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

ZHOU Ming; LU Jianqing; CHEN Jinyin; SHEN Yonggen; ZHU Fengni; WANG Qiang

doi:10.13925/j.cnki.gsxb.20170310

Abstract：[Objective] Bioactive flavonoids,antioxidant capacity and volatile compounds are important quality components in dried citrus peel.'Xiushui Huahong'is a special citrus in Jiangxi province.The aim of this study was to investigate the effects of hot air drying (HAD),vacuum freeze drying (VFD),vacuum drying (VD) and microwave drying (MD) on the contents of total flavonoids and hesperidin,antioxidant capacity and volatile compounds in dried'Xiushui Huahong'peels.The possible reasons for differences among 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 suspended in 10 mL of 70％ methanol in a 50 mL centrifuge tube,and subject to ultrasonic extraction at 40 ℃,160 W for 30 min,and then centrifuged at 5 000 r·min-1 for 10 min.The supernatant was collected and the residue was extracted twice following the same procedure.The pooled supernatant was adjusted to 25 mL for determination for total flavonoids and hesperidin contents and antioxidant capacity.The total flavonoid content was determined with a spectrophotometer at 510 nm according to rutin colorimetry.The hesperidin was determined with the method of Hamdan et al with some modifications.The extract was forced through the 0.22 μm filtration membrane for high performance liquid chromatography (HPLC) analysis.A reverse phase HPLC was used with a Waters C 18 analytical column (4.6 mm×250 mm,5 μm).The binary solvent system consisted of acetonitrile (A) and 0.5％ acetic acid solution (B),and the gradient program was as follows:0-30 min,20％A+80％B.The flow rate was 1.0 mL· min-1 with a column temperature of 40 ℃ and an injection volume of 10 μL in each experiment.Hesperidin was detected at a wavelength of 283 nm.The DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging capacity was determined by the method of Lagha et al.The · OH radical scavenging capacity was determined with the method of Chen et al.The ABTS+ [2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid)] radical scavenging capacity was determined using the method of Li et al.The reducing power was assayed according to Kim et al with some modifications.The volatile flavor compounds in'Xiushui Huahong'peel dried with different methods were determined by headspace gas chromatography-mass spectrometry (HS-GC-MS).The headspace conditions were described by Zhang.Chromatographic separations were carried out with a HP-5MS column (30 m×0.25 mm×0.25 μm) using helium as the carrier gas at 1.0 mL· rmin-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 obtained by electronic impact at 70 eV with a scan range of 35-350 AMU· s-1.Temperature of interface and quadrupole were 280 ℃ and 150 ℃,respectively.Identification of the volatile compounds was achieved by comparing 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 color and 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 Huahong'peel dried by MD had the highest DPPH and ABTS+ radical scavenging capacity and reducing power,while the highest OH radical scavenging capacity was found VFD treatment.35 volatile compounds including terpenes,alchhols,aldehydes,esters and phenols were detected in'Xiushui Huahong'peel dried with the 4 methods by HS-GC-MS.The number of volatile compounds in HAD,VFD,VD and MD treatments was 11,9,12 and 29,respectively.D-limonene was one of the main volatile compounds in'Xiushui 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 in number of volatile compounds and decrease in relative content of D-limonene.[Conclusion] The results of this study indicate drying method should be carefully selected according to processing need.For extraction of bioactive flavonoids from dried'Xiushui Huahong'peel,we should choose VFD.In view of volatile compounds for processing spices,we should choose MD.However,HAD demonstrates the greatest potential for industrial application due to its low cost.