Water Activity Range Research Articles

Moisture adsorption properties and adsorption isosteric heat of dehydrated slices of Pastirma (Turkish dry meat product)

Moisture sorption characteristics of dehydrated pastirma were investigated at 15 °C, 20 °C and 30 °C over a water activity ( a w) range of 0.2–0.9. Sigmoidal (type-II) adsorption isotherms were observed for pastirma. The experimental sorption data obtained were applied to the Halsey, Harkins-Jura, Smith, BET, Henderson, Freundlich and GAB isotherm equations to test fitness of these equations to pastirma. The order of the best fit of sorption data obtained for pastirma at 15 °C, 20 °C and 30 °C in all the range of a w studied (0.2–0.9) was Harkins-Jura > Halsey > BET > Smith > Freundlich > GAB > Henderson, respectively. In the range a w 0.2–0.55, BET model has a better fit than in the range a w 0.2–0.9. Isosteric heats of adsorption were evaluated by applying the Clausius–Clapeyron equation to experimental isotherms and decreased with increasing moisture content.

Effect of surfactants on water sorption and barrier properties of hydroxypropyl methylcellulose films

Moisture sorption isotherms and water barrier properties of films made from hydroxypropyl methylcellulose (HPMC) containing surfactant mixtures of sorbitan monostearate (SPAN 60) and sucrose palmitate (sucrose ester P-1570) were evaluated at 10 °C. The effect of hydrocolloid/surfactant ratio ( H/ S) (0.5, 1.0 and 1.5) and the hydrophilic/lipophilic balance of the mixture (HLB: 4.7, 6.0 and 8.0) was analysed. GAB and BET sorption models were tested to fit the experimental data. The equilibrium moisture content of films increased dramatically above a w=0.6. Films with greater H/ S ratio have greater water binding capacity. For a specific hydrocolloid/surfactant ratio, equilibrium moisture content of the coatings decreased as the HLB of the surfactant mixture increased. Equilibrium moisture contents of the composite films could be predicted from the corresponding values of pure compounds and the respective component mass fractions by a linear model, in the water activity range of 0.11–0.75. At the high relative humidity at which water permeability of the films was evaluated, the water vapour barrier was effective when the films reached a critical amount of surfactants ( H/ S=0.5).

Experimental Determination and Modeling of Sorption Isotherms of Tropical Fruits: Banana, Mango, and Pineapple

Sorption isotherms of banana, mango, and pineapple have been experimentally found at 40, 50, and 60°C by use of the salt method for a range of water activities from 0.056 to 0.85. The sorption capacity of these fruits increases with the temperature for a given water activity. The experimental curves have been simulated by the GAB and BET models. The BET model ensures a better representation of the experimental results for water activity lower than 0.35 with maximum deviation of 1.0, 1.5, and 2.0 kgw · kgdm−1 for respectively banana, mango and pineapple. On the contrary, the GAB model enables the representation of the whole desorption isotherms and the estimation of the water content corresponding to monolayer saturation and of the isosteric heat of sorption. It also takes into account the temperature effect. The isosteric sorption heat of the three studied products are deduced from experimental results and empirical correlations are proposed leading to a satisfactorily representation.

Sorption Isotherms of Vaccum-Fried Carrot Chips

The moisture sorption isotherms for carrot chips after vacuum frying were determined using a gravimetric technique at 10, 25, and 40°C and fitted with BET, GAB, Smith, Halsey, Henderson, and Peleg models. A nonlinear least-squares regression program was used to determine the models constants. The Peleg, Halsey, and GAB models were found to best represent the experimental data throughout the entire water activity range.

In vitro effect of phenolic antioxidants on germination, growth and aflatoxin B1 accumulation by peanut Aspergillus section Flavi

