Sensory and nutritional properties of stinging nettle (Urtica dioica L.) leaves and leaf infusions

Abstract

Stinging nettle/common nettle (U. dioica L.) is known since ancient times as a wild source of food and a herbal medicine, but the plant remains underutilized. Drying of stinging nettle leaves not only allow their use when the plants are not physiologically active but also extend their consumption period and utilization at times of food shortage and for addressing micronutrient malnutrition. However, drying could result in decomposition of heat sensitive metabolites such as fatty acids, amino acids, carotenoids, ascorbic acid and insoluble phenolic compounds present in the fresh stinging nettle leaves. These changes might lead to production of volatile compounds, non-volatile compounds, soluble phenolic compounds etc. The systematic description of the aroma, flavour and colour of cooked stinging nettle leaves and leaf infusions, prepared from fresh or dried leaves has not been published. With this study, the effects of using fresh or oven-dried leaves to cook a relish or to prepare an infusion on sensory and nutritional properties were measured. In addition, the effect of two infusion cycles on the sensory properties of leaf infusions was determined.

Although the colour changed during heat processing, most of the characteristic green-related aroma and flavour notes of fresh nettle leaves were preserved in cooked leaves and leaf infusions prepared from dried leaves. When cooking the leaves, the use of dried leaves resulted in an increase in fermented aroma, earthy, burnt flavour, bitter and also salty taste compared to fresh leaves. In leaf infusions, a decrease of grassy, earthy and mint aromas as well as seafood and green-herblike aroma and flavour notes were observed. The first two brewed infusions from fresh or dried leaves provided similar aroma and flavour intensities. Further, the ∆E (total colour difference) value, showed variation in colour of fresh leaves compared to oven dried leaves. The ∆E value also showed variation in colour between the two infusion cycles as well as in uncooked and cooked leaves.

The change in aroma, flavour and colour of leaf infusions and cooked leaves when oven dried leaves were used compared to fresh leaves, prompted an investigation into the effect of drying methods (i.e. freeze-drying and oven drying) on nutritional properties of stinging nettle leaf food products and food ingredient components. Oven drying of stinging nettle leaves resulted in a higher loss of β-carotene and ascorbic acid content compared to freeze drying. A typical serving portion of either fresh, freeze dried or oven dried nettle leaves could provide more than 20 % of the daily value of vitamin A (e.g. 870 µg per day); therefore, nettle leaves in all these forms are rich sources of vitamin A. In contrast, freeze dried and oven dried nettle leaves were found to be a good source of vitamin C while fresh leaves can be considered as a rich source of vitamin C. In general, dried stinging nettle leaves can be considered as a rich source of Ca, Mg and vitamin A; a good source of vitamin C, Fe, and Mn; and a source of Mg and K. In contrast to a decrease in β-carotene and ascorbic acid content, an increase in total phenol content and antioxidant activity were observed in oven dried leaves compared to fresh stinging nettle leaves.

Dried stinging nettle leaves or leaf powder are used to make infusions and decoctions for human medicinal and nutritional purposes due to the antioxidant properties of its constituent vitamins A and C, and phenolic compounds. This led to further investigation into the effect of the type of extraction (i.e. infusion and decoction) on the ascorbic acid, β-carotene, total phenol content, antioxidant activity of stinging nettle leaf powder manufactured using freeze drying or oven drying. β-carotene and ascorbic acid was found to be higher in infusions compared to decoctions. The total phenol content and antioxidant activity of decoction samples were significantly higher compared to infusions (p < 0.01).

This study provides evidence that stinging nettle leaf food products could potentially contributes to dietary intakes of minerals (i.e. Ca, Mg, Fe, Zn, Mn, Mg and K), protein, vitamins (i.e. A and C) and antioxidants and can potentially be incorporated in the diet for overcoming micronutrient malnutrition. Further consumer research is needed to determine which sensory characteristics of the products from stinging nettles drive liking or disliking by target consumers. All in all, this study contributes to the understanding of the potential of stinging nettle for addressing food and nutrition security.