The Chemistry and Technology of Pectin

Reginald H. Walter, Steve Taylor

• Publisher: Academic Press
• Year: 2012
• Print ISBN: 9780127338705
• eBook ISBN: 9780080926445
• Topic: Technology & Engineering
• Subtopic: Biotechnology
• Index: Technology & Engineering

A fundamental understanding of polymers has evolved in recent years concurrent with advances in analytical instrumentation. The theories and methodologies developed for the galacturonan biopolymers (collectively called pectins) have seldom been discoursed comprehensively in the context of the new knowledge. This text explains the scientific and technical basis of many of the practices followed in processing and preparing foods fabricated with or containing pectin. The material is presented in a very readable fashion for those with limited technical training. - Structural analysis - Commercial extractions methods - Pectin formulations and tropical fruit analysis - Molecular mechanisms of gelatin - Enzymology - Polymer comformation techniques - Analytical methods of polymer analysis

CHAPTER 1

Function of Pectin in Plant Tissue Structure and Firmness

J.P. Van Buren, Department of Food Science and Technology, Cornell University, Geneva, New York

I. Introduction

II. Pectin

A. Classes

B. Structure

C. Degree of Esterification

III. Interactions

A. Entanglement and Cohesion

B. Calcium Cross-Links

C. Ferulate Cross-Links

D. Hydrogen Bonding and Hydrophobic Interaction

E. Electrostatic Effects

IV. Ripening

V. Abscission

VI. Summary

References

I. INTRODUCTION

Pectin substances are complex mixtures of polysaccharides that make up about one third of the cell-wall dry substance of dicotyledonous and some monocotyledonous plants (Hoff and Castro, 1969; Jarvis et al., 1988). Much smaller proportions of these substances are found in the cell walls of grasses (Wada and Ray, 1978). The location of pectin in the cell-wall–middle lamella complex has been known since the earliest work on this material (Kertesz, 1951). Highest concentrations are seen in the middle lamella, with a gradual decrease as one passes through the primary wall toward the plasma membrane (Darvil et al., 1980). Digestion of tissues with pectolytic enzymes leads to dissolution of the middle lamella and cell separation (Vennigerholz and Wales, 1987).

The pectic substances contribute both to the adhesion between the cells and to the mechanical strength of the cell wall, behaving in the manner of stabilized gels (Jarvis, 1984). They are brought into solution more easily than other cell-wall polymers, although their extractability varies widely from species to species. They have a higher degree of chemical reactivity than do other polymeric wall components. Physical changes, such as softening, are frequently accompanied by changes in the properties of the pectic substances.

Tissue firmness can be described as the resistance to a deforming force. Resistance to small deforming forces results from turgor and cell-wall rigidity. When forces are great enough, they can lead to irreversible changes in the conformation of a tissue. At the cellular level, several things may take place. There can be a breakage of cell walls accompanied by release of vacuolar contents. When this type of failure takes place with cells that consist of a large proportion of vacuoles, the tissues can often be described as juicy. Another type of conformation change is a separation of one cell from another. There is a failure at the adhesive layer between the cells. When this loss of adhesion is pronounced, there are extensive cell separations resulting from applied forces, and this type of tissue is frequently described as having a mealy or slippery character.

The type of failure that actually takes place is determined by the relative strengths of the cell wall and of the adhesive layers between the cells. Before the formation of the secondary cell wall, the strength of adhesion between cells is often greater than the strength of the primary cell wall; consequently, cell-wall breakage is the usual result of excessive deforming forces. The primary cell wall is composed of interwoven cellulose fibrils embedded in an amorphous polysaccharide matrix. Its strength is related to its thickness.

Force-induced failures of intercellular adhesion are seen in some ripe or overripe fruits, in heated tissues, and in special areas such as abscission zones. In these cases there have usually been changes that resulted in a weakening of the middle lamella and degradation of its pectin component. Once there has been extensive cell separation, it becomes difficult to cause cell-wall-breakage.

