From Fruit Waste Hydrolysates to Protein-Rich Gluten-Free Staples: A Review and Conceptual Framework for Single-Cell Protein Fortification of Ethiopian Maize Flatbreads

This review synthesizes recent research on strategies to address protein malnutrition in Ethiopia, where diets are predominantly maize-based and fortification with conventional legumes is limited due to antinutritional factors, supply variability, and cost. Concurrently, fruit processing generates abundant sugar-rich residues, including banana, mango, papaya, citrus, and pineapple wastes, which are largely underutilized yet represent attractive renewable nutrient substrates. This review presents an integrated approach to valorizing fruit waste hydrolysates for the production of single-cell protein (SCP) powders, which can enhance the nutritional and functional quality of traditional gluten-free Ethiopian maize flatbreads. Fruit waste hydrolysates have been reported to be rich in glucose (~228 g/L) and fructose (~55.7 g/L) and micronutrients but also contain phenolic compounds that can inhibit fermentation, necessitating controlled microbial cultivation. Food-grade microorganisms, such as Saccharomyces cerevisiae, Candida utilis, and selected filamentous fungi, have been described to produce high protein content (often 60%–82% dry mass) and increase lysine content relative to unfortified maize flour under optimized batch and fed-batch conditions. The resulting SCP powders have been reported to exhibit favorable technofunctional properties, including water absorption and emulsification capacity, making them suitable as dough improvers. Incorporation of 10%–15% fermented microbial protein into maize flatbreads has been reported to improve protein content (~49%–55%), enhance the amino acid profile, increase digestibility by ~40%, and preserve texture, aroma, and cultural acceptability. Although sensory changes and processing constraints remain significant challenges, with fortified, gluten-free products often exhibiting altered texture, mouthfeel, and flavor profiles, as well as demanding precise formulation strategies to balance nutritional benefits with consumer acceptability and manufacturability, this work demonstrates a circular bioeconomy approach transforming fruit waste into high-quality protein powders, offering a scalable, sustainable, and culturally appropriate strategy to alleviate protein deficiency. The findings provide a practical framework for introducing microbial protein into staple diets, advancing food security, and promoting efficient utilization of agroindustrial waste in low-resource settings.