Recovery of protein from green leaves : Overview of crucial steps for utilisation

Plant leaves are a major potential source of novel food proteins. Till now, leaf protein extraction methods mainly focus on the extraction of soluble proteins, like rubisco protein, leaving more than half of all protein unextracted. Here, we report on the total protein extraction from sugar beet leaves (Beta vulgaris L.) by a traditional thermal extraction method consisting of mechanical pressing, heating to 50 °C and centrifugation. The resulting streams (i.e. supernatant, green-protein pellet and fibrous pulp) were characterised in terms of composition, physical structure and processing options. The protein distributed almost equally over the supernatant, pellet and pulp. This shows that thermal precipitation is an unselective process with respect to fractionation between soluble (rubisco) and insoluble (other) proteins. About 6% of the total protein could be extracted as pure rubisco (90% purity) from the supernatant. Surfactants commonly used for protein solubilisation could hardly re-dissolve the precipitated proteins in the pellet phase, which suggested that irreversible association was induced between the co-precipitated proteins and cell debris. Thus, the extraction of this protein will require prevention of their co-precipitation, and should take place in the original juice solution.