Extraction treatment on protein release from dried cassava leaves

Cassava leaves are a promising agro-industrial waste that could be exploited as an alternative protein source. Protein content in cassava leaves ranges between 20% - 40% (d.b.); however, the complex structure of plant cell walls as well as the interactions between proteins and other compounds limit protein extraction. In the present study, five methodologies (alkaline, alcoholic, saline, ultrasound, and enzymatic extractions) were applied separately or combined to study the release of proteins from dried cassava leaves flour (CFL). The temperature was fixed at 90oC for all chemical treatments, while for enzymatic treatment the pH and temperature conditions were set to the optimal condition of each enzyme. The extractions were carried out for 1 hour, except ultrasound treatment, that was carried out for 10 min. For all the extraction, CFL was mixed to distillate water at 1:10 (w/w) proportion. After extraction, the dispersion was centrifuged, and the supernatant was collected to determine protein content. The protein extraction yield (initial protein content on CFL/ protein content in the supernatant) was calculated for all the extraction treatments. The highest protein yield was achieved with ultrasound at 525 W for 10 min followed by alkaline treatment (pH 10, 90C, 1h), which was 43.89% higher than extraction carried out only with ultrasound at the same power and 40.7% higher to alkaline extraction. The disruption of the cell wall by ultrasound due to cavitation effect generates high local temperature and pressure leading to physical damage to the plant cell wall. However, this effect did not result in the solubilization of protein, which could be achieved by increasing the solution pH. The lowest protein yield was reached with enzymatic extraction with Pectinex followed by alkaline extraction. Even though the enzymes shall act by hydrolyzing the polysaccharides responsible to form the cell wall network, favoring the protein release, the enzymatic extraction could have changed the protein interactions, which may have become more sensible to harsh alkaline conditions. Among chemical treatments, decreasing the alcohol proportion from 70:30 to 50:50 (alcohol: water) increased protein yield by around 89%. Also, comparing salt extraction carried out with Ca(OH)2 and NaCl at the same concentration (2mol.mL-1), protein yield increased around 26% when Ca(OH)2 was used. These results evidence the complexity of protein extraction from leaves and indicates that processes involving damage to the plant cell together with protein solubilization are more effective for increasing protein extraction yield of cassava leaves.