Thermal damage in blood orange juice: kinetics of 5-hydroxymethyl-2-furancarboxaldehyde formation

Formation of 5-hydroxymethyl-2-furancarboxaldehyde (HMF) was studied kinetically by HPLC in blood orange juice and model systems of fructose, glucose and sucrose at different acidic pH values and temperatures. The ultraviolet absorption of the solutions changes during reaction by forming an enediol intermediate, which slowly converts into HMF Pseudo first order rate coefficients were used for comparing reactivity of sugars in model systems and in fortified juices. Glucose degrades much more slowly than fructose, while the reactivity of sucrose depends on pH value and temperature. The activation energy for sucrose was about 20 kcal mol(-1) higher than fructose, and such a difference can be ascribed to the rate-determining contribution of the preliminary hydrolysis step. First order rate coefficients in orange juices were similar to those of model systems at comparable pH and temperature, indicating that sugar degradation occurs without any interference of other components, thus excluding the presence of the Maillard's reaction. HMF content constitutes an index of juice deterioration, therefore it should be included among the parameters of quality for commercial blood orange juices.