GAB Equation Research Articles

Thermodynamics of sorption isotherms and storage stability of spray dried sweetened yoghurt powder.

Sorption isotherm is a quantitative approach to predict the shelf life of dried foods. Adsorption isotherms of spray dried sweetened yoghurt powder (SYP) were determined by static gravimetric technique at 20, 30, 40 and 50°C. The data obtained were fitted to eight different sorption models. A non-linear least square regression analysis was adopted to evaluate the model constants. The experimental sorption data were best fitted to four parameter Peleg model. The monolayer moisture contents found from GAB model were 4.88, 4.54, 3.86 and 3.52% at 20, 30, 40 and 50°C, respectively. The maximum net isosteric heat of sorption and sorption entropy of SYP were 9.399kJ/mol and 20.28J/molK, respectively. The Gibb's free energy change for sorption was in the range 3436.19-303.91J/mol. The storage stability in terms of moisture content, thiobarbituric acid, free fatty acid, hydroxymethyl furfural values and starter counts of SYP packed in aluminium laminated polyethylene (ALPE) and low density polyethylene (LDPE) were studied along with their change kinetics. The relationship between the water vapour permeability of packaging materials and adsorbed moisture (determined from GAB equation) in powder was used to predict the shelf life and was predicted as 28 and 44.44days in LDPE and ALPE pouches, respectively.

Static and dynamic methods to determine adsorption isotherms of hemp seed (Cannabis sativa L.) with different percentages of dockage.

Adsorption and desorption isotherms of hemp seeds with 0%, 5%, 10%, 15%, and 20% of dockage were determined using the salt solution static (SSS) method. The wet hemp seeds with 0% dockage were also dried at 30℃ with 50% RH, 35℃ with 30% and 50% RH, and 40℃ with 30% and 50% RH inside a thin‐layer dryer (thin‐layer dynamic method). The hemp seeds with different percentages of dockage showed hysteresis, and this hysteresis became more obvious with the decrease of temperature. At the same condition, the equilibrium moisture content of hemp seeds with 0% dockage was approximately 0.5 percent points lower than that of the hemp seeds with dockage. The best equation to fit the equilibrium moisture content data under constant temperature and RH was the modified GAB equation for both adsorption and desorption isotherms. The constant rate period of drying was observed for <0.75 hr when drying air RH was 30% or when drying air temperature was 40℃. The Henderson and Pabis model was the best model to fit the thin‐layer drying data. The equilibrium moisture contents measured by the SSS method were lower than those measured by the thin‐layer dynamic method when temperature was ≤35℃.

Mathematical modelling of adsorption isotherms of Malaysian variety of purple flesh sweet potato at different temperatures

Purple flesh sweet potato or Ipomea batatas is one of the most important crops in Malaysia. Currently the crop is exclusively consumed and marketed in a dried form as traditional snacks and chips. So, understanding the moisture sorption isotherms of this crop is imperative for determining its storability and optimal shelf life of the dried product. Thus, the objective of this study was to determine the adsorption isotherms of purple flesh sweet potato experimentally and the data obtained were used to model the process at different temperatures by employing several mathematical models using non-linear regression analysis. Several mathematical models (GAB, BET, Oswin, Chung and Pfost, Peleg, Henderson, Langmuir, Caurie and Halsey) were tested and compared with the experimental data. The best fitting mathematical model for all temperatures was Caurie’s equation with highest R2 value within the range of 0.9919 and 0.9949 and lowest chi square (λ2) values of 0.0000151–0.0000229. The monolayer moisture content for optimum moisture level for safe storage can be obtained from BET, GAB and Caurie equations. The monolayer moisture content from the GAB equation were found to be 0.1314, 0.1253 and 0.1182 kgw/kgDM and from Caurie’s equation, the values were 0.071, 0.0707 and 0.0692 kgw/kgDM. Both at 25 °C, 50 °C and 60 °C respectively.

