Development of innovative ingredients from Nettle (Urtica dioica L.)

Urtica dioica L., known as stinging nettle, is an herb mainly used for medicinal and food purposes in salads, pies, soups and tea preparation, and as coloring agent for the extraction of chlorophylls.
Recently, it has received a great interest for its antioxidant activity and other health properties, mainly due to presence and content of chlorophylls, carotenoids and phenolic compounds, and for its easy growth and cultivation.
Despite its wide use as coloring ingredient, limited is the usage of the plant itself and its extracts for other food purposes due to high instability of the bioactive and secondary metabolites. This requires the investigation of stabilization actions including those related to the encapsulation by drying technologies that could also allow to obtain new ingredients with interesting technological functionalities.
To these aims, mechanically extracted nettle juice was subjected to freeze-drying to produce amorphous powders with a higher storage stability. Freeze-drying encapsulation was also applied by using maltodextrin. The differently obtained powders were characterized for their solubility in water, moisture sorption isotherms, coloring power, total phenolic content (TPC), ferric reducing antioxidant power (FRAP) and content of carotenoids, chlorophyll a and b, and their storage stabilities were evaluated at four different temperatures (4°C, 22°C, 35°C and 50°C), during 95 days.
Results highlight that encapsulated nettle juice powder (NJ-MD) presents higher solubility than the non- encapsulated one (NJ), despite the similar moisture sorption isotherm that could be classified as type II. At equal quantity, NJ-MD presented lower values of all the other studied parameters in respect to the NJ mainly due to the dilution effect of the addition of maltodextrin.
In terms of physical and chemical stability no significant differences were observed between NJ and NJ- MD.
During storage, solubility, luminosity L*, hue angle, TPC and FRAP remained constant and this occurred at all temperatures investigated. On the contrary, chroma (C*) and a* chromatic parameters were influenced by temperature of storage; at 50°C with a decrease of the former and an increase of the latter, resulting in a shift of color from the green to a green-yellow. The content of carotenoids and chlorophylls decreased during time depending on storage temperature with the order carotenoids > chlorophyll a > chlorophyll b. In particular, carotenoids and chlorophyll a content remained constant at 4 and 22°C and decreased with a first order kinetic at 35 and 50°C. Chlorophyll b content, instead, remained constant at all temperature excluding 50°C at which decreased with first order kinetic.
These results highlight the possibility to obtain stable derivatives of nettle by applying freeze-drying and freeze-drying encapsulation in presence of maltodextrins offering a new opportunity to use this plant for in innovative food products and nutraceuticals.