Calculation Of Water Activity Research Articles

Calculation of water activity in electrolytic solutions using the Electrolattice and Q-Electrolattice equations of state

Water activity directly influences the deterioration, texture, pigmentation, and preservation of nutrients in food. This property quantifies the availability of water for the enzymatic, chemical, or microbiological activity that defines the shelf life of food. In this way, the food industry needs to measure, predict, and control this property. In this context, the present work aims to evaluate the performance of the Electrolattice and Q-Electrolattice equations of state in predicting and correlating the water activity of solutions containing electrolytes. Parameters available in the literature were used to compare the performance of the Electrolattice and Q-Electrolattice equations in predicting the water activity (in binary, ternary, and quaternary solutions of salts). The Q-Electrolattice model presented better performance than the Electrolattice equation, with a relative mean deviation (RMD) of 1.95%, 0.14%, and 0.06% for binary, ternary, and quaternary aqueous electrolytic solutions, respectively. Then, the Q-Electrolattice equation was used to correlate experimental data of systems containing water + NaCl + sugar (fructose, sucrose, glucose, and xylose) at 25°C and water + sugar (fructose, sucrose, glucose, and xylose) at 35°C in aqueous solutions of salts and sugar. The general weighted average of the RMD% was equal to 0.29%. Finally, the procedure was validated with solutions containing water, NaCl, and sugar (fructose, sucrose, glucose, and xylose) at 35°C, a condition that was not used in the parameter fitting process. The overall weighted average of the RMD for predicting the water activity of water + NaCl + sugar solutions at 35°C using the Q-Electrolattice equation was 0.57%, thus validating the parameters estimated in this work. The results obtained in this study showed the feasibility of using the Q-Electrolattice equation to predict water activity in aqueous solutions of salts and sugars used as a basis for the osmotic drying of food.

A thermodynamic framework for determination of gas hydrate stability conditions and water activity in ionic liquid aqueous solution

The formation of gas hydrates in pipelines and plugging of the gas flow path are a major cause of operating expenses, safety issues, pressure drop, and fatal accidents. Therefore, the inhibition of gas hydrate formation is of paramount importance. The utilization of a new class of inhibitors such as ionic liquids (ILs) is currently of great interest. In this regard, adjusting water activity in the inhibitor blend (water + IL) is a vital factor. Several models have been developed to calculate the activity of water in the presence of IL(s). The major disadvantage of these models is their correlative basis. This study aimed to propose a rigorous predictive model for the calculation of water activity in the presence of IL(s), which would then be applied in the prediction of the gas hydrate stability conditions. The model is made up of a molecular term (the short-range interactions from Free-Volume-Flory-Huggins (FVFH) activity model) as well as a contribution from ionic interactions as a result of IL(s) dissociation in water (the long-range electrostatic interactions from the extended Debye–Hückel (EDH) model). The overall absolute temperature deviation and the average absolute relative deviation percent in the calculated gas (methane and carbon dioxide) hydrate dissociation temperatures for the whole databank (500 data points including 37 ILs) were found to be 0.61 K and 0.22%, respectively. This proves the superiority of the model over the previous correlative-basis ones. Finally, it is concluded that the higher temperature, the higher the IL(s) concentration, and the lower IL(s) molecular weight(s) result in larger model deviations.

Deliquescence lowering in mixtures of NaCl and sucrose powders elucidated by modeling the water activity of corresponding solutions

A significant number of models allowing the calculation of water activity (aw) in aqueous solutions of electrolytes or of organic substances was proposed and verified experimentally in the last few decades. In case of a mixture of organic and inorganic solutes the lowering of water activity with respect to corresponding single solutes was often observed but, to the best of our knowledge, did not find sound physical explanation yet. Present work proposes a thermodynamic model covering wide range of solution concentrations and applicable to multiple solutes, in particular to the mixtures of high total solid contents, situation relevant to powder caking. Deliquescence lowering is evidenced by means of qualitative visual observation and quantitative measurements of water uptake. Excellent agreement between the measured values of aw for different compositions of NaCl–sucrose–water mixtures and the model calculation of the water activity at any composition is obtained. A diagram relating water activity to the composition of a ternary system is proposed. It facilitates foreseeing the conditions of ambient relative humidity where partial or total dissolution of mixtures of NaCl and sucrose will take place. In addition, it furnishes a better understanding of deliquescence lowering reported in the earlier works.

