Approaching the extraction of proteins from brewing wastes using deep eutectic solvents

Deep eutectic solvents (DES) are innovative solvents with promising functionalities, including their use for the sustainable extraction of compounds. This work proposes to evaluate the potential of DES for the extraction of proteins from two residues of the brewing industry with high protein content: brewer's spent grain (BSG) and malt rootlets (MR). Fourteen different DES with pHs ranging from 1.3 to 10.2 were employed and the extraction was assisted by high intensity focussed ultrasounds (HIFU). The DES constituted by guanidinium chloride and urea (1:2) at pH 9.1 provided the highest protein yield for both residues. The extraction conditions (amount of waste, HIFU amplitude, temperature, and extraction time) were optimized by a Box-Behnken experimental design. Long extraction times, high ultrasound amplitudes, and low amounts of residue favoured the extraction of proteins. Under optimal conditions, it was possible to recover 13 +/- 2 g protein/100 g from BSG and 10.2 +/- 0.6 g protein/100 g from MR. Different procedures were evaluated to remove DES from the final extract. This method guaranteed the selective extraction of proteins without the presence of phenolic compounds that are usually coextracted. Moreover, proteins with different bioactivities were identified in purified extracts by proteomic analysis and RP-UHPLC-Q-Orbitrap-MS/MS.