摘要
近十年来,多酚因其具有较好的抗氧化性和自由基清除能力而备受关注。本文以龙眼壳与龙眼核为原料,探讨了多酚的提取与分级纯化工艺,采用FRAP法和DPPH自由基清除法对其抗氧化性进行了评价,利用HPLC-ESI-MS和MALDITOF MS对多酚结构进行了初步分析。
通过对龙眼壳与龙眼核的成分分析,发现龙眼壳中粗纤维含量达11%,龙眼核中淀粉含量约35%;二者多酚含量均达4%以上。通过对福建省十二个龙眼品种的测量发现,不同品种间及不同部位(皮、肉与核)中的总多酚、单宁和原花色素的含量均有明显差异,但多酚含量与FRAP值成正相关性,说明多酚是龙眼具有抗氧化性的主要贡献者。根据FRAP值,“乌龙岭”、“油潭本”、“东壁”、“‘八一'早”和“水涨”五种龙眼具有用于天然抗氧化剂开发的可能性。
通过单因素实验和正交试验优化了龙眼壳与龙眼核多酚的提取工艺条件:丙酮浓度50%(v/v),料液比1:100(g/mL),浸提温度70℃,浸提时间2 h,超声20 min,浸提三次,龙眼壳多酚得率为52.9 mg/g;丙酮浓度50%(v/v),料液比1:140(g/mL),浸提温度50℃,浸提时间2 h,超声20 min,浸提三次,龙眼核多酚得率为62.1 mg/g。
采用Sephadex LH-20柱层析对龙眼壳粗提物(LPCE)和龙眼核粗提物(LSCE)进行了纯化,分别得到LPCE-1～7和LSCE-1～6。利用HPLC测定了各馏分中五倍子酸(GA)和鞣花酸(EA)的含量,发现GA主要存在于LPCE-2和LSCE-3中,含量分别为7.49 mg/g和36.75 mg/g;EA主要存在于LPCE-4～5和LSCE-2～4中,含量最高达81.22 mg/g。
利用HPLC-ESI-MS对各馏分中多酚结构的分析检测出包括GA和EA在内的十七种已知和十种未知酚类化合物,并通过MALDI TOF MS进一步证实了龙眼壳中缩合单宁的存在,其聚合度从二到十。
利用FRAP法和DPPH自由基清除法测定了LPCE-2～7与LSCE-1～6和溶剂萃取法得到的各相LPCE-Ⅱ～Ⅳ与LSCE-Ⅱ～Ⅳ的抗氧化性。其中,LPCE-6和LSCE-2、LPCE-Ⅱ和LSCE-Ⅱ的抗氧化性最好。结合对多酚结构的分析可知,龙眼壳与龙眼核的抗氧化性与其所含的水解单宁有关。
Research interest in polyphenolic antioxidants has increased remarkably in the last decade.In this study polyphenols were extracted and then purified from longan peel and seed.Their antioxidant activity by was determined by FRAP assay and scavenging DPPH free radical ability,and the phenolic structures were identified by HPLC-ESI-MS and MALDI TOF MS.
The compositions of longan peel and seeds were analyzed.Results showed that longan peel contains abundant crude fibre(approximately 11%) and longan seeds contain 35%starch.Both longan peel and seed contain more than 4%polyphenols.A large variation of various phenolic contents and FRAP values in different parts(peel, pulp and seed) among 12 cultivars of longan in Fujian Province were observed.A positive linear correlation between antioxidant activity and contents of total polyphenols was established,which confirmed that polyphenol was a main contributor to its antioxidant activity of longan.Based on the FRAP values,the cultivars Wulongling,Youtanben,Dongbi,Bayizao and Shuizhang can be regarded as potential sources of antioxidants.
Conditions for polyphenols extraction from longan peel and seed were optimized by orthogonal experiment:50%acetone aqueous(v/v),ratio of solid to liquid 1:100(g·mL~(-1))(or 1:140 for seed),extracting for 2h under 70℃(or 50℃for seed) along with ultrasonic pretreatment for 20 min.The optimal yields of polyphenol with three repeated extraction from longan peel and seed were 52.9 and 62.1 mg·g~(-1) of dry weight,respectively.
Purification of crude extract by Sephadex LH-20 column chromatography yielded a total of seven and six fractions(LPCE-1～7 and LSCE-1～6),respectively. GA mainly existed in LPCE-2(7.49 mg·g~(-1)) and LSCE-3(36.75 mg·g~(-1)),and EA mainly existed in LPCE-4～5 and LSCE-2～4,whose maximal content was 81.22 mg·g~(-1).
27 phenolic compounds were identified by HPLC-ESI-MS including 10 unknown structures.MALDI TOF MS reconfirmed that the condensed tannins existed in longan peel,and their drgree of polymerization was from 2 to 10.
The antioxidant activities of LPCE-2～7,LSCE-1～6,LPCE-Ⅱ～Ⅳand LSCE-Ⅱ～Ⅳ(obtained by extracting with different organic solvents) were evaluated by FRAP assay and scavenging DPPH free radical ability.As the results showed, LPCE-6,LSCE-2,LPCE-Ⅱand LSCE-Ⅱhad the highest antioxidant activity, respectively.It was inferred that the potent antioxidant activities of longan peel and seed were mainly attributed to hydrolyzed tannins.
引文
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