Ionic liquid assisted gel casting of cellulose BSG protein mixtures for packaging films

The need for a circular materials economy is driving the search for new biobased plastics with desirable performance. This study generated and characterized biopolymer composite films from microcrystalline cellulose (MCC) and a protein concentrate extracted from brewers' spent grain (BSG), the main by-product of the beer brewing industry. Selected ionic liquids (ILs) were used as solvents for gel casting due to their ability to dissolve cellulose and BSG protein. Formulations with 5:0, 5:1, 5:2.5 and 5:5 (%/% wt) MCC-to-BSG protein ratio were prepared in 1-ethyl-3-methylimidazolium diethyl phosphate, [Emim][DEP], and 1-ethyl-3-methylimidazolium dimethyl phosphate, [Emim][DMP]. The resulting gels were coagulated in water and dried by hot-pressing, generating translucent brown films. The films with 5:2.5 MCC-to-protein ratio had tensile strengths of 106 MPa ([Emim][DEP]) and 118 MPa ([Emim][DMP]), comparing favourably with major commercial packaging materials. The protein cellulose films provided excellent protection against ultraviolet (UV) light, achieving up to 96% blockage for both UV-B and UV-A radiation and exhibited expected thermal stability and moderate antioxidant properties. While addition of BSG protein increased the hydrophobicity of the films' surface and reduced water vapour transmission, the films remained hydrophilic and had high water vapour permeability (approx. 400 g m-2 h-1 at 50% relative humidity). Effective recovery of more than 90% of the alkyl phosphate ILs was demonstrated, with a minimum of six batch washes required to remove the majority of the ILs. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.