The effectiveness of the food-grade antioxidants butylated hydroxytoluene (BHT), trihydroxybutyrophenone (THB), propyl paraben (PP) and butylated hydroxyanisole (BHA) at 1, 10 and 20 mmol l(-1) concentrations on germination, growth, and aflatoxin B(1) (AFB(1)) production by Aspergillus section Flavi strains was determined. Assays on the lag phase of germination, germination percentage, germ tube elongation rate, lag phase, growth rate and AFB(1) production by three strains of Aspergillus flavus and three of Aspergillus parasiticus were carried out in vitro on peanut extract meal agar conditioned at different water activities (a(w): 0.982, 0.971, 0.955, 0.937). The antioxidants PP and BHA efficiently inhibited the germination of the two species tested at the doses 10 and 20 mmol(-1). The antioxidants PP and BHA at 1 mmol l(-1) and THB at 20 mmol l(-1) reduced the germ tube elongation rate most effectively, regardless of a(w) levels. An increase in the lag time and a reduction in the growth rate of 100% of the strains was observed, this was due to the action of BHT at the doses 10 and 20 mmol(-1) at 0.982, 0.971 and 0.955 a(w), although these treatments stimulated the AFB(1) accumulation in most of the fungi tested. The more effective antioxidants were PP and BHA, which increased the lag phase, reduced the growth rate and AFB(1) production in all of the strains at the four a(w) assayed. At concentrations 10 and 20 mmol l(-1), these antioxidants totally inhibited fungal development. The study shows that the antioxidants BHA and PP are effective fungal inhibitors to peanut Aspergillus section Flavi in wide range of water activity. The results suggest that phenolic antioxidants, BHA and PP, can be effective fungitoxicants on aflatoxigenic strains in peanut at industrial level.

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.

Isopiestic Determination of the Osmotic and Activity Coefficients of NaCl + SrCl2 + H2O at 298.15 K and Representation with an Extended Ion-Interaction Model

Isopiestic vapor-pressure measurements were made at 298.15 K for aqueous NaCl + SrCl2 solutions using NaCl(aq) as the reference standard. The measurements for these ternary solutions were made at NaCl ionic strength fractions of y1 = 0.17066, 0.47366, and 0.82682 for the water activity range of 0.9835 = aw = 0.8710. Our results, and those from two previous isopiestic studies, were combined and used with previously determined parameters for NaCl(aq) and those determined here for SrCl2(aq) to evaluate mixing parameters S? Na,Sr = (0.0562 ± 0.0007) kg·mol-1 and ?Na,Sr,Cl = -(0.00705 ± 0.00017) kg2· mol-2 for an extended form of Pitzer's ion-interaction model. These model parameters are valid for ionic strengths of I = 7.0 mol·kg -1, where higher-order electrostatic effects have been included in the mixture model. If the fitting range is extended to the saturated solution molalities, then S?Na,Sr = (0.07885 ± 0.00195) kg·mol-1, and ?Na,Sr,Cl = -(0.01230 ± 0.00033) kg2·mol-2. The extended ion-interaction model parameters obtained from available isopiestic data for SrCl 2(aq) at 298.15 K yield recommended values of the water activities and osmotic and activity coefficients

Equilibrium Moisture Content of Semi-Defatted Pumpkin Seed Flour

Moisture equilibrium data (adsorption and desorption) of semi-defatted (fat 10.6 % wet basis) pumpkin seed flour were determined using the static gravimetric method of saturated salt solutions at three temperatures 10 °C, 25 °C, and 40 °C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to five mathematical models (modified Oswin, modified Halsey, modified Chung-Pfost, modified Henderson and Guggenheim-Anderson-de Boer). The GAB model was found to be the most suitable for describing the sorption data. The monolayer moisture content was estimated using the Brunauer-Emmett-Teller equation.

Influence of water activity and temperature on conidial germination and mycelial growth of ochratoxigenic isolates of Aspergillus ochraceus on grape juice synthetic medium. Predictive models

AbstractThe first stages in the development of Aspergillus ochraceus, an ochratoxin A‐producing fungus that infects grapes and may grow on them, have been studied on a synthetic nutrient medium similar to grape in composition. Spore germination and mycelial growth have been tested over a water activity (aw) and temperature range which could approximate to the real conditions of fungal development on grapes. Optimal germination and growth were observed at 30 °C for all three isolates tested. Maximal germination rates were detected at 0.96–0.99 aw at 20 °C, while at 10 and 30 °C the germination rates were significantly higher at 0.99 aw. Although this abiotic factor (aw) had no significant influence on mycelial growth, growth rates obtained at 0.98 aw were slight higher than those at other aw levels. Predictive models for the lag phase before spore germination as a function of water activity and temperature have been obtained by polynomial multiple linear regression, and the resulting response surface models have been plotted. Copyright © 2005 Society of Chemical Industry

