Vapor pressure and water activity measurements of saturated salt solutions made with D2O at 20°C

The water activity (a_w) values of 10 saturated salt solutions made with D₂O were measured in this study at 20°C: MgCl₂, K₂CO₃, Mg(NO₃)₂, KI, NaCl, KCl, ZnSO₄, BaCl₂, KNO₃, and K₂SO₄. These salts were selected because they represent a wide range of water activity values, from approximately 0.33 to 0.98. The a_w values were measured using two methods: (1) a vapor pressure manometer, and (2) an electronic chilled-mirror (AquaLab) instrument. The a_w values obtained by the manometer method were slightly, but consistently, higher than the a_w values obtained by the electronic chilled-mirror instrument. The a_w values of the saturated salt solutions made with D₂O (as measured by both methods) were also slightly, but consistently, higher than the a_w values of the same saturated salt solutions made with H₂O at 20°C. ©     

Prediction and Measurement of the Water Activity of Selected Saturated Salt Solutions at 5°C and 10°C

A theoretical approach, based on the thermodynamic properties of strong electrolyte aqueous solutions, was used to calculate the a_w at 5°C and 10°C of selected saturated salt solutions frequently used as standards in the range of microbial growth. The results agreed very well with experimental measurements at both temperatures for most salts studied, which included NaBr, NaCl, SrC1₂, (NH₄)₂SO₄, KCl, BaCl₂, and K₂SO₄. For KNO₃ the agreement was somewhat less satisfactory and the reasons are discussed.     

A World Survey of Water Activity of Selected Saturated Salt Solutions used as Standards at 25°C

This paper presents and discusses the results of a world survey of water activity (a_w) of selected saturated salt solutions used as standards by different researchers engaged in a_w determinations for food-related applications. Salt slurries cover the range of a_w about 0.57-0.97 at 25°C and are NaBr, NaCl (NH₄)₂SO, KCl, BaC1₂. 2H₂O, KNO₃ and K₂SO₄. The results indicate that there is a good agreement on the exact value to be assigned to NaBr, NaCl, KC1 and BaC1₂.2H₂O, and to a lesser extent, to K₂SO₄. However, a significant discrepancy exists between researchers on the a_w value assigned to (NH₄)₂SO₄ and KNO₃.     

Theoretical Prediction of the Water Activity of Standard Saturated Salt Solutions at Various Temperatures

A theoretical approach was used to predict the water activity (a_w) at 15°C and 35°C of selected saturated salt solutions used as standards in the range of microbial growth (～0.57–0.97). For this purpose a rigorous thermodynamic model to predict osmotic coefficient was used. The results agreed very well with experimental measurements at 15°C and 35°C for most salts studied, which included NaBr, NaCl, (NH₄)₂SO₄, KCl, BaCl₂, and K₂SO₄. For KNO₃ the agreement at the higher temperature was somewhat less satisfactory and the reasons for this behavior are discussed.     

Measurement of Water Activity of Salt Solutions and Foods by Several Electronic Methods as Compared to Direct Vapor Pressure Measurement

The Kaymont-Rotronics Hygroskop DT, the Beckman-Sina and the Protimeter were tested for their ability to measure water activity (a_w) of five foods over the 0.11 – 0.85 a_w range. Each instrument was calibrated at 30 ± 0.1°C against ten saturated salt solutions using a modification of the AOAC Method 32.004-32.009. Regression of the reading versus true a_w, as determined by the vapor pressure manometer (VPM), gave high r² values for the saturated salt solutions between 0.32 – 0.85 a_w. An error of approximately 0.01 a_w units was found using the 1 hr data but no significantly better regression line was found using the 24-hr data as compared to I hr data. Measurement of the a_w of five foods, however, gave values differing by an average of 0.051 a_w units as compared to the VPM readings. This study demonstrates justification of the FDA cutoff a_w value of 0.85 for low-acid foods as a margin of safety.     

