Studies on a Simple Method for Evaluation of Deterioration Degree of Frying Oils and Fats. II.

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Abstract
In the preceding report, we proposed relative interfacial tension of cyclohexane solution of frying oils and fats as a new index which reflects thermal oxidation of frying oils and fats. In this paper, superiority or inferiority between the relative interfacial tension and several methods proposed previously for the rapid evaluation of deterioration degree of frying oils and fats was discussed. As a result, thiobarbituric acid value and acid value were concluded to be inappropriate indices because of the low correlation with glyceride dimer fraction content (γ=0.252, γ=0.555) and petroleum ether insoluble oxidized fatty acid content (γ=0.333, γ=0.575). Saponification color number showed relatively high correlation with glyceride dimer fraction content (γ=0.705) and petroleum ether insoluble oxidized fatty acid content (γ=0.872). Saponification color number <9.0 and frytest (by E. Merk) <color 3 was correspondent to petroleum ether insoluble oxidized fatty acid content <0.7%. However, one should take account of the fact that the colors of the frying oils and fats themselves exert influence upon saponification color number and frytest to same extent, and the measurement of these indices should be practiced as soon as possible before autoxidation proceed. Relative interfacial tension showed the highest correlation with glyceride dimer fraction content (γ=0.909) among these indices. This index was influenced scarcely by the addition of yolk to the frying oil, similarly to petroleum ether insoluble oxidized fatty acid content. As compared with this, foam spread distance was strongly influenced by the addition of yolk. In conclusion, measurment of relative interfacial tension, saponification color number and frytest should be considerd as an appropriate methods for the rapid evaluation of the formation of injurious products to health by thermal oxidation of frying oils and fats.

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