Impact of carbohydrate constituents on moisture sorption of herbal extracts.

Abstract

Herbal medicines, including traditional Chinese medicines (TCMs) from plants, have been used worldwide for thousands of years in the prophylaxis and treatment of a variety of human diseases. Despite their long standing history and worldwide acceptance, it was not until recent years that the efficacy of certain herbal medicines has been scientifically proven in instances where conventional Western therapies have failed or have not been sufficiently effective (1). This latest development has generated substantial commercial interest in formulating medicinal herbs or their extracts into efficacious, safe, consistent, and stable herbal products, for which oral solid dosage forms (e.g. tablets, capsules, pellets) remain a popular choice. Unlike Western medications, herbal extracts are normally taken in substantially higher doses to yield the desired effects. Thus, from a formulation standpoint, there is a need to incorporate a large quantity of the extracts into a single dosage unit. However, fulfilling this need would be extremely difficult if not impossible in view of the limited dosage unit size that can be used and the difficulty in handling and processing herbal extracts, which tend to be bulky and hygroscopic (2). As has been well documented, residual water associated with drugs in the solid state can exert significant influences on the physical and chemical properties of the latter including chemical degradation, dissolution rate and compressibility (3– 4). While the significance of this impact is well recognized in the herbal drug industry, the generally high hygroscopicity of herbal materials has never been systematically investigated, possibly due to the immense technical difficulties in analyzing their chemical constituents. From a physicochemical perspective, the hygroscopicity of a material depends on its chemical composition and physical state. Carbohydrates, being a major constituent of plants and being soluble in the solvent system (water or waterethanol mixture) commonly used for extracting active principles from herbs, are almost invariably present in crude herbal extracts. Depending on their hydrophilic nature and hygroscopic character, these carbohydrates may be responsible for the commonly observed moisture sorption tendency of herbal extracts. In addition, the complex composition of the extracts will likely hinder their crystallization during the drying process, thus rendering the dried materials amorphous. Based on thermodynamic reasoning, an amorphous solid possesses a high escaping tendency, fugacity or thermodynamic activity, and thus interacts readily with its external environment e.g. water vapour. Thus it can be envisaged that most, if not all, herbal extracts exhibit a significant propensity towards moisture sorption due to their lack of crystallinity. The objective of the present investigation was to determine the impact of carbohydrate constituents and their associated solid state changes on the hygroscopicity of herbal extracts. As a basis for explaining and quantifying hygroscopicity, two wellestablished sorption isotherm equations, namely the Brunauer-Emmett-Teller (BET) and GuggenheimAnderson-de Boer (GAB) models, have been applied to data treatment in the present study.