Determination Of Water Activity Research Articles

Determination of water activities and osmotic and activity coefficients of the system NaCl–BaCl 2–H 2O at 298.15 K

The water activities of the mixed aqueous electrolyte NaCl–BaCl 2(aq) are measured at total molalities from 0.25 mol kg −1 to about saturation for different ionic strength fractions ( y) of NaCl with y=0.33,0.50,0.67. The results allow the deduction of osmotic coefficients. The experimental results are compared with the predictions of the Zdanovskii, Stokes, and Robinson (ZSR), Kusik and Meissner (KM), Robinson and Stokes (RS), Lietzke and Stoughton (LSII), Reilly, Wood, and Robinson (RWR), and Pitzer models. From these measurements, the Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture. The excess Gibbs energy is also determined.

Feasibility of a new sensor for water activity determination

In this paper, we propose a new way for the determination of aw in fresh and salt meat. The principle is founded on the analysis of a small air volume inside the product. First we describe the fundamental definition of water activity, the state‐of‐the‐art of commercial aw meters, the effects of aw on microbiology in food. In the second step, after a description of the sensor, the principle and the procedure of measurement are explained. The measurements of aw are realized in meat at different stages of drying. Reproducibility and the temperature effect are particularly analyzed.

Vapour pressure of water over saturated solutions of tartaric acid, sodium hydrogen tartrate, sodium tartrate, potassium tartrate, calcium tartrate, barium tartrate, citric acid, disodium hydrogen citrate, sodium citrate, and potassium citrate at temperatures from 277 K to 317 K

Vapour pressures of saturated aqueous solutions of tartaric acid, sodium hydrogen tartrate, sodium tartrate, potassium tartrate, calcium tartrate, barium tartrate, citric acid, disodium hydrogen citrate, sodium citrate, and potassium citrate were determined in the temperature range (277 to 317) K using an electronic hygrometer. Only for tartaric acid, sodium tartrate, potassium tartrate and citric acid was it possible to compare the determined vapour pressures with the isoteniscopic results available in the literature. The measured vapour pressures allow the determination of water activities and molar enthalpies of vaporization of solutions at saturation conditions and osmotic coefficients if the solubilities of these compounds in water are known.

BEHAVIOR OF FOODS STUDIED BY THERMAL ANALYSIS Introduction

In the investigation of foods by thermal analysis and calorimetric techniques, many physico-chemical effects can be observed in the temperature range between −50 and 300°C. These thermal phenomena may be either endothermic, such as melting, gelatinization, denaturation, evaporation or exothermic, such as crystallization, oxydation, fermentation. Glass transitions are observed as a shift in the base line; this information, associated with water content and water activity determinations, is of particular interest in relation to storage of food powders but also for gas retention in powders foreseen to foam when dissolved.

Hygrometric determination of water activities, osmotic and activity coefficients of (NaCl + KCl)(aq) at T = 298.15 K

The purpose of this study is to present a model for the prediction of water activity in multicomponent aqueous solutions containing a common ion from available binary data. The hygrometric method has been used to measure relative humidities for the aqueous electrolyte mixture (NaCl + KCl)(aq) at total molalities ranging from 0.2 mol · kg−1to saturation for different molal ratiosr of NaCl(aq) to KCl(aq) with r= (0.2, 0.5, 1, 2, 3, and 4) at T= 298.15 K. The data obtained have been used to determine water activities and osmotic coefficients. The results show that the values of water activities and osmotic coefficients calculated with the proposed model are close to the experimental ones. This model is also compared with four other models (RS, Pitzer, RWR, and LS II) over the range of the studied total molalities. From the measurements, the activity coefficients of NaCl(aq) and KCl(aq) in the mixture have also been determined.

Water activity in aqueous solutions of homogeneous electrolytes: the effect of ions on the structure of water.

An unconventional method, based on the spectrophotometric measurements of solution absorbance during hydration of aldehydes into diols, was applied for the determination of water activity in aqueous electrolyte solutions. The obtained values of the activity coefficient fW as a function of electrolyte concentration were used to estimate water structure ordering around ions. This method does not suffer from the weaknesses of X-ray diffraction and neutron scattering techniques applied in complementary studies.

Hygrometric Determination of Water Activities and Osmotic and Activity Coefficients of NH4Cl–LiCl–H2O at 25°C

The mixed aqueous electrolyte system of ammonium and lithium chlorides has been studied by the hygrometric method at 25°C. The relative humidities of this system are measured at total molalities from 0.3 to 6 mol-kg− 1 for different ionic-strength fractions y of NH4Cl with y = 0.33, 0.50, and 0.67. The data obtained allow the deduction of new water activities and osmotic coefficients. The experimental results are compared with the predictions of the ECA (extended composed additivity) law proposed in our previous work. The Zdanovskii–Stokes–Robinson (ZSR), the Robinson–Stokes (RS), Reilly–Wood–Robinson (RWR), the Pitzer, and the Lietzke–Stoughton (LS II) models are also compared with our results. Predictions made using these models are, in general, consistent with our results. From these measurements, new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture for different ionic-strength fractions.

