Refractance window drying of beetroot: modelling drying behaviour and optimising quality attributes

Refractance window drying (RWD) has gained interest as a mild technology for producing high-value powders, but few studies have focused on beetroot, which is rich in vitamin C and bioactive pigments yet highly perishable and underutilised. This study aimed to find optimal RWD conditions for beetroot slices and to assess powder quality and drying behaviour. An I-optimal experimental design was employed with water-bath temperature (75–95 °C) and slice thickness (1.5, 2.0, and 2.5 mm) as independent variables. Moisture content and vitamin C were the main responses for response surface modelling and multi-response optimisation, while functional properties (water-holding capacity, oil-holding capacity, and rehydration ratio) were evaluated at selected conditions. Powder moisture content decreased as temperature increased and slice thickness decreased, and vitamin C content declined under more severe conditions, with greater losses at higher temperatures and thinner slices. Multi-response optimisation showed that 75 °C and 2.0 mm slice thickness provided the best overall compromise; the resulting beetroot powder had a moisture content of 9.98 ± 0.23% and vitamin C content of 81.90 ± 1.65 mg ascorbic acid/100 g dry matter. Drying curves followed falling-rate drying, and moisture ratio data were best fitted by the Haghai & Ghanadzadeh model for 1.5 mm slices and by the Henderson & Pabis and two-term models for 2.0 and 2.5 mm slices. RWD at 75 °C and 2.0 mm slice thickness appeared a viable way to produce shelf-stable beetroot powder with vitamin C retention and functional properties for use as a natural colourant and functional ingredient in value-added food products.