A rapid method to monitor quality of apple juice during thermal processing

Quality deterioration of apple juice, thermally processed at constant temperatures ranging from 95 to 123 degrees C for 2 to 30 min, was measured by three quality indexes: nonenzymatic browning (NEBI), 5-hydroxymethylfurfural (HMF) and fluorescence relative inner (FLRI). All three indexes followed an apparent zero-order reaction with activation energies of 148.6, 151.1 and 148.6 kJ/mole for NEBI, HMF and FLRI, respectively. FLRI correlated highly with HMF (R(2) = 0.98). The correlation between HMF and FLRI, obtained under constant temperatures, was verified under continuous process conditions of 85 to 135 degrees C and 30 to 180 s. FLRI was found to be a quantitative criterion, which could be applied for 'on-line' monitoring of the deleterious effects of thermal processing of apple juice.