谷物湿热平衡新模型及热力学特性的研究

建立谷物平衡水分与相对湿度在不同温度下的关系模型,并利用小麦、玉米、水稻、大米的静态平衡试验数据进行拟合。结果表明,该方程适合描述小麦、玉米、水稻、大米的等温线,可直接求解出谷物的平衡水分。另外,根据所建立的模型,预测了不同温度条件下的安全储藏水分,并对谷物的热力学性能进行了分析:吸着等热值随着平衡水分值的减小而增加,解吸等热值增加更为显著;解吸和吸附过程中吸着等热值均高于水汽化潜热;平衡水分大于25%后,吸着等热值趋于稳定,与汽化潜热值接近;根据分析所得数据,拟合了小麦、稻谷、玉米解吸与吸附过程的吸着等热曲线回归方程,为以能量化的观念实施粮仓作业管理,实现高效、合理、节能储粮提供了有利依据。     

豆粕平衡水分及吸着等热研究

采用静态称重法测定了国产6个豆粕样品的平衡水分吸附/解吸等温线,并采用修正MCPE、MHAE、MHE、MGAB、MOE及STYE 6个方程进行拟合,得出MGAB、MHAE、MOE均适合描述豆粕平衡水分-平衡相对湿度之间的关系,并可用于分析豆粕水分吸附/解吸等热。当水分小于15%时,豆粕吸附/解吸等热均随水分增大而快速减小,同温度下的解吸吸着等热显著高于吸附吸着等热。当水分大于15%时,豆粕吸附/解吸等热均随水分增大而变化平缓,同温度下的解吸吸着等热趋同于吸附吸着等热。当水分小于15%时,较低温度下的豆粕吸附吸着等热与解吸吸着等热均高于较高温度下的。在水分17.5%的游离水临界点,豆粕的吸着等热(汽化热)接近纯水的潜热,约为2 500 kJ/kg。由等温线分析的25℃豆粕绝对安全水分为12.48%,相对安全水分为13.90%。     

油菜籽平衡水分及吸着等热研究

采用静态称重法测定了我国8个油菜籽品种的平衡水分,指出修正3参数Guggenheim-Anderson-de Boer方程(MGAB)、修正Halsey(MHAE)、修正Oswin(MOE)等方程均适合描述油菜籽平衡含水量(EMC)—平衡相对湿度(ERH)之间的关系,并计算了油菜籽水分吸着(吸附／解吸)等热和安全水分值.在含水率＜12.5％湿基,油菜籽吸着等热均随含水率增加而快速减少,同一温度下的解吸等热显著高于吸附等热.在含水率12.5％以上,油菜籽吸着等热随含水率增加而变化平缓,同一温度下的解吸等热趋同于吸附等热.在含水率＜12.5％条件下,较低温度下的油菜籽吸附等热与解吸等热均高于较高温度.在含水率12.5％的自由水点,油菜籽的吸着等热接近纯水的潜在热.计算的20℃油菜籽储存绝对安全水分是8.09％,相对安全水分是8.98％.     

芝麻平衡水分及吸着等热研究

采用静态称重法测定了我国4个芝麻品种的平衡水分等温线,并采用CAE、修正Chung-Pfost(MCPE)、修正Halsey(MHAE)、修正Henderson(MHE)、修正Guggenheim-Anderson-deBoer(MGAB)、修正Oswin(MOE)及StrohmanYoerger(STYE)7个水分吸着方程进行拟合,指出MOE最适合描述芝麻平衡含水率(EMC)-平衡相对湿度(ERH)之间的关系,并用于计算芝麻吸着等热。在含水率7.5%湿基,芝麻吸着等热均随含水率增大而快速减少,同一温度下的解吸等热显著高于吸附等热。在含水率7.5%以上,芝麻吸着等热随含水率增大而变化平缓,同一温度下的解吸等热趋同于吸附等热。在含水率7.5%湿基条件下,较低温度下的芝麻吸附等热与解吸等热均高于较高温度。在含水率10%湿基的自由水点,芝麻的吸着等热(汽化热)接近纯水的潜热,约是2450kJ/kg。在测定温度10~35℃范围,黑芝麻吸着等热数值类似白芝麻吸着等热数值。计算的25℃芝麻储运绝对安全水分是6.46%,相对安全水分是6.94%。     

