The TBO assay is an established method for quantifying carboxy groups. This assay was performed on the non-treated, Ox, and Plasma samples (Figure ). The number of carboxy groups introduced per unit mass (μmol/g) was calculated from the absorbance at 630 nm (Table ). Approximately 2.6 times more carboxy groups were introduced in Ox-60 and Ox-80 than in Ox-30. This implies that the degree of oxidation increased with the reaction temperature. In contrast, the plasma treatment introduced only a few carboxy groups.