Physicochemical, nutritional, antioxidant, and glycaemic properties of gluten-free cassava bread enriched with nettle leaf and watermelon peel powders

Abstract

Cassava-based bread is naturally gluten-free but nutritionally limited due to low protein, fibre, and bioactive content. This study investigated the incorporation of nettle leaf powder (NLP) and watermelon peel powder (WPP) as sustainable functional ingredients to enhance its nutritional, physicochemical, and health-promoting properties. Composite breads were formulated by partially substituting cassava flour (CF) with increasing levels of NLP and WPP at CWN1 (85% CF: 10% WPP: 5% NLP), CWN2 (70% CF: 20% WPP: 10% NLP), CWN3 (60% CF: 25% WPP: 15% NLP), and CWN4 (50% CF: 30% WPP: 20% NLP). The inclusion of NLP and WPP resulted in expected increases in protein, dietary fibre, and total phenolic content, reflecting the intrinsic composition of the added materials. More importantly, significant ( p < 0.05) increase in antioxidant activity were observed, with CWN4 being highest (DPPH·: 573.49±0.60 µmolTE/g, ABTS·₊: 547.69±0.88 µmolTE/g). In vitro starch hydrolysis revealed a progressive reduction in digestibility, with hydrolysis index decreasing from 73.21 to 66.39 and predicted glycaemic index (pGI, dimensionless) from 71.54 to 62.12, indicating moderated glycaemic potential. Colour analysis revealed darker crumb appearance (L⁎ = 15.75), increased greenness (a⁎ = -8.47), and yellowness (b⁎ = 27.84), attributable to chlorophyll and phenolic pigments. Electronic taste further demonstrated distinct taste profiles, with increased bitterness and umami responses that reflected increasing phenolic and minerals. Overall, while physical bread quality was not improved, the incorporation of NLP and WPP significantly enhanced antioxidant capacity and reduced starch digestibility, highlighting their potential in the development of functional gluten-free products with improved metabolic relevance. • Antioxidant capacity significantly increased in composite breads • Starch digestibility and glycaemic index were reduced • Fibre-rich powders increased crumb firmness and density • Electronic tongue showed increased bitterness and umami • Functional properties improved despite limited physical changes