Different forms of maleic and fumaric acids (cis and trans of 2-butenedioic acid) in honey

The total contents of maleic and fumaric acids (cis and trans-2-butenedioic acids, respectively) were quantified by a high performance liquid chromatography method, in 50 floral honeys of Galicia (north-western Spain). Honey pH, activity coefficients, dissociation constants of the acids (K1 and K2), and the molar concentrations of the forms of maleic and fumaric acids naturally present in honey ([AH2], [AH−] and [A2−]) have been calculated for the first time. The contents of maleic and fumaric acids can be determined either as total maleic and fumaric acids ([AH2]) or as total maleate and fumarate ([A2−]), but there are other forms of these acids in honey. Therefore the calculation of the forms of the maleic and fumaric acids present would illuminate their origin at honey pH. The predominant acid form depends on honey pH value. Maleic acid was quantifiable in 44 honeys. The [AH−] form was found as a major component in all samples. Fumaric acid was quantifiable in 49 honeys. The [A2−] form was found as a major component in most honeys (28 samples) and the [AH−] was predominant in 21 samples. No honey analysed had a [AH2] form as predominant. Although maleic and fumaric acids have the same molecular weight and they are both dicarboxylic acids, their pH relationships differ. The relationships between maleic and fumaric acids and honey pH and between the total content of maleic and fumaric acids and their forms in honey have been calculated.