Effect of Adjusting the Water Activity to 0.95 with Different Solutes on the Kinetics of Thiamin Loss in a Model System

The kinetics of thiamin degradation in a model system of pH 6.0 and water activity (a_w) 0.95 controlled with NaCl, KC1, glycerol or Na₂SO₄, was studied. The type of solute used to adjust a_w, had a dramatic influence on the rate of degradation of thiamin. The loss of vitamin increased in the following order: NaCl > KCl > glycerol > Na₂SO₄. Activation energies, however, were independent of the type of solute and ranged between24–28 kcallmole.     

Determination and correlation of the water activity of unsaturated, supersaturated and saturated trehalose solutions

The water activity (a_w) of unsaturated and supersaturated trehalose solutions (and some sucrose solutions) was determined using a dew point hygrometer, and correlated using an equation originally proposed by Norrish [1966. An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups. Journal of Food Technology, 1,25] for predicting water activity in binary non-electrolyte solutions. The a_w lowering behaviour of trehalose was found to be almost identical to that of sucrose solutions at same concentrations; however, due to the lower solubility of trehalose the a_w of their saturated solutions is higher than that of sucrose ones. The a_w of saturated trehalose solutions at 20, 25, 30, 35 and 40 °C was also determined and found to be between 0.953 and 0.928. These values agreed well with predictions made using a_w data for trehalose solutions calculated at the solubility concentration at the various temperatures.     

Water Adsorption of Peanut Kernels in a Closed System with Respect to Moisture Transfer

Adsorption isotherms of peanut kernels, Tainan # 9 and #11, were determined at 30°C and used as reference to determine the a_w at equilibrium. The changes of the apparent a_w and moisture of three weighed dried kernels in closed systems stored with distilled water were determined in an electric hygrometer and a series of vessels at 30°C. A nonequilibrium state of moisture and apparent a_w gradients of the kernels was observed in the early stages of 144 hr storage. In a 90 min experiment with the hygrometer at 30°C, the vapor adsorption rate and final apparent a_w decreased in order with the increase of the number of kernels, from 1 to 7, stored with saturated K₂Cr₂O₇ salt (a_w= 0.98 at 30°C). Vapor adsorption rate and final apparent a_w of Tainan # 9 were slightly higher than those of # 11.     

Effects of pH and Salts on Physical and Mechanical Properties of Pea Starch Films

To identify the significant contribution of intermolecular hydrogen bonds of starch molecules to the film structure formation, pH of film‐forming solutions was adjusted and also various salts (NaCl, CaCl₂, CaSO₄, and K₂SO₄) were mixed into the glycerol‐plasticized pea starch film. The film made from pH 7 possessed the highest tensile strength‐at‐break (2 times) and elastic modulus (4 to 15 times) and the lowest elongation‐at‐break compared with those of the films made from acid and alkali environments. The pH 7 film also has the highest film density and the lowest total soluble matter. At the level of 0.01 to 0.1 M of CaSO₄ and 0.1 M of K₂SO₄ in a kilogram of starch, the water solubility of the film increased, while chloride salts slightly lowered the solubility. NaCl and CaSO₄ reduced water vapor permeability (WVP), while CaCl₂ slightly increased WVP at 0.01 and 0.06 M concentrations, and K₂SO₄ significantly increased WVP at 0.03 and 0.15 M. Presence of salts increased tensile strength (5 to 14 times than the control films) and elastic modulus (35 to 180 times) of starch film at 0.01 to 0.03 M of CaSO₄ and K₂SO₄. Elongation‐at‐break increased significantly as salt concentration increases to an optimal level. However, when the concentration exceeded above the optimal level, the E of starch films decreased and showed no significant difference from the control film. Overall, the addition of salts modified physical and mechanical properties of pea starch films more than pH adjustment without any salt addition.     

