Influence of Drying Processes and Pretreatments on Nutritional and Bioactive Characteristics of Dried Vegetables: A Review

Drying has been applied to vegetables in order to preserve, store and transport these food products. However, drying implies not only physical changes, easily detectable by the consumer through visual assessment, but also chemical modifications. These are not always visible, but are responsible for alterations in colour, flavour and nutritional value, which compromise the overall quality of the final product. The main chemical changes associated with drying are related to the degradation of phytochemicals, such as vitamins, antioxidants, minerals, pigments and other bioactive compounds sensitive to heat, light and oxygen. Moreover, nutrient losses are inevitably associated with leaching as a result of the water removal from the vegetable during the drying process. In order to prevent or reduce nutrient losses and thus improve the quality of dried products, pretreatments are often applied. In this review, an overview of the procedures developed for dehydration of vegetables applying heat by convection, conduction or radiation is presented. The influence of pretreatments on nutritional and bioactive characteristics of dried vegetables is discussed. Blanching with steam, water or chemical solutions is the most commonly used, but power ultrasound, ohmic blanching, osmotic and edible coatings pretreatments have also been reported. The influence of the drying processes and conditions on nutritional contents and bioactive characteristics is also presented.