Onions and Allied Crops

Volume III: Biochemistry, Food Science, and Minor Crops

James L. Brewster, H.D. Rabinowitch

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9780367403768
• eBook ISBN: 9781000709582
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Originally published in 1990, Onions and Allied Crops, is a comprehensive account of the edible allium, examined across three volumes. The collection examines the major economic and dietary importance of edible alliums in most countries, and brings together contributions from experts across multiple disciplines, including food scientists, economists, agriculturalists and biochemists. The books address selection and breeding of locally adapted cultivars and the development of cultural techniques, allowing for cultivation across the tropics, to the sub-arctic regions. As such the collection examines the allium as a major agricultural asset and the impact this has had on many economies. In this third volume, the analysis and focus is upon biochemistry, food science and minor crops. This volume will be of use and of interest to food scientists, economists, agriculturalists and biochemists alike.

Chapter 1

CARBOHYDRATE BIOCHEMISTRY

Ben Darbyshire and Barrie T. Steer

TABLE OF CONTENTS

I.	Introduction
II.	Compounds and Their Distribution
III.	Nonstructural Carbohydrates during Bulbing. Differences between Cultivars and Species A. Bulb Production B. Cultivar Differences C. Species Differences
IV.	Fructan Synthesis
V.	Concluding Remarks

References

I. INTRODUCTION

This chapter reviews the carbohydrate physiology and biochemistry of Allium species. We limit discussion mainly to issues peculiar to these species, and particular reference will be made to soluble carbohydrates, their distribution, and the synthesis of fructans. Much of our comment refers to work with onion (A. cepa). Some reference is made to investigations of other Allium species, but this is limited, since knowledge of interspecies differences is limited. No reference is made to the broader issues of carbohydrate metabolism, such as sucrose synthesis, which are beyond the scope of this chapter and for which readers are referred to other sources.^{1, 2, 3}

II. COMPOUNDS AND THEIR DISTRIBUTION

The nonstructural carbohydrates in Allium spp. include glucose, fructose, and sucrose together with a series of oligosaccharides, the fructans.^{4, 5, 6, 7, 8, 9, 10} Fructans occur in the Asteraceae,¹¹ but more widely in the monocotyledons, especially in the Amaryllidaceae, Iridaceae, Liliaceae, and Poaceae.¹² In all species fructans are considered to be fructose polymers containing a single glucose residue. Early reports^12-13 suggested only fructose was present, but later Palmer¹⁴ demonstrated the presence of glucose in fructans from a number of sources. The mechanism of fructan synthesis, demonstrated by Edelman and Jefford¹⁵ supports the presence of a glucose residue in fructans because fructose is transferred from one sucrose molecule to another sucrose molecule.

It is important to appreciate that in plants, three different trisaccharides are potentially possible, and all three have been identified in a variety of plants.^5,16,17 They are 1^F-fructosylsucrose, 6^F-fructosylsucrose, and 6^G-fructosylsucrose. These three trisaccharides arise as a result of the transfer of a fructosyl residue from one sucrose molecule to one of three primary hydroxyl groups of another sucrose molecule. The products of such a reaction are one of three possible trisaccharides and glucose. For a simple, diagrammatic representation of the structure of these trisaccharides readers are referred to Henry.⁴³ In onion the trisaccharides 1^F-fructosylsucrose and 6^G-fructosylsucrose have been identified,^5,9,18 and both have been reported in leek.⁵ These isomers have different functions in fructan synthesis (see Section IV). The addition of further fructosyl groups to these trisaccharides leads to the production of tetrasaccharides and higher polymers, giving rise to many possible polymers. The degree of polymerization (DP) of fructans can be quite different in different species. Polymers up to a DP of 260 have been reported in grasses.¹⁹ Figure 1 shows the separation of polymers from onion.

FIGURE 1. The separation of nonstructural carbohydrates from onion bulbs (A. cepa v. Creamgold). Using a 1.6 × 210-cm column of Bio-Gel P-2, minus 400 mesh; collecting 1-ml fractions; with a constant head pressure of 30 kPa and operating temperature of 25°C.⁹

Starch is usually not detectable in onion, but Wilson et al.²⁰ have reported starch inclusions in the chloroplasts of the uniseriate layer of cells around the vascular bundles in onion leaves. Starch was not found in the chloroplasts of the other mesophyll cells. The plants used in their study were 10 weeks old, but it is not clear whether they were grown in bulb-inducing conditions or not. When there is a substantial translocation of carbohydrate to the leaf base, starch may not be present even in the vascular sheath cells.

The soluble carbohydrate concentration per unit of fresh weight of onion leaf is lowest at the top and highest in the basal part (Figure 2). This pattern does not vary substantially with leaf size or level of insertion. Sugar concentration increases in leaf blades, pseudostem, and bulbs with both increasing age and light intensity⁷ (Figure 3).

FIGURE 2. The distribution of nonstructural carbohydrates in sections of a 10-cm onion leaf (A. cepa v. Creamgold), measured from the base: glucose (□); fructose (△); sucrose (○); trisaccharide fructan (•).¹⁰

FIGURE 3. Effect of light intensity during growth on soluble sugar (fructose + glucose + sucrose) % dry weight in the leaf blades (a) and neck (b) of Allium cepa ◯ 100% light at 93.7 J m⁻² s⁻¹; △ 78%; • 35%; ▽ 11%.⁷

Bacon⁵ examined the distribution of the lower molecular weight fructans (up to DP 5) in onion bulbs and found these compounds to be absent from outer, older leaf bases and present in increasing amounts from the outer to the inner leaf bases. De Miniac⁸ reported a similar distribution. These observations were extended by Darby shire and Henry⁹ who reported the distribution of fructans of DP 3 to DP 9 (Figures 4a and b). Free fructose concentration was highest in the outer leaf bases and lowest in the innermost, an observation made also by Rutherford and Whittle.^9a Glucose and sucrose did not change across the bulb.

FIGURE 4. The distribution of nonstructural carbohydrates at harvest across an onion bulb (A. cepa cv. Creamgold) from the outer leaf base (1) to the inner (7). (a): glucose (□), fructose (△), sucrose (•), and total fructans (◯) as a percentage of the total, and (b) the concentration of fructans DP 3 to DP 9.⁹

Under controlled conditions Steer²¹ examined the influence of day/night temperature on the accumulation of nonstructural carbohydrates within a cultivar (Figure 5). No significant differences in fructose, sucrose, or fructan concentrations occurred at the different temperatures. However, differences in the bulb dry-matter percentage may have been related to changes in water content influenced by these compounds. The concentration of monosac charides plus sucrose per unit of tissue water was constant in all temperature treatments, so that the osmolarity remained constant.

FIGURE 5. (a) The wat...

Table of contents

• Cover
• Title Page
• Copyright Page
• Introduction
• The Editors
• Acknowledgments
• Dedication
• Contributors
• Table of Contents
• Chapter 1 Carbohydrate Biochemistry
• Chapter 2 Chemical Composition
• Chapter 3 Flavor Biochemistry
• Chapter 4 Processing of Alliums: Use in Food Manufacture
• Chapter 5 Therapeutic and Medicinal Values of Onions and Garlic
• Chapter 6 Garlic (Allium sativum)
• Chapter 7 Garlic Agronomy
• Chapter 8 Japanese Bunching Onion (Allium fistulosum L.)
• Chapter 9 Leek (Allium ampeloprasum var. porrum)
• Chapter 10 Rakkyo (Allium chinense) G. Don
• Chapter 11 Chinese Chive (Allium tuberosum)
• Chapter 12 Chives (Allium schoenoprasum)
• Index