Comparison of polyphenol content and antioxidative properties of fresh and processed domestic rhubarb

Domestic rhubarb is an edible plant gaining in popularity due to its unique taste and health benefits, especially in the western world. One of major bioactive classes of compounds in rhubarb are polyphenols, commonly responsible for antioxidant properties. Among identified polyphenols present in rhubarb are stilbenes, stilbene-glycosides and flavonoids, such as trans-rhapontigenin, trans-rhaponticin and rutin, repsectively. Among its medical uses, anticholesterolemic, antiseptic, antispasmodic, antitumor, astringent, diuretic, purgative and tonic properties have been reported. The edible part of rhubarb is its stalk, while its leaves are considered toxic, due to relatively high amount of oxalic acid. Polyphenol profile of conserved rhubarb stalk (1 year in sugar solution at room temperature) is significantly different from fresh stalk, presumably due to hydrolytic events and reactions with sugar. In this study we want to: a) compare polyphenol profile and antioxidative properties of rhubarb stalk and leaf, b) measure changes in polyphenol content and antioxidative properties occuring during cooking and storing of rhubarb and c) determine useability of rhubarb waste (leaves) as added-value product. We collected samples from home-grown rhubarb. 1-year old rhubarb stored at room temperature in a sugar solution was also used for analysis. The samples were prepared fresh, and under various storage conditions. Dried samples were prepared for comparative analysis. Polyphenol profile was determined with UPLC chromatography on C18 column (Zorbax) and quantified with gallic acid. Total polyphenols and total flavonoids were determined spectrofotometrically. Antioxidant properties were determined using DPPH and ABTS spectrofotometric methods. Our preliminary data show that: a) rhubarb stalk and leaves showed significantly different polyphenol UPLC profiles, b) significant changes in stalk polyphenol profile were detected after 1 year storage in sugar solution at room temperature.