Determination Of Water Activity Research Articles

Evaluation Of Precision Estimates For Fiber-Dimensional And Electrical Hygrometers For Water Activity Determinations

The precision of instruments used in 3 collaborative studies conducted within the Food and Drug Administration over a 4-year period (1981, 1982, 1984) for water activity (aw) determinations according to the official AOAC method is evaluated. Calibration responses of the instruments were tested for linearity over the aw range from 0.75 to 0.97. Average absolute percent difference between predicted and assigned aw values for the linear model ranged from 0.3 to 0.7% for a fiber-dimensional hygrometer (Abbeon) and 3 electrical hygrometers (Beckman, Rotronics, and Weather Measure). The calibration responses for another electrical hygrometer (Hygrodynamics) were nonlinear. The fiber-dimensional hygrometer yielded mean aw values and precision estimates that did not differ significantly from those obtained with the electrical hygrometers for (NH4)2SO4slush, KNO3 slush, sweetened condensed milk, pancake syrup, and cheese spread. However, the mean aw value for a soy sauce was 0.838 for the electrical hygrometers compared with 0.911 for the fiber-dimensional hygrometer. The fiber-dimensional hygrometer was affected by a volatile component(s) in the soy sauce that caused an erroneously high aw value. Pooled estimates of reproducibility (Sx) in the 3 studies were 0.008 for the fiber-dimensional hygrometer and 0.010 for the electrical hygrometers; these values were not significantly different from those reported in the study that verified the current official AOAC method.

Determination of water activity in water-in-oil microemulsions

The activities of water in water-in-oil microemulsions of Aerosol-OT/water/alkane have been measured using an indicator technique based on the hydration equilibrium of pyruvic acid. Our systems contained discrete water droplets, and the activity was studied as a function of composition (drop letsize) and of the composition of the droplet core (by mixing with an aqueous cosolvent). The water activity is found to be less than unity in all systems. The presence of dioxane as an aqueous cosolvent in the droplet core significantly reduces the activity of the water.

Statistical Calibration of instruments Using Water Activity Determination As An Example

An example is given of a method for establishing the confidence limits for the calibration of an electronic instrument utilized in measuring water activity. The method consists of the inverse use of a regression line with associated Working-Hotelling-Scheffe bounds for the true line and the construction of limits suitable for repeated use of the derived calibration. Interval estimates are given for the Kaymont-Rotronics Hydroskop DT, calibrated with vapor pressure manometer measurements of equilibrated salt solutions as standards. The statistical approach has general applicability and should be applied routinely for all calibration problems.

Determination of Water and Solute Activities in H2SO4–In2(SO4)3–H2O System

Determination of Water and Solute Activities in H<SUB>2</SUB>SO<SUB>4</SUB>&ndash;In<SUB>2</SUB>(SO<SUB>4</SUB>)<SUB>3</SUB>&ndash;H<SUB>2</SUB>O System

Improved Procedure for Determining Water Activity in a High Range

Abstract A procedure for accurate determination of water activity (aw) in a high range (aw between 0.920 and 0.970) using a Beckman Hygroline apparatus was devised because the prescribed calibration procedure was inadequate. This new procedure uses NaCl solutions as standards and the aw values for NaCl solutions reported by Robinson as the reference scale. A quadratic calibration curve: Recorder reading = b0 + b1aw + b2aw2 was established for each sensor. A 3-point calibration scheme, taking measurements of 0.9m (molal), 1.6m, and 2.4m NaCl solutions, was used before and after a series of aw measurements of samples to estimate b0 and verify the constancy of shape of the calibration curve. The equation was solved for aw to convert each recorder reading to an aw value on Robinson’s scale. This procedure yielded precision and accuracy levels for a single determination of 0.0009 and 0.0013 aw unit, respectively. Accuracy of 0.0009 aw unit was obtained by averaging 2 determinations on different sensors.

Determination and Correlation of the Water Activity of Cheese Whey Solutions

ABSTRACTThis work reports on the experimental determination (and correlation) of water activity (aw) in cheese whey solutions of varying total solids content (up to about 50% solids). It was shown that a satisfactory linear correlation between aw and solids (given as g of solids/100g of water) of whey existed. The aw of whey solutions was close to but always below that of lactose, which indicated that other nonlactose constituents also contributed to aw lowering.

