Biblioteca Digital

Desorption isotherms at different operating temperatures provide valuable information for the design and control of drying processes. In the present work the desorption isotherms of pears are determined according to two equivalent forms of the GAB model at temperatures of 20, 25 and 30°C. These cover the range of summer average daily temperatures in Portugal, when the solar drying of pears is carried out. Two different computer fitting methodologies are used, namely, Ordinary Least Squares (OLS) and Orthogonal Distance Regression (ODR), in order to evaluate the above mentioned forms of the GAB model.; http://www.sciencedirect.com/science/article/B8JGD-4RTVWGX-2/1/6c903cd58bc08221bbe47d10c3f56d0a

Brazil is considered one of the largest producers and consumers of tropical fruits. Green coconut (Cocos nucifera L.) stands out not only for its production and consumption, but also for the high amount of waste produced by coconut water industry and in natura consumption. Therefore, there is a need for utilization of this by-product. This study aims to study the adsorption isotherms of green coconut pulp and determine its isosteric heat of sorption. The adsorption isotherms at temperatures of 30, 40, 50, 60, and 70 °C were analyzed, and they exhibit type III behavior, typical of sugar rich foods. The experimental results of equilibrium moisture content were correlated by models present in the literature. The Guggenheim, Anderson and De Boer (GAB) model proved particularly good overall agreement with the experimental data. The heat of sorption determined from the adsorption isotherms increased with the decrease in moisture content. The heat of sorption is considered as indicative of intermolecular attractive forces between the sorption sites and water vapor, which is an important factor to predict the shelf life of dried products.; O Brasil é considerado um dos maiores produtores e consumidores de frutas tropicais. O coco verde (Cocos nucifera L.) se destaca tanto em termos de produção e consumo quanto em quantidade de resíduos gerada por indústrias de água de coco e pelo consumo in natura. Portanto...

The seeds of Araucaria angustifolia, commonly known as pinhão, are widely consumed in both Southern and Southeastern Brazil due to their high nutritious value. Moisture desorption isotherms of cooked pinhão were determined at 15, 25, 30 and 40 oC and modeled using well know isotherm models. Results show that Chirife model most appropriately represents the experimental data as reported previously for raw pinhão. The differential enthalpy was calculated and decreased as moisture content increased, becoming almost constant at 0.3 kg water kg-1 dry solids. Values, for each moisture content, were smaller than those reported for raw pinhão. The enthalpy-entropy compensation theory was applied to desorption isotherms providing the isokinetic temperature (401 ± 17 K). It was found that the desorption process investigated was enthalpy controlled as previously reported for raw pinhão.

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Processo FAPESP: 07/07586-0; Processo FAPESP: 07/00809-3; Processo FAPESP: 04/15550-7; Moisture desorption isotherms of fresh and heat blanched pumpkins ( Cucurbita moschata) were determined at three temperatures ( 30, 50 and 70 degrees C), using the standard, static-gravimetric method. The GAB, Oswin, BET, Halsey, and Henderson models were tested and, with the exception of the Henderson model, showed satisfactory fits to the experimental data. The GAB model was used to analyze the fitting ability to describe the isotherm type. The shape of the desorption isotherms of fresh and blanched pumpkin at 30 and 50 degrees C was intermediate to types II and III, and at 70 degrees C it was of type II for the blanched pumpkin and close to type II for the fresh sample. The influence of blanching on the decrease in equilibrium moisture was very small compared to the fresh samples and it was related to the loss of soluble solids during the pre-treatment. The isosteric heat of sorption measures indicated that a larger amount of heat was required to remove the water from the fresh samples than from the blanched ones.; Isotermas de sorção de umidade de abóboras frescas e branqueadas termicamente foram determinadas em três temperaturas (30...

The net isosteric heat and entropy of water sorption were calculated for kiwifruit, based on sorption isotherms obtained by the static gravimetric method at different temperatures (20 to 70 degreesC). The Guggenheim-Anderson-deBoer equation was fitted to the experimental data, using direct non-linear regression analysis; the agreement between experimental and calculated values was satisfactory. The net isosteric heat of sorption was estimated from equilibrium sorption data, using the Clausius-Clapeyron equation. Isosteric heats of sorption were found to increase with increasing temperature and could be well adjusted by an exponential relationship. The enthalpy-entropy compensation theory was applied to sorption isotherms and plots of DeltaH versus DeltaS provided the isokinetic temperature, T-B = 450.9 +/- 7.7 K, indicating an enthalpy-controlled desorption process over the whole range of moisture content considered.

