Development of dry natural additives from stinging nettle (Urtica dioica) and evaluation of their quality and stability

Urtica dioica L., known as stinging nettle, is a wild plant used mainly as herbal medicine and traditional food purposes, and as colouring agent for the extraction of chlorophylls. Recently, it has gained interest for its nutritional value and as potential source of bioactive molecules, that include, besides chlorophylls, also carotenoids and phenolic compounds. Despite its popular use, limited is the knowledge and information on the preparation and stabilization of nettle derivatives like juices or extracts that could favour its application in food and non-food sectors. Development and optimisation of mild extraction drying and encapsulation processes to produce stable and high quality ingredients easy to handle and rich in nutritional and health compounds could represent an interesting opportunity to valorise nettles. Aim of this study was to develop nettle-based powders additives obtained by freeze-drying. Mechanically extracted nettle juice was freeze-dried without (NJ) and with the addition of maltodextrin (5 %, w/v) as encapsulating agent (NJ-MD). Powders were characterized for water solubility, moisture sorption-isotherms, thermal properties, colouring power, total phenolic content (TPC), ferric reducing antioxidant power (FRAP) and carotenoids, chlorophyll a and b content. Stability at four different temperatures (4°C, 22°C, 35°C and 50°C )for 95 days was evaluated. Results showed that NJ-MD has higher solubility than the non-encapsulated juice (NJ) as well as lower values of all bioactives and antioxidant power, due to “the dilution effect “given by the maltodextrin addition. Both powders showed similar moisture sorption sigmoidal profile (type II). During storage, solubility, luminosity L*, hue angle, TPC and FRAP did not change at all temperatures, while at 50 °C chroma (C*) and a* colour parameter decreased and increased respectively, resulting in a shift of colour from the bright green to a paler green colour. The content of carotenoids and chlorophylls decreased during storage at 35 °C and 50°C, showing first-order degradation kinetics and increasing rate with increasing temperature. Results highlight the feasibility to obtain new nettle powders by freeze-drying and maltodextrins encapsulation to be used as ingredient for innovative food formulated products.