A process for the extraction, purification and enzymatic modification of .beta.-conglycinin .alpha.' subunit, wherein .beta.-conglycinin is selectively extracted from ground, defatted soy, then precipitated by treatment with aqueous ethanol; the enriched fraction is then subjected to Metal Affinity Chromatography (MAC) in denaturant conditions to obtain the .alpha.' subunit, which is treated with chymotrypsin, then subjected to a further MAC to recover the amino-terminal region of this polypeptide (MW 28,000 Da).