Formation and degradation kinetics of organic acids during heating and drying of concentrated tomato juice

Tomato products are often thermally processed or concentrated to obtain their desired shelf life and to facilitate transport. However, processing negatively affects the quality of tomato products. This study focused on the influence of processing on the presence of important tomato taste markers, i.e. citric acid, malic acid, ascorbic acid and pyroglutamic acid (PCA). Isothermal heat treatment of tomato juice was experimentally assessed at varying moisture content (0.18–0.95 kg/kg total), temperature (60–100 °C) and time (0–18 h) combinations. Increasing ascorbic acid degradation (up to 70%) and PCA formation (up to 0.032 mmol/g FT) were measured, while citric acid and malic acid were unaffected. A first order reaction kinetics described the degradation and formation of ascorbic acid (R² = 0.76) and PCA (R² = 0.98), where the coupled effect of both moisture content and temperature on the reaction rates was modelled with an Arrhenius-type equation. Higher temperature enhanced both reaction rates with factors 4.2 and 5.1 for ascorbic acid and PCA, respectively (from 60 to 100 °C at 95 w/w%), while at lower moisture content the rate of the ascorbic acid degradation decreased with factor 3.5 and the rate of the PCA formation increased with factor 3.5 (both from 95 down to 5 w/w% at 90 °C). Finally, by implementation of the kinetic models in a process model it was estimated that 25% of ascorbic acid degrades during cocurrent drying while after countercurrent drying only 21% degrades. Similarly, during cocurrent drying 0.021 mmol/g FT PCA is formed, which is more than during countercurrent drying (0.008 mmol/g FT). This approach yields interesting insight on the effect of processing on presence of ascorbic acid and PCA and thus offers opportunities for process optimization.