Modeling the dynamics of bioactive compounds during continuous UV-C processing of liquid foods with flow recirculation: application to tomato juice

Ultraviolet-C (UVC) processing is a promising non-thermal technology for liquid food preservation, although its impact on bioactive compounds remains a key concern. This study develops a mechanistic model to describe bioactive degradation during continuous UVC processing in recirculating flow systems. The system is represented by a perfectly mixed storage tank coupled with a UV irradiation unit modeled as a series of continuous stirred-tank reactors, capturing hydrodynamics and residence time distribution. Bioactive degradation is formulated as a dose-dependent first-order process, linking intrinsic kinetics to cumulative UV fluence. A dimensionless framework is derived in terms of the Damköhler number, the fraction of irradiated volume, and the cumulative dose, enabling generalized analysis across operating conditions. The resulting linear system admits an analytical solution, clarifying interactions between reaction kinetics, recirculation, and system configuration. Results show that degradation is primarily governed by cumulative UV exposure and volume distribution, while internal hydrodynamics play a secondary role. The model also allows estimation of intrinsic degradation parameters from experimental data. Validation using tomato juice, considering total phenolics and antioxidant capacity, shows good agreement with experimental results. The estimated intrinsic dose-based degradation constants ranged from 1.6 × 10–3 to 3.0 × 10–3 cm2/J for total phenolics and from 6.1 × 10–3 to 7.8 × 10–3 cm2/J for antioxidant capacity. This framework contributes to the rational design of continuous UVC processing systems by enabling prediction of bioactive degradation under recirculating flow conditions.