Rapid Colorimetric Determination of Methylglyoxal Equivalents for Manuka Honey

Abstract

Aims: Realization of a rapid colorimetric assay for monitoring levels of methylglyoxal and other dicarbonyl compounds from Manuka honey. Methods: N-acetyl cysteine (NAC) and 2, 4-dinitrophenylhydrazine (DNPH) were adopted as reagents for methylglyoxal colorimetric analysis of honey at 288 or 525 nm, respectively. Results and Discussion: NAC and DNPH produced linear responses for methylglyoxal with:(i) regression coefficient (R2) equal to 0.99 or 0.97, (ii) molar absorptivity (measure of sensitivity) equal to 287±11 or 14189±498 M-1 cm-1, (iii) a minimum detectable concentration (MDC) of 0.18 mM vs 7.3 µM, (iv) upper linearity limit of linearity (ULL) equal to 4mM or 83 µM, and (v) a day-to-day precision of 16.0 and 18.3%, respectively. Low interferences occurred with reducing sugars, glyoxal or 3-deoxy-D-glucosone. For honey with a unique manuka factor (UMF) rating 5+ to UMF18+, the net concentration of dicarbonyl compounds ranged from 1069 mg-methylglyoxal equivalence per kg (mg MeGEq /kg) to 2208 (mg MeGEq /kg) using the NAC assay. For the DNPH assay, the apparent dicarbonyl concentration was 350 to 1009-mg MeGEq /kg honey. Measures of methylglyoxal equivalences were strongly correlated with the UMF rating for honeys (R2=0.98-0.99). Conclusion: The proposed colorimetric analysis of methylglyoxal equivalence in Manuka honey is feasible proposition. Further work is needed for method validation.