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.     

Desorption isotherms and thermodynamics properties of anchovy in natura and enzymatic modified paste

The thermodynamic properties of anchovy fillets and enzymatic modified pastes in two hydrolysis degrees (3% HD and 14% HD), at 50, 60 and 70 °C were evaluated. The GAB model was used to calculate the values of the monolayer moisture content and the thermodynamic properties of the samples. The enzymatic modification led to the increases of the superficial area and differential enthalpies, and decrease of the differential entropies in relation the samples in natura. The enthalpy–entropy compensation showed that the process was controlled by the enthalpy, it was only spontaneous for the samples in natura. Pore size decreased with enzymatic modification, and all samples were in the limit of region between micropores and mesopores (<2 nm) for moisture content of 15%, and mesopores (from 2 to 50 nm) to moisture content above 15%.     

A new method for predicting sorption isotherms at different temperatures using the BET model

A new methodology to predict sorption isotherms at different temperatures is proposed in this work. The classical BET model is used coupled with the Clausius–Clapeyron equation, and an Arrhenius temperature dependency is assumed for the BET energetic parameter C. When compared to the usual procedure based solely on the use of a model for the sorption isotherm, this methodology presents two main advantages: (i) a single set of parameters may be used to predict isotherms at different temperatures and (ii) fewer experimental data are required to estimate this set of parameters, using only one single sorption isotherm and one single value of differential enthalpy of sorption, at a specific and arbitrary humidity. The new methodology was tested with different food materials and the predicted results were in good agreement with experimental data, attesting the potential of this new approach.     

Moisture adsorption isotherms for mushroom

The moisture adsorption isotherms of Agaricus bisporus and Pleurotus florida mushrooms were determined at temperatures (30-70°C) typically found in drying and storage. The samples were equilibrated above saturated salt solutions. Equilibrium moisture content of mushroom decreased with an increase in temperature at constant water activity. The data was adjusted to 11 sorption models to ascertain the best fit. Comparisons were based on mean relative error, standard deviation and coefficient of determination. Of the models tested, the Chung and Pfost model showed the best fit. The net isosteric heat of sorption was determined using the Clausius-Clapeyron equation. The net isosteric heat of sorption decreased from 13 to about 1 kJ/mol when the moisture content increased from 5 to 50 g /100 g dry matter.     

Moisture sorption properties of chitosan

The moisture equilibrium isotherms of chitosan were determined at 20, 30, 40, 50 and 60 °C, using the gravimetric static method. Experimental data were analyzed by the GAB, Oswin, Halsey and Smith equations. Isosteric heat and differential entropy of sorption were determined from the GAB model using the Clausius-Clapeyron and Gibbs-Helmholtz equations, and pore size distribution was calculated by the Kelvin and Halsey equations. The GAB and Oswin equations showed best fit to the experimental data with R² ≈ 99% and low mean relative deviation values (E% < 10%). Monolayer moisture content values (from 0.12 to 0.20 kg kg⁻¹) and water surface area values (from 450 to 700 m² g⁻¹) decrease with increasing temperature. Isosteric heat and differential entropy of sorption were estimated as a function of moisture content. The Kelvin and Halsey equations were adequate for calculation of pore size distribution, which varied from 0.5 to 30 nm.     

Developments in glass transition determination in foods using moisture sorption isotherms

The food polymer science (FPS) approach has been effectively used to investigate the physical stability of amorphous food materials. The glass transition, a key FPS parameter, has traditionally been determined using thermal techniques that scan temperature while holding the plasticizer (moisture) content constant. Moisture sorption isotherms provide information about the physical properties of food as the plasticizer level is adjusted and temperature is held constant. New automatic isotherm generators can be used to produce high resolution, dynamic isotherms much faster than traditional static methods. Dynamic isotherms for a small selection of amorphous materials have been investigated and shown to experience distinct inflection points in the water activity region where the glass transition temperature is close to the experimental temperature. Several studies on amorphous spray dried milk powder and amorphous polydextrose indicate very good agreement between glass transitions determined using thermal techniques and dynamic isotherm methods. This agreement suggests that dynamic isotherms are a viable alternative to traditional thermal methods for investigating glass transitions of amorphous foods.     