Quality characteristics and moisture sorption isotherm of three varieties of dried sweet potato manufactured by hot air semi-drying followed by hot-pressing

This study compared the quality and moisture sorption properties of dried sweet potato prepared from three varieties (two yellow types and one purple type). Semi-dried sweet potato was subjected to a hot-pressing, which led to a final moisture content of 0.14–0.22 kg/kg solid (dry weight basis). The textural properties of the dried product depended on the sugar content of raw materials. In addition, the sugar content also influenced the MSI properties of the dried product. Based on the mathematical models, the GAB equation was revealed as the best model to explain the MSI properties of dried sweet potato. Among the varieties, Annou-beni (AB) showed the highest sugar content and would be the best variety from a taste point of view. Beni-haruka (BH) showed similar qualities to AB, while the less sugar content of this variety reflected that curing was essential for BH to improve consumers’ preference. Shin-jami (SJ) had a weak point in sweetness since it possessed the lowest sugar content. However, the low sugar content of SJ with an attractive color trait induced an acceptable crispy texture and made the product healthier, and SJ could serve as a good sweet potato variety to make dried snack products.

Moisture sorption isotherms of chitosan-glycerol films: Thermodynamic properties and microstructure

Abstract The sorption isotherms of chitosan-glycerol films and of pure chitosan films using the static gravimetric method were performed. The experimental data were adjusted by several sorption models, and the GAB equation was the one that best represented the experimental data, being chosen to predict the adsorption and desorption behaviors of the films for the ranges studied of water activity (0.043–0.888) and of temperatures (20–50 °C). The chitosan-glycerol films presented higher values of monolayer moisture and lower heat involved in the monolayer compared to the pure chitosan films. The values of net isosteric heat and of differential entropy for the films increased exponentially with the moisture content decrease. Dry films with moisture content (Xe) lower than 0.10 kg kg−1 (dry basis) presented morphological changes according to the isosteric heat and entropy curves, indicating water sorption more easily and the more amorphous material (crystallinity of 9%). While dry films at Xe = 0.40 kg kg−1 (dry basis) maintained the characteristic of semicrystalline material (crystallinity of 33%). The enthalpy–entropy compensation showed that the process was controlled by the enthalpy and, it was only spontaneous for the samples in adsorption. The films were classified as a micro and mesoporous materials, and the average pores sizes were from 0.6 to 17 nm for desorption and, from 0.7 to 21 nm for adsorption.

Drying kinetics and determination of water sorption isotherms of corn

The study was carried out for the purpose of determining the drying kinetics as well as moisture sorption isotherm of hybrid-81 corn. Corn at about 31% moisture content (wb) was dried in a forced convective hot air cabinet dryer at different drying conditions, such as variable air dry bulb temperature (40°C, 50°C and 60°C) and loading density (3.56 kg/m2, 7.12 kg/m2, and 10.68 kg/m2) as well as in shining sun at different layers (3.56 kg/m2, 7.12 kg/m2, and 10.68 kg/m2). The water sorption isotherm of the dried corn was developed using vacuum desiccators, which contained saturated salt solutions in the range of 11-93% RHs. The mono-layer moisture content calculated by the Brunauer–Emmett–Teller (BET) model (6.76 g/100 g solid) was lesser than that calculated by the Guggenheim–Anderson–De Boer (GAB) model (10.53 g/100 g solid). The energy constants were 10.45 and 4.64 as per BET and GAB equation, respectively. Both models gave suitable fits for corn. The activation energy (Ea) for diffusion of water was found to be 11.09 kcal/gm-mole for corn. Furthermore, it was noticed that, with the increase of corn layer, the drying rate decreased in case of both sun and mechanical drying. However, higher loading density resulted in efficient drying, at least up to 10.7 kg/m2. It was shown that the drying time to obtain stability was the lowest for moisture content (12.08 %) corresponding to aw of 0.65 in case of BET or GAB monolayer moisture content. This finding could be helpful in predicting the storage life of corn.J. Bangladesh Agril. Univ. 15(2): 309-317, December 2017

