R. Moreira

SHRINKAGE OF APPLE DISKS DURING DRYING BY WARM AIR CONVECTION AND FREEZE DRYING

ABSTRACT Volumetric and thickness shrinkage evaluated by direct measurement and n-heptane displacement were determined during convective and freeze drying of Golden delicious apples. For convective drying, the influence of blanching and diameter/thickness ratio of the apple disks used were analysed at different levels of moisture content under constant conditions. It was found that shrinkage of dried samples, both by convection and by freeze-drying, is anisotropic to a level which depends on sample geometry (ratio diameter/thickness) used. Blanching did not affect shrinkage results. Based on results obtained a new model to predict bulk density of materials during drying is proposed, showing a better fit to experimental data than previous models reported in the literature. This model was further used to predict changes in apple porosity during drying.

Viscosities of aqueous solutions of sucrose and sodium chloride of interest in osmotic dehydration processes

Abstract Osmotic dehydration of food involves the utilization of solutions with one or several solutes that increase considerably the viscosity of the liquid phase. As commercial sugar and sodium chloride are the common solutes employed in this type of processes, kinematic viscosities of binary and ternary aqueous solutions of these solutes were measured at various concentrations (from 0 up to 4.5 mol kg −1 at 0.5 mol kg −1 intervals) and temperatures (from 20 up to 50 °C). Additionally, densities of each solution at 25 °C are reported. Experimental data were simultaneously correlated with concentration and temperature for binary and ternary solutions with a satisfactory accuracy.

GRAPE DRYING IN A PILOT PLANT WITH A HEAT PUMP

ABSTRACT We describe a pilot-scale drying plant comprising a closed-circuit, hot-air convection chamber with a heat pump. In this plant, food and forest products can be drled with lower energy consumption and over a wider range of air, speeds, temperatures and relative humidities than in conventional driers. Also the dryinq conditions can be controlled independently of external conditions. Under several different sets of drying conditions, we determined the kinetics of dryinq of grapes that had been variously pretreated. The effects of drying conditions and the pretreatment on the drying time and the appearance of the dried product were evaluated. The drying kinetics can be described by a diffusion model.

Evaluation of mass transfer coefficients and volumetric shrinkage during osmotic dehydration of apple using sucrose solutions in static and non-static conditions

Abstract Effects of temperature, solution concentration and solution flow rate on osmotic dehydration/impregnation rate observed during immersion of apple cylinders in sugar solutions at low temperature (⩽25 °C) are presented. Water and sucrose mass transfer coefficients are calculated and correlated using Magee’s model; an extension of the model is proposed to account for solution flowrate; in both cases average relative deviations of less than 3% are obtained. Analysis of results obtained suggests that solute gain by the sample be controlled by diffusion inside the material while water loss is governed by mixed internal–external flow. Additionally, volumetric shrinkage of apple cylinders treated by osmotic solutions is predicted by measuring the change of the moisture content or the net mass loss due to the treatment. Volume changes observed in samples were correlated linearly with moisture content (dry basis) and with the net change in sample weight. These results suggest that shrinkage be essentially due to water removal/solid gain and offer a simple way to predict such changes during industrial processing.

Water adsorption isotherms of carboxymethyl cellulose, guar, locust bean, tragacanth and xanthan gums

Water adsorption isotherms of carboxymethyl cellulose (CMC), guar gum (GG), locust bean gum (LBG), tragacanth gum (TG) and xanthan gum (XG) were determined at different temperatures (20, 35, 50, and 65°C) using a gravimetric method. Several saturated salt solutions were selected to obtain different water activities in the range from 0.09 to 0.91. Water adsorption isotherms of tested hydrocolloids were classified like type II isotherms. In all cases, equilibrium moisture content decreased with increasing temperature at each water activity value. Three-parameter Guggenheim-Anderson-de Boer (GAB) model was employed to fit the experimental data in the water activity range and statistical analysis indicated that this model gave satisfactory results. CMC and GG were the most and the least hygroscopic gums, respectively. Sorption heats decreased with increasing moisture content. Monolayer moisture content evaluated with GAB model was consistent with equilibrium conditions of maximum stability calculated from thermodynamic analysis of net integral entropy. Values of equilibrium relative humidity at 20°C are proposed to storage adequately the tested gums.