Sorption isotherm of corn distillers dried grains with solubles (DDGS) and its prediction using chemical composition

Corn distillers dried grains with solubles (DDGS) has high feed value due to its nutritive contents. The ratio of wet distillers grains (WDG) and condensed distillers solubles (CDS) added during the production process determines the chemical composition of DDGS. Effect of changing this ratio on water sorption behaviour of DDGS, at different temperature, was studied. Five mathematical models were evaluated to explain the sorption behaviour of DDGS. Prediction of sorption isotherm from chemical composition using a four-component model was also investigated. DDGS followed a BET Type III isotherm with higher equilibrium moisture content at increasing relative humidity. Difference in the sorption isotherm curves of DDGS samples reduced as equilibrium relative humidity increased. Equilibrium moisture content of DDGS samples reduced with lowering of CDS quantity added during production process. Modified Halsey equation was found suitable for mathematically explaining the sorption behaviour of DDGS. Binding energy of water molecule with DDGS increased with lowering of CDS level. Sorption behaviour of DDGS can be predicted from the chemical composition of protein, sugar, minerals, starch, fibre and glycerol. Increase in CDS level increased the percent relative deviation of predicted values calculated using the modified four-component model and might be due to the interaction between the chemical compounds present in the samples.     

Adsorption isotherm and heat of sorption of fresh- and osmo-oven dried plantain slices

Firm ripe plantain was transversely cut into 10-mm slices and osmotically pretreated in 52 °, 60 ° and 68 °B sucrose solutions, for 12 h at 25 °C. Fresh- and osmosed plantain slices were air-dried in a cross flow forced draught oven at 60 °C for 72 h. Adsorption isotherms of the products were determined at 20 °C and 40 °C, using a gravimetric-static method. Adsorption data were fitted into eight isotherm models. Isosteric heat of sorption was calculated using the Clausius-Clapeyron equation. Adsorption isotherms of fresh- and osmo-oven dried plantain slices followed type I (J-shaped) isotherms, characteristic of high sugar products. Inversion of 20 C and 40 C isotherms occurred between a_w 0.65 and 0.70. Among the models tested, the Guggenhein Anderson deBoer (GAB) gave the best fit. Isosteric heat of sorption increased with decreasing moisture contents. Negative isosteric heat of sorption occurred at high moisture content. Moreover, isosteric heat of sorption increased with increase in sucrose solution concentration during the osmotic dehydration prior to oven drying.     

Adsorption isotherm and heat of sorption of osmotically pretreated and air-dried pineapple slices.

Slabs (1 ×1.5 ×5 cm) of pineapple were immersed in sucrose solutions of 52°B, 60°B and 68°B (w/w) at 25 °C for 12 h. Osmosed pineapple slices were subsequently air-dried at 60 °C for 72 h. Adsorption isotherms of osmo-dried pineapple slices were determined at 20 °C and 40 °C, using the gravimetric-static method. Isotherms were modeled using Brunnauer, Emmet and Teller (BET) and Guggenheim Anderson de Boer (GAB) models. Heat of moisture adsorptions were calculated from adsorption data using the Clausius-Clapeyron equation. Adsorption isotherms of osmo-oven-dried pineapple followed the characteristic type III (J-shape) isotherms. Equilibrium moisture contents at constant water activity decreased with increasing temperature, but crossing of the isotherms occurred at a_w~ 0.86. GAB and BET monolayer moisture decreased with increased temperature. The net isosteric heat increased with decreased moisture content, and decreased with increased sucrose pretreatment concentration of pineapple slices.     

不同卷烟原料物理保润机理研究

为考察不同卷烟原料的物理保润性能,通过动态水分吸附系统、重量法吸附仪和扫描电子显微镜,分别从失水曲线、水吸附量、水吸附焓和微观物理结构进行分析,从机理上阐明了其保润性能的差异性。水分子吸附研究结果表明,在25.2oC,30mbar时,吸附量大小依次为叶丝>膨胀叶丝>膨胀梗丝 >薄片,采用Sips模型拟合后计算其初始吸附焓大小为膨胀梗丝>膨胀叶丝>叶丝>薄片。结合卷烟原料动态水分评价及微观结构差异分析,得出如下结论:梗丝基于其超大孔结构及亲水性表面,随环境湿度的变化表现出吸湿快、解湿快的特点,保润性能最差;叶丝基于其较丰富的油份及内含物的作用,保润性能最好;膨胀叶丝和薄片保润性能居中。      

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.     

