W. A. M. McMinn’s research while affiliated with Queen's University Belfast and other places

The dielectric properties of pharmaceutical powder–(paracetamol, aspirin, lactose, maize starch, adipic acid) solvent (water) mixtures were measured at 2,450 MHz at a range of moisture contents (0–1.0 kg kg−1, dry basis) and temperatures (20–70 °C). The dielectric constant (ε′), loss factor (ε″) and penetration depth (d p) were found to be dependent on frequency, moisture content, temperature and powder type. For powder–water mixtures, a linear increase in the dielectric properties with moisture content was observed, whilst the temperature dependence was of quadratic form. The penetration depth was also significantly affected by temperature and moisture content. Although, ε″ also increased with increasing temperature, variation with moisture content was temperature dependent. This information on dielectric properties is essential for mathematical description of the pharmaceutical product temperature history during microwave heating and for the design of microwave drying equipment.

Adsorption isotherms of microwave-baked Madeira cake, flour and sugar were determined using a standard static gravimetric method within the temperature range 5–60 °C and relative humidity range of 0.04–0.96. Microwave-baked Madeira cake and sugar samples exhibited Type III isotherm behaviour, whereas flour exhibited Type II characteristics, with the sorption capacity decreasing with increasing temperature for Madeira cake and flour. Conversely, the equilibrium moisture content of the sugar samples increased with increasing temperature at water activity values greater than 0.88. Experimental data obtained were fitted to several isotherm models and their goodness-of-fit evaluated in terms of mean relative error (MRE), standard error of estimate (SEE) and residual sum-of-squares (RSS). Non-linear least square regression software was used to evaluate the models parameters. The Ferro-Fontan model provided the best description of the experimental sorption behaviour of microwave-baked Madeira cake and flour. The net isosteric heat of sorption was determined from the equilibrium adsorption data, using the Clausius–Clapeyron equation. The heat of adsorption decreased with increase in moisture content and approached a constant value. It showed a power law relationship with moisture content for microwave-baked Madeira cake and exponential relationship for flour.

There is a large market for microwave foods, with one of the most prevalent growth areas being low-density bakery products. The dielectric constant and loss factor of Madeira cake batter and its constituents (sugar, cake concentrate, margarine, flour, egg) were measured at a range of moisture contents (0.429–1.000 kg kg−1, dry basis), temperatures (20–80 °C), and over a frequency range of 915–2450 MHz. In general, for a given material, the dielectric properties of samples increased with increasing frequency. Irrespective of frequency and sample moisture content, the dielectric constant of batter and flour was relatively independent of sample temperature, whereas that of the sugar samples increased with increasing temperature, particularly within the lower moisture range. At 2450 MHz the loss factor of batter and flour samples decreased by a limited amount as temperature was increased, whereas at 915 MHz they were relatively unaffected. The sugar samples showed a significant decrease in loss factor with increasing temperature, irrespective of frequency. During microwave baking, the dielectric properties of Madeira cake batter initially increased sharply and then decreased steadily until the end of the baking process.

Starch, as a major component of several food products of plant origin, was used as a simulant for investigation of the basic physical and engineering properties of particulate and porous foods. An experimental tunnel dryer was used to investigate the drying-rate and temperature profiles developed within potato starch gel cylinders during convective drying. The experimental diffusional data were characterised using Fick's second law and an empirical model was developed to predict the effective moisture diffusivity as a function of air temperature, air velocity and sample radius. The experimental and predicted results were in good agreement. The temperature distribution developed within the sample were measured and presented as a function of both drying time and dimensionless distance.

The rehydration characteristics of potato cylinders were examined. Samples were pre-dried in a convective oven (60°C) or in a microwave oven (250, 440 or 600W), and subsequently rehydrated in a water bath at temperatures between 20 and 80°C. Fick's Second Law of Diffusion was used to describe the rehydration kinetics. The process was characterized by two effective diffusion coefficients (D1, D2), or two stages. The effect of temperature on D was interpreted using the Arrhenius relationship. The rehydration kinetics were dependent on temperature, solid-to-liquid ratio, sample dimensions (diameter and diameter-to-length ratio), pre-blanching, drying method (convective or microwave) and pre-soaking (ionic surfactants and NaCl). Agitation and non-ionic surfactants did not effect the rehydration process.

