Selected methods of preparing and processing stinging nettle leaves

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INTRODUCTION

Stinging nettle, Urtica dioica L., is an undervalued wild green vegetable (Khatiwada et al., 2011) distinguished by stinging hairs (Kavalali, 2003; Moskovitz, 2009). The plant require less care and is drought tolerant but it is regarded as a weed in agricultural production systems (Di Virgilio et al., 2015; Khatiwada et al., 2011). Upton (2013) stated that ‘let food be your medicine’, which add value to the potential of incorporating traditional knowledge of stinging nettle. Seasonality, the fear of the stinging hairs, lack of commercial availability and the stigma related to stinging nettles being associate with famine/poor man’s food are some of the reasons for its limited utilization as a source of food. The irritating contents of the stinging hairs are dissipated upon drying (Upton, 2013), blanching and cooking of the leaves (Rutto et al., 2013). As a vegetable, stinging nettle leaves are utilized in the preparation of many dishes such as salads, pies, soups, herbal tea or decocted tea in and combination with noodles, omelets or scrambled eggs (Bisht et al., 2012; Guarrera and Savo, 2013; Guil-Guerrero et al., 2003; Orčić et al., 2014; Sansanelli et al., 2014).

LITERATURE REVIEW

This review will first present an overview of the stinging nettle plant, including its botanical properties, growth characteristics, uses, chemical composition and safety with a focus on the leaves (2.1). The next part (2.2) will highlight selected processing methods of stinging nettle leaves including cooking, infusion, decoction and drying. Part three (2.3) will provide an overview of the nutritional properties of stinging nettle leaves. Further, the chemistry behind the changes in carotenoids, ascorbic acid, total phenol content and antioxidant activity during thermal processing will be discussed. The final part (2.4) will deal with the sensory characteristics of stinging nettle leaves including colour, aroma, flavour and mouthfeel and the chemistry of the formation of flavour compounds during thermal processing.

Stinging nettle plant

Urtica L., the stinging nettle, is an annual and perennial herb distinguished by stinging hairs (Kavalali, 2003). Among the Urticaceae family that comprises 40 genera and greater than 700 species; U. dioica is the most common, followed by U. urens (lesser nettle), U. pilulifera (roman nettle) and U. membranacea, had a wide range of distribution extending from Europe, N. Africa, to Asia, up to North and South America, and South Africa.

Cooking

Stinging nettle leaves based dishes are used in the diet of many countries, particularly among low socioeconomic populations. As an example, nettle soup or potherb in Nepal (Adhikari et al., 2016), boiled nettle and barely flour ‘samma sauce’ in Ethiopia (Shonte and WoldeTsadik, 2013), and nettle leaves added to soups or stews in Nordic countries (e.g. Denmark, Finland, Iceland, Norway and Sweden) (Mithril and Dragsted, 2012). In the areas around the Black sea, nettles are traditional foods consumed for health purposes. It is used in the form of a sour soup in Romania, as a nettle walnut sauce in Georgia and as a herb in Ukraine (Danesi et al., 2013). Maanda and Bhat (2010) reported that stinging nettle has a flavour similar to spinach when cooked, and it is used as a side dish to flavour the meal or used to add a bitter taste to meals.

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DECLARATION
ABSTRACT
DEDICATION
ACKNOWLEDGMENTS
1: INTRODUCTION
2: LITERATURE REVIEW
2.1 Stinging nettle plant
2.2 Selected methods of preparing and processing stinging nettle leaves
2.2.1 Cooking
2.2.2 Infusion and decoction
2.2.3 Drying
2.3 Nutritional properties of stinging nettle leaves and changes during processing
2.3.1 Macronutrients
2.3.2 Mineral composition and anti-nutrients
2.3.3 Carotenoids
2.3.4 Ascorbic acid
2.2.5 Phenolic compounds and antioxidant activity
2.3.6 The potential contribution of stinging nettle leaves to dietary intakes of nutrients in human diet
2.4 Sensory properties of stinging nettle leaves and changes during processing
2.4.1 Colour
2.4.2 Aroma and flavour
2.4.3 Mouthfeel
2.5 Conclusions
3: HYPOTHESES AND OBJECTIVES
3.1 Hypotheses
3.2 Objectives
4: RESEARCH
4.1 Descriptive sensory evaluation of cooked stinging nettle (U. dioica L.) leaves and leaf infusions effect of using fresh or oven dried leaves
4.1.1 Abstract
4.1.2 Introduction
4.1.3 Materials and methods
4.1.3.1 Experimental design
4.1.3.2 Production and harvesting of nettle leaves
4.1.3.3 Preparation of fresh and oven dried leaves
4.1.3.4 Cooking of leaves
4.1.3.5 Leaf infusions
4.1.3.6 Descriptive sensory evaluation
4.1.3.7 Colour measurement
4.1.3.8 Data analyses
4.1.4 Results
4.1.4.1 Effects of species and state of leaves used for cooking on aroma and flavour of cooked leaves
4.1.4.2 The effect of species, state of leaves used and infusion cycles on aroma and flavour infusion
4.1.4.3 Colour values
4.1.5 Discussion
4.1.6 Conclusions
4.2 Oven drying and freeze drying of stinging nettle (U. dioica L.) leaves: effects on nutritional properties
4.2.1 Abstract
4.2.2 Introduction
4.2.3 Materials and methods
4.2.3.1 Drying processes
4.2.3.4 Nutritional properties
4.2.3.5 Data analyses
4.2.4 Results
4.2.4.1 Proximate composition
4.2.4.2 Mineral content
4.2.4.3 The effects of drying methods on β-carotene, ascorbic acid, total phenol content and antioxidant activities
4.2.4.4 The contribution of fresh or dried stinging nettle leaves to dietary intakes of fibre, protein, minerals and vitamins
4.2.5 Discussion of results
4.2.6 Conclusions
4.3 Infusion and decoction of stinging nettle (U. dioica L.) leaf powder manufactured using freeze drying or oven drying: effects on nutritional properties
4.3.1 Abstract
4.3.2 Introduction
4.3.3 Materials and methods
4.3.3.1 Sample preparation
4.3.3.2 Nutritional properties
4.3.3.3 The contribution of infusion and decoction from stinging nettle leaf powder to dietary intakes of vitamins
4.3.3.4 Data analyses
4.3.4 Results
4.3.4.1 Ascorbic acid and β-carotene
4.3.4.2 The contribution of infusion and decoction from stinging nettle leaf powder to dietary intakes of vitamins
4.3.4.3 Total antioxidant activity and phenol content
4.3.5 Discussion of results
4.3.6 Conclusions
5: GENERAL DISCUSSION
5.1 Methodological considerations
5.1.1 Stinging nettle production
5.1.2 Sample preparation
5.1.3 Sensory properties
5.1.4 Nutritional properties
5.1.4.1 β-carotene analysis
5.1.4.2 Ascorbic acid analysis
5.1.4.3 Total phenol content analysis
5.1.4.4 Total antioxidant activity analysis
5.1.4.5 Mineral analyses
5.2 Research findings
5.3 Areas for further research
6: CONCLUSIONS AND RECOMMENDATIONS
7: REFERENCES
8: PUBLICATIONS AND CONFERENCE PRESENTAIONS BASED ON THIS RESEARCH
9: APPENDICES