Impact of Particle Size Reduction and Carbohydrate-Hydrolyzing Enzyme Treatment on Protein Recovery from Rapeseed (Brassica rapa L.) Press Cake

The aims were to assess how particle size reduction and carbohydrate-hydrolyzing enzyme treatment influence protein recovery from rapeseed cold-pressed cake and to determine the effect of these pretreatments in protein extraction procedures varying in ionic strength, pH, and total solid content. Defatted press cake (median particle size 600 µm) was milled to 21-164 µm and 7 µm median particle sizes by pin disc milling and air-flow milling, respectively. The milled press cake samples were treated with a carbohydrate-hydrolyzing enzyme preparation, after which proteins were extracted in saline (pH 6) or alkaline (pH 12) buffer at 5 % solid content, or in water at 20 % solid content. Particle size reduction of the press cake did not influence enzyme action or protein yield, suggesting that protein release from the press cake is not physically limited by cell walls or internal cell structures. As an exception, protein release from the aleuronic cells appeared to be hindered by intact cell walls. Enzyme treatment improved protein recovery, more substantially when the extraction was carried out in water at 20 % solid content than in saline or alkaline conditions at 5 % solid content. The enzyme mediated its positive effect most probably by reducing the water holding capacity of the press cake, thereby facilitating solid-liquid separation, and releasing anionic compounds which improved protein solubility through electrostatic stabilization. The results suggest that carbohydrate-hydrolyzing enzymes are beneficial for rapeseed protein extraction at reduced water content or when no salt or alkali is added to increase protein solubility