Effects of different drying methods on polyphenol profile and antioxidant activities in longan (Ficus carica Linn.)

Abstract: 【Objective】Longan is widely distributed in subtropical zones and usually used to treat chronic diseases in Traditional Chinese Medicine. Longan can be eaten fresh or processed as dried fruit,juice, wine and so on. During processing, drying is the most commonly used method. Since the varia-tion of nutrients and bioactive compounds of different fruits subjected to different drying methods is dis-tinct, in order to process high-quality dried longan fruits, the effects of vacuum drying (VD), freeze dry-ing (FD) and hot air drying (OD) on total polyphenols, phenolic profile and antioxidant activities of lon-gan were studied.【Methods】Fresh longan fruits at commercial maturity stage were collected from the garden in the Institute of China Southern Subtropical Crop Research (Zhanjiang, Guangdong). Longan fruits were manually peeled and enucleated. Then, the longan pulps were dehydrated by VD, FD and OD. The longan pericarps and kernels were dehydrated only by OD. For VD, longan pulps were dried in the vacuum chamber at 70 ℃ for 48 h. For FD, before vacuum freeze drying, the pulps were frozen at -60 ℃ for 12 h. Then, the frozen pulps were dried in an experimental vacuum lyophilizer for 48 h.For OD, longan fruits were placed in a forced air circulation oven at 70 ℃ for 48 h. The dried fruits were grinded rapidly and sieved thorough a 60-mesh screen. Moisture content was measured according to the national standard method. Fresh pulps and dried powder were ultrasonic extracted three times at 40 ℃ for 30 min using 80% aqueous methanol. Folin-Ciocalteu colorimetric method was used to mea-sure the total polyphenols. Ultra high pressure liquid chromatography and triple quadruple mass spec-trometry (6460QQQ- MS, Agilent, California, USA) equipped with an electrospray ionization source (UPLC-QQQ-MS/MS) was used to analyze the polyphenol profile of longan. ZORBAX Eclipse Plus C18 column (100 mm × 2.1 mm i.d., 1.8 µm, Agilent, Waldbronn, Germany) was used. In addition, theantioxidant activities of longan polyphenols were determined with DPPH (2, 2-diphenyl-1-picrylhydra-zyl), ABTS (2, 2’- azino-bis (3-ethylbenzothiazoline-6-sulfonate) and FRAP (ferric ion reducing antiox-idant power) methods. At last, the Sparse Partial Least Squares regression (sPLS) was used to analyze the correlation between polyphenols and antioxidant activities. All the values were presented as means ±SD (n=3). Nonlinear regression and multivariate linear regression analysis were performed by using the IBM SPSS statistics (version 20.0). Statistically significant differences among the treatments were ana-lyzed by one-way analysis of variance (AVOVA), followed by Tukey test. Principle component analysis was performed in R (×64, 3.4.3) using the mixOmics package.【Results】The contents of total polyphe-nols and antioxidant activity in longan pericarp were significantly higher than those in longan pulp and kernel. The total phenolic contents of longan pulp dried by VD, FD and OD were 2.934, 5.288 and2.855 mg·g^-1, respectively. The DPPH radical scavenging ability was 2.537, 5.478 and 1.501 mg·100 g-1, respectively. The ABTS radical scavenging ability was 0.919, 1.216 and 0.870 mg·100 g^-1, respectively,and the FRAP was 5.810, 6.634 and 5.538 mg·100 g^-1, respectively. In this study, 17 polyphenols includ-ing gallic acid, chlorogenic acid, 4-hydroxycinnamic acid, ferulic acid, cinnamic acid, naringin, rutin, quercetin-7-O-β-D-glucopyranoside, isoquercitrin, hesperidin, phloridzin, rhoifolin, methyl hesperidin, quercetin, naringenin, luteolin and phloretin were identified and quantified by UPLC-QQQ- MS/MS,among which hesperidin, phloridzin, methyl hesperidin, naringin, luteolin and phloretin were firstly re-ported in longan fruits. The results also showed that phenolic profile in different parts of longan was dis-tinctly different. The variety and content of polyphenols in pericarp were higher than those in pulp and kernel. Except for gallic acid, longan pericarp contained all the other 16 polyphenols. Quercetin7-O-β-D glucoside was detected only in the pericarp, while gallic acid was detected only in the kernel. Isoquer-cetin, quercetin and luteolin were mainly found in the pericarp and kernel, but not detected in the pulp. 4-Hydroxycinnamic acid, ferulic acid, rutin and hesperidin were the main phenolic compounds in fresh longan pulps. Compared with fresh fruits, contents of most of phenolic components in dried longan pulps were significantly reduced by all the drying methods. Expectedly, contents of most of the polyphe-nols in FD pulps were higher than those in VD and OD pulps. There was no significant difference in polyphenol profile between hot air dried and vacuum dried longan pulps. At last, correlation analysis in-dicated that all the phenolic compounds except for 4-hydroxycinnamic acid and methyl hesperidin were positively correlated with total polyphenols content and antioxidant activity. However, cinnamic acid contributed little to the antioxidant activity of longan polyphenols.【Conclusion】These information in-dicated that longan pericarp and kernel also contained abundant phenolic compounds and possessed high antioxidant activities, which had potential utilization value. Different drying methods showed dif- ferent effects on the phenolic components and the antioxidant activity of longan pulps. Compared with VD and OD, FD can reduce the loss of phenolic components and antioxidant activity in longan.