Net Isosteric Heat Of Sorption Research Articles

INFLUENCE OF ENCAPSULATING MATERIALS ON WATER SORPTION ISOTHERMS OF VACUUM-DRIED PERSIMMON PULP POWDER

Water sorption isotherms for vacuum-dried persimmon pulp (PP) powder were determined in the temperature range of 20–50C, and the effects of maltodextrin (MD) or gum arabic (GA) addition on the water sorption behavior of the dried powders were analyzed. Several models were evaluated to fit the experimental data and the Guggenheim-Anderson-de Boer model was selected as the most adequate to describe the observed behavior. Addition of encapsulants affected the isotherms: at the same water activity, PP powder with added GA (PP + GA) or MD (PP + MD) presented lower equilibrium water content than pure PP and were less affected by temperature variations. Samples of PP + MD presented lower equilibrium moisture content than those of PP + GA. The isosteric heats of sorption of pulp powders with encapsulants were higher (less negative) than those of PP, suggesting that there are more active polar sites in PP than in pulp powder containing encapsulants. PRACTICAL APPLICATIONS The choice of persimmon to carry out this work was due to the large persimmon production available in Brazil; moreover, persimmon pulp is rich in vitamin C, vitamin A and iron, as well as in phenolic compounds. Drying of fruit pulps with high sugar content presents technical difficulties because the hygroscopicity and thermoplasticity of the resulting powders when exposed to high temperature and relative humidity. For this reason, addition of high-molar-mass biopolymers, such as maltodextrin or gum arabic, is a strategy to aid drying and to improve storage stability. Knowledge of water sorption isotherms and net isosteric heats of sorption is important to various food processing operations, including drying, storage and packaging. They are useful in calculating time and energy consumptions during drying, modeling moisture changes during storage and predicting shelf life of food products.

Moisture sorption characteristics of dried acid casein from buffalo skim milk

Adsorption and desorption isotherms of dried acid casein prepared from buffalo skim milk were determined at 25°, 35° and 45 °C over a water activity range of 0.11–0.97 using static moisture gain/loss from test samples. Both the adsorption and desorption isotherms exhibited sigmoid shape corresponding to type II, typical to many foods. There was generally a negative temperature effect on equilibrium moisture content. The effect of temperature was, however, statistically not significant over the temperature range of 25–45 °C. Of the seven sorption models tested for fitting the sorption data, the GAB model gave the best fit at all the three temperatures. The temperature dependence of GAB parameters has been determined in the form of Clausius–Clapeyron equation. The calculated values of monolayer moisture content from BET isotherm equation have been found to be lower than the corresponding values found by using GAB equation. However, in both cases the monolayer moisture was higher in desorption than the adsorption and deceased with increase in temperature. The net isosteric heat of sorption decreased exponentially with increasing moisture content and approached a constant value of 0.331 kJ/mol at moisture content 28 g/100 g (d.b.). The moisture sorption hysteresis observed at 25°, 35° and 45 C was statistically significant. The extent of hysteresis was negligible in monolayer moisture content region, occurred predominantly in the water activity range 0.35–0.60 and decreased at higher water activities. Total hysteresis energy was evaluated from the sorption data using Everett and Whitton plot. The effect of increase in temperature was to decrease the amount of hysteresis.

Moisture adsorption isotherms and thermodynamics properties of sucuk (Turkish dry-fermented sausage)

Moisture adsorption isotherms of sucuk were determined using the isopiestic method at 10 °C, 20 °C and 30 °C and within the range of 0.2–0.9 water activity. The isotherms of sucuk exhibited Type II behavior according to BET classification. The adsorption data were analyzed using mathematical equations of Halsey, Caurie, Peleg, Smith, Oswin, Henderson, Modified-BET, GAB, Ferro-Fontan and Harkins–Jura. The best fit of experimental data was obtained with Peleg equation in the range of temperatures and water activities investigated. Thermodynamic properties such as differential enthalpy and entropy, enthalpy–entropy compensation, spreading pressure, net integral enthalpy and entropy were determined from moisture adsorption isotherm data of sucuk. The net isosteric heat of sorption and differential entropy decreased with increasing moisture contents in an exponential function. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The net integral enthalpy decreased with increased moisture content. The integral entropy increased with moisture content, but it was negative in value. The enthalpy–entropy compensation theory was applied to water adsorption of sucuk.