Hygrometric Determination of Water Activities, Osmotic and Activity Coefficients, and Excess Gibbs Energy of the System MgSO4–K2SO4–H2O

Ternary aqueous solutions of MgSO4 and K2SO4 have been studied by the hygrometric method at 25°C. The relative humidity of this system is measured at total molalities from 0.35 mol-kg−1 to about saturation for three ionic-strength fractions (y = 0.25, 0.50, and 0.80 of MgSO4. The data allow calculation of water activities and osmotic coefficients. From these measurements, the Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fraction y.

Determination of water activities, osmotic and activity coefficients in aqueous solutions using the hygrometric method

In the present study we use the hygrometric method for the determination of relative humidities of aqueous electrolyte solutions. The relative humidities of KCl(aq), Na2SO4(aq), and NaNO3(aq) have been measured for different molalities and at temperatures from (278.15 to 323.15) K. The data on the relative humidities allow the deduction of the water activities and the osmotic coefficients at the temperature of 298.15 K. The solute activity coefficients are also determined for the three salts using Pitzer’s model. The results obtained by this method are compared with those found in the literature.

PHYSICAL AND CHEMICAL CHANGES IN VACUUM PACKAGED PARMIGIANO REGGIANO CHEESE DURING STORAGE AT 25, 2 AND −25C

ABSTRACTParmigiano Reggiano (PR) cheese, packed in single 20g portions under vacuum, was stored at three different temperatures (25C, 2C and −25C) to evaluate chemical and physical changes in the product during 120 days of storage. There was no increase in lipid oxidation, even though the presence of pentane in the head space showed that the PR samples were already slightly oxidized following the 18 months of ripening process. Color and water activity determinations led to the hypothesis that the lipid fraction migrated to the sample surface when stored at 25C. Other changes observed by the authors influenced the modification of the flavor and a decrease in the overall acceptance of the PR samples packed under vacuum and stored at room temperature. However, they did not seem to be related to lipid oxidation.

CHANGES OF SOME THERMAL AND PHYSICAL PROPERTIES IN MODEL SYSTEMS SIMULATING AN ALCOHOLIC FERMENTATION

Highly simplified model systems, simulating the fermentation of 25.00% sugar-water mixture into a 12. 78% ethanol-water mixture, were considered in order to study their thermo-physical properties. Calorimetric data, viscosity and water activity determinations showed that important thermal and physical changes are associated with the steps of a fermentation process. Besides the increase in ethanol content, other concomitant changes are a strong increase in ethanol vapor pressure and a decrease in water activity and viscosity. DSC measurements of the models during scanning from -140C to 25C showed three peaks: peak 1 (exotherm) covered a temperature range from -l10C to -80C and was attributed to the recrystallization of the ethanol monohydrate; peak 2 (endotherm), with peak temperature around -70C, was attributed to the melting of the alcoholic monohydrate; peak 3 (endotherm) in the higher temperature range from -30C to OC, was due to ice melting. The alcoholic monohydrate appeared only in the final simulated step of the fermentation, where only ethanol and water were present. The inhibitory effect of even small amounts of sugars on the formation of the monohydrate might be related to the increase in viscosity. This would confirm the formation of the ethanol hydrate as kinetically hindered, as it is reported in the literature.

Water Activity, Glass Transition and Microbial Stability in Concentrated/Semimoist Food Systems

ABSTRACTThis review examines recent published suggestions that “water dynamics” may be applied instead of water activity (aw) determination to predict microbial stability of concentrated and intermediate moisture food systems. Factors such as the relative effectiveness of additives for antimicrobial stabilization through aw‐lowering (i.e. glucose vs fructose, glycerol vs propylene glycol) and the importance of glass transition temperature as an indicator of microbial stability were examined, based on available experimental data. Determination of the glass‐rubber transition characteristics and/or the use of the “water‐dynamics” map does not enable prediction of the microbial stability of foods (i.e. inhibition of growth) with confidence. These were no more effective alternatives than the concept of water activity as a basis for predicting microbial growth in foods.

Relative Vapor Pressure Measurements Using a Frequency-Modulated Tunable Diode Laser, a Tool for Water Activity Determination in Solutions

Two-tone frequency modulation spectroscopy (TTFMS) is applied to the determination of water activity using an 820-nm GaAlAs diode laser. By measuring the relative light absorption of water vapor over a solution and over a pure water reference, the water activity in the solution is determined. Fast, nonintrusive, and selective measurements are demonstrated. The detection limit for measuring a relative water vapor pressure change was 0.3%. The application of laser modulation spectroscopy to the determination Of water activity is discussed, and suggestions on increasing the sensitivity and accuracy are presented.

