Isolation and characterization of the anthocyanins derived from red radishes (Raphanus sativus L.) and the protective ability of β-lactoglobulin against heat-induced oxidation.

This study employed the "two-step dialysis" method and AB-8 or D101 macroporous resin chromatography to isolate the anthocyanins in red radishes (ARR). The red radish juice was dialyzed twice at 3000 and 500 Da, respectively. UHPLC-QqQ-MS/MS revealed 24 types of ARRs, of which pelargonidin (Pg)-3-diglucoside-5-(malonyl)glucoside (P3D5MG), Pg-3-diglucoside-5-glucoside (P3D5G), Pg-3-(feruloyl)diglucoside-5-(malonyl)glucoside (P3FD5MG), Pg, and malvidin (Mv) represented the main compounds. The total anthocyanin content in the ARR prepared via the "two-step dialysis" method was 29.69% and 18.44% higher than that obtained using AB-8 and D101 macroporous resins, respectively. The ARRs inhibited heat-induced β-lactoglobulin (β-Lg) oxidation. The amino acid residue microenvironment and secondary β-Lg structure were modified via ARR binding. The energy involved in P3D5MG and β-Lg binding was -392 kJ/mol, which was significantly lower than that during the binding process of P3D5M, P3FD5MG, Pg, and Mv to β-Lg (-338 to -168 kJ/mol). These results indicated that "two-step dialysis" was a promising method for deriving natural pigment with strong antioxidant activity from red radishes. PRACTICAL APPLICATION: As a natural food colorant, anthocyanins have attracted increasing attention in the food industry in recent years. This study used "two-step dialysis" to effectively separate ARRs. Moreover, the anthocyanins in ARR can bind to β-Lg to protect against heating-induced oxidation. Therefore, ARRs may not only act as a food pigment but also as antioxidants.