Water Activity Range Research Articles

Assessment of two‐ and three‐parameter Lewicki models for description of sorption phenomena of starch materials

AbstractExperimental moisture sorption isotherm data for raw potato, starch gels (potato, starch–sugar and starch–salt) and starch powders (potato, highly amylopectin and highly amylose) were determined using a gravimetric static method at 30, 45 and 60 °C and over a relative humidity range 0.10–0.84. The raw potato, starch powders and starch gels exhibited type II behaviour, with type III form being observed on incorporation of sugar, or salt, into the starch gel matrix. The two‐parameter Lewicki relationship, and the three‐parameter Lewicki and GAB equations were considered for description of the experimental data. A nonlinear least square regression technique was used to evaluate the constants for the models. The three‐parameter Lewicki model was found to be the most suitable for describing the sorption characteristics of raw potato, potato starch, starch–sugar and starch–salt gels within the investigated temperature and water activity range. However, both Lewicki models gave an inadequate representation of the data for potato, high amylopectin and high amylose starch powders. Copyright © 2004 Society of Chemical Industry

Sorption isotherms for amaranth grains

Compiling of EMC–ERH data for amaranth grains ( Amaranthus cruentus L.) in the range of water activity from 0.029 to 0.979 and temperature from 25 to 90 °C was performed. Included data sets comprised experimental values of EMC–ERH that summarize 78 identified points for desorption, 53 for adsorption and 16 not discerned points that were considered for mean sorption. Five isotherm equations for grains included in the ASAE Standards (Modified Henderson, Modified Chung–Pfost, Modified Halsey, Modified Oswin and GAB) were evaluated for their ability to fit sorption data from the literature ( M e vs. a w for adsorption, desorption and mean sorption). The goodness of fit for each isotherm was quantified through the correlation coefficient ( R 2), the sum of squares (RSS), the standard error of the estimate ( S y ), the mean relative deviation (MRD) and the plots of residuals. The three-parameter GAB isotherm was the best and gave a good correlation ( R 2 > 0.9817, RSS < 0.0293, MRD < 0.1380, S y < 0.0141, and random residuals-plots) for the general data-fit in the range of a w from 0.1 to 0.9, of interest in seed storage and processing. The Modified Halsey equation was rejected because it gave poor statistic parameters of agreement and patterned residual plots. For desorption, the Modified Chung–Pfost model gave the lowest mean relative deviation; the Modified Henderson equation was the second best in describing the EMC–ERH data, followed by the Modified Oswin and GAB models. For adsorption, the GAB equation presented the lesser MRD, followed by the Modified Chung–Pfost, Henderson and Oswin models. When mean sorption data were analyzed, the Modified Chung–Pfost equation was the best. However, when the GAB isotherm was adjusted at each temperature, a higher quality of agreement was obtained compared with the other isotherms, demonstrating the adequacy of GAB model to describe the experimental data of EMC–ERH for amaranth.

Effect of water activity on glass transitions of date pastes

Water sorption isotherms of freeze dried and control (without freeze drying) date paste of cultivar Khalas were obtained at 20 °C using the static saturated salt solution method. The GAB equation fitted adequately for the whole range of water activity for both date paste samples. The monolayer moisture contents ranged between 0.102 and 0.12 for freeze dried and control samples respectively. The glass transition temperatures of saturated salt equilibrated paste samples were measured using a differential scanning calorimetry (DSC). Glass transition temperature of both samples decreased linearly with an increase in water activity. Water adsorption characteristics of freeze dried and control samples differed from each other and were supported by glass transition temperatures.

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.

