Phenolic and protein contents of differently prepared protein co-precipitates from flaxseed and soybean and antioxidant activity and angiotensin inhibitory activity of their phenolic fractions

Abstract The aim of this study was to determine the phenolic and protein contents of differently prepared protein co-precipitates (Co) from flaxseed and soybean and to determine the antioxidant activity and inhibitory activity towards angiotensin-1-converting enzyme (ACE) of their phenolic constituents. Protein co-precipitates were prepared by alkaline extraction involving mixing flaxseed (F) and soybean (S) powders (Mp) or flaxseed and soybean extracts (Me), followed by isoelectric (Ie) precipitation or Ie and heating (Ie-He). The highest protein contents and yields were obtained with Co-Me/Ie (53.28% and 25.58%, respectively) and Co-Me/Ie-He (46.84% and 19.87%), which also contained relatively high amounts of total phenolics (8.10 and 8.57 mg GAE/g). Co-Me/Ie contained more total bound phenolics (4.33 mg GAE/g) than total free phenolics (3.76 mg GAE/g), while Co-Me/Ie-He co-precipitates were richer in free phenolics (4.82 mg/g). Several free phenolic (FP) and bound phenolic (BP) fractions from Co-Me/Ie (FP-50 °C), Co-Mp/Ie (FP-50 °C and BP-base), Co-Me/Ie-He (FP-50 °C) and Co-Mp/Ie-He (FP-50 °C and BP-base) had the highest antioxidant activity, ranging from 24.7% to 29.6%. ACE inhibitory activity of the BP-acid fraction from Co-Me/Ie and FP-25 °C fraction from Co-Mp/Ie-He were the highest (44.6% and 47.5%). Findings suggest that protein co-precipitates from flaxseed and soybean extracts (Co-Me/Ie and Co-Me/Ie-He) have a high potential for use as protein and phenolic-rich functional ingredients that may benefit the formulation of functional foods and nutraceuticals with antioxidant and antihypertensive properties.