The effects of temperature, drying method and pre-drying treatments on water absorption and textural degradation of potatoes during soaking were examined. Samples were pre-dried in a convective oven (60 °C) or in a microwave oven (250, 440 or 600 W), and subsequently rehydrated in a water bath at temperatures between 20 and 80 °C. The texture of potatoes (firmness) was measured using a texture analyser. Fick’s Second Law of Diffusion was used to describe the rehydration kinetics (diffusion coefficient, D and equilibrium moisture content, Xe). Textural degradation kinetics were estimated using the fractional conversion equation. The rehydration characteristics and textural degradation kinetics were dependent on temperature, pre-blanching, drying method (convective or microwave) and pre-soaking (ionic surfactants and NaCl).

Rehydration of pasta at 20, 40, 60 and 80°C was examined using Fick’s Second Law of Diffusion to describe the rehydration kinetics. The process was characterized by two effective diffusion coefficients (D1, D2). The effect of temperature on D was interpreted using the Arrhenius relationship. Application of the Peleg and Weibull models were investigated for predicting water absorption of penne pasta. It was shown that the Peleg constants k1 and k2 decreased with temperature. The Weibull parameter, α, was constant with temperature, while the kinetic parameter, β, decreased with increasing temperature (40–80°C). On rehydration, dry pasta changed from a homogeneous, porous structure with few visible starch granules, to a porous structure containing swollen starch granules held within a protein matrix. At an elevated temperature (80°C), the structure then developed into an open network of coagulated protein. Sample diameter increased on rehydration, with the magnitude of the volume increase increasing with temperature and time.

Common pharmaceutical excipients and active ingredients, wetted with specific solvents, were dried under selected continuous power microwave and pulsed power microwave-vacuum conditions in an experimental system. Irrespective of the drying technique, a typical drying profile, with a constant drying rate stage followed by two falling rate periods, was exhibited. The experimental moisture loss data were fitted to semi-theoretical and empirical thin-layer drying equations and the models compared on the basis of three statistical parameters. The drying characteristics were satisfactorily described by the Lewis, Page, Logarithmic, Chavez-Mendez et al., and Midilli et al. models, with the latter providing the best representation of the data.

The thermal resistance of Bacillus stearothermophilus spores at different stages of rehydration was examined. Inoculation was achieved by rehydrating pasta samples in water containing 1-mL spore crop (106 spores/mL); the product was heat sealed in sterile pouches and processed in a laboratory retort. Thermal inactivation of spores was carried out using 10 time intervals at 121C for moisture contents, 70, 90, 105, 115, 125 and 145% dry basis. The death curves obtained consisted of two phases, an initial rapid decline (1.5 to 2.6 log reductions) followed by a slower, linear decrease of survivors from which the decimal reduction time (D value) was calculated. D121 values were found to decrease with increasing moisture content and ranged from 4.6 to 6.5 min. The thermal resistance constant (z value) was examined to assess the effect of temperature (110–125C) on lethality. z values decreased with increasing moisture content and ranged from 10.7 to 15.6C. The results of this study demonstrate that temperature and moisture content influence the effectiveness of the intended heat process. For products that are rehydrated during heat treatment, the moisture content of the product is a critical factor. Therefore, if complete rehydration is not achieved, this may result in the heat process being less effective for the destruction of microorganisms, which may have implications in regard to food safety.

Adsorption isotherms of oatmeal biscuit (convective-baked and microwave-baked) and oat flakes were determined using a gravimetric static method at 5, 20, 40 and 60 °C (relative humidity range 0.03–0.96). The oatmeal biscuits and oat flakes exhibited Type III and II behaviour, respectively, with the sorption capacity decreasing with increasing temperature. The Ferro-Fontan model provided the best description of the experimental sorption behaviour, followed by the Guggenheim–Anderson de Boer equation. Differential enthalpy showed a power law relation with moisture content, with differential entropy also decreasing with increasing moisture content. The linear relation between differential enthalpy and entropy confirmed the existence of compensation. Spreading pressure increased with increasing water activity and decreasing temperature. Net integral enthalpy decreased with increasing moisture content. However, net integral entropy increased with moisture content, but was negative with respect to the entropy of water.