ReviewCharacterisation of the substituent distribution in starch and cellulose derivatives
Introduction
Starch and cellulose are naturally occurring polysaccharides and the most abundant renewable resources available to man. They are both glucose polymers photosynthesised by solar energy in various plants, where starch serves as the main energy reserve, and cellulose as the structural basis of the plant cell wall. These polymers also constitute a major energy source in human and animal diets. In addition, starch and cellulose are widely used as raw materials in numerous industrial applications, e.g. in the paper, paint, textile, food and pharmaceutical industries. The polymers have great potential for providing a broad range of important functional properties and possess several advantages that make them excellent materials for industrial use; they are non-toxic, renewable, biodegradable and modifiable [1], [2], [3]. As environmental requirements have become of great importance in today’s society, there is an increasing interest in the industrial use of renewable resources, and considerable efforts are now being made in the research and development of starch and cellulose as the basic material in new applications.
In order to increase their industrial use and to fulfil the various demands for functionality of different starch and cellulose products, they are often modified by physical, chemical, enzymic or genetic means. Modification leads to changes in the properties and behaviour of the polymer and consequently, improvement of the positive attributes and/or reduction of the negative characteristics can be achieved [4], [5]. Chemical modification implies the substitution of free hydroxyl groups in the polymer with functional groups, yielding different starch and cellulose derivatives. The properties of a modified polysaccharide depend on several factors, such as the modification reaction, the nature of the substitution group, the degree of substitution (DS) and the distribution of the substitution groups. To direct a modification reaction towards a certain product with the desired properties, it is of importance to have knowledge of the correlations that exist between the modification process, chemical structure and functional properties of the final product. However, the relationships, if any, between these parameters are still far from fully understood, largely due to difficulties in the elucidation of the modified polymer structure, including the distribution of substituents. Therefore, there is a great need for and interest in the development of sensitive and selective methods for the analysis of the chemical structure of starch and cellulose derivatives.
Section snippets
Native starch
After cellulose and hemicellulose the two main polysaccharide components in lignocellulose, starch is the principal carbohydrate found in nature. Native starch occurs in the amyloplasts of seeds, roots and tubers, and in the plastids of green plant leaves as discrete, partially crystalline granules. The major constituent (∼99%) of the starch granule is α-d-linked glucose, which occurs in two different polymeric forms; amylopectin and amylose, but there are also small amounts of phosphorus,
Enzymes
Enzymes are frequently utilised in a variety of industrial applications, such as dairy products (coagulants), detergents, baking, brewing, distilling, fermentation, textiles and medical diagnostics. Another significant application is the starch industry, where enzymes are employed in starch conversion reactions for, e.g. liquefaction, dextrinisation, saccharification and isomerisation, as well as in the biosynthesis of new starch products [35]. Furthermore, as enzymes catalyse reactions that
Analytical techniques
Structural characterisation of starch and cellulose and their derivatives is commonly performed after degradation of the polymer, as the intact polymer, in general, is considered far too large and too complex to be analysed by most analytical techniques. In order to determine the substituent distribution or other structural parameters of these polysaccharides, accurate analysis of the degradation products is a prerequisite. Therefore, various techniques for sample handling, separation and
Analysis of the substituent distribution
The main goal of the chemical modification of starch and cellulose is to alter the properties of the native polysaccharide to produce new materials with desired properties. Consequently, it is important to elucidate the factors that determine which properties can be achieved by chemical modification. It is known that the functional properties of starch and cellulose derivatives depend on the type and number of substituents introduced [1], [2], [32], [78]. However, we still know relatively
Conclusions and future perspectives
The goal of this review was to present a summary of the analytical methods and strategies that are used for structural analysis of starch and chemically modified starch and cellulose, especially those for studies of the substituent distribution in starch and cellulose derivatives.
The complexity of starch and cellulose derivatives requires depolymerisation of the polymer into smaller fragments in order to study the structure. This hydrolysis can be achieved either randomly (chemical hydrolysis)
Acknowledgements
The authors thank The Competence Centre for Amphiphilic Polymers (CAP) for financial support.
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