The aim of the present study was to elucidate previous findings showing that peptide fractions isolated
from yoghurt had antioxidant effects. Therefore, peptides and free amino acids released during fermentation
of milk were characterised. Yoghurt samples were stripped from sugars and lactic acid and subsequently
fractionated by ultra filtration using membranes with cut off sizes of 30, 10 and 3 kDa. The
peptides in these fractions were identified by LC–MS/MS. The identified peptides comprised a few Nterminal
fragments of as1-, as2-, and j-casein, and several fragments from b-casein. Almost all the peptides
identified contained at least one proline residue. Some of the identified peptides included the
hydrophobic amino acid residues Val or Leu at the N-terminus and Pro, His or Tyr in the amino acid
sequence, which is characteristic of antioxidant peptides. In addition, the yoghurt contained a considerable
amount of free amino acids such as His, Tyr, Thr and Lys, which have been reported to have antioxidant
properties. Thus, our findings confirm that the antioxidant effects of the peptide fractions from
yoghurt are due to the presence of certain peptides and free amino acids with recognised antioxidant
activity in these fractions.