The influence of metal ions (Cu (II), Fe (III), Fe (III)) and the effect of
erythro- and
threo-2-methyltartaric, and
meso- and
threo-2, 3-dimethyltartaric acids as chelating agent on the oxygen absorption in O/W-type emulsion were investigated by employing a polarographic Beckman Oxygen Analyzer, Model 777. Soybean oil with different P.O.V. and water were emulsified into O/W-type emulsions using Tween-40, and the decrease of dissolved oxygen was measured in the presence of Fe (II), Fe (III) and Cu (II) ions. In order to evaluate the effect of these acids, an acid solution (10
-1 mol/l, 1 ml) was added to the emulsion. Then, the metallic solution (0.5 × 10
-1 mol/l, 1 ml) was added to the same emulsion, and the variation in the dissolved oxygen was measured. The results of these measurements revealed that the oxygen absorption increased in the order of Fe (II) >> Fe (III) > Cu (II) and the deteriorating effect of these metal ions was suppressed by the addition of these acids.
When the stabilizing effect of these acids was compared each other, the effect increased in the order of non-substituted > monomethyl-substituted > dimenthyl-substituted, that is, tartaric > 2-methyltartaric > 2, 3-dimethyltartaric acids. Moreover, comparing the effect of the isomers of these acids to each other,
erythro-form acids are clearly seen to function as a more effective antioxidant. In the test for antioxidant activity by means of A.O.M. and Shaal Oven Test, the similar results were obtained.
On the basis of these data, it can concluded that ; (1) as for the isomers of these acids, the difference of the stabilizing effect on the oil as chelating agent can be attributed to the equilibrium constants of these acid-metal complexes ; (2) in addition, as for the monomethyl-, dimethyl- and non-substituted acids, it seemed reasonable to assume that these effects as antioxidant may be attributed to the methylene or methin group on the α-position of acids.
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