Fiber-Rich Extruded Snacks Using Millets and Vegetable Powders: Process–Structure–Nutrition Links, Formulation Strategies and Research Gaps

Extruded ready-to-eat snacks are among the most consumed convenience foods globally, yet they are frequently formulated from refined starches that deliver low dietary fiber and limited micronutrient density. This review synthesizes evidence on developing fiber-rich extruded snacks using millet flours as the cereal base and vegetable powders (including pomace-derived powders) as complementary fiber- and phytochemical-rich ingredients. The discussion integrates the compositional advantages of millets, the techno-functional contributions of vegetable powders, and the central processing–structure–property relationships in extrusion cooking that determine expansion, texture, color, shelf stability, and nutritional functionality. Particular attention is given to how extrusion variables (feed moisture, thermal profile, screw configuration and speed, and die conditions) interact with high-fibre matrices to influence starch gelatinization, melt rheology, pore formation, and the distribution of insoluble versus soluble dietary fiber. The review also considers how extrusion can simultaneously enhance digestibility and reduce certain anti-nutritional factors, while risking losses of heat-labile bioactives if conditions are severe. Reported formulation strategies are consolidated, including controlling particle size, optimizing inclusion rates of vegetable powders, balancing protein–starch–fiber ratios, and employing protective or structuring approaches that preserve bioactive stability and sensory acceptance. Finally, the review highlights methodological limitations in the current literature and proposes research directions for scaling fibre–rich millet–vegetable extrudates with consistent quality and validated health-relevant endpoints.