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%。     

谷物平衡水分研究概况

控制吸湿是谷物安全贮藏的基本原则之一,平衡水分测定可用来判断谷物水分变化的趋向.概述了谷物平衡水分测定的原理及数学模型分析,并归纳,发生纳滞后效应的可能理论、影响谷物吸着(包括吸附与解吸)等温线测定的因子,还重点介绍了近年发展起来的、利用平衡水分吸附/解吸数据分析谷物及食品的热动力学函数--水分吸着等热.这些研究进展对完善我国储粮生态区域划分、谷物主产区低温干燥、设计干燥程序和设备、谷物及加工产品的包装及货架期皆有意义.     

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%。     

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％.     

大米安全水分和吸着等热计算

对静态称重法测定的5个大米样品吸着等温线数据,分别采用Brunauer-Emmett-Teller(BET)、CAE、修正Chung-Pfost(MCPE)、修正Guggenhein-Anderson-de Boer(MGAB)、修正Henderson、修正Oswin及Strohman-Yoerger 7个方程进行拟合,MCPE被判定为大米最佳吸着等温线方程。以M=f(hr,t)形式表达的平均解吸等温线方程MCPE的3个参数C1、C2及C3各是492.539、39.846及0.176,在RH 70%下,对应的101、5、202、5、303、5℃条件下大米样品平均解吸值,分别是15.88%、15.49%、15.13%、14.80%、14.50%及14.21%。3个粳米(方正香米、东北普通米、松花江米)在25℃的安全水分是14.92%～15.39%,低于15.5%;2个籼米(泰国香米和江苏米)安全水分则是13.88%～14.43%,低于14.5%,与GB 1354—2009大米限量水分一致。大米吸着等热在含水率<20%干基条件下,随着米粒含水率增加而快速减少,在高于20%干基含水率则随含水率增加而缓慢减少。在含水率<22.5%干基条件下,较低温度下的大米吸附等热与解吸等热均高于较高温度;大米解吸等热高于吸附等热,但是随着含水率增加则差异减少。     

谷物安全水分估算

采用常用的水分吸着等温线模型对测定的14个小麦品种、15个稻谷品种、10个玉米品种及5个大米品种的平衡水分含量（EMC）/平衡相对湿度（ERH）数据进行拟合,Modified-Chung-Pfost（MCPE）被判定为最佳解吸等温线模型。以M=-1/C3×ln[-（t＋C2）C×1ln（r.h.）/C1]形式表达的平均解吸等温线MCPE方程参数C1、C2及C3对小麦分别是635.479、57.093及0.221,对稻谷分别是502.485、45.276及0.186,对玉米分别是389.939、18.792及0.231,对大米分别是492.539、39.846及0.176。根据RH 70%是霉菌生长的界限,推导10、15、20、25、30、35℃条件下各品种的平均解吸值,对小麦分别是14.84%、14.51%、14.21%、13.93%、13.66%及13.41%;对稻谷分别是14.83%、14.49%、14.17%、13.88%、13.61%及13.35%;对玉米分别是15.74%、15.05%、14.45%、13.93%、13.46%及13.04%;对大米分别是15.88%、15.49%、15.13%、14.80%、14.50%及14.21%。这些数据对谷物安全水分标准制定有参考意义。     

