pH-Dependent Extraction of Antioxidant Peptides from Red Seaweed Palmaria palmata: A Sequential Approach

This study employed a diverse approach to extract antioxidant peptides from red seaweed Palmaria palmata, recognized for its comparatively high protein content. Initially, an aqueous extraction of the entire seaweed was performed, followed by enzymatic hydrolysis of the solid residues prepared from the first step. The effects of three different pH levels (3, 6, and 9) during the aqueous extraction were also examined. Results indicated that the solid fraction from the sequential extraction process contained significantly higher levels of proteins and amino acids than other fractions (p < 0.05). Furthermore, the solid fractions (IC₅₀ ranging from 2.29 to 8.15 mg.mL^-1) demonstrated significantly greater free radical scavengers than the liquid fractions (IC₅₀ ranging from 9.03 to 10.41 mg.mL^-1 or not obtained at the highest concentration tested) at both stages of extraction (p < 0.05). Among the solid fractions, those produced fractions under alkaline conditions were less effective in radical scavenging than the produced fractions under acidic or neutral conditions. The fractions with most effective metal ion chelating activity were the solid fractions from the enzymatic stage, particularly at pH 3 (IC₅₀ = 0.63 ± 0.04 mg.mL^-1) and pH 6 (IC₅₀ = 0.89 ± 0.07 mg.mL^-1), which were significantly more effective than those from the initial extraction stage (p < 0.05). Despite no significant difference in the total phenolic content between these solid fractions and their corresponding liquid fractions (3.79 ± 0.05 vs. 3.48 ± 0.02 mg.mL^-1 at pH 3 and 2.43 ± 0.22 vs. 2.51 ± 0.00 mg.mL^-1 at pH 6) (p > 0.05), the observed antioxidant properties may be attributed to bioactive amino acids such as histidine, glutamic acid, aspartic acid, tyrosine, and methionine, either as free amino acids or within proteins and peptides.