Monolayer values at 30°C of various spices as computed by the BET and GAB models

The monolayer values at 30^°C of 17 spices of various botanical families, obtained from different parts of the plant and commercially marketed in diverse forms, were determined from stepwise moisture adsorption data at four relative humidities over the lower 50% of water activity (a _w) values by using the BET and GAB equations. Fitting was similarly good for five spices, better by the BET equation for five other spices, and better by the GAB equation for another three, the experimental results provided by the remaining four species being fitted by neither equation. The monolayer values determined by the three-parameter GAB model were not always larger than those obtained by use of the simpler two-parameter BET equation, which appears to be more accurate in those cases where only sorption data for the low a _w range are available.     

Moisture adsorption behavior of banana flours (Musa paradisiaca) unmodified and modified by acid‐treatment

The moisture sorption isotherms of untreated banana flour (UBF) and acid treated banana flours (ATBFs) were determined using the static gravimetric method of saturated salt solutions at temperatures of 30°C. The range of water activities (a_w) was calculated to be between 0.14 and 0.97. The equilibrium moisture content absorption data were fitted to four sorption models that differ in the information that can be obtained from each one: Brunauer‐Emmett‐Teller equation (BET), Guggenheim, Anderson and de Boer (GAB), Smith, and Iglesias‐Chirife. Monolayer moisture content (X₀) for UBF and ATBFs were found in the range of 4.06–5.47 (BET model) and 3.87–5.88 (GAB model). The GAB model was found to be the most suitable model to describe the isothermal water sorption of UBF and ATBFs in the intervals proposed of a_w. The X₀ values of both models (BET and GAB) increase with increasing a_w. The Banana flour treated for 11 days (ATBF₃) presents the highest value of X₀ compared with all samples. This result suggests that mechanism of adsorption of water and molecular structure in ATBFs was affected, attributed to changes in morphology and crystallinity of the samples with treatment.     

Moisture sorption properties of smoked chicken sausages from spent hen meat

Moisture sorption characteristics of smoked chicken sausages from spent hen meat were investigated at 5, 25 and 50°C over a water activity (a_w) range of 0.11–0.87. Sigmoidal (type-II) desorption isotherms were observed for these samples. The sorption data were analysed using mathematical equations of Caurie, Halsey, GAB and Oswin. The Halsey equation gave the best fit over the entire temperature range. GAB model also described the isotherms fairly well. The GAB and BET monolayer moisture values were not significantly different (P>0.01) and decreased with increasing temperature. The number of adsorbed monolayers, density and amount of bound water and surface area of adsorption also decreased as the sorption temperature increased. The pores, in general, enlarged with rising temperature.     

The derivation of the GAB adsorption equation from the BDDT adsorption theory

The BET‐like adsorption equation, popularly known as the GAB equation, was derived from the BDDT adsorption equation without changes to the bulk liquid properties of multilayer molecules. Without these changes the third parameter K in the GAB equation cannot be said to measure changes in the bulk liquid properties of the multilayer molecules. Instead K measures the range of application of the GAB equation and differentiates multilayer adsorption, when K > 0.5, from singular layer adsorption, when K ≤ 0.5. The GAB equation, derived from and based on the BDDT theory is argued, in this paper, to provide a greater insight into the mechanism of water adsorption than the same equation derived from and based on the BET theory.     

Moisture sorption isotherms and isosteric heat for pistachio

The equilibrium moisture contents (EMC) of pistachio were determined using the standard static-gravimetric method at 15, 25, 35 and 40 °C for pistachio powder at 15, 35 °C for pistachio kernel and pistachio nut for water activity (a _w) ranging from 0.11 to 0.9. At a given water activity, the results show that the moisture content decreases with increasing temperature. The experimental sorption curves are then described by the BET, GAB, Henderson, Oswin, Smith and Halsey models. A nonlinear regression analysis method was used to evaluate the constants of the sorption equations. The Smith model was found to be suitable for describing the sorption curves. The isosteric heat of adsorption of water was determined as a function of moisture content from the equilibrium data at different temperatures using the Clasius–Clapeyron equation.     