Moisture sorption isotherms and modeling for pre-cooked flours of pigeon pea ( Cajanus cajans L millsp) and lima bean ( Canavalia ensiformis)

The moisture sorption isotherms for pre-cooked flours of pigeon pea and lima bean were determined using the static method of saturated salt solution in a range of water activity from 0.20 to 0.97 at 18, 28, 38, and 48 °C and fitted by the Henderson, Chung and Pfost, and Ratti, Crapiste and Rotstein models. The moisture content of the material tested varied from 8% to 60% (dry basis). The agreement between experimental and predicted values is good for three models; nevertheless the Ratti, Crapiste and Rotstein (1989) model gave a better fit of the experimental data. The net isosteric heat of water sorption, estimated by applying Claussius-Clapeyron equation to sorption isotherms at mean temperatures of 23, 33 and 43 °C, was found to be different for pigeon pea flour and lima bean flour. The net isosteric heat of sorption for two flours increased with increasing moisture content and decreased with increasing temperature.

Sorption isotherms and isosteric heat of sorption for grapes, apricots, apples and potatoes

Moisture sorption isotherms of grapes, apricots, apples and potatoes were determined at 30°C, 45°C, and 60°C using the standard, static-gravimetric method. Six two-parameter and five three-parameter sorption models were tested to fit the experimental data. A nonlinear regression analysis method was used to evaluate the constants of the sorption equations. The Halsey equation gave the best fit to the experimental sorption data for all materials tested over the range of temperatures and water activities investigated. The GAB model gave also the closest fit to the sorption data for potatoes and grapes. The agreement between experimental and predicted values of these models was found to be satisfactory. The isosteric heat of desorption and adsorption of water determined from the equilibrium data using the Clausius–Clapeyron equation.

Sorption Equilibrium and Thermodynamic Characteristics of Soya Bean

Desorption equilibrium moisture content and water activity data for soya bean ( Glycine max) TGX 1440-1E were determined using the static gravimetric method. Measurements were taken in the water activity range of 0·07–0·98 at four temperatures (40, 50, 60 and 70 °C). A non-linear regression programme was used to fit five moisture sorption isotherm models [modified Henderson, modified Chung–Pfost, modified Halsey, modified Oswin and modified Guggenheim–Anderson–de Boer (GAB)] to the experimental data. The models were compared using the standard error of estimate, mean relative percent deviation and residual plots. The modified Oswin model, which gave the least standard error of 1·91% and least mean relative percent deviation of 10·15 among the five models, when equilibrium moisture content was taken as the dependent variable, was considered to be the best for predicting the equilibrium moisture content of soya bean. The moisture sorption isotherms of soya bean were sigmoidal in shape, of the type II and were markedly affected by temperature. The modified Halsey model, which gave the least standard error of 0·07 and least means relative percent deviation of 16·67, when water activity was taken as the dependent variable, was considered the best for predicting the water activity of soya bean. The desorption equilibrium moisture content and water activity relationships as expressed by the moisture sorption isotherm models that best predicted the experimental data, were used to determine the thermodynamic characteristics of soya bean. The heat of vaporisation of moisture in soya bean decreased with increase in moisture content and approached the latent heat of pure water at a free water point of between 20 and 22% moisture content (d.b.). The surface potential of moisture in the seed increased with increase in water activity, but was not significantly affected by temperature. Net integral enthalpy decreased from a value of 550 J g −1 as the moisture content increased from 5·20% (d.b.). The trend became asymptotic at a value of 80 J g −1 as the moisture content of 13% (d.b.) was approached. Net integral entropy decreased with increase in moisture content to a minimum value of –0·38 J g −1 K −1 at the moisture content of 11% (d.b.), and thereafter, increased with further increase in moisture content.

Moisture Sorption Isotherms of Walnut Flour at Several Temperatures

ABSTRACTMoisture equilibrium data (adsorption and desorption) of walnut flour were determined using the static gravimetric method of saturated salt solutions at three temperatures 10, 25, and 40 °C. The range of water activities for each temperature was between 0.11 to 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 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.

