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The rate of formation of 5-hydroxymethylfntial (5-HMF) in apple juice with soluble solids ranging from 15 to 69"Brix at 100, 104, 108 or 112°C was experimentally determined. Different mechanisms of reaction were proposed and corresponding kinetic models were derived. The model that provided the best fit to experimental data corresponded to a first-order initiation step, followed by an autocatalytic period limited by consumption of reactants. The resulting kinetic equations were incorporated into the mathematical model of a triple-effect apple-juice evaporator. The simulation values obtained for the content of 5-HMF in the concentrated apple juice were in good agreement with observed industrial levels. From simulation runs 5-HMF generation was strongly related to the temperature level at the first evaporation effect.