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。     

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%。     

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。     

方便米粉的水分吸附和热力学特性

为了给方便米粉的加工和贮藏过程提供理论指导,根据吸附原理,在环境温度分别为15、25 ℃和35 ℃时,采用静态称量法研究方便米粉的吸附等温线。采用7 个常见的非线性回归方程对吸附实验进行拟合,以决定系数、平均相对偏差和标准估计误差为评价指标,确定最佳拟合模型及其参数,探讨方便米粉水分吸附过程中净等量吸附热、微分吸附熵和焓熵互补等热力学性质的变化。结果表明,方便米粉的水分吸附特性属于II型等温线,Peleg和GAB模型都适合描述方便米粉的水分吸附特性。用GAB模型拟合得到的单分子层水分含量X0在15、25 ℃和35 ℃下分别为9.23%、8.34%和7.65%（干基）。在水分吸附过程中,方便米粉的净等量吸附热和微分吸附熵都会随着平衡水分含量的升高而明显下降;同时,存在焓熵补偿现象;根据实验结果绘制净等量吸附热与微分吸附熵的关系图,计算获得方便米粉的吸附过程属于焓驱动和自发过程。本研究对方便米粉贮藏条件选择和进一步评估不同贮藏条件下方便米粉的贮藏期具有指导作用。     

The temperature dependence of isosteric heat of sorption of some cereal grains

An equation predicting the temperature dependence of the isosteric heat of sorption was calculated using the Othmer method. This equation was applied to water vapour pressure isotherms of cereal grains (sorghum, wheat and rough rice) at different temperatures to calculate the isosteric heat and its dependence on moisture content and temperature. The results show a slight decrease of the isosteric heat with increasing temperature. For rough rice grain, the desorption heats calculated by the proposed method at two temperatures were compared with those obtained by the Clausius-Clapeyron method.     

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.     

Sorption equilibrium moisture and isosteric heat of adsorption of Chinese dried wheat noodles

Equilibrium moisture content (EMC) data for dried wheat noodles of ten Chinese varieties were collected by a gravimetric method at 11–96% equilibrium relative humidity (ERH) and 15 °C, 20 °C, 25 °C, 30 °C, and 35 °C. Five models were fitted to the sorption data, namely the modified Chung Pfost equation (MCPE), modified Henderson equation (MHE), modified Guggenheim Anderson deBoer equation (MGAB), modified Oswin equation (MOE), and a polynomial equation. The best fitting equations were MGAB and the polynomial equation. At a constant ERH, the EMC decreased with increasing temperature, despite the minor effect of temperature on the sorption isotherms of dried noodles. Initially, the isosteric heats of adsorption for dried wheat noodles decrease rapidly with increasing sample moisture content (m.c.); however, after the moisture content is more than 15% of the dry basis (d.b.), they decrease slowly with increasing m.c. The heat of vaporization of Chinese dried wheat noodles approaches the latent heat of pure water at a moisture content of ∼20% d.b., which is ∼2500 kJ/kg. The isosteric heats of sorption of Chinese dried noodles predicted by MCPE and MHE models at lower temperatures were higher than those at higher temperatures. When the equilibrium relative humidity (ERH) is 60%, the safe-storage moisture content of Chinese dried wheat noodles are 11.74% and 11.57% d.b. at 25 °C and 35 °C, respectively. Among ten varieties of dried wheat noodles, the egg-flavoured noodle had the highest onset temperature (T_o), peak temperature (T_p), and conclusion temperature (T_c) of gelatinization, but the golden-silk egg noodles had the highest peak enthalpy of gelatinization. The gelatinization T_o, T_p, and T_c of golden-silk egg noodles were the lowest. Most of the ten varieties of dried wheat noodles demonstrated similar thermal properties and hygroscopic behaviour.     

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.     

