W.A.M. McMinn

Water sorption isotherms of starch powders: Part 1: mathematical description of experimental data

Abstract Adsorption and desorption isotherms for potato, highly amylopectin and highly amylose starch powders were determined at 30, 45 and 60 °C using a gravimetric technique. Samples were equilibrated in desiccators containing sulphuric acid solutions of known water activity (0.05–0.95), and placed in temperature-controlled cabinets for approximately three weeks. The starch powders exhibited Type II behaviour. The sorption capacity decreased with increasing temperature. The data obtained were fitted to several models including two parameter relationships (Halsey, Oswin, Henderson, Modified-BET and Smith), three parameter equations (GAB, Ferro-Fontan) and four parameter equation (Peleg). A non-linear least square regression program was used to evaluate the models constants. The empirical Peleg model followed by the kinetic GAB and the semi empirical Ferro-Fontan models were found to best represent the experimental data in the water activity range 0.05–0.95. In the range of water activity 0.35–0.95 the Smith model was shown to give the closest fit to the experimental data.

Moisture Sorption Isotherm Characteristics of Food Products: A Review

Knowledge of the sorption properties of foods is of great importance in food dehydration, especially in the quantitative approach to the prediction of the shelf life of dried foods. Equations for modelling water sorption isotherms are of special interest for many aspects of food preservation by dehydration, including evaluation of the thermodynamic functions of the water sorbed in foods. Knowledge of the thermodynamic properties associated with sorption behaviour of water in foods is important to dehydration in several respects, especially in the design and optimization of unit operation.

Thermodynamic properties of moisture sorption of potato

Abstract The moisture sorption isotherms of potato were determined using a gravimetric static method at 30, 45 and 60 °C, and over a range of relative humidities. The isotherms exhibited Type II behaviour, with the sorption capacity decreased with increasing temperature. The Guggenheim–Anderson–de Boer (GAB) and Halsey models were found to adequately describe the sorption characteristics. Calculation of the thermodynamic properties (differential enthalpy, integral enthalpy, differential entropy and integral entrophy) was further used to provide an understanding of the properties of water and energy requirements associated with the sorption behaviour. Isosteric heats (differential enthalpies) were calculated through direct use of moisture isotherms by applying the Clausisus–Clapeyron equation. The differential enthalpy and entropy decreased with increasing moisture content and were adequately characterised by a power law model. A plot of differential heat versus entropy satisfied the enthalpy–entropy compensation theory. The spreading pressures (adsorption and desorption) increased with increasing water activity, and decreased with increasing temperature. The net integral enthalpy increased with moisture content to a maximum value (around the monolayer moisture content) and then decreased. In a reverse manner, the net integral entropy decreased with moisture content to a minimum value and then increased.

Physical characteristics of dehydrated potatoes — Part II

Moisture removal from solids is an integral part of food processing, with convective drying representing one of the most important techniques for preservation of biological products. However, removal of moisture during drying has detrimental effects on the physiochemical properties of the material. Deterioration of the physical attributes of the system was evaluated on the basis of rehydration characteristics, namely the coefficient of rehydration and rehydration ratio. The rate and degree of rehydration was dependent on the drying conditions, with the extent of cellular and structural disruption dictating the rehydrational capacity.

Principles, Methods and Applications of the Convective Drying of Foodstuffs

Moisture removal from solids is an integral part of food processing. Moreover, convective drying is one of the most important techniques for preservation of biological products. An essential prerequisite for interpretation of the mass transport phenomena during drying is a knowledge of the equilibrium characteristics of the foodstuff. A further necessity is a knowledge of the drying kinetics and a detailed understanding of the moisture transport mechanism. During the drying process, a multiphase system undergoes simultaneous physical and structural modifications. Therefore, for accurate interpretation of this coupled heat and mass transport operation, moisture transport characteristics and physical property data is essential. This paper reviews more salient aspects of convective drying of foodstuffs.

