Two new chemical methods for quantifying carbonyl secondary oxidation products in frying oils and their correlation with the p-anisidine value

Abstract

Thermal decomposition of hydroperoxides during frying generates several secondary oxidation products, with carbonyl compounds predominating. p-Anisidine value (p-AV) is the most prominent official method for quantifying these carbonyl compounds, utilizing the reaction between carbonyls, principally 2-alkenals and 2,4-dienals, and p-anisidine (para-methoxy aniline) reagent. It is well known, however, that despite the success of the p-AV test, the data is generally skewed toward the unsaturated carbonyl compounds and the reagent is highly toxic and presumably, carcinogenic, limiting its acceptability in some food processing settings. A method proposed in the current study utilizes the reaction between carbonyl compounds, specifically aldehydes, and purpald reagent (4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole). Unlike the p-AV, the purpald test is very specific to aldehydes with negligible interference from other functional groups (e.g., ketones, carboxylic acids, esters). Interestingly, the reaction between aldehydes and purpald reagent is skewed toward saturated aldehydes by nearly the same factor that the p-AV is skewed toward unsaturated aldehydes, providing a complimentary information to the p-AV on the nature of secondary aldehydes formed during thermo-oxidative degradation of lipid hydroperoxides. Further, a second method utilizing p-anisidine hydrochloride reagent was optimized for the determination of carbonyl secondary oxidation compounds. A strong correlation was found between each of the proposed alternatives and the p-AV method, providing the opportunity for indirect determination for p-AV in food processing settings where the p-anisidine reagent is considered unacceptable regarding environmental health and safety. Challenges and limitations of the proposed chemical alternative methods are also presented.