Kinds Of Phenolic Compounds In Jujube And Antioxidant Activity

China is the largest producer of jujube in the world. There has been a long history in cultivating and feeding jujube, the fruit of Zizyphus jujube Miller in China. Jujube has been widely used as food, a functional food additive, and a traditional Chinese medicine for many years. Studies on the functions of jujube are mostly on the polysaccharide and polyphenols in jujube. There have been many studies on the polysaccharide, which mechanism has been clarified. Although there has been some published research on the phenolic compounds and antioxidant activity of jujube, they are mostly concentrated on the antioxidant activity of jujube extracts. As far as we know, little has been reported on the distribution of phenolic compounds in jujube and effects of processing on the phenolic compounds and their antioxidant activity.In this study, the composition of phenolic compounds in different tissues of jujube and the correlation analysis of kinds of phenolic compounds and their antioxidant activity evaluated by different methods were analyzed; a simple, sensitive and accurate high performance liquid chromatography with electrochemical detection (HPLC-ECD) method for simultaneous separation and determination of some phenolic compounds has been established; the composition and distribution of phenolic acids in jujube were analyzed; effects of drying and cooking treatment on the composition of phenolic compounds in jujube and their antioxidant activity were discussed.The main results are as follows.1. The methanolic extracts from different tissues of fresh jujube (pulp, seed and peel) all exhibits antioxidant activity, 1,1-diphenyl-2-picrylhydrazyl radical (DPPH·) and hydroxyl radical (·OH) scavenging activity. The extracts of jujube peel show the higher total phenolic and flavanoid contents and the stronger total antioxidant power and DPPH·and·OH scavenging activity than those of pulp and seed. Furthermore, the antioxidant activities are significantly correlated with total phenolic and flavanoid contents, and there is high correlation in the evaluation methods of antioxidant activity. 2. A simple, sensitive and accurate high performance liquid chromatography with electrochemical detection (HPLC-ECD) method for simultaneous separation and determination of six phenolic compounds in jujube has been established by optimizing the separation conditions and selecting the detection conditions. ECD is more sensitive than diode-array detection (DAD) for the detection of the six phenolic compounds. On the HPLC conditions the six phenolic compounds are well separated within 25 min. The limits of detection (LOD) are lower than 8.7×10-3μg/mL and the recoveries are higher than 98 %. This approach has been applied for the simultaneous determination of the six phenolic compounds in jujube, and can provide a reference to determine phenolic compounds in other products.3. The total phenolic contents of free, esterified, glycosided, and insoluble-bound phenolic acids, and the distribution of phenolic acids and their antioxidant activity are different in different tissues (pulp, seed, and peel) of fresh jujube. The free, esterified, glycosided, and insoluble-bound forms of eight phenolic acids in pulp, seed, and peel of jujube are separated and quantified by high performance liquid chromatography with electrochemical detection (HPLC-ECD). In the whole jujube, p-hydroxybenzoic and cinnamic acids are the most abundant phenolic acids. All quantified phenolic acids are mainly present in jujube peel. Phenolic acids in seed and peel are present in the insoluble-bound form, while, in pulp in the glycosided form. The glycosided and insoluble-bound phenolic acid fractions in jujube pulp represent the highest total phenolic content and the strongest antioxidant activity determined by DPPH and FRAP assays. Our results show that most phenolic compounds with antioxidant activity in different tissues of jujube are present as the glycosided and insoluble-bound forms.4. The composition of phenolic compounds in jujube and their antioxidant activity has greatly changed after jujube is dried. The phenolic content significantly decreased after jujube is dried, the total flavonoids contents did not change significantly, but the procyanidin contents increased significantly. The antioxidant activity of jujube is not correlated with the total flavonoids and procyanidin contents, but with the phenolic acids. There is no significant difference in the antioxidant activity of jujube between the hot-air drying and natural drying, but the phenolic acid content in jujube by hot-air drying is lower than that by natural drying.5. The total phenolic content in jujube treated by high-pressure cooking and steam is higher than that by pressure treatments, especially in pulp of jujube. But the jujube treated by high-pressure treatments is a little bitter. The total phenolic acids contents in the peel and pulp of jujube treated by pressure cooking are both the highest, and those in the peel of jujube sufferring from high-pressure cooking and in the pulp sufferring from high-pressure steam are both the lowest. The total flavanoid content in the jujube treated by high-pressure steam is the highest, and that in jujube peel sufferring from pressure cooking and in jujube pulp sufferring from high-pressure treatments is the highest in the jujube peel and pulp, respectively. However, the procyanidin contents in the jujube peel sufferring from any treatments are not detected, and the content in jujube pulp treated by high-pressure steam is the highest.6. The peel of jujube treated by pressure steam exhibits the highest DPPH·scavenging activity and total antioxidant activity. The DPPH·scavenging activity of jujube pulp sufferring from high-pressure steam is greater, and the total antioxidant activity of pulp sufferring from pressure cooking is greater. The total flavanoids in jujube contributes little to its antioxidant activity, but the total phenolics especially the procyanidin and the phenolic acids play an important role in the antioxidant activity.7. If jujube is treated by high-pressure treatments, steam treatment can better preserve the phenolic compounds in jujube and their antioxidant activity. If the pressure treatments are used, cooking treatment is better.