Drying and degradation kinetics of red grape pomace with special emphasis on degradation of anthocyanins using liquid chromatography-orbitrap-mass spectrometry

Red grape pomace, a major byproduct of the grape and wine industry, is rich in different bioactive compounds, particularly anthocyanins, making it a valuable natural resource. However, its high moisture content can lead to deterioration due to fungal and mould growth. This study focuses on extending the shelf life of red grape pomace by using drying as an effective preservation method. The drying of red grape pomace at temperatures 40 to 80 °C was carried out using hot air tray dryer. Mathematical modelling of experimental data with selected seven models showed best fit with Midilli et al. model (R² = 0.9931, RMSE = 0.0124, X² = 0.0117). Effective moisture diffusivity found between 1.54 × 10⁻¹⁰ and 1.12 × 10⁻⁹ m² s⁻¹ across all temperatures and the activation energy 42.79 kJ mol⁻¹. Additionally, the degradation kinetics of total phenolic content (TPC), total flavonoid content (TFC), total anthocyanin content (TAC) and antioxidant activity (AA) were analysed using first-order kinetics. The results indicated an increased rate of degradation with rising drying temperatures. The half-life and D value were lowest for AA, and the highest activation energy observed for TAC (48.019 kJ mol⁻¹), demonstrating the temperature dependence of degradation. Further, anthocyanin profiles were generated using LC-Orbitrap-MS, identifying and quantifying 16 different compounds. The anthocyanin profiling data showed that drying red grape pomace at 50 to 60 °C resulted in lower degradation of anthocyanin compounds compared to other temperatures. Overall, interplay between drying temperatures and phytochemical stability the optimum drying can be carried out between 50 and 60 °C for red grape pomace, which ensures the extended shelf life of red grape pomace for utilization in functional food and nutraceutical industry.