Effect of phenolic compound removal on rheological, thermal and physicochemical properties of soybean and flaxseed proteins

This study aimed to investigate the effect of removal of phenolics on physic chemical properties of protein isolates obtained from flaxseed and soybean. Proteins were isolated (I) from full-fat (F) and defatted (D) soybean (s) and flaxseed (f) using isoelectric precipitation. Free and bound phenolics were removed from the protein isolates. Thermal and gelation properties of protein isolates before and after removal of phenolics were investigated. Protein isolates from defatted soybean after removal of free and bound phenolics were showed a decrease in thermal stability of glycinin. For protein isolate from full-fat soybean, the results showed the removal of free phenolics increase thermal stability of glycinin with increase water holding capacity (WHC) and produce more viscous and less elastic gels as compared to protein isolate after removal of bound phenolics. Removal of free and bound phenolics from flaxseed protein isolates decrease thermal stability, WHC and viscoelastic properties as compared to protein isolate after removal
of free phenolics.  (Authors abstract)

The interactions of phenolic compounds with other food components in complex food systems affect the palatability of foods. Interactions of tannins with other food components such as protein or polysaccharide components are known to occur. The contents of bound phenolic compounds in isolated protein from flaxseed were higher than isolated proteins from soybean.  The presence of protein phenolic interactions in flaxseed and soybean protein isolates improved antioxidant properties of foods. Sinapic acid interacted in the protein isolate in isolated proteins from defatted flaxseed. P Coumaric acid, syringic acid and hesperidin were the major identified bound phenolic compounds identified in soybean protein isolates, while gallic acid, phydroxybenzoic acid, syringic acid, caffeic acid, ferulic acid and p coumaric acid were the major bound phenolic compounds identified in flaxseed. The overall objective of this research was to investigate the effects of removal of phenolics on physic chemical properties of soybean and flaxseed protein isolates by measuring denaturation temperature, water holding capacity (WHC) and rheological properties.

Effect of removal of phenolics on water holding capacity (WHC) of protein gels  shows the effect of removal of free and bound phenolics on WHC of protein gels from full-fat and defatted flaxseed and soybean.  Generally, gels obtained from protein isolates for defatted meals showed higher WHC compared to gels obtained from protein isolates for full-fat except for protein gel after removal of free phenolics from flaxseed and protein gel after removal of free and bound phenolics from soybean. Difference in WHC can be related to the protein content of protein isolates. Removal of free and bound phenolics affected the thermal stability and gelation properties of protein isolates from flaxseed and soybean. The results demonstrated that the thermal denaturation temperature of glycinin from protein isolates for defatted soybean declined slightly after removal of free and bound phenolics. Obtained gel from protein isolate of defatted soybean after removal of free and bound phenolics showed a lower WHC, less viscous and more elastic than protein isolate before removal of phenolics. Results demonstrated that the removal of free phenolics from full-fat soybean increase thermal stability of glycinin, WHC and viscosity of gel compared to protein isolate after removal of bound phenolics. Removal of free and bound phenolics from protein isolates from flaxseed decrease thermal stability, WHC and viscosity and elasticity properties. (Editors comments)