White and red grape juices and their concentrates were subjected to thermal treatments at different temperatures ranging from 50 to 70 degrees C at different times. 5-Hydroxymethylfurfural (5-HMF) and fluorescence relative index (FLRI) were measured. Response surface methodology (RSM) was applied to determine the effect of temperature and time on the HMF and FLRI formation at the different Brix (bx) degrees. An increase in temperature (from 50 to 70 degrees C) and time (from 12 to 192 h) for 15 degrees, 45 degrees and 65 degrees Brix degrees was associated with an increase in HMF and FLRI development of white and red grape juices. HMF formation was higher in white grape juice and concentrates than in red ones. Optimum conditions were confirmed and these fitted the experimental data well. Thus, regression equations can be used to estimate HMF and FLRI values at various Brix degrees for white and red grape juices and concentrates. (c) 2006 Elsevier Ltd. All rights reserved.