Determination of carbonyl value in rancid oils: A critical reconsideration

Different grades of 1-butanol and 2-propanol (2-PrOH) with and without purification were reevaluated for preparation of blank solutions in a modified method for the determination of carbonyl value (CV) in rancid oils (soybean oil and a frying oil). Only purified 2-PrOH with an absorbance lower than 0.35 at 420 nm was considered suitable as a solvent. Good correlations existed between the absorbance at 420 nm, and the molar concentration for 2-decenal and 2,4-decadienal solutions in 2-PrOH reacted with 2,4-dinitrophenylhydrazine. The sensitivity of the method in terms of 2-decenal was higher than that of 2,4-decadienal. The CVs calculated based on 2,4-decadienal were higher than those of 2-decenal, although differences between them were not statistically significant at low levels. Accordingly, the sensitivity of the measurement and the estimated CV content were dependent on the type of standard considered. Triphenylphosphine (TP) exerted a statistically significant effect on the CVs of the soybean oil, unless the peroxide content of the oil sample was very low or much higher than the amount of added TP. The addition of TP to the frying oil was not necessary because the conditions of the CV test did not lead to the production of excess carbonyl compounds caused by the degradation of peroxides.