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Cereal ß-glucan is a water-soluble polysaccharide which is able to form highly viscous solutions even at low concentrations. Due to its health and technological functionalities it has been applied in various foods. Some polysaccharides are used to stabilize emulsions since they inhibit phase separation and oxidation. The proposed mechanisms include viscosity enhancement, metal binding ability and free radical scavenging. In this study, the lipid oxidation retardation and antioxidant effect of high and low molecular weight ß-glucans from oat and barley were evaluated by measuring the production of lipid hydroperoxides and hexanal from lipid oxidation in an emulsion model containing ß-glucans, iron binding capacity, and hydroxyl radical scavenging ability. The studied ß-glucan samples showed various antioxidant activity and the difference was proved to be resulted from residual phytate content, a natural antioxidant, instead of molecular weight and origin of ß-glucan. The ß-glucan samples containing higher content of residual phytate showed higher antioxidant activity, and the antioxidant activity was significantly reduced when phytate was removed. Thus, the study indicated that the residual phytate played a major role in the antioxidant effect of cereal ß-glucans, and the phytate content in plant polysaccharide extracts should be taken into consideration in the studies related to oxidation. In addition, the residual phytate may protect ß-glucan from degradation and maintain its stability and functionality.