Equilibrium moisture content and heat of sorption of Gelidium sesquipedale

Moisture equilibrium data for adsorption and desorption of water from Gelidium sesquipedale were investigated at temperatures in the range of 30-50°C and water activity ranging from 0.05 to 0.9. The experimental procedure used was based on the gravimetric static method. The sorption curves of Gelidium sesquipedale decreased with increase in temperature at constant relative humidity. The hysteresis effect was observed. The experimental data of sorption were described by six models. The GAB and modified BET models were found to be the most suitable for describing the sorption curves. The isosteric heats of desorption and adsorption of water were determined from the equilibrium data at different temperatures.     

Equilibrium moisture content and sorption isosteric heats of five wheat varieties in China

The moisture sorption isotherm data of five Chinese wheat varieties were investigated via the gravimetric static method at five different temperatures (10, 20, 25, 30, and 35 °C) and relative humidity ranging from 11.3 to 96.0%. The sorption data of wheat decreased with an increase in temperature at a constant relative humidity (r.h.). The hysteresis effect was observed between adsorption and desorption isotherms. The width and span of the hysteresis loop decreased with increased temperature, but was not influenced by the hardness of wheat varieties. Six models, namely the Chen-Clayton (CCE), Modified BET (BET), Modified-Chung-Pfost (MCPE), Modified-Henderson (MHE), Modified-Oswin (MOE) and Strohman-Yoerger (SYE), were employed to describe the experimental data. The BET, MCPE and MOE models were selected to best describe the sorption isotherms of wheat in the ranges of 11.3-49.9, 11.3-96.0 and 11.3-96.0% r.h., respectively. The hygroscopic property difference between hard wheat (cv. ‘Longyuan’ and ‘Nanduan’) and soft wheat (cv. ‘Zhaozhuang’ and ‘Lumai’) was very small. The sorption isosteric heat of wheat decreased with an increase in moisture content until the dry bulb moisture content (m.c.) of 20% was reached, and then decreased smoothly with increasing moisture content. A big difference was found between adsorption and desorption isosteric heats of wheat below 20% m.c., but the sorption isosteric heat difference between hard and soft wheat isotherms was small.     

The effect of pre-drying temperature on the sorption properties of microcrystalline cellulose at temperatures up to 90°C

Using a specially-constructed oven and sorbostats the sorption isosteres for microcrystalline cellulose have been obtained at temperatures between 25 and 90°C. Differences are shown between pre-drying at 105°C and pre-drying at the temperature of sorption. The sorption isosteres are not straight lines as expected.     

Study of water sorption behaviour of pectins using a computerised elution gas chromatographic technique

The moisture sorption isotherms of apple and citrus pectins using a computerised inverse gas chromatographic (IGC) method were constructed at 25, 30, 40 and 50°C. Sorption isotherm data are calculated from a single injection of water with the aid of a personal computer and a program developed to perform data handling. Thus sorption isotherms are constructed in < 3 h. Good agreement was observed between moisture sorption isotherms of apple and citrus pectins obtained with the computerised IGC method and the gravimetric static method.     

Moisture adsorption isotherms of Tarhana at 25 °C and 35 °C and the investigation of fitness of various isotherm equations to moisture sorption data of Tarhana

Moisture adsorption isotherms of Tarhana were determined at 25 °C and 35 °C. As the temperature increased, the adsorbed moisture content decreased at water activities between 0.2 and 0.9. The experimental sorption data obtained were applied to the Halsey, Harkins‐Jura, Smith, BET and Freundlich isotherm equations to test fitness of these equations to Tarhana. The order of best fit or sorption data obtained for Tarhana was Halsey > Harkins‐Jura > Smith > Henderson > BET > Freundlich. The obtained sorption data fitted the Halsey, Harkins‐Jura and Smith isotherm equations quite well in the 0.2–0.9 water activity range; However, the best fit was obtained in the Freundlich, BET and Henderson isotherm equations in this water activity range. © 2000 Society of Chemical Industry