Water state diagram and thermal properties of fructans powders

The thermal analyses and the water state diagrams of agavin and inulin were investigated. The thermogravimetric analysis and differential scanning calorimetry (DSC) were used to know the difference between fructans. Linear inulin (220.0 °C) showed a higher thermal stability than the branched agavins (206.7 °C). The samples displayed different physical states and lightness depending on the water activity of storage (a w). The agavin showed different physical states: powder (a w < 0.33), soft lump (a w ≈ 0.43), sticky (0.55 < a w < 0.77), and liquid (a w > 0.85), while inulin showed two physical states: an amorphous powder at a w < 0.55 and a semicrystalline solid at a w > 0.69. These physical states decreased the lightness of fructans. The GAB equation and the Gordon–Taylor equation showed a good fit to the experimental data (R 2 > 0.95). The K constant of GAB equation predicted a lower water affinity of inulin powder, while the agavin powder presented a higher water affinity. The DSC showed a glass transition temperature of 118.1 and 135.3 °C for agavin and inulin anhydrous powders, respectively. An increase in water content in agavin showed a higher depression in the glass transition temperature than in inulin. Also, the inulin exhibited a melting point while agavin not. The water state diagrams showed the critical water activity at which the agavin (0.50) and inulin (0.59) powders were stable to physical changes. This knowledge is used to understand the physical and structural behaviors of agavin and inulin at the different water activities.

Effect of humidity on mechanical, thermal and barrier properties of EVOH films

The moisture content sensitivity of Ethylene-Vinyl Alcohol (EVOH) copolymers is a major problem for food packaging applications. Thus, the relationship between the hydration mechanism at molecular scale and the properties is a critical issue for barrier properties improvement of EVOH films. The aim of this study was to determine material structure-functional property relationships and inter-property correlations.Firstly, mechanical, thermal, water sorption and oxygen barrier properties were determined for a wide range of water activity. The water sorption isotherm modelling with GAB equation, the evolution of the glass transition temperature applying Gordon-Taylor equation as well as the mechanical and O2 barrier properties were studied. The results underlined the plasticization effect of water leading to a decrease of the cohesive energy density and a drastic loss of O2 barrier properties of the EVOH copolymer from a critical point occurring when water activity was close to 0.5.Secondly, an analysis of the experimental water sorption isotherm, expressed as the number of water molecules sorbed per repeating unit in the amorphous phase, associated with an analysis of the evolution of polymer chains mobility allowed to establish a relationship between the hydration mechanism at molecular scale and the properties. It was determined that the amount of water sorbed molecules dramatically increased at a critical point, corresponding to the moment from which one water molecule was linked in average every nine amorphous PVOH units. This point corresponded to the saturation of the monolayer and the beginning of the clustering phenomenon.

Experimental determination of thermophysical properties of unripe banana slices (Musa cavendishii) during convective drying

Thermophysical properties and hygroscopic behavior of unripe banana (Musa cavendishii) were studied. The sorption isotherms were measured at 25 °C, 40 °C, 50 °C and 60 °C and the observed sigmoid curves were modeled with GAB equation which satisfactorily fitted the experimental data. The mean value of monolayer moisture content (Xm) estimated was (5.5 ± 0.6) g/100 g d.b. Drying experiments were conducted at temperatures of 40 °C, 50 °C and 60 °C and the apparent density, specific heat, thermal conductivity and thermal diffusivity – were directly measured as a function of moisture content and porosity. Empirical models obtained by nonlinear regression were successfully fitted to the thermophysical properties, important data to model and simulate the drying process.