Sorption and transport of water vapor in thin polymer films at 35 °C

Using a gravimetric technique, experimental data at 35 °C are reported for isothermal sorption equilibria and sorption kinetics of water vapor in poly(N,N-dimethyl methacrylamide) (PDMAA), poly(2-dimethyl aminoethyl methacrylate) (PDMAEMA), poly(acrylic acid) (PAA), and in a typical membrane of a commercial soft-contact lens made of poly(2-hydroxyethyl methacrylate) (PHEMA). The highest sorption of water vapor is in PDMAA. The Flory–Huggins model and the Zimm–Lundberg clustering theory are used to interpret the equilibrium data. Over the entire range of water activity, the four materials show clustering functions larger than −1 indicating that water molecules cluster together in the polymers. The least hydrophilic polymer, PDMAEMA, and the most hydrophilic one, PDMAA, show respectively, the highest and the lowest tendency of water molecules to form clusters. Water diffusion into the polymer matrix is faster in PDMAEMA (diffusion coefficient, D = 10–20 × 10−8 cm2 s−1) and in PDMAA (D = 4.9–8.7 × 10−8 cm2 s−1) than in PHEMA-lens material (D = 0.55–3.4 × 10−8 cm2 s−1) and PAA (D = 0.98–3.5 × 10−8 cm2 s−1). The measured diffusion coefficients increase with water activity in the PHEMA-lens, decrease with activity in PDMAEMA, and show a more complex concentration dependence in PDMAA and PAA. By writing the diffusion coefficient as a product of an intrinsic mobility, Đ, and a non-ideality thermodynamic factor, Γ, the concentration dependence of D is explained by the interplay of two factors: rising plasticization with water activity, which causes an increase in Đ, and the effect of composition on Γ, decreasing with water activity.

Effect of preservatives on the functional properties of tapioca starch: analysis of interactions

The effect of different levels of potassium sorbate or sodium benzoate addition on the water sorption behavior and on the mechanical properties of freeze-dried tapioca starch gel was studied. It was observed that the addition of potassium sorbate or sodium benzoate to starch before the gelatinization process increased the amount of water sorbed by the system, but experimental moisture content of the mixture was lower than the one predicted by a mass balance, revealing the existence of an interaction between starch and the preservatives. In general, in the range of water activity 0.755–0.976, most of the preservative was bounded to starch. Mechanical properties of the powder obtained after freeze drying of gels were also influenced by the addition of the preservatives: an increase in bulk density, higher cohesiveness and initial Young modulus was observed after antimicrobial addition. Results obtained showed that sorbate and benzoate interact with tapioca starch and that this interaction has a great influence on system behavior.

MOISTURE SORPTION CHARACTERISTICS of STARCH GELS. PART 1: MATHEMATICAL DESCRIPTION of EXPERIMENTAL DATA

ABSTRACTThe moisture sorption isotherms of potato starch, starch‐sugar and starch‐salt gels were determined using a gravimetric static method at 30, 45 and 60C, and over a range of relative humidities from 0.10 to 0.84. the starch gels exhibited Type II behavior, with incorporation of sugar or salt into the samples resulting in Type III form. the sorption capacity decreased with increasing temperature. the experimental data were analyzed using seven models; two‐parameter relationships (Halsey, Henderson, Smith), three‐parameter equations (Ferro‐Fontan, Guggenheim‐Anderson‐de Boer, modified‐Oswin) and four‐parameter equation (Peleg). the empirical Peleg equation and kinetic GAB model (parameters have physical significance) were found to be the most suitable for describing the sorption characteristics within the temperature range 30–60C and water activity range 0.05–0.95.

RP002 Issues concerning mucormycosis. — Comments from the konsiliarlaboratory

The water relations of six species from the Mucoraceae, Rhizopus stolonifer, R. oryzae, R. microsporus var. microsporus, Mucor circinelloides, Absidia corymbifera and Syncephalastrum racemosum were studied at 25 °C and 30° over a water activity (aw) range of 0·995 to 0·80 in glucose-based media. Three isolates of each species were used in a mixed inoculum to minimize isolate to isolate variation. Germination or growth was apparent in all species in less than 12 h from 0·995 to 0·96 aw. At 0·84 aw, R. stolonifer germinated in less than 3 d. R. stolonifer and R. oryzae grew very rapidly (1·5–2·0 mm h−1) at the highest aw, but the growth rate fell as aw was decreased. R. stolonifer and S. racemosum, the most xerophilic of the fungi studied, were able to grow at 0·84 aw. The minimum aw for growth of R. oryzae and A. corymbifera was 0·88 aw, while the lowest aw for growth of R. microsporus var. microsporus and M. circinelloides was 0·90 aw. Growth rates were generally slightly faster at 30° than at 25°, but water relations of most species were similar at both temperatures.