The net isosteric heat and entropy of water sorption were calculated for plum, based on sorption isotherms obtained by the static gravimetric method at different temperatures (20 to 70 degrees C). The Guggenheim-Anderson-deBoer model was applied to the experimental data giving a good agreement between experimental and calculated values. The net isosteric heat of water sorption, estimated by applying Claussius-Clapeyron equation to sorption isotherms, was found to be different for plum skin and pulp, mainly at low moisture contents, and could be well adjusted by an empirical exponential relationship. Plots of enthalpy in contrast to entropy provided the isokinetic temperatures for skin and pulp, indicating an enthalpy-controlled sorption process. Thermodynamic data on water sorption for plums are not found in literature, as opposed to prunes for which the data are abundant.

The aim of this work is to show the importance of the sorption isotherms in the study of the heat and mass fluxes in unsaturated porous media. General forms of the heat and mass fluxes are presented in terms of experimentally accessible quantities. The role of the isotherm slope in the coupling of heat and mass fluxes and its influence on the effective permeability are shown. Separate relations for vapor and liquid fluxes through the porous medium are presented as functions of the temperature and the isotherm slopes. Nonstationary isothermal mass flux is also analyzed, a relaxation time for this process is identifled, and its relation to the isotherm slope is also discussed.

In this study the desorption isotherms of fresh pears were investigated at 30 ºC for fruits of the varieties Amêndoa, Amorim, Carapinheira Branca and S. Bartolomeu, all original from Portugal. This value of temperature was established to represent the summer average daily temperatures in Portugal, where the solar dried of pears is carried out. From the results obtained it was possible to verify that the behaviour of the equilibrium moisture content against the corresponding water activity is very similar for the four varieties of pears, and is of type III according to the Brunauer et al. (1940) classification of isotherms, very characteristic of products rich in carbohydrates (Vásquez-Uña, 2001; Rizvi, 1986). The experimental data were fitted to the Guggenheim-Anderson-deBoer (GAB), Halsey, Hendersen and Oswin models found in literature (Viswanathan, 2003). The performance of these models was evaluated by comparing the coefficient of correlation (R), the residuals of moisture content and the prediction capabilities. From the results obtained it was possible to conclude that the Oswin model is less appropriate for describing the process at study, and that the Chen and Halsey models are also not very adequate. The Henderson and GAB models...

The moisture sorption isotherms of Chilean papaya were determined at 5, 20, and 45 ºC, over a relative humidity range of 10-95%. The GAB, BET, Oswin, Halsey, Henderson, Smith, Caurie and Iglesias-Chirife models were applied to the sorption experimental data. The goodness of fit of the mathematical models was statistically evaluated by means of the determination coefficient, mean relative percentage deviation, sum square error, root-mean-square error, and chi-square values. The GAB, Oswin and Halsey models were found to be the most suitable for the description of the sorption data. The sorption heats calculated using the Clausius-Clapeyron equation were 57.35 and 59.98 kJ·mol-1, for adsorption and desorption isotherms, respectively.

Moisture desorption isotherms of fresh and heat blanched pumpkins (Cucurbita moschata) were determined at three temperatures (30, 50 and 70 °C), using the standard, static-gravimetric method. The GAB, Oswin, BET, Halsey, and Henderson models were tested and, with the exception of the Henderson model, showed satisfactory fits to the experimental data. The GAB model was used to analyze the fitting ability to describe the isotherm type. The shape of the desorption isotherms of fresh and blanched pumpkin at 30 and 50 °C was intermediate to types II and III, and at 70 °C it was of type II for the blanched pumpkin and close to type II for the fresh sample. The influence of blanching on the decrease in equilibrium moisture was very small compared to the fresh samples and it was related to the loss of soluble solids during the pre-treatment. The isosteric heat of sorption measures indicated that a larger amount of heat was required to remove the water from the fresh samples than from the blanched ones.

