Abstract: Proteins isolated from defatted cashew nut flour by salt extraction method (CNSPI) and alkaline extraction and subsequent isoelectric precipitation method (CNPI) were evaluated to compare their structural (molecular weight, secondary structure, amino acid composition), micromorphological and functional (solubility, foaming capacity and foam stability, emulsifying activity index and emulsion stability) properties. The results showed CNSPI and CNPI differed significantly in structure, micromorphology, and functional properties. CNSPI exhibited significantly higher protein content, better solubility and emulsifying properties compared with CNPI, while the foaming properties were lower. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the preparation methods had no impact on the primary structure of protein isolates. The nutritive value of protein isolates was evaluated by amino acid composition analysis. Scanning electron microscopy revealed that CNSPI exhibited a massive flaky microstructure while CNPI appeared in an irregular shape as a combination of only a few globules and some flaky plates. Circular dichroism (CD) spectroscopy demonstrated that β-sheet was the major secondary structure in CNSPI and CNPI. CNPI had relatively more random coil (29.3%) and less α-helix (11.9% vs. 27.0%) than CNSPI, which was possibly because alkaline treatment destroyed protein structure and consequently changed functional properties. Under neutral pH condition, the foaming capacity of CNPI was 119.00%, and the emulsion capacity could reach 30.14 m2/g.

Key words: cashew nut protein, functional properties, micromorphology, secondary structure, alkaline extraction and subsequent isoelectric precipitation method, salt extraction method