Process optimisation and non-destructive quality attributes monitoring during drying: A case study of celeriac slices and characterisation of its powder

This study investigated strategies applied in drying technology to produce an optimised quality of plant-sourced food while considering environmental and sustainability aspects based on a case study of celeriac and powder, respectively. The I-optimal design of response surface methodology with 30 experiment runs was applied for process optimisation. Pre-drying treatments (blanching at 85 °C, 3 min; dipping in 1% citric acid solution, 3 min; no pre-drying treatment), drying temperatures (50, 60, and 70 °C), air velocities (1.5, 2.2, and 2.9 m s-¹), and thickness (3, 5, and 7 mm) were applied. The process parameters that optimised the product quality were 58 °C drying temperature, 2.9 m s-¹ air velocity, and 4.6 mm sample thickness with acid pre-drying treatment. Regarding developing innovative measurement methods, the potential of Visual-NIR hyperspectral imaging (VNIR-HSI, 425–1700 nm) to predict celeriac quality attributes during drying with three different drying temperatures was investigated. The HSI-Gaussian Process Regression (GPR) fusion method excellently predicted moisture content (MC) (R² ≈ 1.00, RMSE = 0.77 gw 100 gₛ-¹) and water activity (aw) (R² = 0.98, RMSE = 0.04). Moreover, the rehydration ratio (RR) and colour parameters were reasonably predicted. However, antioxidant activity (AA) and total phenolic compounds (TPC) were poorly predicted. These results were potentially due to MC variations dominating the NIR region, masking phenolic compounds. Finally, the celeriac-based-trained model was assessed by predicting the MC of apple, cocoyam, and carrot slices. The results were encouraging; however, a GPR model trained on the data of all four commodities was more robust (R² ≈ 1.00, RMSE = 1–2 gw 100 gₛ-¹). Finally, celeriac powder (CP) characteristics processed with an optimised drying process were compared to a commercial product. The CP showed improved colour and water-interaction properties than commercial CP. However, CP had a higher Hausner ratio (HR), indicating its lower flowability, probably because of its higher MC. The commercial CP required less wetting time, probably because its higher density led to faster sinking in water. In addition, both samples have insignificant oil-interaction properties. To sum up, processing chains, especially the drying process and its surrounding unit operations, may have a cumulative effect on product quality.