Moisture sorption isotherms and thermodynamic properties of walnut kernels

The moisture sorption isotherm data of walnut kernels stored in a chamber, the relative humidity (r.h.) of which is regulated by atomizing humidifier, were determined at three different temperatures (25, 35 and 45 °C) and r.h. ranging from 10% to 90%. Eight models, namely the GAB, BET, Henderson, Iglesias and Chirife, Oswin, Peleg, Smith and Caurie equations, were fitted to the sorption data. Several statistical tests were adopted as the criteria to evaluate the fitting performance of the models. Of the models tested, the Peleg model gave the best fit to experimental data. The surface area of a monolayer was calculated. The BET equation was applied to the monolayer moisture content and the corresponding a_w values at which a monolayer forms are presented. The experimental data were also used to determine the thermodynamic functions such as isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The sorption isosteric heats for walnut kernels were determined by the application of the Clausius-Clapeyron equation to sorption isotherms obtained from the best-fitting equation. Isosteric heats decreased with increase in moisture content and approached the latent heat of pure water. Adsorption entropy increased with increasing moisture content, and then it decreased sharply with increase in moisture content. The spreading pressures (adsorption and desorption) increased with increasing water activity. Net integral enthalpy of adsorption increased slightly with moisture content to a maximum value. Thereafter, it remained constant. Net integral entropy of adsorption was negative in value and it decreased with increase in moisture content to a minimum value, and then increased slightly with increase in moisture content.     

The fitting of various models to water sorption isotherms of tea stored in a chamber under controlled temperature and humidity

The moisture sorption characteristics of tea stored in a chamber regulated by an atomizing humidification system were investigated at 25, 35 and 45 °C for water activity ranging from 0.1 to 0.9. The sorption isotherms of tea were typical type II sigmoidal curves according to BET classification. In both adsorption and desorption, an increase in temperature resulted in lower equilibrium moisture contents at corresponding values of water activity. The sorption isotherms exhibited hysteresis over the whole water activity range. GAB, BET, Henderson, Iglesias and Chirife, Oswin, Peleg, Smith and Caurie models were applied for analysing the experimental data. Nonlinear regression analysis was used for the determination of the parameters in the equations. Estimated parameters and fitting ability for sorption models were evaluated. The Peleg model was found to be the most suitable for describing the relationship between equilibrium moisture content and water activity for the whole range of temperatures and relative humidities studied. The surface area of monolayer was calculated. The BET equation was solved for the monolayer moisture content and the corresponding a_w values at which monolayer forms were presented. Sorption isotherm data were used to determine the thermodynamic functions such as isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The Clausius-Clapeyron equation was used to evaluate the isosteric heats of sorption. The isosteric heats of sorption and sorption entropy decreased with increasing moisture content. The heat of desorption was little higher than that of adsorption at low moisture content. The enthalpy-entropy compensation theory could be successfully applied to water sorption by tea. This theory showed that the moisture sorption of tea was governed by enthalpy-controlled mechanisms. The spreading pressure increased with increase in water activity and decreased with increasing temperature. The net integral enthalpy decreased with moisture content while the net integral entropy increased.     

粮堆平衡绝对湿度和露点温度的查定方法

采用我国储粮通风方程CAE拟合测定的小麦、稻谷及玉米平衡水分数据,并绘制这些粮食种类平衡绝对湿度曲线图和平衡相对湿度曲线图,阐明采用平衡绝对湿度图进行通风条件判断较为直观、准确。将参数已知的小麦、稻谷及玉米CAE方程编写成为软件,输入粮食含水率和温度,则可以快速查定粮堆的平衡绝对湿度和露点温度,用作机械通风条件判断。     

Equilibrium moisture content and heat of desorption of saffron

The equilibrium moisture contents of saffron (Crocus sativus L.) stigmas were determined experimentally using the standard gravimetric method at temperatures 30, 45 and 60 °C and water activity ranging from 11% to 83%. The sorption isotherm curves of saffron were sigmoidal in shape and decreased with increased temperature at constant relative humidity. Five selected isotherm models GAB, modified Henderson, modified Chung‐Pfost, modified Halsaey and modified Oswin were tested to fit the experimental isotherm data. Modified Oswin and modified Henderson models were found acceptable for predicting desorption moisture isotherms and fitting to the experimental data, respectively. The isosteric heats of desorption, determined from equilibrium data using the Clausius‐Clapeyron equation, were found to be a function of moisture content. The net isosteric heat of desorption of saffron varied between 1.38 and 5.38 kJ mol^?1 at moisture content varying between 2% and 20% (d.b).