Valorization of Peel Wastes of Purple Turnip (Brassica rapa L.): Extraction of Polyphenolics Through Ultrasonic-Assisted Extraction and Investigation of Changes in Total Phenolic Content, Total Monomeric Anthocyanin Content and Total Antioxidant Capacity During in vitro Gastro-Intestinal Digestion

Abstract

Fruits and vegetables include high levels of bioactive compounds with health-promoting effects. Especially, peels, which are one of the generaly inedible fractions of fruit or vegetable, have higher amount of polyphenolic compounds than their pulps or seeds, since they accumulate in their peels. Purple turnip (Brassica rapa L.) is one of the important members of Brassicaceae family due to its nutritional composition. In this context, the aim of the study was to reveal the potency of peels from purple turnip as a natural antioxidant ingredient To reach this aim, the spectrophotometric assays including the Folin-Ciocalteu method for total phenolic content (TPC), the pH differential method for total monomeric anthocyanin content (TMAC) and CUPRAC and DPPH methods for total antioxidant capacity (TAC) before and after in vitro gastro-intestinal digestion were carried out. According to results of the study, TPC and TMAC of the polyphenolic axtract obtained by ultrasound-assisted extraction were 169.29±6.89 mg GAE/g dw and 159.53±10.82 mg CGE/L, respectively. The TAC of the extracts was 44.19±0.10 mg TE/g dw in CUPRAC assay and 38.52±0.06 mg TE/g dw in DPPH assay. The bioaccessibilities of total phenolics and anthocyanins from extracts after in vitro digestion process were approximately 17% and 6% and the antioxidant capacity of the bioaccessible extract fraction was approximately 28% (CUPRAC) and 0% (DPPH).