Green crop fractionation by twin-screw extrusion: Influence of the screw profile on alfalfa (Medicago sativa) dehydration and protein extraction

• We apply twin-screw extrusion to alfalfa fractionation.
• Protein extraction yields in the green liquid extract are above 50%.
• Twin-screw extruder design is of crucial importance and can be optimised.
• Adding a second filter improves both protein extraction and plant dehydration.

Alfalfa was studied to define a new approach for green plant fractionation using twin-screw extrusion, leading to the production of a green filtrate, rich in proteins and a solid fibrous residue, rich in cellulose. The influence of the screw profile on protein recovery in the liquid extract and on plant dehydration was characterised. Three screw profiles were tested, at different liquid/solid ratios and plant inlet flow rates. A significant difference was determined, at a confidence level of 95%, between the influence of the three profiles on alfalfa dehydration and protein extraction in the raw filtrate. When mechanical action on the plant is stronger, both protein recovery and solid dehydration are better with more than half of the initial proteins (50.8%) recovered in the liquid extract in the best screw configuration (profile 3). However, extrusion is not efficient for the plant dehydration but the interest of adding a second filtration zone was demonstrated; after the first pressing and liquid expression in a first stage, water is added to the residual matter, which leads to a better protein extraction in the second filtration stage.