Protein extraction from agri-food residues for integration in biorefinery: Potential techniques and current status

•Techniques for protein extraction from agrifood residues are critically reviewed.•The protein content is highly variable depending on the agri-food resource.•Several extraction techniques in dry and non-dry conditions are discussed.•The recovery of proteins depends on the type of residue and extraction method.•Some extraction methods have been just included in biorefinery schemes. The biorefinery concept is attracting scientific and policy attention as a promising option for enhancing the benefits of agri-food biomass along with a reduction of the environmental impact. Obtaining bioproducts based on proteins from agri-food residues could help to diversify the revenue stream in a biorefinery. In fact, the extracted proteins can be applied as such or in the form of hydrolyzates due to their nutritional, bioactive and techno-functional properties. In this context, the present review summarizes, exemplifies and discusses conventional extraction methods and current trends to extract proteins from residues of the harvesting, post-harvesting and/or processing of important crops worldwide. Moreover, those extraction methods just integrated in a biorefinery scheme are also described. In conclusion, a plethora of methods exits but only some of them have been applied in biorefinery designs, mostly at laboratory scale. Their economic and technical feasibility at large scale requires further study.