罗非鱼肉干燥过程平衡含水率模型研究

采用静态法测定罗非鱼肉在25、35、45、55℃温度和10%～85%相对湿度范围内的吸湿和解吸平衡含水率,分析温度和相对湿度对罗非鱼平衡含水率的影响。以相关系数和标准误差作为评价指标, 对Henderson模型、Chung-Pfost模型、Halsey模型和Oswin修正模型4个数学模型和平衡含水率曲线进行拟合,比较上述平衡含水率模型对实验数据的拟合精度,发现Oswin修正模型较适合于描述罗非鱼的平衡含水率曲线,并分别给出罗非鱼解吸和吸湿平衡含水率的Oswin表达式。     

Moisture sorption isotherms of green chilli

The adsorption and desorption equilibrium moisture contents of green chilli were determined experimentally in relative humidity range of 11‐97% and at the temperatures of 20, 30, 40 and 50°C. The experimental procedure used was a dynamic method with periodically recording of sample mass. The effect of temperature on adsorption and desorption isotherms was found significant. Hystereses were observed for entire range of relative humidity and hysteresis loops decreased with the increase of temperature. Nine sorption isotherm models were fitted the experimental data at the temperatures of 20, 30, 40 and 50 °C. The modified Smith equation was the best fitted equation to the experimental data for relative humidity range of 11‐97% for the adsorption and desorption isotherms of green chilli.     

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.     

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).     

Moisture sorption isotherms of sorghum malt at 40 and 50 °C

The desorption and adsorption equilibrium moisture isotherms of sorghum malt at the temperatures of 40 and 50 °C, over the water activity range of 0.1-0.9, were determined using the static gravimetric method. A non-linear regression programme was used to fit five moisture sorption isotherm models [Modified Henderson, Modified Chung-Pfost, Modified Guggenheim-Anderson-de Boer (GAB), Modified Halsey and Modified Oswin] to the experimental data. The models were compared using the standard error of estimate, mean relative percentage deviation, fraction explained variation and residual plots.The Modified Chung-Pfost model was found to be the best for predicting the desorption equilibrium moisture content, while the adsorption equilibrium moisture content was best predicted by the Modified Oswin model. The desorption and adsorption water activities were found to be best fitted by the Modified Oswin model.The moisture sorption isotherms were sigmoidal in shape and showed a marked effect of temperature. The span of the moisture sorption hysteresis loop formed, decreased with increase in temperature, while the size increased with increase in temperature.     

Adsorption isotherms for red chilli (Capsicum annum L.)

The adsorption equilibrium moisture contents for red chilli were determined experimentally in relative humidity range of 11–97% at the temperatures of 20, 30, 40, and 50°C. Six equilibrium moisture content models were fitted to the experimental data. The modified Oswin model was the best fitted equation for relative humidity range of 11–97% for the adsorption data of red chilli.     

A mean moisture content-equilibrium relative humidity relationship for nine varieties of wheat

A mean moisture content-equilibrium relative humidity relationship for nine varieties of wheat at 25°C is presented, for which the curves were fitted by eye. The mean safe storage moisture content in equilibrium with 70% relative humidity (r.h.) for all the wheat varieties adsorbing moisture was 14.7% (wet weight basis) but if varietal differences and errors in measurement are taken into account, the moisture content of stored wheat may need to be as low as 13% to be safe from attack by mites. The hysteresis between adsorption and desorption reached a maximum of 0.7% moisture content at 40–50% r.h. Hysteresis was found to be statistically significant up to 75% r.h. Equilibrium moisture contents were calculated from data transformed using the Chung and Pfost equation and compared with the measured moisture contents.     

Moisture sorption characteristics of rice flour

Moisture equilibrium data (adsorption and desorption) of rice flour were determined using the static gravimetric method of saturated salt solutions at three storage temperatures: 10, 20 and 30 degrees C. The range of water activities for each temperature was from 0.11 to 0.85. Equilibrium moisture content decreased with increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Oswin, modified Halsey, modified Chung-Pfost and modified Henderson). The monolayer moisture content was estimated from sorption data using the Brunauer-Emmett-Teller (BET) equation. The isosteric heats of sorption were evaluated using Clausius-Clapeyron equation.     

The influence of the method used for moisture adjustment on the equilibrium relative humidity of stored products

This paper examines some possible causes of differences in the magnitude of the hysteresis effect reported by different workers when studying the moisture content/equilibrium relative humidity relationship of stored products.Three techniques of changing the moisture content during sample preparation were used on Cappelle wheat and the results compared. It was found that the position of the adsorption isotherm was not affected by these techniques, but the position of the desorption isotherm was. At 35% r.h. the moisture content was 0·8 per cent higher on the desorption isotherm by one technique than by the other two. This is equivalent to a difference of 6% r.h.The effect of a fungistat, propylene oxide, on the equilibrium relative humidity, was also included. This caused a ‘reverse’ hysteresis, in that the moisture content at a given relative humidity was lower on the desorption isotherm than on the adsorption isotherm instead of higher, as with previous techniques.     

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.     

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.     

