Valorisation of Pulse Waste: Carboxylic Acid-Based Films from Residual Proteins

Rising environmental concerns have intensified interest in waste valorisation and the development of sustainable, bio-based materials through green chemistry approaches. In this study, proteins extracted from waste lentil and chickpea seeds were used to develop protein-based films using a range of carboxylic acids as cross-linkers. The acids facilitated protein unfolding and promoted intermolecular interactions, allowing tunable control over mechanical strength, barrier performance, and water resistance. In addition to their structural role, the inherent bioactivity of selected carboxylic acids imparted added functionality to the resulting materials. Physical characterisation and FTIR secondary structure analysis revealed that the acid-type, plasticiser, and, in some cases, protein fraction composition influenced the final material performance. Liquid monocarboxylic acids produced cohesive and flexible films, with tensile strength ranging from ~1 to 23 MPa, with formic acid yielding the strongest films. Lactic acid and its blends improved flexibility and reduced permeability, achieving water vapour permeability (WVP) of 5.76 ± 0.7 × 10−12 g m m−2 s−1 Pa−1 and oxygen permeability (OP) of 5.8 ± 0.0 × 10−13 mL m m−2 s−1 Pa−1 at low acid loadings. In contrast, solid di- and polycarboxylic acids tended to crystallise at higher concentrations. Citric acid was an exception, exhibiting behaviour distinct from the other solid acids and producing clear, crystal-free films with excellent flexibility, showing elongation at break (EAB) up to ~326%. Preliminary proof-of-concept application testing demonstrated the suitability of selected films for vegetable shelf-life extension for up to 17 days and for gradual lactic acid release, supporting their potential use as biodegradable cosmetic mask/patch platforms.