Adsorption Studies of Radish Leaf Powder

Radish leaves ( Raphanus sativus ) powder fractions was subjected to moisture adsorption isotherms at different isothermal temperature conditions from 15-45°C with an equal interval of 10°C. The sorption data obtained in gravimetric static method under 0.11–0.90 water activity conditions were subjected for sorption isotherms and found to be typical sigmoid trend. Experimental data were assessed for the applicability in the prediction through sorption models fitting and found that Polynomial and GAB equations performed well over all fitted models in describing equilibrium moisture content – equilibrium relative humidity (EMC–ERH) relationships for shelf stable dehydrated radish leaf powder, over the entire range of temperatures condition under study. The net isosteric heat of sorption, differential entropy and free energy were determined at different temperatures and their dependence was seen with respect to equilibrium moisture content.

Influence of different alkyl‐methylimidazolium tetrafluoroborate ionic liquids on the structure of pebax® films. Consequences on thermal, mechanical, and water sorption and diffusion properties

ABSTRACTIn this work, three ionic liquids (ILs) differing by the length of the alkyl chain linked to their cation were incorporated in a Pebax® copolymer matrix through a solvent cast process for composition from 0 to 70 wt % IL. The copolymer/IL miscibility was investigated via IR Spectroscopy, Differential Scanning Calorimetry and Scanning Electron Microscopy. The three ILs dissolved in the copolymer soft phase for ILs content below 30 wt % whereas they formed segregated dispersed domains at higher loadings. The plasticizing effect of the ILs was examined through DSC and thermomechanical analyses. In the range of IL amount from 0 to 30 wt %, no significant differences were observed in the thermomechanical properties as a function of the IL structure. At higher IL content, the films based on 1‐ethyl‐methylimidazolium tetrafluoroborate sustained better properties. All films exhibited a good thermal stability up to 300 °C. The water sorption isotherms were modeled with GAB equation and both the kinetic and thermodynamic parameters of the sorption mechanism were investigated. A non‐monotonic evolution of the GAB parameters and diffusion coefficient as a function of the IL content was evidenced. Moreover, different behaviors were observed as a function of the IL nature and structuration within the copolymer matrix. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 811–824

THERMODYNAMIC PROPERTIES OF WATER DESORPTION OF SOYBEAN BRAN

The differential and integral thermodynamic properties (enthalpy, entropy, Gibbs free energy, temperature isokinetic and spreading pressure) of soybean bran were determined. Experimental data obtained from the literature were adjusted appropriately by GAB equation (R2&gt;0.95), whose parameters were used to estimate the thermodynamic properties of desorption of soybean bran. The differential enthalpy and entropy increased with decreasing equilibrium moisture content and the isokinetic temperature confirmed the linear chemical compensation between the enthalpy and entropy and the process was enthalpy driven. The spreading pressure increased with increasing water activity. The integral enthalpy decreased with increasing equilibrium moisture content, whereas the integral entropy increased, but negative values, reaching up to -0.02 kJ/mol.K.

Physical properties of maltodextrin DE 10: Water sorption, water plasticization and enthalpy relaxation

Maltodextrins are hydrolysis products of starch. The present research aimed to study physical properties of maltodextrins (water sorption, water plasticization behavior and enthalpy relaxation) and highlighting their functions as food components. The GAB and GT equation were fitted successfully to water sorption and water plasticization data of maltodextrin DE 10. The critical water content and critical water activity values at 25 °C were 14 g water/100 g dry solid and 0.68 aw, respectively. The Kohlrausch–Williams–Watt (KWW model) and extended Adam-Gibbs model fitted well the enthalpy relaxation kinetics data. The non-exponential parameter, β value of maltodextrin DE 10 was within in the range of 0.15–0.93 and its value was dependent on the water activity. At 25 °C, Maltodextrin with aw <0.43 was stable glassy as indicated by very long enthalpy relaxation time. The KWW model gave a more accurate fit than the extended Adam-Gibbs model. Arrhenius model fitted well to enthalpy relaxation data and gave a free activation energy (Ea) of 459.5 kJ/mol.

