The discolouration of sunflower protein isolates which impairs their food use is brought about by the covalent binding of protein with the products of chlorogenic acid (CA) oxidation 1, 21 which proceeds both under the action of polyphenoloxidase (PPO) at pH values near the neutral and by a nonenzymatic way at alkaline pH . The presence of PPO in sunflower seeds was established [4, 5]. Although its activity decreases during oil processing [4, 5], moistening and increased temperature favour the enzymatic oxidation of CA and its coupling with protein. As a result the obtained cake may become unsuitable for food protein isolation. The decrease of free CA (extracted by aqueous ethanol) during the heating of moistened sunflower seeds was described . It was attributed to the thermal decomposition of CA. These results do not exclude the covalent binding of CA with protein.
The determination of both free and bound CA was applied in this work to follow the changes of CA during the processing of sunflower seeds. The decrease of PPO activity was assessed simultaneously and the relationship between these two processes was discussed.