Determination of Water Activities in Aqueous Solutions of Hydrochloric Acid-Metal Chloride

Determination of Water Activities in Aqueous Solutions of Hydrochloric Acid-Metal Chloride

MOLD INHIBITION IN TORTILLA BY DIMETHYL FUMARATE

Nine batches of tortilla containing 0, 0.05, 0.1., 0.2 and 0.4 g of calcium propionate or dimethyl fumarate per 100 g corn flour were prepared. The tortillas were individually packaged in polyethylene bags, stored at room temperature and observed daily for visible signs of mold growth. Samples from each batch were also subjected to the determination of moisture and water activity. The mold-free shelf-lives of tortillas containing calcium propionate ranged from 2–5 days and those of tortillas containing dimethyl fumarate ranged from 2–11 days.

Influence of Ethanol on Determination of Water Activity

The apparent water activity (Aw) of ham treated with ethanol is said to depend upon the method used for measuring it. Samples for examining the influence of ethanol were three kinds of saturated reagent solution (potassium sulfate, ammonium sulfate, sodium chloride and two kinds of salted ground pork. The Aw values of the saturated reagent solutions increased with increase of ethanol concentration as determined by the Japanese official method which is based on the weight equilibrium system, but decreased as determined with an Aw meter (Shibaura Denshi Co., Ltd.) based on the heat conductivity system. However, Aw of the reagent solutions was not influenced by ethanol when determined with a Rotronic Aw meter (immobilized salt sensor); the Aw of the salted ground pork samples decreased slightly with increase of ethanol concentration according to the Rotronic Aw meter.Ethanol was quantitatively detected in 10 out of 11 commercial semi-dried hams. Aw of 3 hams was found by the official method to be more than 0.950, contrary to the Japanese Food Hygiene Law, but was less than 0.940 as determined with the Rotronic Aw meter. Aw determined by the official method showed a statistical correlation with the concentration of ethanol in the commercial hams at the 5% level of significance. Consequently, the official method was concluded to be unsuitable for measuring Aw of hams treated with ethanol.

Water Activity Determination with the Proximity Equilibration Cell

ABSTRACTThe objective was to develop a new method for water activity (aw) determination. A reference material (circle of filter paper) of known sorption isotherm was equilibrated to the sample. Sorption isotherm data for this paper were obtained by equilibrating for 24 hr to each of six salt slushes; these data were described well by a linear equation relating moisture content to log (1 − aw). This equation served as the calibration curve. Results showed that this new method (1) does not require a vacuum, (2) gives a linear calibration equation, (3) is applicable over the aw range from 0.40–0.98, and (4) with minor replication can bring the standard deviation of the mean to less than ± 0.01.

A study of water activity prediction for molasses solutions

This work reports on the experimental determination of water activity in solutions of molasses (“blackstrap”) and its comparison with sucrose. An attempt is made to predict the water activity of molasses from the knowledge of its main nonelectrolyte components, namely, sucrose, invert sugar, and natural acids. The results show that the prediction gives reasonably good results for aw below ~0.92; at higher water activities electrolytes present in molasses also play a role in aw depression; allowance for this contribution has to be made to accurately predict the behavior of molasses. © 1981, American Chemical Society. All rights reserved.

A Proximity Equilibration Cell for Rapid Determination of Sorption Isotherms

ABSTRACTSorption isotherm data are obtained by instrumental determination of water activity at a known moisture content or determination of moisture content after equilibration against a saturated salt solution. The latter method is simpler and the salt solutions are primary standards. However, the equilibration takes a long time, one to ten weeks, depending upon food composition. Thus, the objective of this work was to devise a method that retains the saturated salt solution but accelerates the rate of equilibration. In an equilibration environment between a food product and a saturated salt slurry, the driving force is the difference in vapor pressure; therefore the faster the vapor space reaches equilibrium with the saturated salt slurry, the quicker the maximum driving force for water absorption will be applied to the sample. It was felt that a reduction in the size of the usual large desiccater to a single sample size would provide the necessary area to volume ratio. The vessel chosen was a small plastic chamber (65 mm/53 mm). In this chamber, the surface area to vapor volume ratio was 0.3101 as compared to 0.0335 for the standard desiccator. The sample was contained in a standard aluminum weighing tray modified by removing a 44 mm diameter circular section from the bottom. This was replaced with a 47 mm diameter circle of Whatman No. 1 quantitative filter paper to support the sample and at the same time allow transmission of moisture. This would allow water molecules to travel in a straight line and thus the shortest distance between the saturated salt slurry and the sample. This small vessel with a single sample supported on a filter paper will be referred to as a Proximity Equilibration Cell (PEC). Using this technique, it was found that a 2/mm deep sample of corn starch required only 6 days for complete equilibration, as compared to 21 days for the conventional desiccator. Thus, the PEC satisfied the objective by reducing time by 70%. In making comparisons with the same salts in conventional and the PEC, it was noted that the end point was higher in the latter. Evidently, absorption in case of the conventional desiccator was so slow at the end that no weight gain could be detected in 24 hr but equilibrium had not yet been attained. In contrast, the PEC equilibrated so rapidly that it allowed a closer evaluation of true equilibrium.