Influence of protein concentration on surface composition and physico-chemical properties of spray-dried milk protein concentrate powders

Surface composition, moisture sorption behaviour and glass–rubber transition temperature (Tgr) were determined for spray-dried milk protein concentrate (MPC) powders over a range of protein contents (35–86 g 100 g−1). Surface characterisation of MPC powders indicated that fat and protein were preferentially located on the surface of the powder particles, whereas lactose was located predominantly in the bulk. Moisture sorption analysis at 25 °C showed that MPC35 exhibited lactose crystallisation, whereas powders with higher protein contents did not and continually absorbed moisture upon humidification up to 90% RH. The GAB equation, fitted to sorption isotherms of MPCs, gave increases in monolayer moisture value (mm) with protein content. Tgr, measured with a rheometer, decreased significantly (P < 0.05) with increasing water content and increased with increasing protein content (P < 0.05). In conclusion, increasing protein concentration of MPCs resulted in altered surface composition and increased mm value and Tgr values.

Effect of air humidity on traditional Chinese medicine extract of spray drying process and prediction of its powder stability

In order to solve the adhesion and the softening problems of traditional Chinese medicine extract during spray drying, a new method of adding dehumidified air into spray drying process was proposed, and the storage stability conditions of extract powder could be predicted. Kouyanqing extract was taken as model drug to investigate on the wet air (RH = 70%) and dry air conditions of spray drying. Under the dry air condition, the influence of the spray drying result with different air compression ratio and the spray-dried powder properties (extract powder recovery rate, adhesion percentage, water content, angle of repose, compression ratio, particle size and distribution) with 100, 110, 120, 130, 140 °C inlet temperature were studied. The hygroscopic investigation and Tg value with different moisture content of ideal powder were determined. The water activity-equilibrium moisture content (aw-EMC) and the equilibrium moisture content-Tg (EMC-Tg) relationships were fitted by GAB equation and Gordon-Taylor model respectively, and the state diagram of kouyanqing powder was obtained to guide the rational storage conditions. The study found that in the condition of dry air, the extract powder water content decreased with the increase of air compression ratio and the spray drying effect with air compression ratio of 100% was the best performance; in the condition of wet air, the extract powder with high water content and low yield, and the value were 4.26% and 16.73 °C, while, in the dry air condition the values were 2.43% and 24.86 °C with the same other instru- ment parameters. From the analysis of kouyanqing powder state diagram, in order to keep the stability, the critical water content of 3.42% and the critical water content of 0.188. As the water decreased Tg value of extract powder is the major problem of causing adhesion and softening during spray drying, it is meaningful to aid dehumidified air during the process.

Measurement and Modelling of Water Content Effects on Thermal Properties of Compressed Soil Building Blocks

Aims: The aim of this paper was to determine the thermal conductivity and the thermal effusivity of the compressed soil building blocks as a function of their water content. Study Design: Samples with dimensions 534020 mm 3 were produced for experiment. Eight containers and an oven were used to determine the equilibrium moisture content. The thermal properties of samples were obtained using a symmetrical hot strip method device. Place and Duration of Study: University of Yaounde I and National Advanced School of Engineering, Laboratory of Energizing, Water and Environment, between March 2014 and October 2014. Methodology: Thereafter, sorption isotherm of this building material was determined using the gravimetric static method of saturated salt solutions at 30°C (ambient average temperature), and GAB equation was applied to discuss the results. Then, a symmetrical hot strip method device was used to measure the thermal conductivity and the thermal effusivity of these samples, with their Original Research Article Andre Talla; BJAST, 6(5): 520-533, 2015; Article no.BJAST.2015.107 521 water content varying from 0 to a maximum value of 0.139 kgw.kgdb .An adapted device was developed to prevent water evaporation on the lateral faces of the samples. Results:Both thermal conductivity and thermal effusivity were modelled and the experimental results were processed to evaluate these thermal properties of compressed soil building blocks.A new simplified model, based on a physical approach with assumption of an ideal shrinkage of the material during the evaporation of water, was built. Conclusion: Calculated and experimental values of thermal properties were in good agreement, with a maximum standard error of 1.671 WmKs for thermal effusivity and of 0.024 WmK for thermal conductivity. The suitability of this model for other buildings material will be further studied.