Effect of dioxane on the structure and hydration–dehydration of α-chymotrypsin as measured by FTIR spectroscopy

A new experimental approach based on FTIR spectroscopic measurements was proposed to study simultaneously the adsorption/desorption of water and organic solvent on solid enzyme and corresponding changes in the enzyme secondary structure in the water activity range from 0 to 1.0 at 25 °C. The effect of dioxane on the hydration/dehydration and structure of bovine pancreatic α-chymotrypsin (CT) was characterized by means of this approach. Dioxane sorption exhibits pronounced hysteresis. No sorbed dioxane was observed at low water activities ( a w < 0.5) during hydration. At a w about 0.5, a sharp increase in the amount of sorbed dioxane was observed. Dioxane sorption isotherm obtained during dehydration resembles a smooth curve. In this case, CT binds about 150 mol dioxane/mol enzyme at the lowest water activities. Three different effects of dioxane on the water binding by the initially dried CT were observed. At a w < 0.5, water adsorption is similar in the presence and absence of dioxane. It was concluded that the presence of dioxane has little effect on the interaction between enzyme and tightly bound water at low a w. At a w > 0.5, dioxane increases the amount of water bound by CT during hydration. This behavior was interpreted as a dioxane-assisted effect on water binding. Upon dehydration at low water activities, dioxane decreases the water content at a given a w. This behavior suggests that the suppression in the uptake of water during dehydration may be due to a competition for water-binding sites on chymotrypsin by dioxane. Changes in the secondary structure of CT were determined from infrared spectra by analyzing the structure of amide I band. Dioxane induced a strong band at 1628 cm −1 that was assigned to the intermolecular β-sheet aggregation. Changes in the intensity of the 1628 cm −1 band agree well with changes in the dioxane sorption by CT. An explanation of the dioxane effect on the CT hydration and structure was provided on the basis of hypothesis on water-assisted disruption of polar contacts in the solid enzyme. The reported results demonstrate that the hydration and structure of α-chymotrypsin depend markedly on how enzyme has been hydrated — whether in the presence or in the absence of organic solvent. A qualitative model was proposed to classify the effect of hydration history on the enzyme activity- a w profiles.

Drying characteristics and sorption isotherm of tomato slices

The drying behaviour of tomato slices was investigated at 45, 60 and 75 °C. Three falling rate periods were observed with diffusion coefficients in the range 3.72–12.27 × 10 −9 m 2 s −1. The water vapour sorption isotherm of dehydrated tomato slices in the water activity ( a w) range of 0.08–0.85 was also determined at three temperature levels, i.e., 25, 30 and 40 °C. Five sorption models were fitted with the adsorption data generated from the gravimetric method. GAB and Oswin models describe the adsorption characteristics of dehydrated tomato at 25 °C better than other models with GAB model being the best applicable model. The isosteric heat of adsorption decreases with increasing moisture.

Vapor pressures and isopiestic molalities of concentrated CaCl 2(aq), CaBr 2(aq), and NaCl(aq) to T = 523 K

The Oak Ridge National Laboratory high-temperature isopiestic apparatus was outfitted with precise pressure gauges to allow for direct vapor pressure measurements. Vapor pressures over concentrated solutions of CaCl 2(aq), and CaBr 2(aq) were measured at temperatures between (380.15 and 523.15) K in the range of water activities between 0.2 and 0.85. Isopiestic molalities were used to determine osmotic coefficients at the conditions where NaCl reference standard solutions remained undersaturated. The main goal of this work was to improve the accuracy of isopiestic comparisons based on the calcium chloride reference standard. Osmotic coefficients for CaCl 2(aq) and CaBr 2(aq) calculated from both isopiestic and direct vapor pressure results were combined with the literature data and used to build general thermodynamic models based on a variant of extended Pitzer ion-interaction equations and valid at the saturation pressure of water. While these empirical models approach the accuracy of the experimental data in a wider range of concentrations and temperatures than any previously published equations, considerable amounts of accurate data and a substantial effort are required in order to obtain a satisfactory representation using power series-based virial equations. The effect of experimental uncertainties on the accuracy of the direct vapor pressure results is discussed, including in particular the error caused by the presence in the apparatus of a small amount of CO 2. The substantial decrease of the solubility product of CaCO 3 in concentrated chloride solutions at temperatures above 423 K is a serious defect of calcium chloride as a water activity reference standard.