Brazil is considered one of the largest producers and consumers of tropical fruits. Green coconut (Cocos nucifera L.) stands out not only for its production and consumption, but also for the high amount of waste produced by coconut water industry and in natura consumption. Therefore, there is a need for utilization of this by-product. This study aims to study the adsorption isotherms of green coconut pulp and determine its isosteric heat of sorption. The adsorption isotherms at temperatures of 30, 40, 50, 60, and 70 °C were analyzed, and they exhibit type III behavior, typical of sugar rich foods. The experimental results of equilibrium moisture content were correlated by models present in the literature. The Guggenheim, Anderson and De Boer (GAB) model proved particularly good overall agreement with the experimental data. The heat of sorption determined from the adsorption isotherms increased with the decrease in moisture content. The heat of sorption is considered as indicative of intermolecular attractive forces between the sorption sites and water vapor, which is an important factor to predict the shelf life of dried products.

The aim of this work was to study the drying curves and equilibrium isotherms of extruded fish feed. The drying curves were determined at air temperatures of 50, 60, 70 and 80°C and airflow velocities of 1.5, 2.5 and 3.5 m/s). The equilibrium isotherms of relative humidity of air were obtained between 10 and 80% at 30, 40, 50, and 70°C. The experimental data were fitted for non-linear regression by using Statistica® to the models reported in the literature. The results showed that the drying curves and the equilibrium isotherms were significantly influenced by variations of the air temperature in a similar way to solid materials as described in the literature. The statistical results for models of Page and Peleg showed that the fitting of the experimental drying curve and isotherm data were satisfactory.

Biscuits (sweet, strongly sweet, semi-sweet biscuits, crackers, wafers) are characterized by a low moisture content in the final product and high levels of fat and sugar [1]. Dehydrated foods, such as biscuits, are very sensitive to gain moisture from the surrounding atmosphere, resulting in a consequent deterioration. When, at constant temperature, the product's moisture increases from the atmosphere, is obtained the adsorption isotherm and when it loses moisture is obtained the desorption isotherm. These curves are very important when studying the storage of dehydrated foods and the specification of packaging that is best suited for them. This study aimed to determine the adsorption curves of Maria biscuits from different brands at temperatures of 25 ºC and 40 °C.The equilibrium between the sample and the atmosphere was achieved by placing the samples inside flasks containing saturated salts, at temperatures of 25 ˚C and 40 ˚C, for a period long enough to let reach equilibrium (5 days). Though the determination of moisture content (W) and water activity (aw) the adsorption isotherms were plotted. Moisture was determined by a Hallogen Moisture Analyser (HG 53 from Mettler Toledo) and the water activity was determined by a hygrometer (Hygroskop Bt-RS1 from Rotronic). The results obtained were later fitted to the Guggenheim-Anderson-deBoer (GAB) model [2-4]...

The adsorption of 3,5-dimethyltetrahydro-1,3,5-thiadiazine-2-thione (dazomet) on bentonite and clinoptilolite has been studied at four temperatures (288, 293, 298 and 303 K) and two different solutions (water and water-ethyl alcohol mixture, 50% v/v). The adsorption rates at low concentrations of dazomet were found to fit the first-order kinetic equation. For each system at constant concentration, the rate constants increased with increasing temperature in aqueous solution, but these constants decreased with increasing temperature for 50% (v/v) water-ethyl alcohol mixture solution. By using the Arrhenius equation, the activation energies for each system were calculated. Thermodynamic parameters were evaluated according to Eyring's equation. The values of enthalpy of activation, deltaH*, are lower than TdeltaS*. The results indicated that the adsorption process was entropy-controlled for each system. Adsorption isotherms were determined at 288 and 303 K. These isotherms were fitted to Freundlich equation for aqueous solution at 15 °C, but adsorption from the two different solutions at 30 °C were modeled according to the Langmuir and BET isotherms.

Solid mixtures for refreshment are already totally integrated to the Brazilian consumers' daily routine, because of their quick preparation method, yield and reasonable price - quite lower if compared to 'ready-to-drink' products or products for prompt consumption, what makes them economically more accessible to low-income populations. Within such a context, the aim of this work was to evaluate the physicochemical and mineral composition, as well as the hygroscopic behavior of four different brands of solid mixture for mango refreshment. The BET, GAB, Oswim and Henderson mathematical models were built through the adjustment of experimental data to the isotherms of adsorption. Results from the physiochemical evaluation showed that the solid mixtures for refreshments are considerable sources of ascorbic acid and reductor sugar; and regarding mineral compounds, they are significant sources of calcium, sodium and potassium. It was also verified that the solid mixtures for refreshments of the four studied brands are considered highly hygroscopic.

