Steviol Glycosides

Cultivation, Processing, Analysis and Applications in Food

Ursula Wölwer-Rieck

• Publisher: Royal Society of Chemistry
• Year: 2018
• ISBN:
  9781788015660
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

The popularity of the plant Stevia ( Stevia rebaudiana ) has risen due to increasing use and interest in its sweet constituents called steviol glycosides. In recent years, these have been approved all over the world as food additives in the category of sweetener, hence they have received more attention and their use in food formulations has increased significantly. New techniques in growing stevia have resulted in new varieties with interesting steviol glycoside profiles. Also, new techniques to analyse the content of sweeteners in different matrices and the detection of new steviol glycosides with very pleasant sensory profiles has followed. The aim of this book is to present novel uses and manufacturing developments as well as to gather together up-to-date information across the whole developing area of steviol glycosides research.

CHAPTER 1

Crop and Steviol Glycoside Improvement in Stevia by Breeding

S. Tavarini, B. Passeraand L. G. Angelini^*

Department of Agriculture, Food and Environment (DAFE), University of Pisa, Pisa, Italy
^*E-mail: [email protected]

Worldwide demand for purified steviol glycosides (SVglys) from Stevia rebaudiana Bertoni leaves is steadily increasing. In the future, it is expected that the agricultural production capacity will be lower than the market demand. To improve the competitiveness of stevia production it is important to produce higher-performing crops in terms of yield and quality, resource use efficiency, greater resistance and resilience against drought stress, extreme weather conditions and a range of biotic stresses. Therefore, the development of new varieties/cultivars of S. rebaudiana with higher leaf and SVglys yields with greater resistance to abiotic and biotic stresses is the topmost priority and the primary aim for plant breeders. Since genotypic and phenotypic variation in the characters, in which we are interested, was reportedly quite high, researchers have attempted to enhance knowledge about this variability, and to develop breeding strategies, starting to study the feasibility and relevance of the characters being measured under different environmental and agro-climatic conditions. The chapter, aims to provide up-to-date scientific information regarding the breeding and selection programs carried out on stevia, describing both conventional plant breeding approaches and new biotechnological approaches, in order to provide new varieties/cultivars with improving quality traits.

1.1 Introduction

Worldwide demand for both stevia leaves and purified steviol glycosides (SVglys) is steadily increasing,¹ and it is expected to increase further in the future, as metabolic disorders such as type-II diabetes and obesity are becoming ever more prevalent. Despite the stevia global market size being characterised by rapid progress, the agricultural production of this crop is still problematic and insufficient to meet the growing global demand. Stevia yields still remain low and unstable in many countries, where the crop is of recent domestication and/or performed by small- and medium-sized farmers, because of high input costs (in particular for planting and establishment), lack of suitable adapted and available varieties, limited expertise in the cultivation, poor disease control and lack of irrigation. To improve the competitiveness of stevia production it is important to produce improved, higher-performing and yielding crops, that are more resistant to drought, extreme weather conditions and a range of biotic stresses.

Therefore, genetic improvement with the development of varieties with higher leaf and SVgly yields and greater resistance to abiotic and biotic stresses, in comparison with the most and currently known cultivars, are important goals in stevia breeding.

1.2 Objectives of Stevia Selection and Breeding

The general objectives of stevia selection and breeding are: (1) to increase the leaf yield and its stability, (2) to ensure high quality value in terms of SVgly content and composition, and (3) to produce types with greater resistance and resilience against disturbance and stress, and targeted to specific growing conditions and farming needs.

The main desired traits, to reach through breeding program strategies in stevia, can be summarised as:

• high leaf yield per unit area
• high leaf-to-stem ratio
• rapid growth rate and regrowth capacity
• enhanced photosynthetic activity
• high SVgly content in the leaves
• high content of specific SVglys (e.g. rebaudioside A – Reb A)
• high adaptability to a wide range of pedoclimatic conditions
• resistance to environmental stresses, pest and diseases
• photoperiod insensitivity
• self-compatibility for viable seed production.

1.2.1 Dry Leaf Yield per Plant and Unit Area

Different characteristics are associated with dry leaf yield such as leaf thickness, number of leaves per plant, leaf surface area, number of branches.^2,5 S. rebaudiana leaves are relatively small, simple, opposite, subsessile, with large variability both in shape and size, ranging from oblanceolate to lanceolate or ovate.⁶ It is possible to select high yielding varieties or hybrids characterised by longer and broader leaves, higher specific leaf area and thickness. The plants should have compact stature and green erect stems, upright and multiple branched. A reduced apical dominance with a higher number of branches, instead of a long slender plant with a single shoot, is preferred. Stem internodes should be closely spaced, to accommodate more leaves on the stems. Plants bearing upright leaves, instead of horizontal, have a better interception and utilisation of light, and are preferred in high plant density crop cultivation.