Effect of acetonitrile on the hydration of human serum albumin films: a calorimetric and spectroscopic study

A new experimental approach based on the combination of calorimetric and FTIR spectroscopic measurements was proposed to study simultaneously the sorption of water and organic solvent, and corresponding changes in the structure of protein films in the water activity range from 0 to 1.0. Enthalpy changes (Δ H tot) on the interaction of water with the dried human serum albumin (HSA) in the presence and absence of acetonitrile (AN) have been measured using a Setaram BT-2.15 calorimeter at 298 K. Spectroscopic data on water and organic solvent vapor sorption by the HSA films and the corresponding changes in the protein secondary structure were determined by means of a Bruker Vector-22 FTIR spectrometer. By using a water activity-based comparison we characterised the effect of acetonitrile on the hydration and structure of the HSA films. Acetonitrile (AN) sorption isotherm resembles a smooth curve. HSA film binds about 250 mol AN/mol protein at the lowest water activities. As the water activity increases from 0 to 0.8, the sorption of AN gradually decreases from 250 to 150 mol AN/mol HSA. At a w > 0.8, the sorption of AN sharply decreases to zero. Acetonitrile decreases markedly the water content at a given a w. This behavior suggests that the suppression in the uptake of water is due to a competition for water-binding sites on the HSA films by acetonitrile. Changes in the secondary structure of HSA were determined from infrared spectra by analyzing the structure of amide I band. Acetonitrile increases the intensity of the 1654 cm −1 band that was assigned to the α-helix structure. Changes in the intensity of the 1654 cm −1 band agree well with the decrease in water uptake in the presence of AN. An explanation of the acetonitrile effect on the hydration and structure of the HSA films was provided on the basis of hypothesis on water-assisted disruption of polar contacts in the initially dried protein.

Sorption Isotherms and Moisture Diffusivity in Fried Cassava-Shrimp Chips

Equilibrium moisture sorption isotherms of fried cassava-shrimp chips were investigated by measuring water activity at 20, 30, and 40°C for modeling transient diffusion of moisture. The sorption isotherms followed a type III shape in water activity range of 0.1–0.9 showing negligible influence of temperature, and were best represented by the GAB, Henderson, and modified Henderson equations. Effective moisture diffusivity of chips based on analytical solution of Fick's law ranged from 0.8 × 10−11 to 2.1 × 10−10 m2/s during exposure to fixed relative humidity (50, 70, and 90%) and temperature (20, 30, and 40°C) environments. An Arrhenius type relationship adequately described the dependence of effective moisture diffusivity both on temperature and relative humidity, and led to good agreement between experimental and estimated transient moisture sorption data.

Enthalpy–entropy compensation in sorption phenomena of starch materials

The enthalpy–entropy compensation theory was applied to the experimental moisture adsorption and desorption isotherm data (water activity ( a w) range 0.006–0.982) of raw potato, potato starch gel, potato starch powder, highly amylopectin corn starch powder and highly amylose corn starch powder in the temperature range 30–60 °C. A linear relation existed between differential enthalpy (Δ H) and differential entropy (Δ S) for all the experimental data considered, thus satisfying the enthalpy–entropy compensation theory. Further analysis of the data indicated an enthalpy-controlled (isokinetic temperature ( T β ) > harmonic mean temperature ( T hm)) and spontaneous (−Δ G) sorption process.

Thermodynamic properties and sorption equilibrium of pestil (grape leather)

Moisture sorption isotherms (MSI) of pestil, a well-known grape leather in Turkey, were determined at 15, 25 and 35 °C over a water activity ( a w) range of 0.06–0.98, using gravimetric static method. The Gugghenheim–Anderson–de Boer (GAB), Halsey, Oswin, and Peleg models were tested to fit the experimental data by using nonlinear regression analysis method. The MSI exhibited type III behaviour, and the inverse temperature effect was observed for the sample stored at 35 °C at higher a w values (>0.65). The Halsey and Peleg models adequately described the MSI. The thermodynamic properties of MSI (net isosteric heat, differential entropy, spreading pressure, net integral enthalpy and net integral entropy) were also evaluated for understanding of the properties of water and calculating the energy requirements for transport phenomena during processing. The net isosteric heat and differential entropy decreased with increasing moisture content and were characterized by two-parameter exponential decay and power-law models respectively. The spreading pressure of pestil increased with increase in water activity and decreased when the temperature increased to 35 °C. Net integral enthalpy increased to a maximum value of 20.25 kJ/mol and net integral entropy decreased to a minimum value at around monolayer moisture content. Thereafter, with increasing moisture content the enthalpy decreased and the entropy increased gradually.