II. PECTIN

A. Classes

Pectins have frequently been classified by the procedures used to extract them from cell walls. In general, three types have been distinguished: water-soluble pectins extractable with water or dilute salt solutions; chelator-soluble pectins extractable with solutions of calcium chelating agents such as ethylenediaminetetraacetic acid (EDTA), (cyclohexanediaminotetraacetic acid (CDTA), or hexametaphosphate; and protopectins that are brought into solution with alkali solutions or hot dilute acids. The difficulty in extracting protopectin may be owing to acid-and/or alkali-labile bonds that secure the protopectin in the primary cell wall matrix. A large part of the protopectin can be solubilized by 0.05M Na₂CO₃, but a small fraction remains insoluble after the use of extractants as strong as 4M KOH (Massiot et al., 1988; Ryden and Selvendran, 1990). Selvendran (1985) has suggested that the water-soluble and chelator-soluble pectins are derived from the middle lamella. It is possible that a part of the protopectin chain is imbedded in the cell wall, with the rest extending into the middle lamella.

The proportions of these pectin types vary considerably between different tissues. In carrots and snap-bean pods (Sajjaanantakul et al., 1989) most of the pectin is of the chelator-soluble type. In ripe and even senescent apples, most is of the protopectin type (Massey et al., 1964; O’Beirne et al., 1982). In some other ripe fruits, such as freestone peaches (Postlmayr et al., 1956), most of the pectin is of the water-soluble type, while in ripe clingstone peaches, approximately equal proportions of all three types were found. In tissues such as carrots, potatoes, and snap-bean pods with high proportions of chelator-soluble pectin, the infusion of chelators into the tissue results in dramatic losses of cohesion (Linehan and Hughes, 1969; Van Buren et al., 1988). Tissues such as beet root, with a high proportion of protopectin, show little loss of cohesion when treated with chelating agents (S. Shannon, unpublished data, 1975).

The water-soluble and chelator-soluble pectins are typically composed mainly of galacturonic acid residues with about 2% rhamnose and 10–20% neutral sugar. The distribution as well as the number of free carboxyl groups may be important in affecting whether a pectin is water soluble or chelator soluble. The protopectins, particularly if they are extracted with alkali, have high neutral sugar content (Selvendran, 1985), mainly galactose and arabinose. Commercially prepared pectins often resemble water-soluble and chelator-soluble pectins in their composition, but it is likely that many of their neutral sugars have been removed by hydrolysis during extraction.

It seems that the major contribution to intercellular adhesion comes from the chelator-soluble fraction and the protopectin. In general, softening during ripening (Massey et al., 1964; Gross, 1984) or heating (Van Buren et al., 1960b) is accompanied by a loss of protopectin and an increase in water-soluble pectin.

B. Structure

The principal constituent of the pectin polysaccharides is D-galacturonic acid, joined in chains by means of α...

Table of contents

• Cover image
• Title page
• Table of Contents
• FOOD SCIENCE AND TECHNOLOGY
• Copyright
• CONTRIBUTORS
• PREFACE
• Chapter 1: Function of Pectin in Plant Tissue Structure and Firmness
• Chapter 2: Jams, Jellies, and Preserves
• Chapter 3: Other Pectin Food Products
• Chapter 4: Tropical Fruit Products
• Chapter 5: The Chemistry of High-Methoxyl Pectins
• Chapter 6: The Chemistry of Low-Methoxyl Pectin Gelation
• Chapter 7: Gelation of Sugar Beet Pectin by Oxidative Coupling
• Chapter 8: Pectinesterase
• Chapter 9: The Polygalacturonases and Lyases
• Chapter 10: Analytical and Graphical Methods for Pectin
• Chapter 11: Rheology of Pectin Dispersions and Gels
• Chapter 12: Nonfood Uses of Pectin
• INDEX
• FOOD SCIENCE AND TECHNOLOGY