Modelling of 5-hydroxymethylfurfural photo-degradation by UV irradiation. Influence of temperature and pH

The main aims of this study were (a) to prove that the UV irradiation of juices prevents the formation of 5-hydroxymethylfurfural (HMF), (b) to know the influence of temperature and pH on the UV photo-degradation of the HMF when it is unfortunately present in the juice (for example, after a previous thermal treatment or after a long storage period) and (c) to model this photo-degradation, proposing a reaction mechanism related to the power absorbed by the solution that depends on the HMF concentration.For these purposes a mid-pressure mercury lamp with emission wavelengths between 250 and 740 nm was used.Firstly, nectarine juice was irradiated to be sure that HMF was not synthesised. Then, aqueous solutions of 100 mg·L^? 1 HMF at pH 3, 4 and 5 were irradiated at 12, 25, 35 and 45 °C for 120 min. Aliquots were analysed to measure their HMF contents and absorption spectra.The photo-degradation data fitted well to both zero-order and pseudo-first-order kinetic models and the constant values were similar. The increases in both temperature and pH enhanced the photo-degradation, the optimal conditions inside the ranges studied being 45 °C and pH = 5, when a reduction of 60% of the initial content of HMF was reached.The spectral radiant power absorbed by the whole solution and the incident spectral radiant power reaching any depth of the reactor were evaluated taking into account the linear spherical emission model and using the Simpson integration method. Its dependence on the HMF concentration was also studied. A three stage degradation mechanism was proposed, matching both the zero-order and pseudo-first-order kinetic models previously obtained.