Water Activity Measurements Research Articles

Measurement of water activity of saturated salt solutions with the vapour pressure manometer

The factors which can affect the results of water activity measurement with the vapour pressure manometer (VPM) are analysed. The manometer reading (h l ) is dependent on the volume (V c ) and water activity (a w ) of the sample. The value of h 2, on the other hand, is affected by the solubility of gases in the solution and the volume of the sample. Hence, it is recommended that the volume of the sample be kept constant when using the VPM technique. The time to reach equilibrium is dependent on the a w of the solution and the temperature of measurement. However, these variables do not explain all the variability of the equilibration time observed in this work. A marked effect of the evacuation operation on the solution temperature as a result of adiabatic evaporation of water was observed. The effect of the evacuation of the manometer on the water content of the sample is not clear. However, it is recommended that the water content of the sample be measured after its a w measurement, and not before. The reading of h 2 is shown to have a negligible effect on the value of the correction factor and so a simplified equation for the correction factor is developed.

Enzymatic ester synthesis with continuous measurement of water activity

Ester synthesis from aliphatic monoalcohols and organic acids was investigated by using a microbial lipase. The reaction medium only contained the substrates and the enzyme without addition of water or organic solvent. During the reaction, water was produced and the water activity ( a w) increased. Batch reactors and continuous-flow reactors were used. In batch, the a w was 0.13 at the beginning of the reaction and increased to reach a plateau at 0.77, after which ester synthesis continued without modification of the a w. Different alcohols and acids were tried in solid-liquid reactors, and in all cases synthesis occurred, leading to a significant increase in the water activity. For continuous-flow reactors, the use of silica beads retaining water inside the reactor where the enzymatic reaction took place resulted in some control of the enzymatic reaction by changing the a w.

Mixed Saturated Salt Solutions as Standards for Water Activity Measurement in the Microbiological Growth Range

ABSTRACTPresent work explores the feasibility of using mixed saturated salt solutions to provide additional solutions of known water activity (aw). The mixed saturated salt solutions studied were: K2SO4‐KNO3, K2SO4‐MgSO4, KCl‐KNO3, NaCl‐KCl and NaCl‐NaNO3. The aw of the ternary saturated salt mixtures was predicted using a theoretical model of the thermodynamic properties of mixed aqueous electrolyte solutions. Prediction of aw compared very well with experimental determinations.

Storage stability of coconut milk powder

AbstractSpray‐dried coconut milk powder prepared with additions of skim milk and dextrin was stored in packages for 6 months at room temperature (30°C and 80% RH). The packaging materials used were aluminum laminate (vacuum and air packed) and polypropylene. The sorption isotherm of the powder showed that the monolayer of water molecules was complete when the relative humidity was 27% which corresponded to a moisture content of 5%. Measurements of water activity, moisture content, peroxide value and solubility index indicated that lipid oxidation is the limiting factor in the stability of the powder. Its maximum shelf‐life was 4 months when air or vacuum packaged in aluminum laminates and 1 1/2 months in polypropylene.

Revised PEC method for measuring water activity

SummaryThe Proximity Equilibration Cell (PEC) method of Lenart & Flink (1983) for measuring water activity (aw) in foods was revised by improving equilibration cell design and cell handling procedures. Calibration methods were changed to eliminate time and temperature variation effects on measured (aw). The PEC cells are inexpensive, easy to use and can measure aw over the range 0.4–1.0, although accuracy is greatly improved above 0.6. In a study of osmotic concentration of apple (Palacha & Flink, 1987) over 1000 aw measurements were made over a 3‐month period. Values of aw for raw apple (0.985) and sucrose osmosis solutions were in good agreement with literature values.

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.

Sorption equilibrium and hydration studies of lysozyme: water activity and 360-MHz proton NMR measurements

Sorption equilibrium and hydration studies of lysozyme: water activity and 360-MHz proton NMR measurements

Cryoscopic approach to water activity measurement of non-liquid foods: Application to cheese

The initial freezing points (fp) of aqueous cheese extracts were related (r = 0·99) to the water activity (A w ) of the starting non-extracted cheeses by the equation A w = 1·0162 + 0·0981 fp. Comparisons of the cryoscopic procedure for A w evaluation in cheese with a reference method (thermocouple psychrometer) show that differences between the methods were, in the majority of cases, lower than 0·02 A w units and, consequently, the cryoscopic approach may be used in the cheese industry as a quality control technique. Cryoscopic measurements gave better estimates of A w than chemical analysis.

Critical review of water activities and microbiology of drying of meats.

The drying of meat is one of the least expensive and most effective ways to preserve meat. This article reviews the influence of intrinsic and extrinsic parameters of foods in relation to the drying of meat and the role of microbial activities in the drying and storage of meat. The critical role of water activity and its relationship with other physical and chemical parameters and microbial survival in the drying of meat is also reviewed in detail. This article ends with a description of the various methods to measure water activity in foods. The conclusion is that the drying of meat is a highly complex phenomenon which is influenced by the composition of meat, its water holding capacity, the processing of meat, and various microorganisms found on meat before and after the drying process. There is no doubt that the drying of meat is an effective preservation process, but attention should be placed on the possible survival of microorganisms in the process in order to prevent spoilage and food-borne diseases.