Equilibrium Water Sorption in Human Stratum Corneum

The water content of the stratum corneum (SC) is a key factor in skin barrier homeostasis; it is intimately related to both skin condition and skin permeability. Studies of water uptake in excised human SC show strong similarities and allow characterization of the equilibrium SC water sorption isotherm in terms of widely used theoretical models. At low water activities, SC water sorption resembles that in other keratinized tissues (i.e., wool and horn), whereas at high water activities, it resembles that in polymeric hydrogels. In this paper, theoretical water sorption models [Brunauer-Emmett-Teller (BET), D'Arcy-Watt, and Frenkel-Halsey-Hill] are fit to the combined human SC water sorption data from our laboratories and others. Each of these models provides a satisfactory description of the equilibrium water content of human SC over the water activity range 0.03-1.0. An accompanying paper discusses the implications of SC water sorption on water mobility in corneocytes and on SC permeability.

Properties and sorption studies of chitosan–polyvinyl alcohol blend films

Chitosan–polyvinyl alcohol (PVA) blend films were prepared and characterized for mechanical and optical properties. Intermolecular interaction between chitosan–PVA blend was investigated using FTIR. Moisture sorption characteristics of chitosan–PVA blend at 25 °C were studied for water activities ranging from 0.11 to 0.92. The moisture content increased with increase in PVA concentration. The moisture sorption data were used to fit eight sorption models and constants were determined by linear fitting. The co-efficient of linear regression (0.98–0.99) and root mean square error values (1.08–15.32) confirmed the best fit for all the models. GAB model was applicable for a wide range of water activity.

Water sorption isotherms of starch powders: Part 1: mathematical description of experimental data

Adsorption and desorption isotherms for potato, highly amylopectin and highly amylose starch powders were determined at 30, 45 and 60 °C using a gravimetric technique. Samples were equilibrated in desiccators containing sulphuric acid solutions of known water activity (0.05–0.95), and placed in temperature-controlled cabinets for approximately three weeks. The starch powders exhibited Type II behaviour. The sorption capacity decreased with increasing temperature. The data obtained were fitted to several models including two parameter relationships (Halsey, Oswin, Henderson, Modified-BET and Smith), three parameter equations (GAB, Ferro-Fontan) and four parameter equation (Peleg). A non-linear least square regression program was used to evaluate the models constants. The empirical Peleg model followed by the kinetic GAB and the semi empirical Ferro-Fontan models were found to best represent the experimental data in the water activity range 0.05–0.95. In the range of water activity 0.35–0.95 the Smith model was shown to give the closest fit to the experimental data.

Application of the Guggenheim, Anderson and De Boer model to correlate water activity and moisture content during osmotic dehydration of apples

Water activity and moisture content are the most valuable characteristics for assessing the stability of dried foods. The aim of this work was to evaluate if a desorption isotherm model could be used to describe the relationship between the water activity and moisture content in the product during osmotic dehydration. This model can lead to a better understanding of how water activity of a product can be reduced during osmotic dehydration. The Guggenheim, Anderson and De Boer (GAB) model was chosen because it has been proven to best fit the desorption and adsorption of foods over a large range of water activities. The studied material was apple pieces, and the osmotic medium with a water activity of 0.939 was prepared with sucrose. The osmotic dehydration process was studied by collecting moisture content and water activity data at different temperatures and times. The model was shown to fit well the experimental points at temperatures 25, 45 and 55 °C. At 65 °C, however, the model is satisfactory if a w is below 0.970 but not above. The effect of temperature can be expressed on the C-parameter of the GAB model.