A STUDY OF THE WATER ACTIVITY LOWERING BEHAVIOR OF POLYETHYLENE GLYCOLS IN THE INTERMEDIATE MOISTURE RANGE

ABSTRACTThis work reports on the experimental determination of water activity (aw) in aqueous solutions of polyethylene glycol 200, 400, and 600, respectively. The measurements were made in the aw range of most interest to intermediate moisture foods formulation (about 0.80–0.95). The observed behavior was very well fitted by Norrish's (1966) equation and a simple relationship was found between Norrish's correlating constant and molecular weight of polyethylene glycols. It was found that this relationship may be explained through a theoretical analysis of the thermodynamic behavior of macromolecules in solution.

Activity of water in frozen systems

Some fundamental thermodynamic aspects of water activity in general, and of water activity in a frozen medium especially, are reviewed briefly. Two types of calculations for determination of water activity in a frozen state will be mainly discussed. It is evident from these equations, that the water activity in a frozen state, that is, a three phase system, solid-liquid-gas, depends only on the temperature. This means that the dissolved substances have no influence on the water activity, but for the limitation that the water crystallizes out as pure ice. This was also assumed in our paper. In part two of this work, the influence of water activity in frozen systems on diffusion phenomena, enzymatic reactions and growth of microorganisms, is discussed. The aim, here, is to try to relate the said phenomena to common physical parameters.

Psychrometric Determination of Water Activity in the High aw Range

A method is described to measure the water activity (aw) of foods in the range of 0.935 to 1.0 using a psychrometer. Samples (0.15 to 0.35 g) were equilibrated for 2 h in the sample chamber before cooling the thermocouple and recording the subsequent voltage on the instrument. The aw was read off a standard curve prepared from sodium chloride solutions of known water activity values. Results show that the method was considerably better than the graphical interpolation method and comparable to the microcrystalline method but was much quicker and required less effort.

WATER ACTIVITY DETERMINATION: A COLLABORATIVE STUDY OF DIFFERENT METHODS

ABSTRACTSeven methods of water activity (aw) measurement were tested in a collaborative study between three laboratories. These methods included the direct measurement of vapor pressure, an isopiestic technique and several electronic devices. Both saturated salt solutions and foods were measured. When compared to standards, the vapor pressure manometric (VPM) technique gave the best results. However, the absolute values of the standards are questionable. The results of the comparison of the values for foods showed a range of ##0.02 aW units. Thus, one can question the validity of literature values reported to three decimal places or the absolute values for limits on microbial growth. This research suggests that some standards must be set for all research groups and shows that overall, the VPM is probably the best and most accurate method

Determination of water activities of dilute electrolyte solutions

Determination of water activities of dilute electrolyte solutions

Studies on the Liquid-Liquid Partition Systems. II. The Determination of Water Activity by the Solvent Extraction Technique

Abstract An attempt has been made to apply the solvent extraction technique to the determination of the water activity in various electrolyte solutions. Benzene was agitated with a sample solution until the distribution equilibrium was attained, the water content in the benzene was then measured by Karl Fischer titration. The water activity was calculated on the basis of the water content. The results agree with the data obtained by the vapor pressure method. On the basis of these, a solvent extraction procedure has been recommended as an easy and rapid method to determine the water activity in various electrolyte solutions.

Water Activity of Flour at High Moisture Contents as Measured with a Pressure Membrane Cell

SUMMARYA method was developed for the determination of water activities of flour at high moisture contents. In this method a flour sample was allowed to equilibrate with water contained in a porous membrane while under a certain constant mechanical pressure. The cell was fitted with a porous membrane which allowed the transfer. of moisture from and to the sample. After equilibrium was achieved the sample was analyzed for moisture content. A thermodynamic relationship enabled calculation of the water activity corresponding to the pressure used in the cell. Results obtained with this method were in agreement with those obtained with the constant‐relative‐humidity desiccator technique in the over‐lapping range of water activities. The two methods were also in agreement that the water activity of flour was not affected by the particle size distribution.