Method of Acid Value Determination for Oils Containing Alkali-Labile Esters

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Acid value (AV) determination for identical samples of oil extracts from non-pungent pepper (Capsicum annuum L. var. longum cv. CH-19 sweet) by titrimetry using indicator dye and a potentiometer was found to vary considerably from one laboratory to another. Free fatty acid generation by alkali hydrolysis of capsinoids during titration, with consequent false endpoints was found the major reasons for this. The structure of capsinoid, the principle constituent in non-pungent pepper, is similar to that of capsaicinoid, the difference being that capsinoid is the ester of vanillyl alcohol and fatty acid, whereas capsaicinoid, an acid amide consisting of vanillyl amine and fatty acid. The major members of the capsinoid family are capsiate (4-hydroxy-3-methoxybenzyl-8-methyl-6-nonenoate) and dihydrocapsiate (4-hydroxy-3-methoxybenzyl-8-methylnonanoate). In consideration of the hydrolysis of alkali-labile esters such as capsinoids in oils during AV determination, the authors established an alternative method by which AV can be found with no involvement of artifacts. AV is an index of oil deterioration directly reflecting free fatty acid content. AV was thus determined by quantifying free carboxyl residues subsequent to specific and quantitative modification with the fluorescent labeling reagent 9-anthryldiazomethane (ADAM). Fluorescence-labeled carboxyls were quantified by HPLC, with carboxyls determined by calibrating fluorescence intensity of chromatographic peaks with stearic acid as the control. Relative fluorescent intensity of labeled fatty acids differing in carbon-chain length from 8 to 20 was essentially the same with respect to molar concentration. The present method thus serves as an accurate means for AV determination of oils containing alkali-labile esters.
2004-07-01

Method of Acid Value Determination for Oils Containing Alkali-Labile Esters

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