R. H. Driscoll

The thin-layer drying characteristics of garlic slices

Abstract The thin-layer drying characteristics of garlic slices (2–4 mm) were investigated for a temperature range 50–90 °C, a relative humidity range 8–24%, and an airflow range 0.5–1 m/s. An analysis of variance (ANOVA) revealed that temperature and slice thickness significantly affected the drying rate while relative humidity and airflow rate were insignificant factors during drying. Effective diffusivity of water varied from 2 to 4.2 × 10−10 m2/s over the temperature range investigated, with an energy of activation of 989 kJ/kg. Four mathematical models available in the literature were fitted to the experimental data, with the Page and the two-compartment models giving better predictions than the single-term exponential and Thompsons model. The temperature dependence of the diffusivity coefficients was described satisfactorily by a simple Arrhenius-type relationship.

Moisture sorption characteristics of in-shell macadamia nuts

Abstract Adsorption and desorption isotherms of macadamia in-shell nuts at nine relative humidity levels and six temperatures ranging from 11 to 97% and 20 to 60°C respectively were established using the gravimetric static method. The goodness of fit of the experimental data to four sorption models was evaluated. Based on sorption data, the heats of adsorption and desorption as a function of moisture content were determined. The Guggenheim, Anderson and de Boer (GAB) equation with six constants where the monolayer moisture content also varied with temperature according to an Arrhenius-type relationship gave the best fit for both adsorption and desorption data.

Enthalpy-entropy compensation models for sorption and browning of garlic

The enthalpy-entropy compensation theory was applied to both the sorption and non-enzymic browning of garlic. The theory was found to explain sorption well but gave a non-linear relationship for browning. The resulting power law model for sorption was compared with the Guggenheim-Anderson-de Boer (GAB) model and found to predict higher moisture contents at high water activity levels.

Shrinkage, density and porosity of garlic during drying

Abstract The apparent density of garlic cloves (with a moisture content range of 10–65% wet basis) was evaluated by weighing the product in air and determining the corresponding buoyancy force in toluene. An analysis of variance (ANOVA) revealed a significant effect (P

Density, shrinkage and porosity of calamari mantle meat during air drying in a cabinet dryer as a function of water content

Abstract The particle density of calamari mantle meat powder decreased with increasing water content and apparent density gave a peak at low water content, then decreased with increasing water content. The shrinkage and porosity were derived from the experimental density data and were correlated empirically. The empirical models were used to predict the density data. An attempt was made to apply the theoretical model proposed by Rahman (1991, Ph.D. Thesis, University of New South Wales, Australia) based on conservation of mass and volume, and modified to include excess volume and air pore formation. The uncertainty in the true density measurement, which is a common difficulty in this area, has made it impossible to predict accurately the excess volume, thus limiting Rahmans model.

The thin-layer drying characteristics of macadamia in-shell nuts and kernels

Abstract The thin-layer drying behaviour of macadamia in-shell nuts and kernels for a temperature range of 26–56°C and 21–48°C, respectively, and a relative humidity range of 15–75% and 14–63%, respectively, was examined. The two-term exponential model predicted accurately the drying behaviour of in-shell nuts and kernels. The temperature dependence of the drying rate constants was best explained by an Arrhenius-type relationship. At low moistures, the removal of moisture from the kernel was faster when in-shell nuts were dried than when extracted kernels were dried.

Thermophysical properties of fresh and dried white onion slices

Abstract The variation of the specific heat, effective thermal conductivity and density of onion (Allium cepa L., cv. Southport White Globe) slices with moisture content at near ambient temperature was determined by the method of mixtures, the line heat-source probe and the liquid displacement method respectively. The specific heat, the effective thermal conductivity and the calculated effective thermal diffusivity were found to vary linearly with moisture content. The effective thermal conductivity of the onion slices did not change significantly in the measurement temperature range 30.7–33.0 °C and the porosity range 0.09-0.27. None of the additive models tested accurately predicted the specific heat and the effective thermal conductivity of onion slices at different moisture contents. The true density of the onion slices increased linearly from 1066–1416 kg/m3 as the slices were dried. The apparent density and internal porosity as a function of moisture content were correlated by third-order polynomials.

BULK DENSITY, POROSITY AND RESISTANCE TO AIRFLOW OF GARLIC SLICES

ABSTRACT The bulk density of garlic slices at different moisture levels (ranging from approximately 3 to 65% MC wet basis) was determined by weighing the contents of a container of known volume. The porosity was calculated using its relationship with bulk and apparent densities. An analysis of variance (ANOVA) revealed that bulk density and porosity were affected significantly by moisture and slice thickness as well as the interaction of these variables. Bulk density varied in a positive linear fashion with moisture and thickness while a negative linear correlation was found for the calculated bulk porosity. The linear model met me adequacy criterion for characterising the behaviour of garlic. Using a laboratory unit, the vertical resistance to airflow through the product and die effect of moisture and slice thickness were investigated for an airflow rates of 0.09 to 1.2 m3/s-m2. A higher resistance to airflow was noted for the wet product with the experimental data fitting to Shedds model when the airfl...

Kinetics of non-enzymatic browning in onion slices during isothermal heating

Abstract A static method involving the exposure of onion (Allium cepa L., cv. Southport White Globe) slices at different moisture contents to atmospheres of constant temperature (40–80 °C) and water activity (0.26-0.96) was employed to obtain kinetic data on non-enzymatic browning. The results showed that browning in onion slices could be modelled as a zero-order reaction. The reaction was temperature dependent, following the Arrhenius relation, and the reaction rate varied as a quadratic of water activity. The maximum browning rate occurred in the 0.60-0.70 range of water activity. Activation energy and Q10 values for browning in onion slices ranged over 121–139 kJ mole and 4–5, respectively. A generalised kinetic model of onion browning as a function of time, water activity and temperature of the product was established.

PREDICTING THE SORPTION BEHAVIOUR OF GARLIC SLICES

ABSTRACT Moisture desorption properties of fresh garlic (cv. Early Californian) were investigated at 20 °C to 70°C and relative humidities ranging from 11% to 85% using the gravimetric static method. The slices were allowed to equilibrate in a constant humidity environment (hygrostats) maintained by salt solutions. The effect of temperature and relative humidity on the equilibrium moisture content was highly significant (P<0.0001). Four models available in the literature, namely the Chung-Pfost, modified Halsey, modified Henderson and the Guggenheim-Anderson-de Boer (GAB) were evaluated (the correlation coefficient, F-ratio and plots of residuals) to determine the best fit for the experimental data. On the basis of the F-test, The modified Henderson model was found adequate and the GAB model superior for characterising the sorption behaviour of garlic slices in the temperature and water activity range investigated.