Physico-chemical properties, antioxidant properties, and <i>in-vitro</i> digestibility of quinoa and amaranth grain flours and water-extracted starches

Quinoa and amaranth pseudo cereals are utilised as grain and flours in baked foods, where starch is a cost-effective functional ingredient compared to protein. However, flour components (protein, fibre, lipids) interact with starch during gelatinisation and retrogradation, weakening gel strength and increasing syneresis in frozen matrices. This study investigated the pasting behaviour, antioxidant properties, and in-vitro digestibility of hydro-extracted starches from quinoa and amaranth, alongside their flours, and benchmarked them against commercial corn starch for food applications. Starch was isolated from quinoa and amaranth flours using a modified water-based protocol involving incubation, wet milling, sieving, centrifugation, and oven drying to preserve native granule integrity. Phenolic compounds, flavonoids, and antioxidant capacity (DPPH assay) were quantified, while paste clarity and freeze-thaw stability were assessed spectrophotometrically and by syneresis. Total starch content was determined using the AOAC Megazyme kit (K-TSTA-100A). In-vitro digestibility, including Rapid Digestible Starch (RDS) and Slowly Digestible Starch (SDS) fractions, was evaluated using AACC methods and enzymatic hydrolysis. Results revealed marked differences in bioactive content and functionality. Quinoa flour (7.85 mg GAE/100 g) exhibited the highest total phenolics, followed by corn starch (0.59 mg GAE/100 g), while amaranth starch had none. Quinoa starch showed the greatest improvement in paste clarity (9.33–62.83%), highest free-thaw stability (47.35–31.09%), and exhibited SDS properties, making it suitable for reduced glycaemic response. These findings highlight the health-promoting potential and favourable physico-chemical attributes of quinoa and amaranth flours and starches, making them valuable ingredients in food formulations.