Effects of different drying methods on flavonoid content, antioxidant capacity and volatile compounds in the peel of‘Xiushui Huahong'sweet orange

Astract:【Objective】Bioactive flavonoids, antioxidant capacity and volatile compounds are importantquality components in dried citrus peel.‘Xiushui Huahong'is a special citrus in Jiangxi province. Theaim of this study was to investigate the effects of hot air drying (HAD) , vacuum freeze drying (VFD) , vacu-um drying (VD) and microwave drying (MD) on the contents of total flavonoids and hesperidin, antioxidantcapacity and volatile compounds in dried‘Xiushui Huahong'peels. The possible reasons for differencesamong the four drying treatments were analyzed, and the suitable drying method for‘Xiushui Huahong'orange peel was selected via comprehensive evaluation.【Methods】0.1 gram of powdered sample was sus-pended in 10 m L of 70% methanol in a 50 m L centrifuge tube, and subject to ultrasonic extraction at40 ℃, 160 W for 30 min, and then centrifuged at 5 000 r·min^-1 for 10 min. The supernatant was collectedand the residue was extracted twice following the same procedure. The pooled supernatant was adjusted to25 mL for determination for total flavonoids and hesperidin contents and antioxidant capacity. The totalflavonoid content was determined with a spectrophotometer at 510 nm according to rutin colorimetry. Thehesperidin was determined with the method of Hamdan et al with some modifications. The extract wasforced through the 0.22 μm filtration membrane for high performance liquid chromatography (HPLC) anal-ysis. A reverse phase HPLC was used with a Waters C18 analytical column (4.6 mm×250 mm, 5 μm) . Thebinary solvent system consisted of acetonitrile (A) and 0.5% acetic acid solution (B) , and the gradient pro-gram was as follows:0-30 min, 20%A+80%B. The flow rate was 1.0 m L·min-1 with a column temperatureof 40 ℃ and an injection volume of 10 μL in each experiment. Hesperidin was detected at a wavelength of283 nm. The DPPH (2, 2-diphenyl-1-picrylhydrazyl) radical scavenging capacity was determined by themethod of Lagha et al. The ·OH radical scavenging capacity was determined with the method of Chen etal. The ABTS+[2, 2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) ] radical scavenging capacity wasdetermined using the method of Li et al. The reducing power was assayed according to Kim et al with somemodifications. The volatile flavor compounds in‘Xiushui Huahong'peel dried with different methodswere determined by headspace gas chromatography-mass spectrometry (HS-GC-MS) . The headspace con-ditions were described by Zhang. Chromatographic separations were carried out with a HP-5 MS column (30 m×0.25 mm×0.25 μm) using helium as the carrier gas at 1.0 m L·min-1 in the constant flow mode.Split ratio was 50∶1, and injector temperature was 250 ℃. The oven temperature program was as follow:initial 40 ℃ for 3 min, raised to 140 ℃ at 10 ℃·min-1, then to 250 ℃ at 20 ℃·min-1 and maintained for 5 min. The electron ionization source temperature was maintained at 230 ℃ and mass spectra were obtainedby electronic impact at 70 e V with a scan range of 35-350 AMU·s-1. Temperature of interface and quadru-pole were 280 ℃ and 150 ℃, respectively. Identification of the volatile compounds was achieved by com-paring the GC retention time and mass spectra NIST11.L standard library.【Results】‘Xiushui Huahong'peel dried with different methods had a great difference in appearance. VFD was good for retaining colorand shape; MD could cause large browning. The total flavonoids and hesperidin contents were higher in‘Xiushui Huahong'peel dried with VFD than those dried by the other drying methods.‘Xiushui Hua-hong'peel dried by MD had the highest DPPH and ABTS+radical scavenging capacity and reducing pow-er, while the highest.OH radical scavenging capacity was found VFD treatment. 35 volatile compounds in-cluding terpenes, alchhols, aldehydes, esters and phenols were detected in‘Xiushui Huahong'peel driedwith the 4 methods by HS-GC-MS. The number of volatile compounds in HAD, VFD, VD and MD treat-ments was 11, 9, 12 and 29, respectively. D-limonene was one of the main volatile compounds in‘Xiush-ui Huahong'peel. Its relative content followed the order of HAD (86.618%) >VFD (82.715%) >VD (79.022%) > MD (57.663%) . Results showed that higher temperature drying (MD) could lead to increase innumber of volatile compounds and decrease in relative content of D-limonene.【Conclusion】The resultsof this study indicate drying method should be carefully selected according to processing need. For extrac-tion of bioactive flavonoids from dried‘Xiushui Huahong'peel, we should choose VFD. In view of vola-tile compounds for processing spices, we should choose MD. However, HAD demonstrates the greatest po-tential for industrial application due to its low cost.