Effects of convective air drying temperature on nutritional quality and colour of watercress (Nasturtium officinale)

Watercress (Nasturtium officinale) is one of the most popular leafy vegetables consumed in the world, either fresh or cooked, presenting high contents of phytochemicals and bioactive compounds. However, due to its high moisture content (91% w.b.), this vegetable is easily perishable. To prolong shelf life and provide the convenient transportation, dried watercress might be a novel product for consumption in soups and other recipes. Convective air drying is an affordable process, but may cause significant nutritional degradation in vegetables, given their sensitivity to heat, light, oxygen and leaching phenomena. Therefore, the purpose of this study was to evaluate the drying characteristics and the effects of drying temperature on quality attributes of watercress, without any pre-treatments. Drying at temperatures of 40, 55 and 70 ⁰C resulted in drying times of 230, 119 and 92 minutes, respectively. The nutritional parameters were negatively correlated with the drying temperature. Retentions of 12.5±3.8 - 72.8±4.2%, 51.1±3.3 - 80.4±4.8%, and 44.3±6.1 - 92.1±12.2% were obtained for vitamin C, total phenolic compounds and total antioxidant activity, respectively. Increased values of total colour difference (TCD), browning index (BI) and higher chlorophylls losses (26.1±3.9-55.6±14.9%) were also observed as drying temperature increased. In conclusion, convective air drying affected considerably both watercress’s nutritional values and quality characteristics. Moreover, more pronounced deterioration was observed when higher drying temperatures were employed. Alternative drying methods or application of pre-treatments prior to drying are suggested for further research on dehydrated watercress, which is expected to be a novel product for food recipes or as a food supplement powder.