Moisture sorption isotherms and net isosteric heat of sorption for spray-dried pure orange juice powder

The aim of this study was to measure the moisture sorption isotherms of spray-dried pure orange-juice powders. These isotherms have been fitted using eleven different sorption equations. The GAB model was found to be the best fit. The GAB parameters were estimated both by direct and indirect regression methods. The net isosteric heat of sorption was also determined using the Clausius–Clapeyron equation and was successfully used as a limiting condition for estimating the six unknown parameters of the GAB equation using the direct regression method. The net isosteric heat of sorption decreased from 9.05 kJ/mol at 0.05 kg/kg dry basis to approximately 0.0 kJ/mol at 0.7 kg/kg dry basis. These values, as expected, were comparable to the enthalpy difference between the monolayer and multilayer moisture (8.16 kJ/mol), and the difference between the latent heat of condensation for pure water and the heat of sorption of multilayer moisture (=−0.88 ± 0.69 kJ/mol).

Convective Drying and Water Adsorption Behavior of Unripe Banana: Mathematical Modeling

In this study, we modeled the convective drying of unripe banana slices and the water adsorption behavior of the banana flour. Drying experiments were conducted at temperatures from 30 to 60C with an air velocity of 1.5 m/s, and adsorption isotherms were measured at 15, 25 and 35C. A four-parameter empirical model that we proposed fitted quite satisfactorily with the experimental drying data. In addition, the drying behavior during the first hours was described by an effective diffusivity coefficient, which ranges from 1.2 × 10−10 m2/s to 6.6 × 10−10 m2/s. The observed sigmoid adsorption isotherms of unripe banana flour were modeled quite well by the GAB equation. Estimated monolayer moisture content has values between 0.08 and 0.09 g/g. Practical Applications Since unripe banana is easily transported and stored, it has become a potential food product for industrialization. Drying is one of the most important unit operations and classical methods for preserving foods, which extends their shelf-life, decreases their weight for transportation, and reduces their volume for storage. Studying the drying kinetics of fruits is necessary in order to provide information about the time required for attaining low and safe moisture content. On the other hand, adsorption isotherm predicts the shelf-life stability, which involves moisture changes that may occur during storage, and thus it is useful for selection of packaging materials. Sorption isotherms are important for modelling drying processes, as well for designing and optimizing drying equipment helping us to understand how a food material behaves during drying, but also how a dried product behaves after the drying process to maintain its quality and acceptability is essential.

