Evaluation of the non-enzymatic browning in thermally processed apple juice by front-face fluorescence spectroscopy

Fresh apple juice was heated at 95 °C for 30 and 60 min, and then stored for 6 days for obtaining different extent of non-enzymatic browning. Front-face fluorescence excitation–emission-matrix (EEM) with excitation at 355 and 400 nm and emission ranges of 385–600, and 430–600 nm was used to measure the juice samples. The sign test pointed out an enhanced sensitivity of EEM compared to commonly used browning indicators, such as non-enzymatic browning index (NEBI), color readings in L∗a∗b∗ space, and the concentration of 5-hydroxymethylfurfural (HMF). Using the fluorescence data, the correctness of classifications of fresh juice, heated juice, and stored juice were >85%. Multivariate analyses were applied to study the relations of fluorescence emission spectra and common indicators. For predicting NEBI, b∗, and HMF, the correlation coefficients were >0.80. Resulting, the spectroscopic analyses of fluorescent neoformed products such as HMF provide a feasible approach to monitor non-enzymatic browning of juice.