Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Influence of ions on the critical behavior of a binary mixture near the consolute point

The lower critical point of stratification of a 3-methylpyridine (MP)+heavy water (D₂O) mixture in the presence of Na⁺ and CI^– ions has been studied by the Toepler shadow method. Addition of 0.3% ions lowered the critical temperature and reduced the equilibration time and the gradient of the refractive index (compressibillity). The analysis of the form of the near-critical isotherm demonstrated the ionic mixture to be described by the index = 3.05 ± 0.15. which corresponds to the classical mean-field theory. The results obtained provide evidence that even small admixtures of charged particles result in a substantial suppression of fluctuations near the critical point by the long-range Coulomb interaction.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

液氮温度下分子筛的真空吸附特性试验研究

为探究低温容器夹层所用分子筛吸附剂的吸附特性,采用静态膨胀法进行试验,获得了平衡压力为10-3Pa～103Pa范围内4A、5A和13X分子筛对N2、O2单一组分以及空气的吸附等温线,比较了不同分子筛对气体的吸附能力差异,探究了分子筛的吸附机理.研究结果表明:液氮温度下,5A和13X分子筛在真空条件下对N2及O2的吸附性...     

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.     

Heat and mass transfer during the coffee drying process

For a better comprehension of heat and mass transfer during the coffee drying process and optimization of the industrial application transport coefficients and coffee properties were determined. Heat transfer coefficients were measured for different air velocities and were found to follow the known dimensionless equations for the flow surrounding a sphere. Thermal conductivities and effective diffusion coefficients were measured as a function of moisture content as well as volume and densities of the coffee beans. The mentioned properties depend directly on the humidity of the coffee beans rather than on the drying conditions. Sorption behaviour was investigated and temperature dependent parameters for the Guggenheimer–Anderson–deBoer-isotherm (GAB) were determined according to the Arrhenius relationship.     

Moisture sorption isotherm of fresh lean beef and external beef fat

The moisture sorption isotherm (MSI) of lean beef and fat beef was experimentally determined. The experimental procedure used was that of the COST 90 project with some modifications to accelerate equilibration. The procedure was validated with the standard reference material microcrystalline cellulose. The MSI of the beef at the highest humidity range was obtained by accelerating equilibration with changes of salts, using a low water activity salt for some time. This procedure was reliable for beef samples but not for the fat samples. No significant changes were found for lean beef in the temperature range 5–40 °C. Three models, GAB, Peleg and Lewicki, were used to fit the experimental data. The best fit was obtained with the GAB equation. The fat MSI was determined at 5, 15 and 25 °C and it was best fitted with the Lewicki model.     

Model analysis of sorption isotherms

The equation developed by Blahovec and Yanniotis, which is based on surface adsorption and solution water, was applied for fitting experimental sorption data for starchy and high protein foods, fruits and vegetables, nuts, legumes, and seeds. Analysis of sorption isotherms shows that surface adsorption is more important than solution water in the isotherms for cereals, potatoes, legumes and seeds, while in vegetables, fruits, meat, milk products and some nuts solution water is more important. The ratio of solution water to surface adsorption increases as water activity increases, it is higher for the adsorption isotherm than the desorption isotherm at any water activity value and decreases as temperature increases.     

Water Adsorption Isotherms and Storage Stability of Freeze-Dried Bovine Colostrum Powder

Moisture adsorption isotherms of freeze‐dried bovine colostrum powder were obtained using the gravimetric method at three temperatures (15, 25, and 35°C) in the water activity range of 0.11–0.86. The moisture adsorption isotherms of freeze‐dried bovine colostrum powder showed a sigmoid isotherm curve typical of the type II BET classification shape. The Guggenheim-Anderson-de Boer and Brunauer-Emmett-Teller models were recommended to fit experimental data well. The shelf life of freeze‐dried bovine colostrum powder packaged in aluminium-laminated polyethylene pouches was evaluated for 243 days, and the storage stability was investigated when stored at 25°C and 50% relative humidity. A slight increase in moisture content, a linear increase in color difference, and a slight decrease in IgG concentration were found within 60 days storage under the above-mentioned conditions.     

D301阴离子交换树脂对大豆异黄酮分离特性的影响

为更好地将离子交换树脂应用于大豆异黄酮的分离纯化,研究了乙醇含量、pH、温度、初始浓度等条件对D301阴离子交换树脂吸附分离大豆异黄酮的影响。静态吸附实验结果表明,该树脂在以水为溶剂、pH 8左右、40℃时对大豆异黄酮有最好的吸附效果。穿透曲线实验表明,动态饱和吸附量受温度影响较大,当柱温为50℃、初始浓度C07.03 mg/mL时,饱和吸附量为105.0 mg/g干树脂。洗脱曲线实验表明,采用体积分数75%乙醇洗脱,异黄酮回收率为89.3%,产品中大豆异黄酮含量高达56.0%,含量比原料提高了十几倍。表明D301阴离子交换树脂对大豆异黄酮的分离纯化具有较好的选择性和应用前景。