Hydrolyzing Stevioside With Acid Catalyst And Antioxidant Activity Of Steviol Glycosides

In this thesis, preparing steviol or isosteviol by hydrolyzing stevioside with acid catalystwas studied. Catalysts such as inorganic acid, organic acid, acidic ionic liquids, super solidacid and acid resins were screened and the appropriate catalyst was found which could beapplied for producing steviol or isosteviol. In addition, the reaction condition was optimized.The results showed that steviol was obtained from hydrolysis of stevioside at low acidconcentration, while isosteviol was attained from the same hydrolysis with higher acidconcentration due to the rearrangement of steviol. The antioxidant ativity of glycosides andtheir hydrolysis derivatives were also tested. The main conclusions are as followed:1. Experimented the effect of the factors of temperature, St concentration, catalyst amountand reaction time on the hydrolysis of stevioside catalyzed by hydrochloric acid orsulfamic acid. The results showed that stevioside conversion was99.8%, a yield of steviolreached83.6%and a yield of isosteviol reached12.4%catalyzed by hydrochloric acidwith the optimum conditions. At the same time, the active energy was obtained as68.84kJ/mol from hydrolysis using hydrochloric acid as catalyst. With the sulfamic acidcatalyst, a conversion of stevioside of99.6%, a yield of steviol of81.5%and a yield ofisosteviol of11.7%were obtained under the optimum conditions.2. Selected [MIMPS]3PW12O40ionic liquid as the catalyst for preparing isosteviol.Thorough optimizing factors, the process conditions were:300g/L of St,95oC reactiontemperature, catalyst concentration0.0075mol/L, in20h. The results showed thatstevioside conversion was99.7%, a yield of isosteviol reached65.3%and a yield ofsteviol reached26.7%catalyzed by [MIMPS]3PW12O40with the optimum conditions.3.732acidic resin and smaller particles001×7.5(Ca/Na) acidic resin were respectivelyserved as catalysts to investigate the effect of different particle size of resin on theconversion of stevioside for isosteviol. With732acidic resin catalyst, a conversion ofstevioside of99.8%, a yield of isosteviol of88.8%and a yield of steviol of5.1%wereobtained with the optimum condition. Moreover, the conversion of stevioside was95.1%and the yield of isosteviol was70.6%after the732acidic resin was applied in thereaction for5times. The results showed that stevioside conversion was99.7%, a yield ofisosteviol reached91.9%and a yield of steviol reached4.8%catalyzed by001×7.5(Ca/Na) acidic resin catalyst. After10times reuse of the catalyst, the conversionof stevioside still reached97.6%and a yield of isosteviol still was75.3%.4. Resin catalyst chitosan bead was used for developing a continuous flow reactor for thehydrolysis of stevioside. Identified that the larger particles732acidic resin was moresuitable in a packed bed reactor. Packed bed reactor containing6g resin catalyst withfeeding of stevioside solution(20g/L) at a flow rate of5.0mL/h supported maximumproductivity of isosteviol at95oC. The results showed that stevioside conversion wasnearly97.0%, a productivity of isosteviol reached1.54g/L/h. Using120hourscontinuously, stevioside conversion remained above90.0%and isosteviol productionstrenght was0.48g/L/h.5. Under the same molar concentrations, antioxidant activity of steviol glycosides were preliminarily explored. The experiment results showed that stevioside had the strongestability to remove DPPH free radical and hydroxyl free radical, also with the strongestreducing ability. With concentration0.0057mmol/mL, clearance ratio of DPPH freeradical and hydroxyl free radical were63.6%,13.6%respectivly. Experimented theeliminating of superoxide anion and the ability of anti-lipid oxidation, rebaudioside Awas the best, while steviol and isosteviol has no anti-lipid oxidation activity.