Application of Mid- and Near-Infrared Spectroscopy for the Control and Chemical Evaluation of Brine Solutions and Traditional Sea Salts

Mid- and near-infrared spectroscopy methodologies were explored for the analysis of brine solutions and traditional sea salt samples. Brine solutions from different salt pans, corresponding to different stages of sodium chloride crystallisation, were collected. A total of 61 dried and non-dried traditional sea salts were also analysed. Partial least squares regression with leave-one-out cross-validation strategy was applied for the calibration of inorganic constituents Ca²⁺, Mg²⁺ and K⁺, alkalinity as HCO ₃ ^? , SO ₄ ^2? , NO ₂ ^? and NO ₃ ^? and phosphate in brine solutions. Promising results were obtained with the near-infrared (NIR) methodology for brine solutions with coefficients of determination R ²?>?0.90 for Mg⁺², K⁺, HCO ₃ ^? and SO ₄ ^2? . Using mid-infrared, the calibration for H₂PO ₄ ^? was R ²?=?0.85. In relation to the sea salt samples, the strategy adopted was the re-sampling based cross-validation using different spectral pre-processing treatments. In this case, the calibrations using the two IR methodologies fell bellow acceptable levels for the techniques; however, by comparing the R ² coefficient, the results were slightly better when using the NIR spectra of dried sea samples. In general, these results open a new possibility for the IR applications and also bring an opportunity for continuing with the NIR characterization for dried sea salt samples.     

Effect of Water Activity on the Sensory Crispness and Mechanical Deformation of Snack Food Products

Potato chips, popcorn, puffed corn curls, and saltines were equilibrated to various water activities (a_w) over saturated salt solutions. Sensory panels determined the crispness and textural hedonic quality of the humidified products by the magnitude estimation technique coupled to a verbal concept scale. Critical water activities (a_c), where the products became organoleptically unacceptable, generally fell in the 0.35–0.50 a_w range. Instron analyses showed that the force-deformation curve changed distinctly near the a_c for saltines and puffed corn curls, while the curve changed more gradually with increasing a_w for popcorn. Potato chips did not produce a consistently shaped force-deformation curve. The cohesiveness value of popcorn was found to be a good indicator of its sensory crispness.     

Characterization of the Effect of Solutes on the Water-Binding and Gel Strength Properties of Carrageenan

The water activity of carrageenan gels with incorporated solutes was determined by cryoscopic osmometry. Kappa (k), kappalambda (kλ) and iota (κ) carrageenan were utilized at concentrations of from 0.1 2% solids producing a sol, viscous sol, and gel. The solutes used were sucrose, NaCl, Na₂SO₄, KCl, NH₄Cl and urea at concentrations ranging from 0.05–1.0 kinetic units per kg H₂O. All the solutes were found to give a_w values as measured cryoscopically withhin 0.0005 units of literature values. The water activity of pure carrageenans at 0.1–2% solids was greater than 0.999. Addition of some solutes increased the water binding of the carrageenan-solute-water system as was found by a lower a_w than expected. This interaction effect increased with increasing concentration of solute and carrageenan and was greater for solutes with lower activity coefficients. The interaction effect increased in the following order: Na₂SO₄, NaCl, KCl, and NH₄ Cl. The force required to rupture the gel was measured using the Instron Universal Testing Machine. Solutes were found to influence the gel strength of k and kλ carrageenans, but not ι carrageenan. The solutes increased kλ carrageenan's gel strength in increasing order: urea, sucrose, Na₂SO₄, NaCl, NH₄Cl, and KCl and k carrageenan's gel strength in increasing order: sucrose, Na₂SO₄ and NaCl. There appears to be no simple relationship between gel strength and water-binding when solutes are added to carrageenan.     

