| The rambutan(Nephelium lappaceum) belongs to the subtropical fruits, which is a tropical species common to Southeast Asia. This fruit is an importantly commercial crop in Asia, where it is consumed fresh, canned or juiced, and appreciated for its exotic appearance and refreshing flavor. However, the residues resulted from consuming or processing was always discarded, which consisted principally peels. In this strudy, the rambutan peels (RP) was selected as research material, and the parameters for the extraction techniques of soluble polyphenols from RP were optimized to obtain the maximum yield of soluble phenolic compounds. Moreover, the free, soluble conjugate, and insoluble-bound phenolic compounds from RP were prepared and their components were analyzed. At last, their antioxidant activities were determined in vitro in the hope of providing a theoretical basis for the rational utilization of rambutan peel resources. The major findings were presented as follows:1. The optimal conditions obtained for the extraction techniques of soluble polyphenols from RP were ethanol concentration of80.85%, extraction time of58.39s, and liquid-to-solid ratio (Liq/sol) of24.51:1. A mean value of213.76mg GAE/g DW was observed from real experiments, which was in agreement with the theoretical value of213.94mg GAE/g DW predicated by the model equation. It confirmed that the response model was adequate to reflect the expected optimization. Ethanol concentration and Liq/sol had significant effects at p<0.01, and the interaction term of ethanol concentration and Liq/sol had significant effects at p<0.01. The final predictive equation obtained was given below:Y=210.90+15.06A+21.59C-4.28AC-29.06A2-20.19B2-20.31C2where Y, A, B and C were soluble phenolic content, ethanol concentration, extraction time, and Liq/sol respectively.2. The results demonstrated that free, soluble conjugate and insoluble-bound fractions of phenolics could be detected in RP. The contents of free, soluble conjugate and insoluble-bound phenolics were185.12,27.98and9.37mg GAE/g DW respectively, and the free fraction accounded for83.21%of the total phenolics. The content of free phenolics was significantly higher than that of soluble conjugate and insoluble-bound fractions (p<0.05). The contents of4-hydroxybenzoic acid and ellagic acid were higher than other phenolic acids in the free fraction. The soluble conjugate fraction mainly contained gallic acid and caffeic acid. The insoluble-bound phenolic fraction was composed of gallic acid, caffeic acid and protocatechuic acid.3. The free fraction of phenolics from RP exhibited strong antioxidant activities in the five in vitro assaying models of reducing power, DPPH-scavenging activities, hydroxide radical scavenging activities, nitrite scavenging activities and lipid peroxidation inhibiting ability. The values of IC50for the five assaying models were4.21,3.55,85.53,17.06and19.12μg/mL respectively. Excepting for the hydroxide radical scavenging activities, the soluble conjugate and insoluble-bound phenolics were also exhibited strong activities, and their values of IC50in the other four assaying models were67.65and81.17μg/mL,41.96and18.78μg/mL,17.24and18.56μg/mL,20.45and14.13μg/mL, respectively. The free fraction showed the similiar IC50with Vc for the reducing power and DPPH-scavenging activities, and significantly lower than that of the other two fractions’ and BHT’s (p<0.05). The insoluble-bound fraction of phenolics exhibited significant higher activity in lipid peroxidation inhibiting ability model than BHT (p<0.05).In conclusion, the free fraction of phenolics from RP showed the highest content, and was more easily extracted compared with the soluble conjugate and insoluble-bound phenolics. Moreover, it had strong antioxidant activities in vitro. So it will be very useful for the utilization of RP as antioxidant in food and medicinal preparations. |
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