MOISTURE SORPTION CHARACTERISTICS OF TARHANA, A FERMENTED TURKISH CEREAL FOOD

The adsorption and desorption isotherms of three types of commercial tarhana, a popular Turkish cereal food used in soup-making, were determined as 10, 20 and 30C, and water activities (a_a) ranging from 0.08 to 0.92 using the gravimetric static method. The results showed that commercial tarhana has a type HI BET classification. Sieve size affected hygroscopicity. Hysteresis effects were not detected among the three formulations examined, but because of the limited number of formulations, that conclusion is only tentative. Moisture up-take accelerated around a_w 0.6.     

Moisture sorption and the applicability of the Brunauer-Emmet-Teller (BET) equation for blanched and unblanched mushrooms

Moisture sorption isotherms of blanched and unblanched mushrooms over 0.11–0.75 a_w were determined at 27°C and 37°C by using the static gravimetric method. Adsorption and desorption behaviors of blanched and unblanched mushrooms were compared. The unblanched material adsorbed more water than that of the blanched. In desorption isotherms, the equilibrium moisture contents of the unblanched material were found to be higher than those of the blanched throughout the entire a_w range. The BET equation was tested to fit the experimental moisture sorption data over the 0.11–0.43 a_w, 0.11–0.55 a_w, 0.11–0.64 a_w, and 0.11–0.75 a_w. Nonlinear regression analysis was used for the determination of the parameters in the equation. The quality of the fit of the BET model over each a_w range was judged from the value of the relative percent root mean square (% RMS). The moisture sorption behavior over 0.11–0.43 a_w of mushrooms has indicated that the BET equation is applicable generally up to 0.43 a_w. Monolayer moisture contents and C constants in the BET equation obtained for each a_w interval were reported. The water activities corresponding to the monolayer values have been determined and discussed related to mushroom storage. The net isosteric heats of adsorption and desorption were estimated from equilibrium sorption data, using the integrated form of the Clausius-Clapeyron equation. The heats of adsorption/desorption decreased with increase in moisture content and approached to a constant value. It was concluded that moisture adsorption/desorption occurred by physical mechanisms at high moisture contents, but at low moisture contents, besides physical adsorption, chemisorption was also observed.     

Sorption isotherms and thermodynamic properties of freeze‐dried colostral whey powders with different additives

Moisture adsorption isotherms of colostral whey (CW) powders with different additives (maltodextrin and sucrose) were determined using gravimetric static method at 15–35 °C in the water activity range of 0.067–0.76. The moisture adsorption isotherms obtained were typical sigmoid curves, and the modified‐Halsey and Guggenheim–Anderson–de Boer (GAB) equations gave the best fit to experimental data among five well‐known equations. Addition with maltodextrin into CW powders could effectively decrease equilibrium moisture content (EMC), whereas addition with sucrose increased EMC in the water activity of 0.43–0.76 at 15 and 25 °C and in the water activity of 0.21–0.76 at 35 °C, respectively. Thermodynamic properties including net isosteric heat of sorption and differential entropy were determined from adsorption data using Clausius–Clapeyron equation. The results showed that net isosteric heat of sorption of all the samples decreased exponentially with increasing EMC. Enthalpy–entropy compensation theory was applicable for adsorption process of all the samples, and the adsorption processes were enthalpy‐driven.     

Sorption isotherms of tapioca flour

The adsorption and desorption isotherms of tapioca flour was performed in the range of water activity (a_w) from 0.22 to 0.92 at 25 °C, and the applicability of six mathematical models in data prediction was evaluated. The tapioca flour presented Type II isotherm (sigmoid), and the hysteresis effect was not observed. For the microbiological stability, the moisture of tapioca flour should not exceed 10.1%. The monolayer moisture content value indicates that the drying process of tapioca flour should not be conducted to obtain moisture levels lower than 4.92% to avoid unnecessary power consumption. The data fit showed that all mathematical models are able to predict the adsorption isotherm of tapioca flour, and the models of Handerson, Smith, GAB and BET are able to predict the desorption isotherm.     

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 a_w range of 0.22–0.85. However, the BET model was more applicable between a_w 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 a_w at 303 and 313K, respectively, were rated higher in terms of textural properties investigated.     