Moisture adsorption behavior of banana flours (Musa paradisiaca) unmodified and modified by acid‐treatment

The moisture sorption isotherms of untreated banana flour (UBF) and acid treated banana flours (ATBFs) were determined using the static gravimetric method of saturated salt solutions at temperatures of 30°C. The range of water activities (a_w) was calculated to be between 0.14 and 0.97. The equilibrium moisture content absorption data were fitted to four sorption models that differ in the information that can be obtained from each one: Brunauer‐Emmett‐Teller equation (BET), Guggenheim, Anderson and de Boer (GAB), Smith, and Iglesias‐Chirife. Monolayer moisture content (X₀) for UBF and ATBFs were found in the range of 4.06–5.47 (BET model) and 3.87–5.88 (GAB model). The GAB model was found to be the most suitable model to describe the isothermal water sorption of UBF and ATBFs in the intervals proposed of a_w. The X₀ values of both models (BET and GAB) increase with increasing a_w. The Banana flour treated for 11 days (ATBF₃) presents the highest value of X₀ compared with all samples. This result suggests that mechanism of adsorption of water and molecular structure in ATBFs was affected, attributed to changes in morphology and crystallinity of the samples with treatment.     

Influence of water activity on microbial development in minced,cured pork,in relation to gas headspace composition

The water activity (a_w) of a cured minced meat mixture was reduced with NaCl or glycerol + NaCl and the products stored in air, N₂ or CO₂ at 4°C. The lower the a_w, the lower was the growth rate of the total microflora except when a_w was reduced below 0·98 with glycerol and products were stored in CO₂, when the growth rate was independent of a_w. At a_w 0·94 NaCl was more effective than glycerol + NaCl in reducing the growth rate. In air the final microflora of products stored at a_w ≤0·96 consisted primarily of yeasts while at 0.98 a_w, both Lactobacillus (62%) and yeasts (38%) were found. In CO₂ and N₂Lactobacillus predominated at 0·94–0·98 a_w when 2% NaCl or NaCl + glycerol were used. At 0·94 a_w, produced using 6% NaCl, a heterogenous final microflora was found, including Micrococcus and Staphylococcus. An increase in the microbial shelf-life of meat products may be obtained by reducing the a_w, particularly for use with storage in N₂- and CO₂-atmospheres. The type of solute chosen to regulate a_w may affect the solubility of CO₂, which in turn affects the shelf-life.     

Moisture Sorption Isotherms for Un-Modified and Osan-Substituted Dextrin and Gum Acacia Used as Carrier Materials for Spray Dried Encapsulation of Flavoring Materials

Moisture sorption isotherms of ten un-modified and octenyl succinic anhydride substituted dextrin and gum acacia samples were determined gravimetrically via equilibration over saturated salt solutions (a_w ranged from 0.11 to 0.85) at 23 and 35°C. The degree of octenyl succinic anhydride substitution did not affect moisture sorption characteristics for dextrin samples. For gum acacia, increased levels of octenyl succinic anhydride substitution resulted in a decreased affinity for moisture in the range of 0.3 to 0.7 a_w. The sorption data were well described by the BET and GAB models.     

A Factorial Approach to Modeling aw of a Multicomponent Food in the High Moisture Range (aw 0.90–1.00)

Water activity (a_w) depression in a multicomponent food such as fish sausage was investigated using a 4x3x3 factorial design on combinations of NaCl, nonfat dry milk (NFDM) and lard added to a fixed amount of minced fish. The effects of NaCl, NFDM, lard and the interaction NaCl x NFDM, were significant. The responses across levels of the above factors were linear. From the magnitude of effect of each factor, two extended models for predicting a_w were derived in terms of molal salt concentration and two empirical parameters called water activity depressing capacity, d_i, and water binding capacity, WBC, respectively. Both were demonstrated to predict accurately a_w of newly formulated fish sausage.     

