Reaction order, activation energy (E,) and average rate constant (k,) were evaluated by dynamic modeling for color degradation of grapefruit juice during thermal processing. Simulated aseptic conditions were: 85-135"C, 40-175 set processing time, and average velocities 5-30 cm/set. Quality was assessed by nonenzymatic browning index (NEBI) and S-hydroxymethyl-furfural (HMF). Average values for reaction-order and energy of activation were: 0 and 21.4 kcal/g-mol (NEBI) and 0.3 and 23.2 kcal/g-mol (HMF), respectively. These values were compared with those contained in the deformed ellipsoid which define the 90% joint confidence interval of all possible parameter combinations. Based on initial and final concentrations, the method can predict effects of processing on liquid foods.