Ensiling of the pulp fraction after biorefining of grass into pulp and protein juice

Biorefining of green biomass such as grass and grass-clover is an emerging field with extensive potentials for new value-chains and cascading use of biomasses. Grass biomass can be biorefined into a protein-rich juice fraction for monogastric feed purposes and a fibrous pulp fraction for ruminant feed or biogas production. This study investigated the ensiling of the pulp fraction after extraction of protein juice from two types of grass biomass, namely grass-clover and ryegrass with 28.8 and 33.6% total solid (TS) in the two pulp fractions, respectively.Lab-scale ensiling experiments for 3 to 5 months showed that increased time from processing of grass biomass to initiation of the ensiling of the pulp fraction resulted in higher final pH of the silage, particularly for the ryegrass pulp, and reduced concentration of lactic acid, total solid (TS), total amino acids (AAs) and free AAs. Addition of sucrose could, to some extent, counteract the effect of delayed ensiling on pH.Pilot-scale ensiling experiments for 7–11 months demonstrated limited effluent run-off with 1.8% for grass-clover pulp and 0.0% for ryegrass pulp. The ensiling of pulp had significant impact on a number of chemical constituents, e.g. with a reduction in neutral detergent fibre (NDF), in-vitro digestibility of organic matter (IVOS), total AAs and true protein but with an increase in free AAs. Also, ensiling significantly increased the biochemical methane potential (BMP) for anaerobic digestion durations up to 20 or 30 days but not at longer durations. Mass balances demonstrated a high recovery of volatile solids (97–98%), NDF (92–94%) and methane production potential (98–116%). However, recovery was lower for true protein (67–85%) and total AAs (84–90%), indicating some degradation of protein and AAs during ensiling. In conclusion, ensiling may offer an efficient way of conserving the quality of grass pulp, provided the process and logistics are optimized, particularly with a rapid sealing of the pulp after processing.