From protein sources to fit-for-purpose ingredients: benchmarking side-stream valorised and emerging alternative proteins against milk and soy

Background Population growth and climate change are creating an urgent demand for additional protein sources. Milk and soy proteins currently dominate, however, their production is associated with environmental, resource, and productive land-use challenges. In response, research and innovation have increasingly focused on alternative proteins that can enhance sustainability, reduce environmental impact, and support long-term food security, whilst meeting existing nutritional and functional requirements. Scope and approach This review compares oilseed side-stream proteins, aquafaba, green biomass proteins, insect proteins, and microbial proteins, using milk and soy as practical reference proteins. These sources are compared across four distinct dimensions: carbon footprint, nutritional quality, techno-functional properties and food safety. Key challenges to their use in food applications are synthesised together with strategies to improve ingredient performance and safety. Key findings and conclusions The evidence supports fit-for-purpose ingredient selection rather than a universal ranking. Side-stream valorised and emerging alternative proteins cannot be ranked through a single performance criterion. Milk remains the strongest nutritional benchmark, while soy provides a balanced plant-based reference. Insect proteins show strong nutritional potential; microbial proteins offer promising opportunities for scalable, tailored production; aquafaba offers application-specific functionality; green biomass and oilseed side-streams are particularly relevant from a circularity perspective. Protein blending, mild extraction, fractionation, fermentation, and extrusion can address particular limitations but may also increase resource use or introduce new safety concerns. The proposed application-specific framework guides ingredient selection for beverages, dairy and meat analogues, egg replacement, sauces, bakery products and nutritional fortification, while highlighting gaps in the available evidence.