red pitaya juice,and the isomerization,decarbonization,deacylation and decarboxylation of betainin,phyllocactin,2′-O-apiosyl-betanin and 2′-O-apiosyl-isophyllocactin led to the decomposition of betacyanins during TS treatment.The increase in 5-hydroxymethylfurfural and browning degree suggested non-enzymatic browning was also one of the important factors leading to color change.Polyphenol oxidase was inactivated by TS,though more than92.27%total polyphenol retained speculating that enzymatic browning caused by polyphenol oxidase wasn’t a main cause of color change.Hence,the decomposition of betacyanins and non-enzymatic browning were major contributors to color change during TS.Thirdly,storage experiments were performed.The obtained results showed that growth of aerobic bacteria,yeasts and molds in juices that stored in glass bottles after treated by TS at475 W and 56℃for 20 min was observed.The shelf life of them were 28 days and 4 days at 4℃and 25℃,respectively,which conformed to the national standard and equalled to samples treated by thermal processing at 83℃for 1.5 min.There were no significant changes on soluble solid content,pH and browning degree of juice during storage(p>0.05).The L*,a*and b*values decreased linearly,resulted toΔE=7 units,that was similar with samples treated at 83℃for 1.5 min(ΔE=7.58).Betacyanins content reduced linearly and was consistent with change of a*value.The residual activities of polyphenol oxidase and peroxidase were low and stable during the whole storage period.In all,speculated that discoloration of juice during storage mainly resulted from decomposition of betacyanins.Pearson analysis revealed thatΔEwas highly correlated with the betacyanins contents(0.95),furtherly suggested that decomposition of betacyanins and growth of native microbiota resulted to color change of red pitaya juice during storage.In addition,there were no significant differences between quality of juices that stored in glass and PET bottles during storage,and juice would be preservedwith better quality at 4℃than that at 25℃.Finally,based on the main causes of discoloration of clear red pitaya juice during TS treatment and followed storage,antioxidants were added for color retention.Results showed that 5 mM V_C had best protective effect for TS treatment juice,the retention of betacyanins increased 2.7%,and a*value was smallest andΔE value was 2.81 units.5 mM V_C could improve 3.5%～5.9%betacynins retention in TS treatment juice,ΔE value changed smaller,suggested V_C can be used for color retention.