The annually renewable agricultural residues represent an abundant, inexpensive and readily available source of renewable lignocellulosic biomass. Each year, farming and agricultural processing generate millions of tonnes of residues, such as corn cobs and husks, groundnut shells, rice straw, banana stems, soy hulls and sugar beet pulp (Ruan et al. 1996). These materials can be obtained at a low cost from a variety of sources, but the content and quality of the three major structural polymeric components (lignin, cellulose and hemicellulose) depend on the type of material (Taherzadeh and Niklasson 2004). Their utilizations are attracting increased interests around the world, particularly for the production of novel materials for environmentally friendly industrial applications after chemical modification (Pandey et al. 2000; Richardson and Gorton 2003).
Cellulose, one of the major component of lignocellulosic biomass, is a polydisperse polymer of high molecular weight and comprised long chains of d-glucose units joined together by β-1, 4-glucosidic bonds. (Krassig 1985). Significant advances in cellulosic modifications (mechanically, chemically and even enzymatically) and the resultant production of derivatives with unique chemical, physical and physiological properties, have dramatically increased interest in cellulose research over the past decade. This renewed focus on cellulose and its derivatives has resulted in the production of cellulose and its derivatives with varied physicochemical and functional properties (Azubuike et al. 2012; Ohwoavworhua and Adelakun 2005).
Among other uses, cellulose is widely employed as a raw material to prepare a number of excipients. Microcrystalline cellulose is described as ‘purified, partially depolymerized cellulose’ prepared by treating alpha cellulose obtained as a pulp from fibrous plant material with mineral acids (Brittain et al. 1993). Commercially available microcrystalline cellulose is derived from highly costly hard wood and also purified cotton. Several methods covered by patents have been described in the literature for the manufacture of cellulose powder. The need for cheaper sources of microcrystalline cellulose has led to the investigation of other lignocellulosic materials based on agricultural residues (Paralikar and Bhatawdekar 1988; Uesu et al. 2000; El-Sakhawy and Hassan 2007; Ejikeme 2008, Ohwoavworhua et al.
2009; Suesat and Suwanruji 2011). The chemical composition and physical structure of microcrystalline cellulose depend significantly on the characteristics of the raw material employed and the manufacturing conditions (Landín et al. 1993). As a result, several types of microcrystalline cellulose are available in the market with different physicochemical and thermal properties, and therefore, they will have different functional parameters and applications. These differences can affect their functional properties when employed in pharmaceutical formulations.
Apart from promoting the manufacturability of drug product, excipients are expected to guarantee the stability and bioavailability of the drug substance from the drug product. As a consequence, their characterisation must go beyond the simple tests for identity, purity and strength as prescribed in general by the Pharmacopoeia monographs. Full physical characterisation of solid materials is now made possible with the help of high-resolution analytical techniques (infrared spectroscopy, powder X-ray diffraction, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)) on the molecular, particulate and bulk levels (Pifferi et al. 1999). This systematic approach is necessary to guarantee the behaviour of the excipient during the formulation and production phases. For example, infrared spectroscopy can determine the molecular structure and possible chemical interactions. TGA and DSC analyses are often adopted to clarify the stability, compatibility and transitions of phase of the excipients. The structure of the single crystal or the powder can be examined with absolute certainty by X-ray diffraction. It provides information on the degree of crystallinity of the powder and also polymorphic forms. The degree of crystallinity in the cellulose is important because it influences the various properties including the compactibility and absorption of water, which in turn influence the flowability and the stability of the medicinal product (Pifferi et al. 1999).
As part of the on-going efforts to develop low-cost local raw materials from lignocellulosic materials (agricultural residues) with desired physicochemical properties for the industrial applications, we have in this study reported some physical properties of microcrystalline cellulose prepared from corn cob (an agricultural residue) and also evaluate their structural and thermal properties with view of ascertaining its potential as a pharmaceutical excipient. An attempt was also made to find out if the period of the hydrolysis has effect on these properties. These properties were also compared to a commercial brand Avicel® PH 101 (Fluka, New South Wales, Australia) that was obtained from highly costly hard wood.