花生平衡水分及吸着等热研究

采用静态称重法测定了我国2个花生品种仁和荚果的平衡水分,指出修正Halsey方程(MHAE)最适合描述花生EMC-ERH之间的关系,并用于计算花生仁和荚果吸着等热。在含水率<12.5%湿基,花生仁和荚果吸着等热均随含水率增加而快速减少,同一温度下的解吸等热显著高于吸附等热。在含水率12.5%以上,花生仁和果吸着等热随含水率增加而变化平缓,同一温度下的解吸等热趋同于吸附等热。在含水率<12.5%湿基条件下,较低温度下的花生仁和荚果吸附等热与解吸等热均高于较高温度。在含水率12.5%湿基的自由水点,花生仁和荚果的吸着等热接近纯水的潜在热。计算的20℃绝对安全储存水分对花生仁是8.15%,对花生荚果是9.22%。     

采用一个多项式方程拟合粮食水分吸着等温线

假定粮食平衡水分(EMC)是相对湿度(RH)的多项式函数,也是温度(t)的线性函数,提出了一个方程EMC=C1RH3+C2RH2+C3RH+C4RH2t+C5RHt+C6t+C7,C1~C7是方程参数。此方程对我国主要粮种水分吸着等温线的拟合度指标决定系数R20.994,平均相对误差MRE%3.46,这些指标优于修正Chung-Pfost(MCPE)、修正Halsey(MHAE)、修正Henderson(MHE)、修正Guggenheim-Anderson-de Boer(MGAB)及修正Oswin(MOE)的拟合度指标。分析该方程拟合出的粮食吸着等温线,谷物解吸与吸附之间存在明显的滞后现象,而大豆滞后现象不明显。由拟合的解吸等温线分析30℃相对安全水分值,谷物低于14%,大豆为11.47%。     

我国小麦水分吸着等温线4-参数GAB方程研究

在水分活度(aw)>0.85条件下,小麦样品易发生霉菌生长而影响平衡水分测定的准确性,本研究采用Blahovec和Yanniotis 2009年发表的适合aw在0～1的修正4-参数GAB方程(MGAB),对测定的我国13个小麦品种的水分吸着等温线数据进行拟合和分类。结果表明,MGAB方程拟合的决定系数(R2)>0.97,平均相对百分率误差(MRE)<8.34%,方程的每个参数在红麦与白麦之间、硬麦与软麦之间、春麦与冬麦之间,差异不显著。但是,同一小麦品种吸附等温线的方程参数,不同于解吸等温线的对应方程参数。另外,根据D10、Rfi、awn指标判断小麦水分吸着等温线类型,13个小麦品种的吸附和解吸等温线均属于接近Langmuir类型的S型等温线。结论是小麦水分吸附与解吸的MGAB方程参数可用于小麦收获后干燥及储藏通风操作。     

糯米粉的水分等温解吸及孔特性

糯米粉的等温解吸特性一方面对分析其与周围环境之间的水分传递十分必要,另一方面,还可用于计算糯米粉内孔的特性,借此加深对水分吸附机理的了解。本实验采用静态称质量法在10、20、30℃条件下测定糯米粉在10个水分活度解吸后的平衡水分含量,然后采用4个等温吸附模型对实验结果进行拟合,并分析糯米粉的孔特性。研究结果表明:糯米粉中水分的解吸属于典型的Ⅱ型吸附;Lewicki模型最能描述同一温度条件下的等温解吸过程,而GDW模型能够同时描述温度和水分活度对平衡水分含量的影响。糯米粉中同时存在微孔和介孔;孔径分布为单态分布,仅在微孔区域出现一单峰,温度变化仅对该峰的峰值产生影响;微孔体积随温度降低而增大;糯米粉的吸附面具有分形特性,随温度降低吸附表面的多孔性增强,单位吸附面积增大,最终导致吸附能力增强。     

根据Vrentas-Vrentas理论模拟小麦粉的水分吸附滞后：模型参数χ及k表示为Fermi函数的形式

水分吸附及其滞后现象广泛存在并对食品加工及食品品质具有重要影响。采用动态水蒸气吸附分析仪测定了小麦粉在3个温度条件下的吸湿、解吸平衡水分,然后基于Vrentas-Vrentas(VV)理论进行模拟分析。结果显示,小麦粉的水分吸附滞后程度随温度升高而减小,吸湿与解吸曲线在高水分区域重合,据此提出一个确定玻璃化转变水分的方法。不论吸湿还是解吸,其Flory-Huggins相互作用系数χ皆随水分体积分数而变化,本实验提出采用Peleg改进的Fermi函数表示这种关系。在根据吸湿的χ参数模拟解吸时,VV模型中的参数k随体积分数变化可以表示为Fermi函数的形式。     

