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.     

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

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.     

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

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.     

An Experimental Study on Drying of Green Apples

Drying behavior of green apples in a laboratory dryer was examined. Prior to drying, the apples were cut in 8 mm thick slices, which were then treated with citric acid solution and blanched hot water at 80°C. Next, they were dried at 65°C with an air velocity of 2.0 m/s. The shortest drying time (270 min) was obtained with apples pretreated with citric acid solution. The drying data were fitted with 11 mathematical models available in the literature. Selection of the best model was investigated by comparing the determination of coefficient (R ²), reduced chi-square (χ²), root means square error ( RMSE ), and mean relative percentage error (P) between the experimental and predicted values. The results showed that the Wang and Singh, logarithmic, and Verma et al. models gave the best results in describing thin-layer drying of apple slices. The effective moisture diffusivity of pretreated samples with citric acid solution was higher than the other samples.     

Dehydration of Garlic Slices by Combined Microwave-Vacuum and Air Drying

Abstract

Combination of microwave-vacuum drying and air drying was investigated as a potential mean for drying garlic slices. The sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional hot-air drying at the temperature of 45°C to final moisture content less than 5% (wet basis). Pungency, color, texture, and rehydration ratio of garlic slices dried by this method were evaluated and compared with those dried by freeze drying and conventional hot-air drying. The comparison showed that the quality of garlic slices dried by the current method was close to that of freeze dried garlic slices and much better than that of conventional hot-air dried ones. The lab microwave-vacuum dryer which the materials to be dried could be rotated in the cavity was developed by the authors.     

Mathematical Modeling of Withering Characteristics of Tea Leaves

The withering characteristics of tea leaves were examined for different temperatures. Tea leaves were withered at a temperature range of 20–45°C with a constant air velocity of 1.1 m/s. The experimental results illustrated the absence of constant-rate drying period and withering took place only in the falling-rate period. During the falling-rate period, at constant drying air flow rate, the drying rate increased and drying time decreased with the increase in drying air temperature. Drying models of Henderson and Pabis and Page were evaluated based on mean bias error (E_MB), root mean square error (E_RMS), correlation coefficient (R²), and the chi square (χ²). The Henderson and Pabis model was found to be a better model for describing the withering characteristics of tea leaves for each of the temperatures of 20, 25, 30, and 35°C. The values obtained from Page model were found to be more reasonable for temperatures of 40 and 45°C than the other model. Both the models closely fitted the withering data within a certain range of temperature. The Henderson and Pabis model gave better prediction and satisfactorily described the withering characteristics of tea leaves at temperatures lower than 40°C whereas the Page model fitted well at temperatures greater than 40°C.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Drying Kinetics and Bioactivity of Beetroot Slices Pretreated in Concentrated Chokeberry Juice and Dried with Vacuum Microwaves

Beetroot slices 18 mm in diameter and with a thickness of 9.6, 6.3, 3.35, or 2.6 mm were pretreated in 40°Bx chokeberry juice at a temperature of 50°C for 2 h and then dried by a vacuum-microwave (VM) method at different microwave power levels, such as 120, 240, 360, 480, and 480/120 W. The drying kinetics were described by a fitted model that incorporated contributions from the surface area of the samples, microwave power, and VM processing time. As the microwave power during VM drying increased, the bioactive potential decreased for untreated samples and increased for pretreated samples. Increasing the samples' specific surface area resulted in shorter drying time, lower temperature of the material during drying, and enhanced quality of the dried product. For osmotically pretreated beetroot slices with a specific surface area of 827 ± 18 m²m^?3, a final VM drying step at 240 W is recommended to produce high quality vegetable snacks.     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

ABSTRACT

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

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.     

EXPERIMENTAL STUDY OF SORPTION ISOTHERMS AND DRYING KINETICS OF MOROCCAN EUCALYPTUS GLOBULUS

ABSTRACT

The paper deals with sorption isotherms and drying kinetics of Eucalyptus globulus. The sorption isotherms have been determined using a static gravimetric method and then fitted by GAB equation at T = 30, 40, and 50°C over a range of relative humidity from 0.05 to 0.9. The drying kinetics in a solar dryer with an auxiliary heating system are determined. The results show with good reproducibility that temperature is the main factor in controlling the drying rate. The drying rate equation is determined empirically from the characteristic curve of drying.     

Microstructural Changes during Drying of Apple Slices

Abstract

In recent years some effort has been done trying to relate microstructural changes of dehydrated foods with macroscopical properties and quality factors of the processed product. This work assesses the microstructural changes of apple slices submitted to convective drying. Apple slices (25.7 mm diameter, 2.6 mm thickness) were dried in an oven at 70°C on a screen sample holder in order to allow mass transfer in any side of the sample. Microphotographs of the same sample were taken at determined time intervals using a stereomicroscope. The objects in these photographs were identified and classified in three size groups: (a) the cells group, containing objects with an area less than 0.03 mm²; (b) the mixed cells-intercellular spaces group, with an area between 0.03 and 0.06 mm², and (c) the intercellular spaces group, including objects with an area higher than 0.06 mm². Geometrical parameters of such objects, namely size (area, perimeter, equivalent diameter, and major and minor axis length) and shape parameters (compactness, elongation, and roundness) were evaluated. Shrinkage of cells and intercellular spaces was very clear during drying, observing a decrease of size with moisture content. Concerning shape factors, compactness remained constant, roundness slightly decreased during drying, and elongation increased in the final stage of the process. Macroscopical changes of the entire disc followed the same behavior as microscopical ones, suggesting that changes at the two scale levels are strongly related.     