Adsorption isotherms of carbon tetrachloride at temperatures between 273 and 323 K have been determined on the pure silica form of MCM-41 of pore diameter ca. 3.4 nm. All isotherms were of Type V, the isotherms at 273, 288 and 303 K showing hysteresis loops, whereas the isotherm at 323 K was completely reversible. Despite the questionable validity of the Kelvin equation when applied to narrow mesopores, changes in the relative pressure positions of capillary condensation and evaporation as a function of the temperature appear to be well described. Neutron diffraction measurements at 200 and 273 K show significant changes in the physical properties of the adsorbed CCl4 in the MCM-41 from those of bulk adsorbate. The results also suggest a highly heterogeneous surface and appear to show some flexibility in the pore walls upon pore filling. The conditions required for first order reversible capillary condensation are discussed.

The knowledge of sorption isotherms is important for establishing conditions of storage and of processes like drying. There are several models for fitting sorption isotherms. This work presents a study about the agreement of 40 mathematical models of sorption isotherms to experimental data of 53 food products. The quadratic residual sum and the standard error were the criteria of evaluation. For the major part of the products, the best agreement was obtained with equation of Jaafar and Michalowski, if temperature or saturation pressure were not considered as a variable. For cases where temperature or saturation pressure were considered, the equation of Strohman and Yoerger was the one with the best agreement for most of the products. Ross equation, based on thermodynamics aspects, was also tested for some products, but the agreement was just satisfactory.

The equilibrium moisture content of malting barley grains (cv. Scarlett) was determined by gravimetric-static method in the range 20-50°C using saturated salt solutions (LiCl, MgCl2.6H2O, K2CO3, NaBr, NaNO2, KI, NaNO3, NaCl, (NH4)2SO4) to provide water activities for each temperature between 0.11 and 0.813. Desorption isotherms were obtained at 20, 30, 40, and 50°C. All the sorption curves were found to be Type II, increasing moisture content with the increase of water activity from approximately 5.5-7.45% d.b. (dry basis) for different temperatures when the water activity was 0.111. Different models (Modified Henderson, Modified Chung-Pfost, Modified Halsey, Modified Oswin and GAB) were fit to the data and it was found that the best results corresponded to the Modified Henderson equation. The isosteric sorption heat varied from 3479 kJ/kg (20°C) to 2393 kJ/kg (50°C) decreasing with increasing moisture content, thus indicating that reducing the moisture content increases the energy required to remove water.

Textured soy proteins (TSP) are functional ingredients used in food applications. This study aims to compare the sorption isotherms of two commercial types of TSP at 10, 20, 30 and 40 °C; the best fit to the experimental data was selected from eight classical models. The curves obtained for TSP type I showed that the equilibrium moisture content decreases as temperature increases for water activities up to 90%. At higher water activities, the moisture content showed an inverse behavior. This inversion did not occur for TSP type II due to the absence of sugars in this type of TSP. The Chirife, GAB and Peleg equations presented the best fit for the curves.

Moisture adsorption isotherms for yellow pitahaya fruits were studied at 15, 25, and 35 °C using the gravimetric method for a 0.111-0.901 water activity range. Experimental values were adjusted by using the GAB, BET, Henderson, Smith, Caurie, and Peleg models. The isosteric heat of adsorption (Qst) was determined using the Clausius-Clapeyron equation. The results showed that yellow pitahaya fruits exhibit type III adsorption isotherms and that the equilibrium moisture content (EMC) is temperature-dependent. For the same water activity value, the EMC decreased when temperature increased. The GAB model presented the best fitto the experimental values. The Qst dropped from 57.154 to 45.79 kJ/mol when the EMC increased from 0.05 to 0.40 g water/g dry matter, respectively. These results are important for establishing the best storage conditions for dehydrated pitahaya fruits as well as for shelf-life prediction and the design of packaging materials.