Water sorption and the plasticization effect in wafers

Wafers are low moisture food products whose crunchiness or crispness is considered a primary textural attribute, highly affected by the product's physical state (glassy or rubbery). The water activity–water content–glass transition relationships for commercial wafers are reported, using Gordon and Taylor's equation to model the water plasticization effect and also Brunnaver‐Emmett‐Teller (BET) and Guggenheim‐Anderson‐DeBoer (GAB) sorption models. BET monolayer moisture content was 6.2% and the moisture limit to fit this model was about 11.5%. Critical water activity and critical water content for the glass transition of the product at 20 °C were 0.591 and 0.118 (mass fraction) respectively. Abrupt changes in the mechanical product properties, evaluated from a three‐point bend test, could be observed at these limits. Moisture levels between 6 and 11% give rise to a glassy state matrix in the product and so to acceptable product crispness. Below this range, the glassy matrix seems to turn fragile and, above this range, the product becomes rubbery.     

Desorption isotherms,net isosteric heat and the effect of temperature and water activity on the antioxidant activity of two varieties of onion (Allium cepa L)

The standard static gravimetric method was used to determine moisture desorption isotherms (MDIs) of two onion varieties (Goudami and Galmi Violet) at 30 °C, 45 °C and 60 °C in the water activity ranging from 0.055 to 0.83. The combined effects of temperature and water activity on the antioxidant activities of the onion varieties were also studied. GAB, Oswin, Smith and BET equations were tested to fit the experimental data. The net isosteric heat of sorption was calculated. Equilibrium moisture content (EMC), total phenolic content (TPC) and antiradical activity were also measured. The isotherm and the EMC vary significantly with the onion variety and drying temperature, irrespective of water activity (a_w). Desorption isotherms were best described by the GAB model. The maximum net isosteric heats for Galmi Violet (32.58 kJ mol^?1) were greater than those of Goudami (23.50 kJ mol^?1) at each EMC. The TPC and antiradical activity of the Galmi Violet were significantly (P ≤ 0.05) higher than that of the Goudami at all investigated temperatures and water activities.     

The dehydration of paprika with ambient and heated air and the kinetics of colour degradation during storage

The hygroscopic behaviour of dried red pepper is characterized by means of water vapour adsorption isotherms at 5, 20 and 35°C; from the GAB equation, a monolayer water content of 0·0816 kg H₂O per kg dry matter has been deduced. The effect of temperature on the drying rate of pepper is considered and yields are compared when the drying is carried out with ambient air and with different loading densities (10–40 kg m⁻²). The kinetics of paprika colour degradation during storage at different temperatures and with a moisture content corresponding to the monolayer have been studied. A sharp change in the rate of colour loss is observed at 15°C: the value Q₁₀ (°C) changes from 1·62 to 2·82 when the temperature rises above 15°C.     

Experimental determination and modelling of water desorption isotherms of tropical woods: afzelia, ebony, iroko, moabi and obeche

Desorption isotherms of Afzelia, Ebony, Iroko, Moabi and Obeche have been experimentally found at 20, 30, 40, 50 and 60 °C by use of the salt method for a range of water activities from 0.06 to 0.97. The sorption capacity of these woods increases with the temperature for a given water activity. The experimental curves have been simulated by the GAB and BET models. The GAB model enables the representation of the whole desorption isotherms. Meanwhile, the BET model ensures a better representation of the experimental results for water activity lower than 0.35 with mean relative deviation of 2.9, 2.1, 1.3, 3.2 and 3.3% for respectively Afzelia, Ebony, Iroko, Moabi and Obecheand a better estimation of the water content corresponding to monolayer saturation.     

Desorption Isotherms and Isosteric Heat of Three Tunisian Date Cultivars

The sorption isotherm of three Tunisian cultivars, Allig, Kentichi, and Deglet Nour, were obtained at 40, 60 and 80 °C. The static gravimetric method was used, according to the COST 90 recommendations. The overall shape of the curves describing the water content as a function of water activity was typical of sugar-rich materials. A difference between varieties exists for the sorption isotherm, thus for the drying behavior. Data were compared with respect to several isotherm models. It was found that the GAB and the BET equations could satisfactorily represent the sorption data up to about 0.9 relative humidity. Through the BET equation, monolayer moisture content was calculated for the isotherm zone corresponding to monomolecular adsorption. In addition, isosteric heat of sorption was determined from sorption data using the Clausius–Clapeyron equation.     

