Effect of air-drying temperature on physico-chemical properties of dietary fibre and antioxidant capacity of orange (Citrus aurantium v. Canoneta) by-products

Dehydration promoted important modifications affecting both the physico-chemical properties of dietary fibre (DF) and the antioxidant capacity of orange by-products (peel and pulp remaining after juice extraction). The significance of such changes was largely dependent on the air-drying temperature used (from 30 °C to 90 °C). The major modifications on the DF components were observed when either extended drying periods, i.e. at lower temperatures, or elevated drying temperatures were applied. Dehydration around 50–60 °C apparently promoted the minor disruption of cell wall polymers, in particular of pectic substances. Pulp samples exhibited higher values of swelling (SW) and fat adsorption capacity (FAC) than those derived from orange peel. Although, significant decreases in water retention capacity (WRC), FAC and solubility values were detected for both by-products as the air-drying temperature increased. The antioxidant capacity associated to dehydrated citrus by-products was significantly higher for orange peel than for pulp samples. In general, the by-products studied proved to be quite resistant to the different heat treatments applied within the range of 40–70 °C. In overall, the study shows that, in order to preserve either the DF quality and/or the antioxidant capacity, air-drying temperature should be controlled since both types of compounds, DF components and antioxidants, might be degraded or modified either when extended drying periods and/or high drying temperatures are applied.