Determination of Phytochemicals, Antioxidant and Pigment Stability of Selected Agro-waste for Application as Food Colorants. P

Abstract

Valorization of agro-wastes is an eco-friendly method for the conversion of agricultural wastes into valuable products or energy, aimed at reducing waste, creating economic value, and promoting sustainability. Valorisation of agro-waste for natural colourants was carried out through the acidifiedmethanol extraction of sugarcane peel, pumpkin peel, onion peel, and sorghum stalks. Extracts were analyzed for phytochemicals, DPPH antioxidant activity (62.5–500 µg mL⁻¹) and pigment stability across pH 2–8, light (6 h) and heat (60°C, 1 h). Sorghum stalk extract contained the highest phenolics and tannins, giving the strongest DPPH scavenging (83% at 500 µg mL⁻¹) and superior light stability (4% loss). Sugarcane peel yielded comparable phenolics/flavonoids (≈ 21 µg g⁻¹) and 80% DPPH activity, yet lost 14% colour under light. Pumpkin peel, rich in flavonoids (22 µg g⁻¹) and in carotenoids, displayed the highest pH-independent absorbance and an unusual photo-intensification, but 20% thermal loss. Onion peel, despite having the highest flavonoids, showed the lowest DPPH activity (62%), indicating that glycoside-linked quercetin is less redox-active. All pigments were most intense at pH 2, declining sharply at neutral/alkaline pH. Sorghum stalk emerges as the most balanced antioxidant-colorant for acidic foods, whereas pumpkin peel suits wider pH formulations. The study confirms these wastes as sustainable, functional sources of natural pigments that may find applications in the food industry.