G. Vázquez

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.

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.

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.

EFFECTS OF VARIOUS TREATMENTS ON THE DRYING KINETICS OF Muscatel GRAPES

ABSTRACT The drying kinetics of Muscatel grapes at 60°C and a relative air humidity of 22% were determined after various pretreatments had been applied to the grapes. The pretreatment parameters varied were the chemical agent (potassium carbonate solution with or without olive oil, or sodium hydroxide solution); the duration of pretreatment; and the temperature and concentration of the solution. A 3 minute pretreatment with an aqueous solution of K2CO3 and olive oil at 60°C reduced drying time from about 80 h to 20 h. Optimal drying load corresponded to a single layer of almost close-packed grapes on each tray. The experimental drying kinetics were satisfactorily reproduced by a model in which both grape size and the effective coefficient of diffusion are functions of moisture content.

THE DEHYDRATION OF GARLIC. 2. THE EFFECTS OF PRETREATMENTS ON DRYING KINETICS

Abstract The kinetics of drying of pretreated garlic 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. Pretreatmenl consisted in immersion of the slices in water or solutions of K2CO3, K2CO3 with olive oil, Na2S2O5 or NaOH, at 25°C for 60 or ISO s. The effects of each pretreatmenl on the drying kinetics and the quality of the dried product were evaluated, in the latter case by determining its rehydration ratio. Pretreatment with water or the Na2S2O5 solution gave good results regardless of the thickness of the garlic slices, whereas pretreatment with a dilute NaOH solution gave good resuhs for thin slices (≤2.5 mm). The experimental kinetic data were satisfactorily fitted by a difiusional kinetic model developed in Part 1 of this work.