Water adsorption and desorption isotherms of chestnut and wheat flours

Adsorption and desorption isotherms of chestnut and wheat flour at different temperatures (20, 35, 50 and 65 °C) were determined over a range of water activity of 0.09–0.91 using a static gravimetric method. According to BET classification the obtained isotherms for both flours were of type II. Taking into account IUPAC classification the hysteresis loops were of type H3. Experimental data were fitted by means of GAB and Chung–Pfost models. GAB model gave the best results for both flours, but both models can be considered adequate by the analysis of some statistical parameters such as the mean relative percentage deviation modulus, root mean square error and coefficient of determination. The net isosteric heats of sorption, estimated by the Clausius–Clapeyron equation, decreased in both flours when moisture content of samples increased. Heats of desorption were higher than the heats of adsorption at each moisture content and temperature in both flours, but in the case of wheat flour strong differences at low moisture contents were found.

Hydration of Cellulose/Silica Hybrids Assessed by Sorption Isotherms

The hydration of cellulose/silica hybrids (CSH) containing 13, 35, and 46% (w/w) silica synthesized in situ by a mild sol-gel aqueous process was studied employing sorption isotherms and surface energy measurements. Water sorption is governed by two simultaneous equilibria at the silica-vapor and cellulose-vapor interfaces due to the presence of cellulose regions covered and uncovered with silica as confirmed by X-ray scattering analysis. The important contribution of a water impermeable cellulose-silica interface to the surface properties of CSH is highlighted. CSH exhibit type II isotherms in the temperature range 15-40 degrees C, more appropriately described by the Guggenheim-Anderson-de Boer (GAB) model than by the Brunauer-Emmett-Teller (BET) model. Specific surface areas calculated using both models (S(BET) and S(GAB)) increase linearly with the amount of silica influencing the higher moisture content of CSH for the same water activity. However, the presence of silica reduces the strength of interaction between monolayer water molecules and the CSH surface as revealed by the net isosteric heat of sorption (35 kJ/mol for cellulosic pulp and 26 kJ/mol for CSH with 35% silica). Silica confers a higher surface energy to CSH contributing to its polar component and surface wettability (contact angle with water) when compared to cellulosic pulp. The average diameter of cellulose microfibrils and the interfacial cellulose-silica areas are assessed on the basis of the analysis of sorption isotherms.

Moisture adsorption isotherms of microwave-baked Madeira cake

Adsorption isotherms of microwave-baked Madeira cake, flour and sugar were determined using a standard static gravimetric method within the temperature range 5–60 °C and relative humidity range of 0.04–0.96. Microwave-baked Madeira cake and sugar samples exhibited Type III isotherm behaviour, whereas flour exhibited Type II characteristics, with the sorption capacity decreasing with increasing temperature for Madeira cake and flour. Conversely, the equilibrium moisture content of the sugar samples increased with increasing temperature at water activity values greater than 0.88. Experimental data obtained were fitted to several isotherm models and their goodness-of-fit evaluated in terms of mean relative error (MRE), standard error of estimate (SEE) and residual sum-of-squares (RSS). Non-linear least square regression software was used to evaluate the models parameters. The Ferro-Fontan model provided the best description of the experimental sorption behaviour of microwave-baked Madeira cake and flour. The net isosteric heat of sorption was determined from the equilibrium adsorption data, using the Clausius–Clapeyron equation. The heat of adsorption decreased with increase in moisture content and approached a constant value. It showed a power law relationship with moisture content for microwave-baked Madeira cake and exponential relationship for flour.

Thermodynamic Analysis of Water Vapor Sorption Isotherms and Mechanical Properties of Selected Paper‐Based Food Packaging Materials

Adsorption isotherms of 3 selected paper-based packaging materials, that is, vegetable parchment (VP) paper, Kraft paper, and solid-bleached-sulfate (SBS) paperboard, were determined at 3 different temperatures (25, 40, and 50 degrees C). The GAB isotherm model was found to fit adequately for describing experimental adsorption isotherm data for the paper samples. The monolayer moisture content of the paper samples decreased with increase in temperature, which is in the range of 0.0345 to 0.0246, 0.0301 to 0.0238, and 0.0318 to 0.0243 g water/g solid for the MG paper, the Kraft paper, and the SBS paperboard, respectively. The net isosteric heats of sorption (q(st)) for the paper samples decreased exponentially with increase in moisture content after reaching the maximum values of 18.51, 27.39, and 26.80 kJ/mol for the VP paper, the Kraft paper, and the SBS paperboard, respectively, at low-moisture content. The differential enthalpy and entropy of 3 paper samples showed compensation phenomenon with the isokinetic temperature of 399.7 K indicating that water vapor had been adsorbed onto the paper samples with the same mechanism. Depending on the paper material, tensile strength of paper samples was affected by moisture content.

