Functionality and structure of sunflower protein isolates as affected by the extraction process and esterase hydrolysis

Abstract: Sunflower kernels are a promising source of plant-based protein but applying such proteins in the food industry requires improved protein functionalities. Sunflower proteins are usually extracted in an aqueous alkaline solution, followed by protein isolation using acidic precipitation, membrane processing, or chromatographic procedures. Sunflower kernels also contain a significant amount of chlorogenic acid, which reacts to produce a green pigment under alkaline conditions. Ideally, extracting chlorogenic acid from the kernels prior to protein extraction would reduce the interaction of these polyphenols with the proteins. In this study, we investigated using an esterase enzyme to hydrolyze the chlorogenic acid in sunflower before protein extraction and evaluated its effect on protein structure and functionality. Sunflower protein isolates (SPI) were extracted from sunflower kernels by alkaline extraction isoelectric precipitation method, followed by lyophilization. Hydrolysis of chlorogenic acid was confirmed by HPLC analysis which showed an increase in caffeic acid. The characteristic green color associated with chlorogenic acid was not observed following enzyme treatment; however, an increase in browning was observed in the SPI. Removal of fat from the kernels increased protein yield from an average of 39.97 % to 64.14 %. Hydrolysis of chlorogenic acid in the kernels resulted in a decrease in protein yield to 42.38 %. Protein solubility also increased with the hydrolysis of chlorogenic acid to 0.70 % compared to 0.37 % when the polyphenols were removed with an organic solvent. Esterase addition also increased oil absorption while decreasing water absorption in the absence of lipids. The protein structural characteristics were then evaluated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, size exclusion chromatography, quantification of sulfhydryl/disulfide group, and Fourier transform infrared spectroscopy analyses. The protein content exposed sulfhydryl group and beta-turn structure differed among SPI following defatting and removing polyphenols from the sunflower meal.