Kinetics and Influencing Factors of Nonenzymatic Browning in Apple Juice Concentrate

The effects of various processing operation units on browning and the rate of nonenzymatic browning in commercial apple juice concentrates (70.5 °Brix) stored at 5, 20 or 37 ℃ were investigated. The changes of color value (T440), browning index (A420nm), amino nitrogen content and 5-hydroxymethylfurfural content were evaluated. The rates of browning at all three temperatures obeyed the first-order kinetic model. Browning reaction occurred more obviously at high temperature. There was a general decrease in amino nitrogen content and an increase in 5-hydroxymethylfurfural content with prolonged storage; this effect was most pronounced for samples stored at 37 ℃. The relation between 5-hydroxymethylfurfural content and fruit browning could be described as an exponential explanation for the reason why browning occurs in apple juice concentrate during storage is Maillard reaction, which could also be validated by the linear relation between amino nitrogen and browning. During the early processing stages, including crushing, pressing, enzymolysis, ultrafiltration and first pasteurization, no obvious Maillard reaction was found. The follow-up operation unit, adsorption, resulted in the removal of amino nitrogen and other dark colored compounds. However, Maillard reaction was undesirably promoted, leading to an obvious reduction of amino nitrogen in apple juice.