Assessment of the impact of drying processes on orange peel quality characteristics

Orange peel was dried applying three different processes: convective drying, microwave drying, and freeze-drying. Some physicochemical properties (moisture content, water activity, and color parameters) and phytochemical characteristics (total phenolic compounds and total antioxidant activity) were assessed in fresh and dried samples. Four kinetic models were tested aiming for convenient modeling of the convective drying curves. The Page and Approximation of diffusion models were adequate in data fitting, which was assessed by the magnitude of the adjusted coefficient of determination ((Formula presented.)) and reduced χ2. The moisture content and water activity of peel dried with the three processes were within acceptable levels for safe storage of products. All processes affected peel color. However, the browning index of freeze-dried samples was equivalent to fresh ones, being higher when convective and microwave drying were applied. Higher chroma parameters were obtained in dried samples indicating a more intense color than fresh peel. Freeze-drying enhanced total phenolic compounds and total antioxidant activity of orange peel when compared to convective and microwave drying. Therefore, from a quality point of view this process seems to be the most appropriate to produce dried orange peel. Practical Applications: Dried orange peel transformed in flour, or slices, cubes, or different geometrical features, may be suitable for incorporation as an ingredient in many types of foods such as cakes, biscuits, or bread, enriching their content in phenolic compounds and total antioxidant activity, therefore creating a value-added product. Within the drying conditions used in this study, it was found that the freeze-drying process is preferable when a lighter and more yellow dried orange peel is desired, with improved phytochemical characteristics. On the other hand, when a darker product with color similar to the fresh peel needs to be attained, the microwave drying process should be chosen.