Unexpected thermal destruction of dried, glass bead-immobilized microorganisms as a function of water activity.

To help us understand the factors and mechanisms implicated in the death of microorganisms or their resistance to temperature in a low water activity environment, microorganisms were dried on the surface of glass beads before being subjected to high temperatures for a short period followed by rapid cooling. Two microorganisms were studied: the yeast Saccharomyces cerevisiae and the bacterium Lactobacillus plantarum. Experiments were carried out at 150, 200, and 250 degrees C, with four durations of heat treatment and seven levels of initial water activity between 0.10 and 0.70. We observed an unexpected range of water activity, between 0.30 and 0.50, at which microorganisms were more resistant to the various treatments, with maximal viability at 0.35 for L. plantarum and 0.40 for S. cerevisiae.

Thermodynamic Analysis of Water Vapour Sorption of Edible Starch Based Films

Moisture sorption isotherms of high amylose corn starch (HACS) films at 5, 25, 35 and 45 °C within an approximate water activity ( aw) range of 0.11-0.85 were obtained using a static gravimetric method. HACS films fitted a type III BET model at all temperatures studied. Water sorption process in high amylose based films was exothermic within the range of water activities investigated. There was a continuous decrease of the affinity between film sorbed structure with moisture as water content increased. Negative values of Δ G in the whole range of moisture content studied were indicative of a spontaneous process. The entropy curve exhibited a well-defined minimum corresponding at the completion of the monolayer.

Evaluating the migration of ingredients from active packaging and development of dedicated methods: a study of two iron-based oxygen absorbers

The behaviour of two commercial oxygen-scavenging products with respect to migration of active ingredients into foodstuffs was investigated. Migrants were identified, and by using appropriate analytical methods, migration was determined in a variety of liquid, solid or gelled food simulants and foods. were chosen to cover a range of water activities and viscosities. Foods and the gelled food simulant agar were packed with and without vacuum, and with the oxygen scavenger in various locations relative to the packed food. The main migrants, as identified by X-ray fluorescence spectrometry, infrared spectroscopy and scanning electron microscopy with energy-dispersive spectrometry were Na+ and Cl− in non-acidic aqueous simulants, and Na+, Cl − and Fe2+ in 3% acetic acid. Migration into aqueous simulants exceeded the current European Union limit for total migration from plastic materials (assumed to be currently applicable to these systems) and was probably excessive by any reasonable standard. However, neither oxygen scavenger appeared to release significant quantities of migrants into solid foods when the scavenger was properly located in the package and the packing process does not favour the contents becoming wet by water released from the food.

Evaluation of sodium lactate as a replacement for conventional chemical preservatives in comminuted sausages inoculated with Listeria monocytogenes

Sodium lactate (SL) as a potential replacer for potassium sorbate (PS) or sodium benzoate (SB) in comminuted sausages was evaluated. Sausages manufactured with 3.3% SL were compared with a control and 0.05 or 0.1% of PS and SB with regard to its influence on changes of chemical composition, physico-chemical and textural properties, and the growth of inoculated Listeria monocytogenes (LM) stored at 4 °C for up to 8 weeks. The sausages contained 62–64% moisture, 15–17% fat and 12–14% protein with pH range of 6.10–6.15 and water activity ( a w) range of 0.936–0.941. Sausages containing 3.3% SL alone had lower ( P<0.05) thiobarbituric acid reacting substances (TBARS) values than the control and those of PS (0.05–0.1%). Lightness values of sausages varied ( P<0.05) among preservatives and storage times, while yellowness values tended to increase with storage time. Textural attributes (springiness and hardness) were reduced after 2 and 6 weeks storage, respectively. Sodium lactate at an incorporation level of 3.3% to sausage formulation had an antilisterial effect similar to those of 0.05–1.0% of PS or SB and delayed the lag phase for the growth of Listeria monocytogenes at least 2 weeks, compared with the control.