Bioactive peptides (BPs) are protein fragments with biological activity. They have a length of two to twenty amino acid residues and their molecular weight is less than 3 kDa. They can be easily absorbed into the intestinal track and can reach specific enzyme sites due to their small size. These peptides are usually obtained from food proteins under special conditions, such as hydrolysis processing. They are widely used in the food industry for functional purposes. For example, whey protein peptides can suppress oxidative deterioration of meat products, which decreases the formation of odors and extends the storage life of the product (183).
Peptides with functional properties are isolated by using chromatographic techniques. After the separation and purification, the peptide sequence identification can be performed by Edman degradation with some cleavage reagents, such as phenyl isothiocyanate. In addition, mass spectrometry is also an effective method for peptide identification [76]. Two kinds of methods are available for peptide sequencing, including database matching and de novo sequencing. The latter is suitable for materials that do not have a database and enables the identification of peptides with modified amino acids.
Some peptides have antioxidant characteristics and have been found in plant-based proteins, such as quinoa seed and soybean cake protein isolates. The antioxidant property of these peptides is likely linked to the aromatic groups of tyrosine, tryptophan, and histidine, which can donate hydrogen atoms to electron-deficient radicals, whereas cysteines have a sulfhydryl group that can trap oxygen molecules.