稻谷平衡水分的测定及EMC/ERH等温线方程的选择

采用静态称重法测定7个籼稻和1个粳稻品种的平衡水分,并利用常用的6个EMC/ERH方程(修正BET、修正Chung-Pfost、修正Guggenheim-Anderson-deBoer、修正Henderson、修正Oswin及Strohman-Yoerger)拟合吸附/解吸等温线。在ERH 11.3%~96%范围内,修正Chung-Pfost(MCPE)和Strohman-Yoerger(STYE)拟合最佳。采用含有3个系数、易于转化为M=f(r.h.,t)或r.h.=f(M,t)表达形式的MCPE方程,对8个稻谷品种的吸着等温线进行拟合。以M=f(r.h.,t)形式表达的MCPE方程系数C1、C2、C3,在拟合吸附等温线时分别是784.894、143.337、0.174,在拟合解吸等温线时分别是588.376、59.026、0.180,吸着平均值分别是638.444、87.074、0.177。稻谷吸附与解吸等温线之间存在滞后现象,随着温度增加,滞后环宽度减少。     

Moisture desorption isotherms of medium-grain rough rice

Moisture desorption isotherms (EMC/ERH) of medium-grain rough rice (Japonica variety) were determined using a constant environment chamber for various combinations of air temperature (11.8-51.0 degrees C) and relative humidity (37.1-89.7%). The initial moisture contents were in the range of 24.7-41.6% dry basis. A thin-layer technique was used to achieve uniform drying. Each test was continued until the moisture content change in 24h was less than 0.1% dry basis. The final moisture content was considered as the dynamic equilibrium moisture content. Four three-parameter EMC/ERH equations, the modified Henderson, modified Chung-Pfost, modified Oswin and modified Halsey equations, were compared for their ability to fit the experimental EMC/ERH data. The residual sum of squares (RSS) and standard error of estimate (SEE) were adopted as the criteria to evaluate the fitting performance of the models. The modified Chung-Pfost equation was identified as the most appropriate equation for representing the EMC/ERH desorption isotherms of rough rice. Coefficients for equilibrium moisture content as a function of equilibrium relative humidity and temperature are given. The EMC/ERH data obtained in this study agreed well with previously published data. However the average isotherm, combining desorption and adsorption data, of ASAE does not predict the desorption EMC of rough rice accurately.     

玉米吸湿特性及其等温线类型研究

对测定的我国16个玉米品种的水分吸着等温线数据,采用9个水分吸着方程进行拟合,并根据修正4-参数Guggenheim-Anderson-de Boer方程(4-MGAB)派生的指标划分等温线类型。结果表明,CAE、修正Chung-Pfost(MCPE)、Strohman-Yoerger(STYE)、Brunauer-Emmett-Teller(BET)、3-MGAB、4-MGAB、修正Henderson(MHE)及修正的Oswin(MOE)方程均适合拟合测定的玉米水分吸着等温线,而Modi-fied Halsey(MHAE)不适合。MCPE和CAE被认为是玉米较佳水分吸着方程,方程参数MCPE的C1和C2、CAE的b1在不同玉米品种之间、黄玉米和白玉米之间差异不明显。但是,同一玉米品种吸附等温线的方程参数不同于解吸等温线的对应方程参数。另外,根据D10、Rfi、awn、X4指标判断玉米水分吸着等温线类型,16个玉米品种的吸附和解吸等温线均属于S型等温线(Ⅱ)。结果表明玉米水分吸附与解吸的MCPE和CAE方程参数可用于玉米收获后干燥及储藏通风操作。     

Sorption isotherms,GAB parameters and isosteric heat of sorption

The diffusion–sorption drying model has been developed as a physics‐based way to model the decreasing drying rate at low moisture contents. This new model is founded on the existence of different classes of water: free and bound water. The transition between these classes and the corresponding thermodynamics form distinct components of the drying model. This paper shows that the characteristics of the different classes of water and of the transition between them can be deduced from the GAB sorption isotherm. The parameters in the GAB sorption isotherm support the theory of localised sorption, establishing the existence of different classes of water. Moreover, the sorption mechanism retrieved from the GAB parameters is in accordance with the sorption mechanism, which is obtained from the moisture dependence of the net isosteric heat of sorption. This holds for experimental sorption data of corn and starch as well as for literature data on five vegetables and four fortified cassava products. An extremum in the net isosteric heat of sorption coincides with the transition between bound and free water, and the partition moisture content corresponds with the monolayer value derived from the GAB equation. This confirms that the GAB monolayer value can be chosen as model boundary between bound and free water. Moreover, it reveals that this method can be developed into a technique to estimate the bound water content in foods. Copyright © 2005 Society of Chemical Industry     

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.     