Vapour–liquid equilibrium of ternary aqueous polymer solutions containing polyethylene glycol and polyethylene glycol dimethyl ether at different temperatures

Water activities of ternary aqueous polymer solutions containing polyethylene glycol and polyethylene glycol dimethyl ether were determined experimentally using an improved isopiestic method at T=298.15, 308.15 and 318.15K. The applicability of the modified Wilson model was tested for correlation of water activity of ternary polymer–polymer–solvent solutions studied in this work and others studied previously. The parameters determined in binary solutions were used in the calculation of water activity of ternary polymer solutions.

Measurements of particle masses of inorganic salt particles for calibration of cloud condensation nuclei counters

Abstract. We measured the mobility equivalent critical dry diameter for cloud condensation nuclei (CCN) activation (dc_me) and the particle mass of size-selected (NH4)2SO4 and NaCl particles to calibrate a CCN counter (CCNC) precisely. The CCNC was operated downstream of a differential mobility analyzer (DMA) for the measurement of dc_me. The particle mass was measured using an aerosol particle mass analyzer (APM) operated downstream of the DMA. The measurement of particle mass was conducted for 50–150-nm particles. Effective densities (ρeff) of (NH4)2SO4 particles were 1.67–1.75 g cm−3, which correspond to dynamic shape factors (χ) of 1.01–1.04. This shows that (NH4)2SO4 particles are not completely spherical. In the case of NaCl particles, ρeff was 1.75–1.99 g cm−3 and χ was 1.05–1.14, demonstrating that the particle shape was non-spherical. Using these experimental data, the volume equivalent critical dry diameter (dc_ve) was calculated, and it was used as an input parameter for calculations of critical supersaturation (S). Several thermodynamics models were used for the calculation of water activity. When the Pitzer model was employed for the calculations, the critical S calculated for (NH4)2SO4 and NaCl agreed to well within the uncertainty of 2% (relative). This result demonstrates that the use of the Pitzer model for the calibration of CCNCs gives the most accurate value of S.

A new formula for accurate calculation of water activity in sorption calorimetric experiments

A sorption calorimetric method proposed by Wadsö and Markova allows determination of water activity. Calculation of water activity is based on Fick’s first law. The correct form of the Fick’s law takes into account not only vapour diffusion itself but also bulk flow of vapour. A new formula, which uses the correct form of the diffusion law for direct calculation of water activity, is proposed.

An accurate model for prediction of gas hydrate formation conditions in mixtures of aqueous electrolyte solutions and alcohol

AbstractA new thermodynamic model for calculating the hydrate formation temperature of different hydrate formers in aqueous solutions of both electrolytes and a single alcohol has been presented. This method uses a generalization of the Aasberg‐Petersen model for water activity. For calculation of water activity in the presence of electrolytes, the effect of alcohols was taken into account without using any new fitting parameters. The results are in good agreement with published experimental data. Calculated values of the hydrate formation temperature in the presence of alcohols and electrolytes are compared with those obtained by other existing models. The average absolute error of hydrate formation temperature for 524 available experimental data points is about 0.87 K.

Quantitative estimation of seed desiccation sensitivity using a quantal response model: application to nine species of the genus Coffea L.

AbstractSeed desiccation sensitivity was studied in nine species of the genus Coffeaby measuring seed viability after equilibration over various saturated salt solutions. A quantal response model based on the logistic distribution was developed in order to describe the typical S-shaped patterns observed. The closeness of fit of the desiccation sensitivity model was shown, and the assumption that seed desiccation sensitivity follows a continuous distribution within species was verified. For each species, the water content at which 50% of initial viability was reached, WC50, and a specific parameter describing the intra-specific variability, β, were calculated using a non-linear regression. A simplified water sorption model was developed which allowed easy calculation of water activity and water potential corresponding to WC50 (aw50 and Ψ50) for relative humidities ranging between 10 and 100%. Distribution of WC50and Ψ50 (or aw50) in the genus Coffea was homogeneous within the following intervals: from 0.05 to 0.38 g H2O.g−1dw for WC50 and from −168 to −11 MPa for Ψ50. Different classifications of the coffee species studied as regards to their desiccation sensitivity were obtained depending on whether WC50 or Ψ50was used for classification. The continuum for desiccation sensitivity observed within the nine species studied confirmed that coffee is an appropriate material for studying desiccation sensitivity.