Determining salt concentrations for equivalent water activity in reduced-sodium cheese by use of a model system

The range of sodium chloride (salt)-to-moisture ratio is critical in producing high-quality cheese products. The salt-to-moisture ratio has numerous effects on cheese quality, including controlling water activity (a_w). Therefore, when attempting to decrease the sodium content of natural cheese it is important to calculate the amount of replacement salts necessary to create the same a_w as the full-sodium target (when using the same cheese making procedure). Most attempts to decrease sodium using replacement salts have used concentrations too low to create the equivalent a_w due to the differences in the molecular weight of the replacers compared with salt. This could be because of the desire to minimize off-flavors inherent in the replacement salts, but it complicates the ability to conclude that the replacement salts are the cause of off-flavors such as bitter. The objective of this study was to develop a model system that could be used to measure a_w directly, without manufacturing cheese, to allow cheese makers to determine the salt and salt replacer concentrations needed to achieve the equivalent a_w for their existing full-sodium control formulas. All-purpose flour, salt, and salt replacers (potassium chloride, modified potassium chloride, magnesium chloride, and calcium chloride) were blended with butter and water at concentrations that approximated the solids, fat, and moisture contents of typical Cheddar cheese. Salt and salt replacers were applied to the model systems at concentrations predicted by Raoult's law. The a_w of the model samples was measured on a water activity meter, and concentrations were adjusted using Raoult's law if they differed from those of the full-sodium model. Based on the results determined using the model system, stirred-curd pilot-scale batches of reduced- and full-sodium Cheddar cheese were manufactured in duplicate. Water activity, pH, and gross composition were measured and evaluated statistically by linear mixed model. The model system method accurately determined the concentrations of salt and salt replacer necessary to achieve the same a_w as the full-sodium control in pilot-scale cheese using different replacement salts.     

Development of a validated model to describe the individual and combined water activity depressing effects of water soluble salt, sugar and fat replacers

The relationship between water activity (a_w) and concentration of NaCl, sucrose and various simple and commercial salt, sugar and fat replacers was determined. The model originally developed by Favetto and Chirife (1985) and extended by Roa and Tapia (1998) was successfully used to model the relationship between a_w and molality of the binary and multi-component aqueous solutions. These models were then reparameterized to enable the a_w to be modeled as function of the water phase concentrations of binary and multi-component solutions. This reparameterization enables the modeling of the a_w depressing effects of compounds for which sufficient data required to calculate the molecular weight cannot be obtained. Both models were successfully used to predict the a_w of some food products including meat and fish products and mayonnaises with average deviations of 0.0052 from the measured a_w, and bias and accuracy factors of 0.995 and 1.0057, respectively.     

Reduction of nitrate prior to kjeldahl digestion

A new method is described for reducing nitrate prior to Kjeldahl digestion of soil and crop samples. Zinc is used to reduce CrIII to CrII, which in turn reduces NO₃—N. Zinc powder is mixed with the sample, a solution of CrK(SO₄)₂ in H₂SO₄ added and the mixture allowed to stand at room temperature for 2h. The usual Kjeldahl reagents (K₂SO₄, CuSO₄ and H₂SO₄) are then added and the digestion completed in a digestion block at 360°C. Nitrate is quantitatively converted to NH₄—N, whether on its own, in plant material or in soil.     

Uncertainty analysis of hygrometer-obtained water activity measurements of saturated salt slurries and food materials

Water activity is a critical food stability parameter. Thus, assessment of the uncertainty associated with the instruments and measurement procedures used to obtain a_w data is critical and can be accomplished using uncertainty analysis. Uncertainty analysis was applied to a_w data obtained for two saturated salt slurries (potassium acetate and potassium chloride) and two food materials (corn flakes and grape jam) at 25 °C using a commercially available dew point hygrometer, the AquaLab 3TE. Sources of uncertainty investigated included sample and dew point temperatures, sample lot, cup volume, and sample preparation (“as is” or ground), and day to day changes. Uncertainty analysis revealed that the magnitude of the different sources of uncertainty depended on material type, but that the dew point hygrometer instrument uncertainty was always significantly smaller than each of the other sources of uncertainty investigated, except for jam cup volume, which was exceeding small. Uncertainty analysis also revealed that dew point hygrometer accuracy was better than the ±0.003 a_w value reported by the manufacturer. Measurement procedures were identified that could be implemented to reduce a_w measurement uncertainty for corn flake and jam food materials. The uncertainty analysis used herein can also be applied to investigate other food materials and/or instruments.