Molecular structure of exudate gums with special reference to gums of the sterculia genus

Abstract

The term 'gum', in its broadest sense, refers to both hydrophobic and hydrophilic substances of high molecular weight which usually exhibit colloidal properties when dispersed in an appropriate solvent. Hydrophobic substances often called gums include high molecular weight hydrocarbons and other petroleum products, rubbers, certain synthetic polymers and resinous saps which often exude from evergreens.

More specifically, the term gum applies to plant polysaccharides or their derivatives which are dispersible in either cold or hot water to produce viscous solutions or suspensions. As much as three- quarters of the dry weight of plants may be polysaccharide and, consequently, such substances are of wide occurrence. The most important gums, however, are those which are readily obtainable in large amounts from the plant. Some of these gums are used industrially and, indeed, many have been known since ancient times. One of the chief sources of such polysaccharides is seaweed which furnishes agar, algin and carrageenin while seed gums, such as gum guar and locust bean gum, are also important, particularly from the point of view that the plant which produces the seeds is often grown extensively as a food crop. In contrast to these naturally occurring gums, other gums are obtained from cellulose, one of the main components of the plant cell wall, and starch, a food reserve polysaccharide, by esterification and etherification. Commercially, however, the most important gums are plant exudates and most plant families have been found to include species which exude gums to a greater or lesser degree.

In this context, the term 'exudate gum' strictly refers to those commercially important gums which exude in copious amounts from shrubs or low- growing trees, forming, on exposure to the atmosphere, glossy nodules or flakes which are usually brown or yellow in colour. These gum producing trees grow predominantly in Africa or Asia indicating the climatic requirements for their growth.

The most likely function of gum,formation is to prevent infection of the plant tissue and to prevent loss of moisture (1). This is evident from the fact that tapping of the trees immediately results in the sealing off of the exposed area by the gum and the tree continues to grow without apparent damage.

Several suggestions have been made as to the origin of these gums. They may be products of normal plant metabolism but healthy Acacia trees, grown under favourable conditions of moisture, soil and temperature do not produce any gum, while trees which are grown under adverse conditions do secrete gum (2).

A most challenging problem to the chemist is the mode of formation of these gums. This requires a knowledge of the complex processes whereby they are first of all synthesised in the plant from simple compounds and then transported as required from the site of formation to the injured site. Work on the early stages of biosynthesis concerning the reactions involved in the transformations of monosaccharides, the building units of polysaccharides, into one another and into derivatives capable of enzymic polymerisation has been reviewed by J.K.N. Jones (3) and by Neufeld and Hassid (4). At present, little is known of the reactions involved in the enzymic polymerisation stage of complex heteropolysaccharide biosynthesis although, in this respect, recent advances have been made in the biosynthesis of complex lipopolysaccharides from the cell wall of Gram -negative bacteria (5). Clearly, a better understanding of the problem would be obtained from a knowledge of the structural relationships between different polysaccharides such as cellulose, hemicelluloses, pectins and gum exudates and between individual polysaccharides at different stages in their formation.