폴리아크릴산 나트륨 염이 함침된 흡수성 고분자 복합 필름의 제조 및 특성 연구

수분은 제품의 물리적 변화, 미생물 성장, 화학적 반응을 일으켜 품질을 저하시키는 주요 요인이 된다. 본 연구는 기능성 포장 필름으로 응용하기 위하여 수분을 흡착력이 우수한 폴리아크릴산 나트륨 염(polyacrylic acid partial sodium salt, PAPSS)을 선상저밀도폴리에틸렌(LLDPE) 고분자 수지에 함침된 수분 흡착 기능을 가진 복합 필름을 제조하였다. 제조한 수분 흡착 복합 필름은 PAPSS 물질 입자가 LLDPE 고분자 매트릭스에 잘 분포되어 있음을 확인하였다. 또한 PAPSS 첨가량이 증가됨에 따라 필름의 투명도가 저하되었다. 고분자에 첨가되는 PAPSS 물질의 증가는 복합 필름의 인장강도와 신장률 값이 감소됨을 확인할 수 있었으나 각각 0.5, 1, 그리고 2% PAPSS 필름들 간에는 큰 유의적 차이를 보이지 않았다. 반면 4% PAPSS 함침 필름은 다른 농도로 PAPSS 물질이 함침된 필름과 비교하여 인장강도 및 신장률 값의 급격한 감소를 나타냈다. PAPSS 함량의 증가는 필름의 산소 및 수분 투과도 값이 감소됨이 관찰되어 고분자에 PAPSS 첨가는 필름의 가스 차단의 개선을 나타냈다. PAPSS 함침 필름들은 상대적으로 높은 습도 환경 조건에서 우수한 수분 흡착율을 나타내는 것을 확인하였으며, 특히 4% PAPSS 함침 필름이 다른 PAPSS 농도 함침 필름들에 비하여 높은 수분 흡착율을 보였다. <TEX>$25^{\circ}C$</TEX> 조건에서 필름 샘플의 수분 흡착 등온 곡선에 가장 적합하게 묘사되는 최적의 모델 방정식은 GAB 모델로 나타냈다. PAPSS 첨가는 고분자 필름의 결정화 및 녹는 온도 변화에 영향을 보였으나, 열중량 변화에는 큰 차이가 없이 안정된 열적 특성을 나타냈다. Moisture is a major factor causing the deteriorative physical change, microbial growth, and chemical reaction of the products. In this study, the moisture absorbing composite films have been prepared with moisture absorbing material of polyacrylic acid partial sodium salt (PAPSS) impregnated on LLDPE polymer for the functional packaging applications. The results showed that PAPSS impregnated film illustrated uniformly dispersed PAPSS particles in the LLDPE polymer matrix. The transparency of the PAPSS impregnated film decreased slightly at higher PAPSS concentrations. An increase in the PAPSS content for moisture-absorbing films showed a similar decrease in tensile strength, percent elongation at break, and tear strength. Their values of films impregnated with PAPSS of 0.5, 1, and 2% showed no significant difference. Meanwhile, 4% PAPSS films significantly decreased the values of mechanical properties compared to the films impregnated with different PAPSS levels. Values of the oxygen permeability and water vapor permeability for PAPSS impregnated films decreased significantly with greater PAPSS. The results indicate that 4% PAPSS impregnated in LLDPE films had high affinity of moisture absorbencies compared to the other films. The mathematical equation that best described the moisture sorption isotherm of each film sample was the GAB equation at <TEX>$25^{\circ}C$</TEX>. The crystallization and melting temperatures of PAPSS films were influenced by the addition of PAPSS material, but showed good thermal stability.

Predicting Sorption Isotherms and Net Isosteric Heats of Sorption of Maize Grains at Different Temperatures

Abstract In Sub-Saharan Africa, drying maize on their stem was the traditional technique frequently used; this technique must be improved to avoid contaminations and to increase the quality of drying. However, the method of storage is accountable for the most significant losses after harvest, because mildew develops when the conditions of storage (too high temperature and moisture of the air) do not tally with the final content of the dried product. Sorption isotherms of products are most important to model moisture uptake during storage and distribution. Sorption isotherms of intermediate moisture content maize grains were determined using the gravimetric static method of saturated salt solutions at 30°C, 40°C, 50°C, and 60°C, and GAB equation was applied to discuss the results. This model correctly describes the evolutions of maize sorption isotherms, with maximum deviation of 0.0080 kg water/kg db. The net isosteric heat of sorption was determined also, using the Clausius–Clapeyron equation, and it was varied from 463 kJ/kg to 1,264 kJ/kg, decreasing with increasing moisture content. This effect was well described by an exponential function with a regression coefficient R 2 &gt; 97%. The monolayer moisture content was found to decrease with increasing temperature. These results can be used to predict the potential changes in the stability of maize grains and later for the development of a system of suitable drying.