Determination of Water Activity in Ammonium Sulfate and Sulfuric Acid Mixtures Using Levitated Single Particles

Water activities of ammonium sulfate-sulfuric acid mixtures with ammonium to sulfate molar ratios between 0 and 2 were measured by a spherical void electrodynamic levitator at relative humidities (RH) of 0.18–0.90. Since the composition of the solid particles is subject to uncertainty, solution properties were determined relative to the known properties at RH about 0.75–0.90. The data were compared with other measurements and the estimates from the Zdanovskii-Stokes-Robinson (ZSR) method and SCAPE, a newly developed gas-particle equilibrium model, and generally were found to be in good agreement.

SENSORY, CHEMICAL and BACTERIOLOGICAL STABILITY of FRANKFURTERS CONTAINING MILK PROTEINS and CORN GERM PROTEIN FLOUR

Shelf stability of frankfurters extended with milk proteins and corn germ protein flour (CGPF) stored at 1-2C for 45 days was studied by water activity, sensory evaluation, bacteriological tests, and chemical determinations of total volatile nitrogen (TVN), ammonia nitrogen, and TBA values. the incorporation of milk proteins and CGPF affected shelf stability of frankfurters as determined by sensory characteristics, bacteriological test, and chemical analysis. After 45 days of storage, the frankfurters with CGPF added had a stronger atypical aroma and atypical flavor than the all-meat control and products containing milk proteins. However, the atypical notes were relatively low, up to 10.2 on a 60-point scale, and should be considered as weak and acceptable. the all-meat control had a juicier mouthfeel than the other frankfurters. CGPF-containing frankfurters contained an intermediate count (log 3-4 CFU/g) of psychrotrophic bacteria, whereas the other products contained a low bacteria count (log 0-2 CFU/g) after 45 days of storage. There was no formulation treatment effect on TBA values. TVN in frankfurters increased with increasing storage time. the ammonia contents of treatment and control frankfurters were not different during 45 days of storage.

Comparison of methods for determination of high water activities. Application to dairy products and juices: M.A. Esteban, M. Alcalá, A. Marcos and J. Fernández-Salgvero, Food Chem. 35, 153–158

Comparison of methods for determination of high water activities. Application to dairy products and juices: M.A. Esteban, M. Alcalá, A. Marcos and J. Fernández-Salgvero, Food Chem. 35, 153–158

Determination of water activity in brie and camembert cheese varieties by four different methods

The water activity (A w) of single samples from 12 brands of Brie cheese and 12 brands of Camembert cheese was measured at 20°C by using four methods based on different principles, namely psychrometry, cryoscopy, dew-point hygrometry and isopiestic equilibration. The average A w values found (± standard deviation) for the Brie and Camembert cheese were 0·965 ± 0·005 and 0·967 ± 0·009, respectively. The mean difference between methods was less than 0·005 A w units, while the mean deviation from the average A w values was ± 0·005 A w units. The mean deviations from the average A w for the entire population of samples of the two cheese varieties were 0·000, +0·001, +0·001 and −0·003 A w units for the gravimetric (isopiestic), psychrometric, cryoscopic and hygrometric methods, respectively. The average pH values measured (± standard deviation) were 6·96 ± 0·44 and 7·01 ± 0·47 for the Brie and Camembert cheese, respectively.

Comparison of methods for determination of high water activities. Application to dairy products and juices

Water activity was determined in various dairy products (ice creams, yoghourts and flavoured milks) and several juices by three different methods: psychrometric, cryoscopic and gravimetric. The differences between the A W values provided by the three methods were also calculated. Such differences were always smaller than 0·005 A w units, which allows the use of a straightforward method (gravimetric) together with two instrumental methods (psychrometric and cryoscopic) for the determination of high A w values (in the range 0·999–0·900) in the aforesaid types of products.

Determination of water activity of canned fish using gravimetric, hygrometric and psychrometric methods

SummaryThe water activity (aw) of canned mackerel, tuna and sardine was determined using three methods: a gravimetric and two instrumental methods. The aw values obtained by the gravimetric method, which only requires simple laboratory apparatus, were not significantly different from the values obtained by the other two methods, and can be used to determine high aw values.

Effects of Initial Moisture Content and Storage Relative Humidity on Textural Changes of Layer Cakes During Storage

ABSTRACTA study was undertaken to examine the effects of initial moisture content and storage relative humidity on certain textural changes occurring in layer cakes during a 42‐day storage period at 20°C. Water activity and moisture determinations were carried out. Force compression curves from the Instron Universal Testing Machine allowed the estimations of cake firmness, adhesiveness, cohesiveness, and elasticity. Results indicated that increased cake moisture contents led to a decrease in initial cake firmness but did not reduce final cake firmness. Cakes stored at increased relative humidities lost a negligible amount of moisture but continued to increase in firmness and adhesiveness throughout the entire 42‐day period.