Water Sorption Isotherms of Raisins, Currants, Figs, Prunes and Apricots

Moisture sorption isotherms of dried fruits [Sultana raisins, Corinthian (black) currants, figs, prunes, and apricots] were determined at 15, 30, 45, and 60°C, using the standard static gravimetric method developed by the European Cooperation Project COST 90. Experimental curves showed an inverse effect of temperature at high moisture content due to high sugar content of dried fruits. The hysteresis between adsorption and desorption was verified experimentally. The GAB equation was used to predict experimental data for water activity range 0–0.95.     

Moisture adsorption properties and adsorption isosteric heat of dehydrated slices of Pastirma (Turkish dry meat product)

Moisture sorption characteristics of dehydrated pastirma were investigated at 15°C, 20°C and 30°C over a water activity (a(w)) range of 0.2-0.9. Sigmoidal (type-II) adsorption isotherms were observed for pastirma. The experimental sorption data obtained were applied to the Halsey, Harkins-Jura, Smith, BET, Henderson, Freundlich and GAB isotherm equations to test fitness of these equations to pastirma. The order of the best fit of sorption data obtained for pastirma at 15°C, 20°C and 30°C in all the range of a(w) studied (0.2-0.9) was Harkins-Jura>Halsey>BET>Smith>Freundlich>GAB>Henderson, respectively. In the range a(w) 0.2-0.55, BET model has a better fit than in the range a(w) 0.2-0.9. Isosteric heats of adsorption were evaluated by applying the Clausius-Clapeyron equation to experimental isotherms and decreased with increasing moisture content.     

Moisture sorption isotherms and thermodynamic properties of apple Fuji and garlic

The moisture equilibrium isotherms of garlic and apple were determined at 50, 60 and 70 °C using the gravimetric static method. The experimental data were analysed using GAB, BET, Henderson–Thompson and Oswin equations. The isosteric heat and the differential entropy of desorption were determined by applying Clausius–Clapeyron and Gibbs–Helmholtz equations, respectively. The GAB equation showed the best fitting to the experimental data (R² > 99% and E% < 10%). The monolayer moisture content values for apple were higher than those for garlic at the studied temperatures; the values varied from 0.050 to 0.056 and from 0.107 to 0.168 for garlic and apple, respectively. The isosteric heat and the differential entropy of desorption were estimated in function of the moisture content. The values of these thermodynamic properties were higher for apple (in range 48–100 kJ mol^?1 and 14–150 J mol^?1 K^?1) than for garlic (in range 43–68 kJ mol^?1 and 0–66 J mol^?1 K^?1). The water surface area values decreased with increasing temperature. The Kelvin and the Halsey equations were used to calculate the pore size distribution.     

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

The 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 and modeling of energy sorghum (Sorghum bicolor (L.) Moench)

Long-term low cost storage of whole-stalk lignocellulosic energy sorghum biomass (specialized forage varieties of Sorghum bicolor (L.) Moench) is essential for the feedstock's successful role as a dedicated energy crop for ethanol production. As an alternative to expensive ensiling methods, aerobic storage of S. bicolor (L.) Moench biomass in traditional rectangular bale formats could alleviate feedstock supply costs if material deterioration in storage could be minimized. Moisture desorption and adsorption isotherms for S. bicolor (L.) Moench were created at 15 °C, 20 °C, 30 °C, and 40 °C with water activities from 0.1 to 0.9 using the dynamic dew-point method. Sorption isotherms were modeled using four temperature dependent and three temperature independent equations. The relationship between equilibrium moisture content and water activity was found to decrease with increasing temperatures. GAB (Guggenheim–Anderson–de Boer) monolayer moisture content and the moisture content at which microbial activity becomes limited were found to range from 5.6% db to 10.4% dry basis (db) and 12.0% db to 18.4% db, respectively. The net isosteric heat of sorption was calculated using the Clausius–Clapeyron equation and determined to be higher for desorption than adsorption with both trends decreasing exponentially at increasing levels of moisture content. The differential entropy of S. bicolor (L.) Moench was shown to exhibit a log normal relationship with moisture; peaking near the monolayer moisture content. The results of the study indicate that aerobic storage of energy sorghum biomass may be similar to other herbaceous feedstocks should extensive drying occur before entering storage.