1.2.2 Leaf-to-stem Ratio

The stems of stevia plants do not contain appreciable quantities of SVglys, which are mainly accumulated in the leaves.^7,8 Hence, varieties that have a higher leaf-to-stem ratio are desirable because they yield higher quantities of SVglys per unit of harvested plant biomass.

1.2.3 Growth and Photosynthetic Activity

Equally relevant, is a higher relative growth rate and a better capacity to regrow after each harvest, giving new leafy shoots rapidly, thus allowing multiple harvests per year. Other desirable agronomic characteristics include lodging resistance.

Dark green leaves, with a higher chlorophyll content and photosynthetic rate per unit leaf area, that show better efficiency in converting solar energy in leaf biomass and SVglys is another important characteristic⁹ that should be considered in breeding strategies.

1.2.4 Wider Crop Adaptability, Yield Stability and Abiotic Stress Resistance

Although originating from the Paraguayan highland of Amambay, between 22° and 25° S latitude and 55° to 56° W longitude,^10,15 stevia can be grown over a wide range of climatic conditions, from semi-humid, subtropical to temperate zones.¹⁶ Once established, it can tolerate mild frosts, but not long periods of hard frosts, which kill the roots of the plant. This lack of winter hardiness means that stevia can be grown as a perennial (three to five years) in temperate to warm climates but as an annual in colder regions.¹⁶

Despite its suitability for general cultivation over a wide range of environmental conditions, a wider adaptability with particular attention to cold and drought resistance, is still considered to be an important objective in stevia plant breeding, because it helps in stabilising the crop production over regions and seasons.

Stevia plants suffer from several abiotic factors such as drought, extreme temperatures like heat, cold and frost, wind, flooding, soil salinity, excessive light intensity and UV exposition. Stevia plants vary considerably in their tolerance to abiotic stresses. The development of drought-resistant cultivars of stevia through selection and breeding is of considerable economic value for increasing crop production in areas with low precipitation or without proper irrigation. The development of drought tolerance in plants is the result of the overall expression of many traits in a specific environment, including both drought escape (rapid development to complete a life cycle before drought) and drought avoidance (reducing water loss to prevent dehydration) strategies. Water stress reduces photosynthesis in the leaves of stevia due to stomata closure in the short-term, and due to photo-inhibition damage and to the inactivation of RuBisCO enzyme (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the long-term.⁹ In this context, physiological and biochemical traits that contribute to enhance water use efficiency and photosynthesis at an early stage can be very useful. In semi-arid regions, resistance/tolerance to salinity is one of the major priorities to be addressed. Salinity reduces plant growth and yield, as well as SVgly content of the leaves. Since stevia shows variability in what concerns the sensitivity or tolerance to salt stress,^17,19 future research should focus on molecular, physiological and metabolic aspects of this stress tolerance to facilitate the development of cultivars with an inherent capacity to withstand salinity stress.

In temperate and cool regions, low temperature is the primary abiotic stress that limits crop productivity, since stevia is very sensitive to hard frosts especi...

Table of contents

• Cover
• halftitle
• Title
• Copyright
• series_editors
• Preface
• Contents
• Chapter 1 Crop and Steviol Glycoside Improvement in Stevia by Breeding
• Chapter 2 Biosynthesis of Steviol Glycosides and Related Diterpenes in Leaves and Glandular Trichomes of Stevia rebaudiana Bertoni
• Chapter 3 Steviol Glycosides Production: Traditional Versus New Technologies
• Chapter 4 Analysis of Steviol Glycosides
• Chapter 5 Presentation and Analysis of Other Constituents in the Leaves: Polyphenolics in Stevia rebaudiana Leaves
• Chapter 6 Presentation and Analysis of Other Constituents in the Leaves: Analysis of Lipids and Volatile Terpenes in Stevia rebaudiana
• Chapter 7 Antioxidant Capacity of Stevia Leaves
• Chapter 8 Stevia rebaudiana Bertoni: Beyond Its Use as a Sweetener. Pharmacological and Toxicological Profile of Steviol Glycosides of Stevia rebaudiana Bertoni
• Chapter 9 Steviol Glycosides in Dentistry
• Chapter 10 Sensory Effects of Steviol Glycosides: Taste Perception and Beyond
• Subject Index