Phenolic-protein interactions in oilseed protein isolates

- Higher content of bound phenolics was found in FsI compared to DsI.
- Soybean and flaxseed proteins had protein-phenolic interactions.
- Full-fat protein isolate from flaxseed showed protein-lipid-phenolic interactions.
- No antioxidant activity was found for bound phenolics from DsI.
- Sinapic was present in DfI protein but not in FfI.

The objective of this work was to investigate the phenolic-protein interactions in protein isolates from soybean and flaxseed. Proteins were isolated from full-fat and defatted soybean and flaxseed using sodium hydroxide extraction and isoelectric precipitation. Free phenolics were extracted from the protein isolates using methanol; for bound phenolics, the proteins were subjected to basic and acidic hydrolysis followed by methanol extraction. Total phenolic contents in free and bound phenolic extracts were determined by spectrophotometric analysis. Reversed phase-HPLC was used for separation of individual phenolic compounds, which were identified by mass spectrometry (MS). Protein isolates from full-fat soybean showed higher content of phenolics in bound phenolic extract (20% of total phenolic content) compared to protein isolates from defatted soybean (11% of total phenolic content). Generally, bound phenolics (20%-30% of total phenolic content) were higher in protein isolates from flaxseed than in protein isolates from soybean (10%-20% of total phenolic content). RP-HPLC chromatogram showed that the soybean and flaxseed protein isolates had free phenolic compound profile which was different than bound phenolic compound profile. The phenolics in free extracts exhibited greater antioxidant activity than phenolics in bound extracts of protein isolated from soybean and flaxseed. Results suggest the presence of primarily protein-phenolic interactions in protein isolates from soybean and primarily protein-lipid-phenolic interactions in protein isolates from flaxseed.