Subcritical water fractionation of proteins and free amino acids from Brewer’s Spent Grain (BSG)

Abstract

Brewer’s spent grain (BSG) has been traditionally used only in animal feed despite its high nutritional value, with a protein content of ~20% in dry weight basis [1]. This work is part of a wider project for a complete biomass valorization by using pressurized fluids, namely SC-CO2 to recover the lipophilic fraction and water. In this work we proposed the use of subcritical water at 50 bar and 4 ml/min in a semicontinuous reactor at different temperatures (125 to 185ºC) to extract and hydrolyze BSG proteins. Figure1 shows protein and amino acid values on SubCW extracts accumulated after 240 minutes of extraction. The highest protein content, 15.6 g/100g BSG, dry, was obtained at 185ºC. On the other hand, the highest free amino acid content in extracts, 77 μmol aa/g BSG, dry, was achieved at 160ºC due to amino acid decomposition at this operation conditions. This trend has been also observed in the literature for SCWF of oyster at 150 ºC [2]. Free amino acid formed during BSG protein hydrolysis in SubCW are listed in Table 1 together with the amino acid composition of the BSG protein fraction. Results appears to show a trend related to the hydrophobicity of each group of amino acid since an increase in temperature improves the yield of aliphatic amino acids extraction, while the highest yield for charged amino acids, was reached at 145ºC, and neutral amino acids with a polar side chain, had the highest yield at 160ºC. This agrees with the fact that aliphatic amino acids are stable at higher temperatures. The decrease of water polarity with temperature may favor the affinity for these amino acids due to their hydrophobic character. In addition, small aliphatic amino acids are formed during the decomposition of the other amino acids [3]. BSG offers a great potential as raw material to obtain protein hydrolyzates and amino acids due to its high protein content (~20%) and the elevated rate of EAA. Despite different amino acids has found to be a key parameter on the yield obtained for each amino acid as function of operating temperature. Results obtained in this work suggest that SCWF of BSG is able to recover and hydrolyze BSG proteins.