MODELLING OF DESORPTION ISOTHERMS OF CHESTNUT: INFLUENCE OF TEMPERATURE AND EVALUATION OF ISOSTERIC HEATS

The desorption isotherms of chestnuts (Castanea sativa Mill) were determined at 278, 293, 303, 313, and 323K by a gravimetric technique. The range of water activity was from 0.08 at 0.91. The sorption isotherms were of the type II according to Brunauers classification. The effect of the temperature shows a cross over of the desorption isotherms at water activity 0.6. The experimental data are fitted with several models (Oswin, Smith, Henderson, Halsey, and G.A.B.). Oswin and G.A.B. models gave the best fittings. Particularly, with G.A.B. model parameters some interesting information as monolayer moisture content, heat of sorption of the first and nth layer adsorbed and net isosteric heat were obtained.

Desorption isotherms of muscatel and aledo grapes, and the influence of pretreatments on muscatel isotherms

Abstract Desorption isotherms were determined at 25°C for Aledo grapes and at between 25°C and 50°C for Muscatel grapes. The efficacy of two physico–chemical pretreatments designed to accelerate grape-drying kinetics was also studied for Muscatel grapes. The isotherms obtained were fitted with various models, among which the GAB equation gave the best fits, and was subsequently used to calculate the monolayer moisture content for each grape variety. Pretreating Muscatel grapes did not greatly alter the optimal conditions for their storage and handling, suggesting that treated and untreated grapes can safely be stored under the same conditions.

THE DEHYDRATION OF GARLIC. 1.DESORPTION ISOTHERMS AND MODELLING OF DRYING KINETICS

Abstract Desorption isotherms for 1 mm thick garlic slices were determined at between 25 and 50°C and satisfactorily fitted with the G.A.B. equation. The kinetics of drying of slices of thickness 1.5 - 5.0 mm were determined at between 40 and 60°C using an air flow rate of 2.5 m-s−, and the conditions affording a high-quality product m the shortest time were identified. After establishing how the effective diffusion coefficient of the garlic slices varied with their moisture content during drying, a diffusional model accounting for these variations was developed and satisfactorily fitted to the experimental kinetic data.

Osmotic Dehydration Kinetics of Pumpkin Fruits Using Ternary Solutions of Sodium Chloride and Sucrose

The objective of this work was to obtain experimental data and modeling of osmotic dehydration kinetics of pumpkin fruits (Cucurbita pepo L.) with aqueous NaCl/sucrose solutions. For this purpose, effective diffusion coefficients for water, sucrose, and NaCl were calculated by means of a simple model based on Ficks second law. Water loss achieved 80%, sucrose 13%, and NaCl 6% of the initial sample weight. Effective diffusion coefficients ranged from 0.58–1.40 × 10−9 m2/s, 0.75–1.23 × 10−9 m2/s, and 2.60–4.11 × 10−9 m2/s for water, sucrose, and NaCl, respectively. The proposed model gave a good correlation of the experimental data. The quality of the operation was evaluated by analysis of the values of WL/SG ratio.

Sorption isotherms of lupine at different temperatures

Abstract Sorption isotherms of lupine at 25, 35, 45 °C were determined. Adsorption and desorption data were obtained at each temperature and no hysteresis was observed. The experimental method was a gravimetric static method. The results were fitted by the Guggenheim–Anderson–deBoer model. The monolayer moisture content (parameter of the model) was constant in the range of temperatures employed with a value of 0.0497 dry basis (d.b.). The isosteric heat of lupine was evaluated at several moisture content using the Clausius–Clapeyron equation and below a moisture content of 0.15 (d.b.) increases appreciably up to around 20 kJ/mol at very low moisture content.