Extraction, Composition Analysis, And Thermal Stability Of Anthocyanins From Purple-fleshed Sweet Potato

Purple-fleshed sweet potato(Ipomoea batatas L.) is an annual hebaceous plant within Ipomoea of Convolvulaceae. It is a highly nutritious vegetable that is rich in vitamins, minerals, dietary fiber and dietary carbohydrates. In addition to the listed nutritive components, purple-fleshed sweet potato contains large amount of anthocyanins, which have excellent biological properties. In this dissertation, the extraction technology of purple-fleshed sweet potato anthocyanins was studied, and then the composition was analyzed using LC-ESI-MS technology, finally the thermal stability in various pH values aqueous solutions and fruit juices was investigated. The main results and findings were summarized as follows:With citric acid as reagent, the optimum process parameters of extraction temperature73.7℃, ratio of solid to liquid1:55and citric acid concentration1.2%were obtained for anthocyanins extraction from purple-fleshed sweet potato powder by RSM. The maximum predicted anthocyanins yield was480.6mg/100g dry base, but actual anthocyanins yield was479.8mg/100g dry base.In the Jihei NO.1cultivar, twelve anthocyanins were isolated and identified using LC-ESI-MS, they were Cyanidin-3-sophoroside-5-glucoside, Peonidin-3-sophoroside-5-glucoside, Cyanidin-3-p-hydroxybenzoyI sophoroside-5-glucoside, Peonidin-3-p-hydroxybenzoyl sophoroside-5-glucoside, Cyanidin-3-(6"-feruloyl sophoroside)-5-glucoside, Peonidin-3-(6"-feruloyl sophoroside)-5-glucoside, Cyanidin-3-(6"-caffeoyl sophoroside)-5-glucoside, Cyanidin-3-caffeoyl-p-hydroxybenzoy1sophoroside-5-glucoside, Cyanidin-3-(6"-caffeoyl-6’"-feruloyl sophoroside)-5-glucoside, Peonidin-3-caffeoyl sophoroside-5-glucoside, Peonidin-3-caffeoyl-p-hydroxybenzoyl sophoroside-5-glucoside, and Peonidin-3-(6"-caffeoyl-6’"-feruloyl sophoroside)-5-glucoside. The most abundant anthocyanin was Peonidin-3-caffeoyl sophoroside-5-glucoside. Almost all the anthocyanins in the Jihei NO.1cultivar were acylated, peonidins and cyanidins were the predominant forms. At the same time, one delphinidin which may be a unique anthocyanin was isolated and found, its impossible molecular structure was delphinidin-3,5-diglucoside, or delphinidin-3-sophoroside, or delphinidin-3-caffeoyl glucoside. The anthocyanins from Jihei NO.1were glycosylated with glucose or sophorose, and acylated with p-hydroxybenzoic acid, ferulic acid or caffeic acid.Kinetics of thermal degradation results showed that the degradation of purple-fleshed sweet potato anthocyanins in various pH values aqueous solutions and fruit juices followed a first-order reaction kinetics model at80,90and100℃. The stability of anthocyanins depended on temperature, pH and solvent matrix. The anthocyanins were most stable at pH3, the half-lives at80,90and100℃were96.3,25.6and12.6h respectively, and the activation energy was111.57kJ/mol. Especially, higher stability was obtained at pH3and4than at pH2. The anthocyanins were most stable in apple juice, followed by pear, grapefruit, orange, tangerine, kiwifruit and lemon juices. The half-lives of anthocyanins in apple juice at80,90and100℃were32.4,17.0and8.1h respectively, the activation energy was75.68kJ/mol. Worst thermal stability was obtained in lemon juice (pH2.23), this may be contributed to the lower pH value and higher ascorbic acid content of the juice. The anthocyanins were more heat-resistant but easier to be influenced by small temperature changes in simple aqueous solutions than in complex juices.