Moisture adsorption isotherms of guar (Cyamposis tetragonoloba) grain and guar gum splits

Moisture adsorption isotherms of guar grain and guar gum splits were determined at 10, 20, 30 and 40 °C and 23-96% relative humidities using gravimetric method. The sorption data were fitted to six well-known sorption isotherm models (modified Chung-Pfost, modified Halsey, modified Henderson, modified Oswin, Chen-Clayton, and GAB models) using non-linear least square method. The GAB model was found the most satisfactory for representation of the equilibrium moisture content data for guar grain and guar gum splits. The equilibrium moisture content of guar gum splits was found to be significantly higher (p < 0.05) than that of guar grain. The isosteric heat of sorption was determined from the equilibrium moisture adsorption data using Clausius-Clapeyron type equation. Exponential relationship described well the dependence of isosteric heat of sorption on the equilibrium moisture content. The enthalpy-entropy compensation theory applied to sorption isotherms indicated enthalpy controlled sorption process.     

Effect of drying temperature and beeswax content on moisture isotherms of whey protein emulsion film

The objective of this investigation was to study the effect of drying temperature and beeswax (BW) content on moisture sorption behavior of whey proteins emulsion films. For this purpose, films were obtained by the casting method and dried at two selected temperatures (5 and 25 °C) and constant relative humidity (RH) (58%). After drying, films were removed from the casting plates and were conditioned in the environmental chamber set at 25 °C and 58% RH for 3 days. Subsequently, portions of 400 mg of film were placed in glass bottles and pre-dried in desiccators containing drierite (a_w=0) during 10 days. Then, the bottles were placed in hermetically sealed glass jars containing 10 different desiccants to achieve a_w ranging from 0.11 to 0.90, and allowed to reach equilibrium. The analyses were made in quintuplicate at 25 °C. The equilibrium moisture content (EMC) was determined by drying samples in an oven and the experimental data were fitted by the Brunauer-Emmett-Teller (BET) and the Guggenheim-Anderson-De Boer (GAB) models. The results showed that both models were effective to describe the moisture sorption behavior of the films. The GAB model gave better fit than the BET model. The increase of the drying temperature of 5 to 25 °C and the incorporation of lipids reduced the EMC of whey protein emulsion films. Finally, data from experimental sorption isotherms are a useful tool to predict the effect of the environmental conditions that surround the film on its properties; particularly considering that the stability of an edible film is function of its mechanical and moisture barrier properties and both are strongly influenced by the presence of water, film formulation and drying and storage conditions.     

Water sorption isotherms for lemon peel at different temperatures and isosteric heats

Lemon peel constitutes a potential source of dietary fiber to formulate new and healthier products, as well as a source of essential oils. The relationship between moisture content and water activity provides useful information for lemon peel processing, especially for drying and storage. Water sorption isotherms of lemon peel were obtained using a standardized conductivity hygrometer at four different temperatures (20, 30, 40 and 50 °C) and wide ranges of moisture content (5.381-0.002 kg water/kg dry solid) and water activity (0.984-0.106). One theoretical (GAB) and four empirical equations (Oswin, Henderson, Halsey and Ratti) were used for modelling sorption isotherms. After evaluating the models according to several criteria, the GAB model appeared as the best option. Isosteric heats of sorption were assessed from experimental sorption isotherm data using different methods.     

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.     

Molecular thermodynamics modeling of equilibrium moisture in foods

Moisture content in foods affects many aspects of processing and storage, as well as quality and pathological activity. Understanding its behavior is very important in the food industry for designing effective long-term food management strategies, in particular transportation and storage. Most approaches found in the literature dealing with moisture in foods consist of measuring it as a function of relative humidity and temperature and then fitting the data to empirical or semi-empirical models for interpolation or correlation purposes. Most semi-empirical models are modifications of early gas adsorption models on hard surfaces, which fundamentally are not well suited to describe water in softer materials such as foods. In this work, we propose and use a more general framework based on equilibrium molecular thermodynamics to model and correlate equilibrium moisture content (EMC) in foods that takes into account other non-adsorption phenomena such as mixing and swelling of the biopolymers in foods. The framework is general, and the current model shows very good results correlating and modeling EMC. The model parameters provide insight into the nature of the water activity in food products, providing potentially useful information for food processing and modeling.