Mathematical modelling of moisture sorption isotherms and determination of isosteric heat of blueberry variety O′Neil

SummaryThe sorption isotherms of blueberry variety O’Neil were determined at 20, 40 and 60 °C, for a range of water activity of 0.10–0.95. The isotherms showed that the equilibrium moisture content increased when temperature decreased at constant water activity. The BET, GAB, Halsey, Henderson, Caurie, Smith, Oswin and Iglesias‐Chirife equations were tested for modelling the sorption isotherms. The results showed that GAB, BET and Halsey models gave the best fit quality for the experimental desorption data, and BET, Oswin and Henderson for adsorption data as suggested by the statistical tests employed. The net sorption heat was calculated using the Clausius–Clapeyron equation giving 38.62 kJ mol−1 (desorption) and 30.88 kJ mol−1 (adsorption) at a moisture content of 0.01 g water (g d.m.−1). Tsami equation was applied to estimate the net isosteric heat of sorption as function of equilibrium moisture content with satisfactory results.

Moisture Sorption Characteristics of Starchy Products: Oat Flour and Rice Flour

Moisture sorption isotherms for oat flour and rice flour were determined at 5, 23, and 45 °C using a gravimetric technique in an aw range of 0.08–0.98. The results obtained showed that, for aw values lower than around 0.75, in both flours, the sorption capacity decreased with increasing temperature, while the opposite behavior was observed at aw greater than this value. It was found that the experimental data in the water activity range considered were better represented by the four-parameter Peleg equation. The monolayer water contents for both materials were determined by the BET and GAB models. The net isosteric heats of sorption were estimated using the Clausius–Clapeyron equation. For both materials, the monolayer water content and the isosteric heat of sorption were found to decrease with increasing temperature and increasing moisture content, respectively.

Neural network modeling of sorption isotherms of longan ( Dimocarpus longan Lour.)

A multilayer neural network model was developed to predict the equilibrium moisture content of longan ( Dimocarpus longan Lour.) and the model was trained using a back-propagation algorithm. The predictive power of the model was found to be high ( R 2 = 0.9998) after it was adequately trained. The artificial neural network (ANN) model was better than the well known phenomenological Guggenheim, Anderson, and de Boer (GAB) model previously developed by the authors [Janjai, S., Bala, B.K., Tohsing, K., Mahayothee, B., Heawsungcharern, M., Muhlbauer, W., Muller, J., 2006. Equilibrium moisture content heat of sorption of longan ( Dimocarpus longan). Drying Technology 24, 1691–1696]. The ANN model was programmed in C++. The isosteric heat of sorption of longan is predicted by a power law model developed in this study, which was found to have better fit than the exponential model previously developed by the authors [Janjai, S., Bala, B.K., Tohsing, K., Mahayothee, B., Heawsungcharern, M., Muhlbauer, W., Muller, J., 2006. Equilibrium moisture content heat of sorption of longan ( Dimocarpus longan). Drying Technology 24, 1691–1696]. Also a power law model was developed for entropy of sorption. The net isosteric heats of sorption were compared for longan ( D. longan Lour.), litchi ( Litchi chinensis Sonn.) and mango ( Mangifera indica L. cv. Nam Dok Mai). Longan and litchi have the same pattern of variation in heat of sorption with moisture contents which might be due to similar biological structure of both of these two fruits. However, at a moisture content of above 50% (d.b.) the isosteric heat of sorption of longan is lower than and it is higher at a moisture content below 50% (d.b.) when compared with that of mango. This set of two equations (isosteric heat and entropy) would be useful in the simulation of storage of dried longan. The artificial neural network model predicts equilibrium moisture contents more accurately and hence better equations for heat of sorption and entropy are developed based on data from the neural network model.

Equilibrium Sorption Isotherms of Aloe Vera Gel Powder

The equilibrium moisture content of dehumidified air-dried Aloe vera (Aloe barbadensis Miller) gel powder was measured by the static gravimetric method. The equilibrium was achieved after 10 days for adsorption at 25 °C, 32 °C, and 39°C with water activity ranging from 0.10 to 0.90. The sorption isotherm ofAloe vera gel powder decreased with increase in temperature at constant relative humidity. The experimental data of sorption were fitted by four models (GAB, Smith, Oswin, and Peleg). The models were compared using the sum of squares error (SSE), mean relative percent deviation modulus (P), coefficient of correlation (R 2 ), root mean square error (RMSE), chi-square statistic (χ 2 ), and the trend of residual plots. Results proved that the GAB model was more appropriated for predicting the equilibrium moisture content of Aloe vera gel powder for the range of temperatures and water activities studied. The net isosteric heat of sorption ranged from 2.23% to 22.16% kJ mol -1 for moisture contents ranging from 2% to 18% (d.b.).

