Chlorogenic acid (CGA) oxidation induced formation of green trihydroxy benzacridine (TBA) derivatives can occur in alkaline isolated sunflower protein resulting in undesirable discoloration. Since cysteine can preferentially react with oxidized CGA to form colorless thiol-CGA conjugates, cysteine was investigated as an anti-greening strategy for sunflower protein isolation without de-phenolization. Alkaline supernatant of buffered sunflower protein solutions with 0- 5.6 mM cysteine was acidified to pH 5.6, the isoelectric point of helianthinin sunflower proteins. After isoelectric precipitation, the precipitate was lyophilized for 12 hours. Color intensity and composition were determined by Hunter L*a*b*, HPLC and LC-MS respectively, along with nitrogen solubility. Conformational changes of sunflower protein were measured by FTIR. Cysteine was found to limit TBA formation in the lyophilized isolates as green discoloration was lowered with increasing cysteine. Addition of cysteine had a protective effect on CGA during protein isolations, as more CGA was retained in protein isolate at higher cysteine levels. Addition of cysteine during isolation increased the beta-sheet: alpha-helical structure ratio in a concentration-dependent manner. Formations of green TBA and cysteinyl-CGA conjugates were confirmed by LC-MS. Findings suggested that addition of cysteine could be a potential anti-greening strategy for sunflower protein isolation, which could retain a higher antioxidant phenolic content while simultaneously enhancing protein solubility.
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