Antioxidant Properties And Inhibiting Browning Technology Of Raisins

Raisins have attracted much attention because of their high antioxidant capacities. Thispaper mainly investigated the antioxidant activities of30raisins, analyzed the relationshipbetween antioxidant activities and their appearance, evaluated the effect the browning duringdrying on antioxidant activity of raisins, and explored the solution to inhibit browningphenomenon. The detailed contents are as following:(1) Thirty different samples, including black (9varieties), reddish-brown (8), andyellowish-green (13) raisins, were analyzed. Each color group included both seed-bearing andseedless varieties. The CIELAB color parameters(L~*, a~*and b~*),Total phenolic (TPC) andtotal flavonoid (TFC) contents were determined, and antioxidant capacity was assayed asreducing power and as scavenging capacity of2,2-diphenyl-1-picrylhydrazyl (DPPH) and2,2’-azinobis (3-ethylbenzenothiazoline-6-sulfonic acid). It was indicated that TPC and TFCof these raisins ranged from6.79±0.66to26.58±2.71μmol gallic acid equivalents (GAE)/graisin weight (RW), and1.20±0.14to6.90±0.68μmol rutin equivalents (RE)/g RW,respectively. In all samples tested in this study, Otilia had the highest amount of TPC, whileJiahuangputao had the highest amount of TFC. Overall, black and seeded raisins had highervalues for all five parameters than reddish-brown or yellowish-green and seedless ones,respectively.(2) The changes of color parameters, TPC, and antioxident activity valued by DPPH,ABTS radical scavenging capacity and reducing power during the air drying of Thompsonseedless at30℃were investigated. The browning started from the8thday and then increasedwith the drying time, which was indicated as the increase of a~*value and decrease of L~*value,b~*value and c~*value. The total polyphenol content showed an overall decrease during thedrying processing with a sharp increase at the8thday, coincident with the start of browningand the occurrence of a peak of a~*value. The polyphenol content and three antioxidationactivities showed a significant positive correlation ship. It was indicated that polyphenol is themajor antioxidant in Thompson seedless grape and the increase of browning could result inthe decrease of polyphenol content and antioxidation activity of the grape.(3) This study explored the technology condition of blanching and soaking with zincchloride solution to protecting green during drying of green raisins, and ascertained theoptimum conditions: select moderately mature（soluble solid20%~23%） and dark green grape as raw material, blanch in hot water of80℃for6min, then soak with10000mg/kg zincchloride dissolved in80%ethanol solution for20h, lastly dry at30℃.