Moisture sorption and the applicability of the Brunauer-Emmet-Teller (BET) equation for blanched and unblanched mushrooms

Moisture sorption isotherms of blanched and unblanched mushrooms over 0.11–0.75 a_w were determined at 27°C and 37°C by using the static gravimetric method. Adsorption and desorption behaviors of blanched and unblanched mushrooms were compared. The unblanched material adsorbed more water than that of the blanched. In desorption isotherms, the equilibrium moisture contents of the unblanched material were found to be higher than those of the blanched throughout the entire a_w range. The BET equation was tested to fit the experimental moisture sorption data over the 0.11–0.43 a_w, 0.11–0.55 a_w, 0.11–0.64 a_w, and 0.11–0.75 a_w. Nonlinear regression analysis was used for the determination of the parameters in the equation. The quality of the fit of the BET model over each a_w range was judged from the value of the relative percent root mean square (% RMS). The moisture sorption behavior over 0.11–0.43 a_w of mushrooms has indicated that the BET equation is applicable generally up to 0.43 a_w. Monolayer moisture contents and C constants in the BET equation obtained for each a_w interval were reported. The water activities corresponding to the monolayer values have been determined and discussed related to mushroom storage. The net isosteric heats of adsorption and desorption were estimated from equilibrium sorption data, using the integrated form of the Clausius-Clapeyron equation. The heats of adsorption/desorption decreased with increase in moisture content and approached to a constant value. It was concluded that moisture adsorption/desorption occurred by physical mechanisms at high moisture contents, but at low moisture contents, besides physical adsorption, chemisorption was also observed.     

MOISTURE SORPTION ISOTHERMS FOR KARINGDA (CITRULLUS LANATUS (THUMB) MANSF) SEED, KERNEL AND HULL

Adsorption-desorption behaviors of karingda (Citrullus lanatus (Thumb) Mansf) seed, kernel and hull for nine equilibrium relative humidities (ERH) ranging between 11 and 96% at temperatures of 10, 20, 30, 40 and 50C were studied following a static equilibriation technique using saturated solutions of various salts. Under both the adsorption and desorption processes, the equilibrium moisture contents (EMC) of the hull were found to be highest followed by those of the seed and the kernel at all the corresponding temperatures and ERH values. Analysis of these data using four sorption models (modified Henderson, modified Halsey, modified Chung-Pfost and Guggenheim-Anderson-de Boer) and taking the temperature dependence of the respective coefficient into consideration, it revealed that both the Chung-Pfost and the GAB models were acceptable in describing EMC-ERH relationships for karingda seed, kernel and hull over the entire range of temperatures. The excess heat of sorption of all the samples, estimated from the Clausius-Clapeyron equation, decreased exponentially with the increase in moisture content of the same.     

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.     

贮藏温湿度对糙米平衡含水率的影响

为解决糙米贮藏过程中含水率随贮藏温湿度变化的问题,应用静态称重法对糙米籽粒进行了不同温度和相对湿度条件下的吸附与解吸平衡含水率试验,分析了温度、相对湿度对糙米平衡含水率的影响规律.利用SAS软件处理试验结果,拟合了Henderson等5种经典模型的参数并评价了拟合效果,确定了最佳拟合模型及其优化参数.结果表明,修正GAB模型及其优化参数组合最适合描述糙米籽粒的吸附和解吸平衡含水率,其相关系数分别为0.998 3和0.9977.     

Mathematical modelling of moisture sorption isotherms and determination of isosteric heat of blueberry variety O'Neil

The sorption isotherms of blueberry variety O'Neil were determined at 20, 40 and 60 °C, for a range of water activity of 0.10–0.95. The isotherms showed that the equilibrium moisture content increased when temperature decreased at constant water activity. The BET, GAB, Halsey, Henderson, Caurie, Smith, Oswin and Iglesias-Chirife equations were tested for modelling the sorption isotherms. The results showed that GAB, BET and Halsey models gave the best fit quality for the experimental desorption data, and BET, Oswin and Henderson for adsorption data as suggested by the statistical tests employed. The net sorption heat was calculated using the Clausius–Clapeyron equation giving 38.62 kJ mol⁻¹ (desorption) and 30.88 kJ mol⁻¹ (adsorption) at a moisture content of 0.01 g water (g d.m.⁻¹). Tsami equation was applied to estimate the net isosteric heat of sorption as function of equilibrium moisture content with satisfactory results.     

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.     

Effect of sodium chloride, sodium nitrite and temperature on desorption isotherms of previously frozen beef

Moisture desorption isotherms of beef were determined in the relative humidity range of 23 to 90% at 5, 15 and 25 °C and at 2.5% NaCl and 2.5% NaCl + 150 ppm NaNO₂ content. Desorption isotherms were found to be typical type II sigmoid. The water content at equilibrium was higher in beef with NaCl and NaCl + NaNO₂ than control samples. Experimental data were fitted to various mathematical models and it was found that the Peleg model was best in describing the equilibrium moisture content relationship for beef samples over the entire range of temperatures. The net isosteric heat of sorption was estimated from equilibrium desorption data, using the Clausius-Clapeyron equation. Isosteric heats of desorption were found to increase with decreasing moisture content.