Water activity measurements with single suspended droplets: The NaCl-H 2O and KCl-H 2O systems

The levitation of a single micrometer-sized particle in a controlled water vapor environment is shown to be a novel experimental technique for measuring the thermodynamic properties of concentrated electrolyte solutions. Water activity data are obtained for the aqueous solutions of NaCl and KCl at 25°C over the entire concentration range up to the critical supersaturation concentration. Empirical constants are derived from a best fit of the data to the extended form of the Debye-Hückel equation.

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).

Effectiveness of Available Filters for an Electric Hygrometer for Measurement of Water Activity in the Food Industry

ABSTRACTThe effectiveness of the filters available for the Novasina Hygrometer for measurement of water activity (aw) in intermediate moisture foods, most of them suspected of being potential sources of sensor contamination, was investigated. Foods studied were apple preserve, chocolate topping, honey, milk jam, salted anchovy, intermediate moisture beef, raw ham, dry salami, smoked pork sausage, provolone and roquefort cheese, tomato ketchup, semimoist pet food, concentrated tomato paste and Worcestershire sauce. For most foods, the use of an adequate filter permitted an acceptable precision in aw measurement; however, neither of the filters studied allowed measurement of aw in the pet food containing propylene glycol.

Technical note: Water activity measurement near to 1.00

Technical note: Water activity measurement near to 1.00

Improving Accuracy and Precision of Water Activity Measurements with a Water Vapor Pressure Manometer

ABSTRACTAccuracy and precision of water activity measurements using vapor pressure manometry can be significantly improved by taking into account the volume change that occurs when water is eliminated from the air‐water vapor mixture. It is shown that this can be done either by designing the apparatus according to a design constraint equation presented in this paper or by using a new equation to calculate water activity correcting for volume change.

A novel isopiestic measurement of water activity in concentrated and supersaturated lithium halide solutions

Dans cette methode des gouttes microscopiques de solution aqueuse de LiBr et LiI a temperature ordinaire sont limitees electrodynamiquement dans une chambre close. La temperature de l'eau varie entre −74 et +21°C

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

ABSTRACTThe Kaymont‐Rotronics Hygroskop DT, the Beckman‐Sina and the Protimeter were tested for their ability to measure water activity (aw) of five foods over the 0.11 – 0.85 aw 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 aw, as determined by the vapor pressure manometer (VPM), gave high r2 values for the saturated salt solutions between 0.32 – 0.85 aw. An error of approximately 0.01 aw 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 aw of five foods, however, gave values differing by an average of 0.051 aw units as compared to the VPM readings. This study demonstrates justification of the FDA cutoff aw value of 0.85 for low‐acid foods as a margin of safety.

Sensor Contamination with Organic Volatiles During Water Activity Measurements with an Electric Hygrometer

ABSTRACTThis work investigates sensor contamination of an electric hygrometer by organic volatiles during water activity (aw) measurement in foods and volatile‐containing model systems. Various aromatic and flavor chemicals in aqueous solutions, at a concentration of O.1‐0.5%, may contaminate the sensor leading to erroneous results in aw measurement. Various condiments and seasonings also contain volatile compounds which produced a significant effect on sensor performance. Some glycols were also found to contaminate the sensor during aw measurement.

Filament Hygrometer for Water Activity Measurement: Interlaboratory Evaluation

A method is presented for measuring water activity (aw) of foods with a filament hygrometer. An interlaboratory evaluation carried out for the Nordic Committee on Food Analysis demonstrated that measurements can be made with a repeatability and reproducibility of 0.005 and 0.010 aw, respectively. The critical parameters of aw measurements are defined and discussed in relation to the accuracy and reproducibility required. The need for strictly standardized procedures for maintenance of the equipment, sample treatment, calibration, and measuring is emphasized.

Water Activity Measurements with A Capacitance Manometer

ABSTRACTThe design, use and performance of a capacitance manometer for the measurement of water activity (aw) are evaluated. The device consists of a sensitive capacitance sensor for measuring the vapor pressure of a sample. This value is then divided by a known value for the vapor pressure of pure water at a specific temperature to obtain aw. Precision and accuracy of this Instrument are determined and compared to a conventional electric hygrometer. These parameters are similar to the hygrometer; however, the principal advantages of the capacitance manometer may occur in circumstances in which direct measurements of aw are required and situations in which contamination of hygrometric sensors could occur.

Prediction of Water Activity of San Simón Cheese

In San Simón cheese water activity was highly related to moisture content and, consequently, to the aqueous concentration of many solid components. The following linear regression equations aw = .7662 + .0046g moisture/100g cheese, and aw = 1.0234 - .0070g ash/100g moisture, as well as a simple nomograph based on both chemical parameters, was used to predict water activity of San Simón cheese with an absolute error not greater than±.01 often found in measurements of water activity by different methods.