Modeling Moisture Desorption Isotherms and Thermodynamic Properties of Fermented Tea Dhool (Camellia sinensis var. assamica)

The desorption isotherms of fermented tea dhool were determined at 50, 55, 60, and 70°C by using static gravimetric method of saturated salt solutions within the water activity range of 0.05–0.81. Results indicated that the equilibrium moisture contents were decreased with the increase in temperature. A modified Oswin model was well fitted with the experimental data. Desorption isotherm curves obtained were type II sygmoidal. Thermodynamic properties like net isosteric heat of sorption and differential entropy were found to decrease exponentially with the increase in moisture content. Monolayer moisture content was also decreased linearly with the increase in temperature. The enthalpy-entropy compensation theory could be successfully applied for the moisture desorption behavior of fermented tea dhool samples and it was an enthalpy-driven spontaneous process.

Drying Characteristics, Sorption Isotherms, and Lycopene Retention of Tomato Pulp

Tomato pulp separated by centrifugation of crushed tomatoes was dried in cabinet, tray, fluidized-bed, and freeze dryers at 50–70°C using 4–12 kgm−2 tray loads. Drying behavior was well described by the Page's model with coefficient of determination greater than 0.99 and standard error of 0.004–0.019. A fluidized-bed dryer was more efficient to dry tomato pulp than tray, freeze, and cabinet dryers. Retention of lycopene in tomato pulp was maximum in a freeze dryer (72.483–78.093 mg 100 g−1) followed by fluidized-bed (42.273–57.926 mg 100 g−1), cabinet (35.532–49.271 mg 100 g−1) and tray dryer (24.454–45.842 mg 100 g−1) under experimental conditions (tray load 4–12 kgm−2, temperature 50–70°C). The moisture sorption isotherms of tomato pulp obtained by equilibrating saturated salts solution of known water activity (0.113–0.92) at 30–60°C were sigmoid. A modified Henderson model was the best model for the tomato pulp with coefficient of determination more than 0.99 and standard error less than 0.270. The net isosteric heat of sorption, estimated using the Clausius-Clapeyron equation, was 21.451–5.541 Jmol−1 at 25–5% moisture content (dry basis).

Sorption isotherms and thermodynamics of water sorption of ready-to-use Basundi mix

Basundi mix representing powdered Basundi (an Indian traditional milk product somewhat similar to sweetened condensed milk) is a convenience product developed for enhanced shelf life at ambient temperature. Water sorption isotherms were obtained for this ready-to-use mix over the temperature range of 5–45 °C. There was definite inversion of the temperature effect at high water activity (>0.7). GAB model gave excellent agreement between experimental and predicted values. Other properties of sorbed water viz., monolayer water, number of adsorbed monolayers, density and amount of bound water and surface area of adsorption were also obtained. The net isosteric heat of sorption had strong dependence on moisture content. The enthalpy–entropy compensation suggested that the sorption mechanism involved is enthalpy driven over the entire range of moisture content studied.

Thermodynamic and Quality Properties of Lemon Juice Powder as Affected by Maltodextrin and Arabic Gum

Sorption isotherms of lemon juice (LE) powders with and without additives—18% maltodextrin (MA) or 18% gum Arabic (GA)—were determined at 20–50°C. Addition of additives was shown to affect the isotherms in such a way that, at the same water activity, samples LE + GA and LE + MA presented lower equilibrium moisture content and were not so affected by varying temperature. The net isosteric heats of sorption of juice powders with additives were higher (less negative) than those of lemon juice powders, suggesting that there are more active polar sites in the product without addition of GA or MA. In general, the quality properties decreased with the addition of maltodextrin and gum arabic and it was obtained similar values for LE + GA and LE + MA.