Calculation of water activity in surface mould-ripened soft cheeses from their chemical composition

The water activity (A w) of 24 samples of 12 different brands of Brie cheese and another 12 of Camembert cheese was measured at 20°C by four methods (psychrometric, cryoscopic, dew-point hygrometric and isopiestic equilibration). The cheeses were also analysed chemically for their moisture, salt (NaCl), ash and non-protein nitrogen (NPN) contents. A linear regression analysis of data pairs of the variables [NaCl], [Ash] or [NPN] (in g/100 g moisture) and A w (average values of the four methods) yielded the following relations A w=0·9813−0·0045 [NaCl] A w=0·9769−0·0019 [Ash] A w=0·9793−0·0101 [NPN] On the other hand, a multiple linear regression analysis of sets of g ash/100 g moisture, g NPN/100 g moisture and average A w at 20°C led to the equation A w=0·996−0·0029 [Ash] − 0·0106 [NPN] All four equations can be used alone or in conjunction to predict the water activity of surface mould-ripened soft cheeses. The differences between the calculated and measured (average) A w values were similar to those yielded by the four measurement methods used.

Water Activity Calculation by Direct Measurement of Vapor Pressure

ABSTRACTAn improved method for calculation of water activity from vapor pressure manometric data, which takes into account the volume expansion of nonadsorbing gases, the initial pressure in the desiccant flask and the pressure exerted by the desiccant, was developed. The water activity of pure water was measured over a range of nonadsorbing gas pressures and initial pressures in the desiccant flask. Standard deviations in aw of pure water calculated using no corrections, corrections for volume expansion only, and full corrections were 0.221, 0.011, and 0.005, respectively. Correction only for volume expansion may under some conditions result in larger errors than when no corrections are made at all. Based on the improved equation, an experimental modification which reduces loss of water from samples during measurement was proposed.

Measurement and calculation of water activity in Blue cheese

SummaryWateractivity (aw), pH, and several chemical components were determined in 20 samples of Blue cheese belonging to the varieties Bleu de Bresse, Cabrales, Danablu, Edelpilzkäse, Gorgonzola and Roquefort. The aw in these cheeses could be calculated from chemical data on a water content basis (g/100 g H2O) by two simple equations:where SN is the soluble nitrogen.Differences between calculated and measured aw using these relationships were similar to those obtained with a more complex equation described by Rüegg & Blanc (1983).

Role of water activity in ethanol fermentations

A separate role for water activity in the conversion of sugars to ethanol by two strains of yeast is identified. During fermentation of both single and mixed sugar substrates, the water activity was shown to remain constant during the logarithmic growth phase. This is despite the changes in concentration of substrates and product, the constancy reflecting the fact that the greater influence of ethanol on the solution activity is counterbalanced, in the early stages of the fermentation, by its low yield. The end of the log phase of growth coincides with the start of a period of gradually decreasing water activity. For the more ethanol-tolerant strain UQM66Y, growth was found to cease at a constant value of water activity while that for the less tolerant strain UQM70Y depended on both ethanol concentration and water activity. It is argued that water activity is a more appropriate variable than ethanol concentration for describing some of the nonspecific inhibitory effects apparent in ethanol fermentations. A straightforward method for the calculation of water activity during such fermentations based on the use of solution osmolality is presented.

DETERMINATION OF FACTORS CONTROLLING ACCURATE MEASUREMENT OF Aw BY THE VAPOR PRESSURE MANOMETRIC TECHNIQUE

ABSTRACTThe precision and reproducibility of the VPM method has been improved by the simultaneous measurement of water vapor pressure and temperature of the food sample. It has been shown that evacuation of the system substantially lowers food temperature due to evaporation of water. The drop in sample temperature is dependent on water activity of the material and is large at high water activities. Even at steady readings of the manometer, the temperature of the sample is not equal to room temperature. Therefore, calculation of water activity based on room temperature gives erroneous results which are significantly lower than the results based on the measured sample temperature.