Increasing protein recovery from lucerne by a second pressing step: opportunities and challenges for process and feasibility

Green leafy biomass is a promising source for extraction of protein for food and feed, but low yields in the process affects sustainability and feasibility. Here, opportunities and challenges concerning increased recovery of soluble protein were evaluated by chemical, enzymatic and mechanical treatments combined with re-pressing of pulp (P), whereafter the process was assessed environmentally and economically. The treatments affected the recovery of soluble proteins by impacting the nitrogen flow from P to green juice (GJ) and the recovery of soluble protein from GJ, with the latter most strongly correlating to the recovery of soluble protein. Treatments, such as freezing of P, addition of water, NaOH, SDS, or brown juice, and repeated ball milling, increased the nitrogen extraction from P to GJ significantly (>35% compared to control). However, several treatments caused protein degradation and co-extraction of cell wall components and particles containing insoluble protein, thus negatively affecting the recovery of soluble proteins. Treatments contributing to cell disruption was most beneficial for recovery of soluble protein, with the highest recovery (similar to 14%) obtained by NaOH treatment at >= 0.1 M, corresponding to a five-fold increase as compared to water treatment. Assessment at demo-scale showed that repressing of P with 0.1 M NaOH was environmentally and economically more costly per protein unit than pressing original biomass, the same cost was seen for every additional re-pressing, but with reduced protein recovery. Thus, the initial pressing should be followed by maximum one re-press with 0.1 M NaOH, and produced heat should be recovered, if implementing the process industrially.