Moisture sorption isotherms and heat of sorption of instant (soluble) green tea powder and green tea granules

Moisture adsorption isotherms of green instant tea powder and green tea granules were determined at 20, 30, 40 and 50 °C. A gravimetric static method was used under 0.11–0.90 water activity ranges for the determination of sorption isotherms that were found to be typical type II sigmoid. Experimental data were fitted to various mathematical models and found that Peleg model suits best in describing equilibrium moisture content–equilibrium relative humidity (EMC–ERH) relationships for instant green tea samples as well as green tea granules, over the entire range of temperatures. The net isosteric heat of sorption was determined from the equilibrium data at different temperatures. The isosteric heat of sorption varied between 48.54 and 44.71 kJ/mol at moisture levels varying between 1 and 9 g/g dry matter for instant green tea powder and 47.96 and 44.10 kJ/mol at moisture level varying between 0.2 and 1.4 g/g dry matter for green tea granules.

Moisture Sorption Isotherms of Osmotically Dehydrated Sweet Pepper

Moisture sorption isotherms of osmotically dehydrated sweet peppers were determined at 15oC, 30oC and 40oC using static desiccator techniques. In the study, seven levels of water activity in range of 0.08 to 0.91 were used by the use of saturated salt solutions. The isotherms were found to be sigmoid type and of BET classification II. Out of seven sorption models i.e. BET, modified BET, Hasley, Caurie, GAB, Oswin and Smith, fitted to the experimental data, Oswin model was found to be the best for accurate prediction of moisture sorption isotherm with highest value of coefficient of determination (R2) and lowest values of standard error (SE) and relative deviation parentage (Rd). The value of monolayer moisture content of the osmotically dehydrated sweet pepper was found to be 3.037%, 3.934% and 4.432% (db) at 15oC, 30oC and 40oC respectively. The values of net isosteric heat of sorption as calculated by Clausius–Clapeyron equation showed a regular fall with increase in moisture content.

Sorption isotherms and moisture sorption hysteresis of intermediate moisture content banana

Sorption isotherms of dried or intermediate moisture content (IMC) products are most important to model moisture uptake during storage and distribution. Working isotherms of IMC bananas were determined at five different temperatures (10, 15, 20, 30 and 40 °C). Desorption and adsorption isotherms were determined at 10 and 40 °C. All the sorption curves were found to be Type II, with non-zero moisture content (approximately 3–9% db for different temperatures) when the water activity was zero. Different models were fitted to the data and it was found that the best results were obtained with a modified Freundlich equation (valid from 0.06 to 0.76 water activity). The dependence of the models constants on temperature was investigated and the secondary models were built accounting for the effect of a w and temperature on the samples moisture content ( R 2 = 99%). Hysteresis was observed at 10 °C and 40 °C, but the effect was greater at 10 °C. The net isosteric heat of sorption (working isotherms) varied from 0.85 kJ g −1 mol −1 to 7.67 kJ g −1 mol −1, decreasing with increasing moisture content. This effect was well described by an exponential function ( R 2 > 99%). These results provide reliable experimental data on sorption isotherms which is important for designing an optimized packaging system.

Effect of drying method on the moisture sorption isotherms for Inonotus obliquus mushroom

Moisture sorption isotherms of Inonotus obliquus mushroom were studied over a selected temperature range (20–50 °C). Sigmoid sorption isotherms were observed for these samples. The sorption data were analyzed using various conventional models. The Oswin model was found to be the best model for predicting the equilibrium moisture content of mushroom in the range of water activity 0.08–0.96. The monolayer moisture content decreased as temperature increased and was affected by the drying method used. The net isosteric heat of sorption was determined using the Clausius–Clapeyron equation and the value decreased with increase in moisture content of mushroom.

Water Sorption Isotherms and Crispness of Fried Yam Chips in the Temperature Range from 293K to 313K

Water sorption isotherms of fried yam chips were determined using a static gravimetric method with saturated salt solutions in the range of water activity between 0.22 and 0.85 at 293, 303 and 313K. Four sorption models namely GAB, Peleg, modified Mizrahi, and BET were fitted with the sorption data generated. The GAB model followed by Peleg and modified Mizrahi models were found to best represent the experimental data in the aw range of 0.22–0.85. However, the BET model was more applicable between aw range of 0.22–0.55. The adsorption isotherm of fried yam chips clearly showed the influence of temperature, decreasing the moisture content at a fixed water activity value with higher temperature. The net isosteric heats of sorption of water were estimated by applying the Clausius–clapeyron equation to the adsorption isotherms at different temperatures. The net isosteric heat of sorption was observed to be decreasing as moisture content increases. Samples stored in desiccators of 0.44 and 0.55 aw at 303 and 313K, respectively, were rated higher in terms of textural properties investigated.