Antioxidant abillity and total phenolic content of stevia rebaudiana bert extract

V zadnjem času se je povečalo zanimanje za liste Stevie rebaudiana Bert, saj ti predstavljajo vir nekaloričnih sladil, znanih kot steviolni glikozidi, ki v prehrambeni industriji vse pogosteje nadomeščajo tradicionalna sladila in omogočajo zmanjšanje vnosa sladkorja v telo. Predvsem listi rastline vsebujejo visoke vrednosti fenolov, vitamina C, karotenoidov ter klorofilov z antimikrobnimi in antioksidativnimi lastnostmi, kar je povezano z blagodejnimi vplivi na človeško zdravje, npr. z zmanjšanjem pojavnosti kroničnih bolezni in zaviranjem procesa celične oksidacije. Nekatere raziskave so namreč dokazale, da so vodni ekstrakti Stevie rebaudiana Bert naravni zaviralci polifenol oksidaze in peroksidaze. Hkrati steviol glikozidi vplivajo na boljše izločanje hormona inzulina in dvig tolerance na glukozo. Vse to kaže številne možnosti njene uporabe kot naravnega antioksidanta, hkrati pa potrjuje uporabnost Stevie za preventivne in terapevtske namene. Namen diplomske naloge je bil pridobiti ekstrakte iz listov Stevie z različnimi ekstrakcijskimi postopki ob uporabi več organskih topil in obratovalnih pogojev ter preučiti njihov vpliv na kakovost ekstrakta. Poleg klasičnih ekstrakcij smo izvedli tudi ekstrakcijo z ultrazvokom in superkritično ekstrakcijo. Sledile so kemijske analize pridobljenih ekstraktov, ki smo jih izvajali s spektrofotometričnimi metodami. Preučevali smo vsebnost skupnih fenolov in antioksidativno učinkovitost. Na koncu smo izvedli še analizo ekstraktov s HPLC-metodo, s katero smo želeli identificirati nekatere najbolj tipične fenolne spojine, prisotne v ekstraktu Stevie.There has been a great interest lately for the leaves of Stevia rebaudiana Bert, because they present a source for non-caloric sweeteners known as Steviol glycosides, which are frequently used in the food industry as replacements for traditional sweeteners and enable a low sugar intake. The leaves of the plant contain a high amount of phenolic acids, vitamin C, carotenoids and chlorophylls. Those compounds are known of their antimicrobial and antioxidant properties, which results in beneficial effect on the human health. The outcomes represent aslower occurrences of chronical diseases and the slower process of cellular oxidation. Different researches have proven that the water extracts of Stevia rebaudiana Bert are natural inhibitors of polyphenol oxidase and peroxidase. At the same time, the steviol glycosides influence a better insulin secretion in addition to an increment of glucose tolerance. This leades to an increased application of Steviaas a natural antioxidant andits usage for preventive and therapeutic purposes. The aim of the thesis was to obtain extracts from the Stevia leaves, with different extract procedures, while using multiple organic solvents and operational conditions in addition to study their effect on the quality of the extract. In addition to the classic extraction techniques we also performed ultrasound and supercritical extraction. The procedure was followed up by a chemical analysis of the extracts, which were performed by spectrophotometric methods. The content of phenolic compounds and the antioxidant effectiveness have been considered. Finally, the composition of the extracts has been determined by HPLC-method. The most typical phenolic compounds present in the Stevia extract have been identified