Ultrasound-assisted extraction of anthocyanins from red onion skin, and determination antioxidant activities in red onion

Anthocyanins, a class of polyphenols, are the pigments responsible for the red, purple and blue colors of most fruits and vegetables. Interest in using anthocyanins as value added natural alternatives to the use of synthetic food colorings has increased in recent years, especially with the recognition of potential adverse health effects from the consumption of synthetic colorings[2]. It is known that anthocyanin pigments act as strong antioxidants and are antiinflammatory, with antimutagenic and cancer chemopreventive activities [1]. Several technologies for the extraction of anthocyanin pigments from various plants sources have been proposed in literature [1]. conventional solvent extraction techniques have some limitations. These include mass transfer resistances, the large amount of solvent used in classical extraction methods, long extraction time, high extraction temperature, health related risks and low efficiency and extraction yield[3,4]. Ultrasound-assisted extraction can be used as a tool to overcome the drawbacks of conventional solvent extraction methods and to improve some benefits of the solvent extraction process [5]. In this study, Ultrasound-assisted extraction of anthocyanin compounds was optimized, Among the experimental range used for testing the variables, extraction with 80% solvent concentration, 50 mL/g liquid solid ratio, 65⁰C extraction temperature and 11.5 min extraction time resulted in the highest level of TAC (13.22 mg C3G/ g DW). The total anthocyanins content in onion was utilized by the pH differential method. Anthocyanin content of freeze dried onion sample was 13.22 (mg anthocyanidin-3-glucoside/100 g dried onion). The total flavonols content in red onion skin was measured by spectrophotometric method. The total polyphenol contents were also measured by means of Folin- Ciocalteu method. Moreover, all the concentrations of anthocyanins, flavonoids and polyphenols showed significantly positive correlations with antioxidant activities measured by DPPH , and FRAP assays.