Characterization and Techno-Functional Properties of High Protein Walnut Flour from an Oil by-Product

Abstract

A high protein walnut flour (HPWF) wasobtained by defatting walnut flour (WF), which is a by-product of the oilindustry. The objective of this study was the chemical and techno-functional characterizationof HPWF. Composition, amino acid content, protein secondary structure, proteinsolubility and thermal transitions were measured. Besides, the techno-functionalproperties, emulsion activity and stability, and water holding and oilabsorption capacities, of HPWF were evaluated. Also, the molecular mass ofproteins under denaturing conditions and the microstructure of HPWF wereevaluated by electrophoresis and confocal scanning laser microscopy,respectively. HPWF had 55.4% protein content and 21.5% total dietary fibre. Interms of HPWF amino acid composition, the limiting amino acids were thesulphurated cysteine and methionine. By FTIR analysis, the main secondarystructures were β-sheet (49%) followed by α-helix (24%); both structures areconsidered to be ordered. Likewise, HPWF soluble proteins increased at basic pHand HPWF proteins were separated in 11 bands with molecular masses ranging from97 kDa to 18 kDa by electrophoresis. With respect to techno-functionalproperties, HPWF presented good emulsion activity (51%) and high thermal emulsionstability (46%). In addition, HPWF retained 571% and 242% of water and oil byweight, respectively. Finally, the micrograph showed the predominance ofprotein structures and fibre fragments, and the presence of few lipids mostlytrapped. These results showed that HPWF is an interesting source of plant-basedproteins and walnut flour can be used to obtain high protein ingredients fromnon-traditional sources.