Surface activity of protein extracts from seed oil by-products and wettability of developed bioplastics

Abstract

Protein concentrates, extracted from the by-products of Hemp (Cannabis sativa), Cardoon (Cynara cardunculus), and Argan (Argania spinosa L.) oilseeds after oil extraction, were utilized to produce sustainable and eco-friendly protein-based bioplastics. The surface wettability and roughness of these bioplastics were investigated due to the pivotal role of these parameters in their application in food packaging sector. Advancing contact angle value revealed that Argan protein-based bioplastic is the most hydrophobic (θ > 90°) while Cardoon protein-based film is the most hydrophilic (θ < 40°). These results are then supported by the roughness of wet film surfaces and their swelling ratio, where Cardoon-based bioplastics and Argan-based bioplastics have also the lowest and the highest roughness average, respectively. The measured hydrophilicity of the produced bioplastics motivates a detailed investigation of the surface activity of protein concentrates used for film formation which can determine the packaging by coating. Adsorption/desorption kinetics, surface tension and dilatational rheology are reported at two different pHs values (10 and 12). The three protein concentrates are surface active and develop stable adsorbed layers at the air-water interface after 1 h. Cardoon proteins showed the same final surface tension (>45 mN m−1) at both pHs, while Argan and Hemp proteins displayed lower values than Cardoon, especially at pH 12 (<40 mN m−1), indicative of higher surface activity. Finally, Argan protein adsorbed more irreversibly, followed by Hemp protein while Cardoon protein provides the largest partial desorption from the surface. Combined analysis of the data features a correlation between the higher advancing contact angle of Hemp protein-based bioplastic and higher hydrophobicity, less swelling, faster adsorption kinetics, lower surface tension and more irreversible adsorption of protein. These apply for Hemp and Cardoon protein concentrates and their driven bioplastics while the presence of free oil in Argan protein concentrate and its bioplastic hinders the correlation between different phenomena. The new interconnections highlighted between film properties and surface adsorption could potentially be employed in food packaging and coating technology.