DRYING CHARACTERISTICS OF THE HAZELNUT

Abstract

Equilibrium moisture content isotherms for Spanish hazelnut (Corylus avellana L.) at different temperatures (30°C-80°C) were determined using static gravimetric method. Thin layer drying experiments were done with forced air circulation and were conducted with different operating conditions to determine the drying characteristics of hazelnuts. The effect of air temperature (30°C-70°C), air velocity (0.5 m/s - 2 m/s) and drying bed loading density (50 kg/m² - 150 kg/m²) on drying of unshelled and shelled hazelnuts was studied. Six mathematical models were used to fit the experimental equilibrium moisture content data, from which the G.A.B. model was found to give the best fit. Diffusion coefficients were determined by fitting experimental thin-layer drying curves to the Fick's diffusion model. Variation of the effective diffusion coefficient with temperature was of the Arrhenius type. The Page equation was found to describe adequately the thin layer drying of hazelnut. Page equation drying parameters k and n were correlated with air temperature and relative humidity.     

LOW TEMPERATURE MUSHROOM (A. bisporus) DRYING WITH DESICCANT DEHUMIDIFIERS

ABSTRACT

Cultivated mushrooms (Agaricus bisporus) slices of 2·5 and 5 mm thick were dried with dehumidified air at 20°, 30° and 40°C. Rehydration ability of dried mushrooms was used as criteria for the evaluation and determination of optimum conditions. Drying mechanism of the mushroom slices was expressed by unsteady state diffusion and the results were interpreted by Fickian model. Drying temperatures lower than 40°C promoted the production of light coloured mushrooms with high rehydration ratios. Diffusivity constants were in the range of 2·6?12×l0^-11 m²/s and the activation energies varied in the range of 23·5?30·3 kJ/gmol depending on the temperature and sample thickness.     

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.     

DETERMINATION OF MOISTURE DIFFUSIVITY AND BEHAVIOR OF TOMATO CONCENTRATE DROPLETS DURING DRYING IN AIR

ABSTRACT

The drying mechanism and diffusion coefficient of water in spherical droplets (1.73 – 2.08 mm diameter) of tomato concentrates were successfully interpreted and modelled by using Fick's law. Solids content of the initial concentrate (5–15% w/w), and drying temperature (60^° – 100^° C) were varied but the drying air was kept at constant velocity and humidity.

The effective moisture diffusivity was estimated from the drying rate curves and expressed by an Arrhenius relation. Further, it was observed that case hardening has a large effect on the diffusion process causing the effective diffusional distance and the rate of moisture accumulation in the hardened crust to vary with the moisture content, according to a sorption controlled mechanism.     

DETERMINATION OF MOISTURE DIFFUSIVITY OF RED DELICIOUS APPLE TISSUES BY THERMOGRAVIMETRIC ANALYSIS

ABSTRACT

Moisture diffusivity is an important parameter needed in the analysis, design and optimization of drying processes for food and other materials. Published data on moisture diffusivities of food materials are scarce and, sometimes, inconsistent due to a lack of a precise and repeatable experimental technique. Most experimental data are limited to low and moderate drying temperature (<70°C), whereas in the food industry hot air of up to 100°C is usually used in the falling rate period to speed up the drying processes. In this study, the effective moisture diffusivities of Red Delicious apple tissues were determined from drying curves produced with a Perkin Elmer thermogravimetric analyzer, using the slope method. The experiments were conducted at lour temperatures 60, 80, 100 and 120°C. Two well defined falling rate periods were observed. The effective moisture diffusivity, for the four temperature levels ranged from 3.2 × 10^?7 to 7.9 × 10^?8 m²/s for the first falling rate period and 3.8 × 10^?8 to 4.7 × 10^?8 m²/s for the second falling rate period. The temperature dependence of the effective diffusivity can be described with an Arrhenius-type equation.     

Thin-Layer Drying Characteristics and Quality Evaluation of Air-Dried Ganoderma Tsugae Murrill

Ganoderma is normally dried to extend its shelf life without using chemical preservative and to concentrate the medicinal value in the fruiting body. Convective hot air drying characteristics of Ganoderma tsugae Murrill were evaluated in hot air circulated oven at different drying temperatures, sizes, and air flow rates. The drying kinetics of Ganoderma tsugae in kidney shape and slices were investigated and compared at different drying conditions. The variation of effective moisture diffusivity values at decreasing moisture contents during drying was determined from the drying data. Four well-known thin-layer drying models were fitted to the experimental data and the Midilli model was found to satisfactory describe the drying characteristics of kidney-shaped Ganoderma tsugae. Ganoderma tsugae dried at 50°C with air velocity of 1.401 ms^?1 showed the highest retention of crude ganoderic acid content compared to other drying conditions.     

PREDICTING THE SORPTION BEHAVIOUR OF GARLIC SLICES

ABSTRACT

Moisture desorption properties of fresh garlic (cv. Early Californian) were investigated at 20 °C to 7O° 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 thc 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 deterinhe the best fit for the experimental data. On the basis of the F-test, The modifled 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.     

Drying Characteristics of Purslane (Portulaca oleraceae L.)

Abstract

Thin layer drying rates of purslane were determined experimentally as a function of temperature with air velocity kept constant at 1.1 m/s and relative humidity below 5%. Thin layer drying data were obtained for purslane at four drying air temperatures (35, 70, 95, and 120°C). Five thin layer-drying models (Henderson and Pabis, exponential, Page, two-term exponential, and Thompson models) were fitted to the drying data. The color of purslane was determined after drying using a spectro-colorimeter (Hunter Lab) in terms of Hunter L, a, and b values. The Page model was found to be most suitable in describing the drying characteristics of purslane. New parameters developed for the model resulted in a good fit at different temperatures. Color measurement indicated that greenness decreased with an increase in drying air temperature. Typical drying times were 88.41, 138.53, 416.38, and 1371.85 min at 120, 95, 70, and 35°C, respectively.