Physical and dielectric properties of pharmaceutical powders

Abstract The limited availability of published physical and dielectric property data for pharmaceutical powders hinders the design of processing systems, particularly dryers. In this study, the physical properties (solubility and boiling point) and dielectric properties, in terms of temperature rise, dielectric constant, and dielectric loss factor, of selected pharmaceutical powders were measured. The pharmaceutical actives, paracetamol and aspirin, and selected common bulk excipients, together with a range of solvents including water, ethanol, methanol, and acetone, were used. The solubility of the powders was found to be solvent dependent and increased with increasing temperature. Solute concentration was also shown to affect the solvent boiling point, with the boiling points for certain systems being elevated or reduced by up to 11 and 33 °C, respectively. Research into the dielectric properties of pharmaceutical materials has, in particular, been prompted by widespread interest in microwave radiation as an alternative energy source for drying. The temperature rise induced in the powders and solvents during microwave heating was shown to give an effective indication of the relative dielectric properties. Greater temperature rises were observed in liquid and ionic materials. The dielectric constant and loss factor of a dry mixture was found to be dependent on the relative volumetric fraction and dielectric properties of the components. The temperature rise increased with increasing moisture content; however, the solubility of the solute in the solvent had a significant effect on the dielectric properties of the mixture. In general, the dielectric constant and loss factor increased with increasing moisture content. However, for selected powders, the dielectric properties at the critical moisture content were significantly greater than those at higher moisture contents.

Microwave-vacuum drying of pharmaceutical powders

Abstract Due to their temperature sensitive nature, many pharmaceutical products are dried under vacuum to facilitate solvent evaporation at reduced temperatures. However, this necessitates long drying times and represents a processing bottleneck. Microwave heating of such materials at reduced pressures offers a more rapid method of moisture removal, without the risk of product damage. Within this study, the effect of vacuum on the rate of solvent evaporation was investigated, using a range of powders (pharmaceutical actives Paracetamol and Aspirin, and a range of bulk excipients) and solvents including water, ethanol, methanol, and acetone. In general, drying rate increased as system pressure decreased, however, the magnitude and duration was system specific. As expected, the diffusion coefficient decreased from the first to the second falling rate period, with the diffusivity in each drying period increasing with increased vacuum. The findings in this research have significant implications for drying operations in the pharmaceutical industry.

A multiple regression approach to the combined microwave and air drying process

A semi-empirical approach was used to develop mathematical models to evaluate the performance of various combinations of microwave power and air conditions during the drying of Solanum tuberosum (potatoes). An experimental system was developed whereby air was introduced into the cavity of a domestic microwave oven. A multiple linear regression technique was employed to relate the drying rate constant to the design and operational parameters, which included air temperature, air velocity and microwave power as well as sample specifications. This allows the models to take into account the microwave characteristics as well as the surrounding air effects. The modelling equations verified the experimental results and proved to be an important tool in predicting the drying rate under any drying conditions. The models indicated that microwave power levels and sample specifications are the most important parameters, with statistically significant effects on the drying constants. Air temperature and velocity were of less significance within the range considered in this study.

Thin-Layer Modeling of Microwave, Microwave-Convective, and Microwave-Vacuum Drying of Pharmaceutical Powders

Abstract Common pharmaceutical excipients and active ingredients, wetted with specific solvents, were dried under selected microwave, microwave-convective, and microwave-vacuum conditions in an experimental drying system (2.45 GHz, 90 W). The generalized drying profile was independent of drying technique and material characteristics, with a constant rate stage and two falling rate periods being observed. Moisture removal rate was, however, dependent on the drying technique (pressure and presence/absence of external heating source), and powder and solvent properties (dielectric, physical and thermal). The experimental moisture loss data were fitted to selected semi-theoretical and empirical thin-layer drying equations. The correlations were compared according to two statistical tests, namely root mean square error and reduced chi-square. The microwave, microwave-convective, and microwave-vacuum drying behaviors were adequately represented by the Page, Logarithmic, Chavez-Mendez et al. and Midilli et al. equations, with the latter providing the best description of the experimental results.

Microwave drying of pharmaceutical powders

Although the application of microwave dryers has proven successful in many industries, the uptake rate of microwave technology by pharmaceutical companies has been slow, particularly in the primary manufacturing sector. Pharmaceutical powders are temperature sensitive thus, drying conditions are primarily dictated by stringent quality criteria. The heating effect of microwaves on a material is controlled by the dielectric loss factor. The loss factors of single and two-component systems of some common pharmaceutical excipients and active ingredients were determined within an experimental atmospheric microwave drying system. Formulations with a high percentage of ammonium acetate exhibited comparatively high loss factors, whilst the Aspirin-based samples presented low loss factors. However, both were significantly less than that of water; the solvent used in this work. The global loss factor of a multi-component system was found to be dependent on the loss factors of the individual components and their relative concentrations. The effect of moisture content on dielectric properties was also examined. Results showed that the loss factor of bound moisture is significantly less than that of free water, however, as moisture content increases, the loss factor of a wetted powder approached that of pure water. The effect of agitation on the drying behaviour of the powders was also examined, with the results revealing that the rate of drying increases with increasing frequency of agitation.