Antioxidant activity of natural extracts measured with a novel solid‐state crocin bleaching assay by hot melt extrusion

Abstract

A solid-state adaptation of the crocin bleaching assay was developed by a hot melt extrusion apparatus. Hot melt extrusion was used to prepare pellets from starch, saffron, a radical initiator (AAPH), and different concentrations of antioxidants or plant extracts. The color changes of the extruded pellets provided the antioxidant capacity of the samples by adapting a competition kinetic equation. Without radical initiators, the pellets extruded were reddish, while those with AAPH but without antioxidants were yellow. Instead, when antioxidants were present, the resulting color intensity of the pellets varied proportionally to the antioxidant concentration and type. The relative rate constant (kAH/kCrocin), indicative of antioxidant capacity, appeared to be much higher for Trolox (0.462) and ascorbate (0.276) than for olive leaf (0.019) and clove (0.023) extracts, while the latter showed higher antioxidant activity in a classical liquid assay (ORAC), highlighting the lack of correlation (R2 = 0.448) between the results from the classical ORAC assay and those of the proposed assay, likely because of the harsher processing conditions and stronger matrix interactions. Practical applications This study proposes a novel methodological approach to test the antioxidant activity of natural extracts in complex food formulations under harsh process conditions, like those found in hot melt extrusion. The effect of extrusion process on the remaining antioxidant activity can be tested from pellets made of starch, crocin, a radical initiator, and antioxidants. The color changes of the resulting pellets provide a practical, objective, and direct evidence of the antioxidant activity of the extracts to preserve the crocin substrate loss.