The prediction of moisture sorption isotherms for dairy powders

Abstract Moisture sorption isotherms were measured for whey protein isolate, high micellar casein and a milk protein concentrate powder. No temperature dependence was observed over the temperature range of 4–37 °C. At 50 °C the powders absorbed less moisture than observed at the lower temperatures. These isotherms were used to predict the isotherms for freeze-dried amorphous lactose/casein/whey protein powders. An isotherm for micellar casein was predicted using a simple additive isotherm model and was used along with isotherms for whey protein and amorphous lactose to predict moisture sorption isotherms for commercial dairy powders. Predicted isotherms compared well with measured isotherms indicating that this simple additive isotherm model is suitable for predicting moisture sorption isotherms of dairy powders. Delayed lactose crystallisation was observed in lactose/whey protein powders when compared to lactose/casein powders over the same water activity range.

An improved experimental and regression methodology for sorption isotherms

AbstractSorption isotherms of corn and starch cylinders with immobilised catalase are experimentally determined at different temperatures for use in drying models in optimal control studies. This application of the sorption isotherm requires an accurate prediction of the sorption data at different temperatures for the low water activity range. The GAB equation is used for the prediction of the sorption isotherms. Two major problems are encountered by employing standard procedures, ie prediction of sorption at aw &lt; 0.11 and sensitivity of the GAB parameters to the applied data range. An improved methodology is developed, consisting of extending the standard experimental procedure with additional data points in the low water activity range and changing the criterion in the regression procedure in the sum of squares, which is weighed by the variance of the experimental data. The new methodology leads to accurate, consistent and physically relevant parameters of the GAB equation, which are independent of the applied data range in the regression analysis and which result in accurate predictions of the sorption behaviour at low water activity. The sorption data at different temperatures at low water activity can be predicted in the best way with parameters obtained after direct regression based on weighed SSQ. Copyright © 2004 Society of Chemical Industry

Moisture adsorption behaviour of semolina and farina

Moisture adsorption isotherms of semolina (from hard wheat) and farina (from soft wheat) were determined at 20, 35, 50 and 60 °C using the isopiestic method. The adsorbed moisture content significantly affected ( p < 0.05) product type, temperature and water activity. Moisture up-take accelerated about 0.75 water activity. The experimental sorption data obtained in this study were applied to Halsey, Harkins-Jura, Henderson, Iglesias-Chirife, Smith BET, GAB isotherm equations to predict the sorption behaviour of semolina and farina. Henderson, Halsey and GAB equations were found to be the most suitable model to describe the isothermal water sorption of semolina and farina at 0.1–0.9 water activity range. Monolayer moisture contents ( m 0) for two products were calculated from BET and GAB equations. The m 0 values of both models decreased with increasing temperature. The values of m 0 of semolina and farina at 20 °C were 11.8 and 9.16 g/100 g calculated with GAB, respectively. Adsorption isosteric heat ( Q s) decreased; the maximum heat of adsorption was obtained in the moisture content 6–7% and was higher for semolina (15.2 kJ/mol) than farina (14.34 kJ/mol). The Q s value quickly decreased with increase in moisture content to approximately 15% and then was plateau on axis of moisture content. Semolina and farina must be storage below 75% relative humidity at 20 °C prevents caking and deterioration because moisture sorption acceleration was increased after 0.75 a w. The moisture content of the products in this storage conditions could be approximately 12.5%.

Moisture sorption characteristics of chickpea flour

Moisture equilibrium data (adsorption and desorption) of chickpea flour were determined using the static gravimetric method of saturated salt solutions at four storage temperatures: 10, 20, 30 and 40 °C. The range of water activities for each temperature was between 0.11 and 0.85. Equilibrium moisture content decreased with the increase in storage temperature at any given water activity. The experimental data were fitted to four mathematical models (modified Chung–Pfost, modified Oswin, modified Halsey 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.