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.     

Water Activity Measurements with A Capacitance Manometer

The design, use and performance of a capacitance manometer for the measurement of water activity (a_w) 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 a_w. 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 a_w are required and situations in which contamination of hygrometric sensors could occur.     

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. ©     

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.     

Impact of temperature and water activity on enzymatic and non-enzymatic reactions in reconstituted dried mango model system

Mango fruit (Mangifera indica L.) is highly perishable and must therefore be quickly stabilized. Drying appears to be one of the most promising stabilization solutions. The traditional mango drying process can be divided into two phases: the first, which is performed at ~80 °C for 10 to 12 h, sees water activity (a _w) decline from 0.98 to 0.8. During the second phase, the drying temperature is reduced to 50 °C until a _w reaches ~0.6. These conditions are highly favorable to enzymatic and non-enzymatic reactions (1st and 2nd step, respectively). The focus of this work was therefore to highlight the effects resulting from the coupling of a _w and temperature on the development of these classes of reactions during the whole process. Several precursors (reducing sugars and amino acids) and intermediary products of the Maillard reaction were analyzed to estimate non-enzymatic reaction intensity when polyphenoloxidase (PPO) was associated with enzymatic browning. We were able to show that the highest production of 5-hydroxymethyl furfural (5-HMF) as a Maillard reaction marker occurs mainly for an a _w of 0.6. Kinetic analysis of Maillard reaction precursors and intermediate products in model mango system helped us identify asparagine and arginine as limiting precursors, and 5-HMF as a significant a _w-dependent product. The impact of the a _w on the thermal degradation of PPO proved to be radically different. After 8 h at 40 °C, the PPO was almost totally inhibited if this treatment is conducted on a sample at a _w 0.65, while residual activity was estimated to be 69.4 % for a sample at a _w 0.98.     

Predicting suitable storage conditions for spray-dried microcapsules formed with different biopolymer matrices

Canola oil (CO)-in-soy protein isolate (SPI, 12.5 g per 100 ml of solution), CO-in-whey protein concentrate (WPC, 30 g per 100 ml of solution) and CO-in-mesquite gum (MG, 30 g per 100 ml of solution) emulsions were obtained (all with CO to biopolymers solids ratio of 25 g of canola oil per 100 g of wall material), spray dried and stored at water activities (a_w) of 0.12–0.89 at temperatures of 15, 25 and 35 °C. The powders adsorption isotherms experimental data were fitted to the GAB model. Their differential and integral enthalpies and entropies were estimated. The microcapsules point of minimum integral entropy (maximum stability) kept at 25 °C occurred at 10.40 g water per 100 g of dry solids (a_w of 0.55) for MG, 10.59 g water per 100 g of dry solids (a_w of 0.68) for SPI, and 6.38 g water per 100 g of dry solids (a_w of 0.46) for WPC. The integral enthalpy–entropy compensation indicated that the water vapour adsorption was controlled by entropic mechanisms at low a_w, but by enthalpic mechanisms at high a_w.     

Evaluation of sample preparation methods for water activity determination in jerky and kippered beef: A research note

Commercially available packaged whole muscle beef jerky, chopped and formed beef jerky, and kippered beef steak were obtained from retail stores to determine the effect of two sample preparation methods on water activity (a_w). Intact samples were prepared by cutting product into a hexagonal shape with a 3.2 cm diameter. Diced samples were prepared by cutting the product into 0.4 × 0.4 cm squares. Whole muscle jerky a_w was higher (0.016 units; P < 0.001) using the intact method compared to the diced method. There was no difference (P > 0.05) in a_w levels between the two preparation methods for chopped and formed jerky or kippered beef steak. An intact sample preparation method is recommended for a_w determination of whole muscle jerky to obtain a more conservative value, especially if a_w is near the margin of safety.     

An attempt to minimize potassium sorbate concentration in sponge cakes by modified atmosphere packaging combination to prevent fungal spoilage

The combined effect of modified atmosphere packaging (MAP) and potassium sorbate (PS) on the preservation of a typical Spanish cake (sponge cake) was studied. A full factorial experiment design including other factors, such as water activity (a_w) (0.80, 0.85 and 0.90) and pH (6 and 7.5) was applied. Eurotium amstelodami, E. herbariorum, E. repens and E. rubrum colony radii were measured daily for 28 days. All different combinations of PS concentration and MAP were more effective at pH 6 than at 7.5. As a_w increased, the amount of PS or carbon dioxide (CO₂) necessary to prevent or delay fungal growth increased. The antifungal activity of PS could be improved by packaging in a suitable modified atmosphere (MA). At pH 6, 0.1% of PS effectively prevented mould spoilage upto 28 days in the entire a_w range, by combining with 50–70% CO₂ level. On the other hand, 0.2% of PS in 30% N₂:70% CO₂ MA packages prevented spoilage of neutral cake analogues at all a_w levels. Control air-packaged bags with 0.2% of PS showed visible growth after 6 days, at pH 6 and 0.90 a_w. If no MAP was used, low a_w was the main preservation factor.     

A stochastic approach for integrating strain variability in modeling Salmonella enterica growth as a function of pH and water activity

Strain variability of the growth behavior of foodborne pathogens has been acknowledged as an important issue in food safety management. A stochastic model providing predictions of the maximum specific growth rate (μ_max) of Salmonella enterica as a function of pH and water activity (a_w) and integrating intra-species variability data was developed. For this purpose, growth kinetic data of 60 S. enterica isolates, generated during monitoring of growth in tryptone soy broth of different pH (4.0-7.0) and a_w (0.964-0.992) values, were used. The effects of the environmental parameters on μ_max were modeled for each tested S. enterica strain using cardinal type and gamma concept models for pH and a_w, respectively. A multiplicative without interaction-type model, combining the models for pH and a_w, was used to describe the combined effect of these two environmental parameters on μ_max. The strain variability of the growth behavior of S. enterica was incorporated in the modeling procedure by using the cumulative probability distributions of the values of pH_min, pH_opt and a_wmin as inputs to the growth model. The cumulative probability distribution of the observed μ_max values corresponding to growth at pH 7.0-a_w 0.992 was introduced in the place of the model's parameter μ_opt. The introduction of the above distributions into the growth model resulted, using Monte Carlo simulation, in a stochastic model with its predictions being distributions of μ_max values characterizing the strain variability. The developed model was further validated using independent growth kinetic data (μ_max values) generated for the 60 strains of the pathogen at pH 5.0-a_w 0.977, and exhibited a satisfactory performance. The mean, standard deviation, and the 5th and 95th percentiles of the predicted μ_max distribution were 0.83, 0.08, and 0.69 and 0.96 h^− 1, respectively, while the corresponding values of the observed distribution were 0.73, 0.09, and 0.61 and 0.85 h^− 1. The stochastic modeling approach developed in this study can be useful in describing and integrating the strain variability of S. enterica growth kinetic behavior in quantitative microbiology and microbial risk assessment.     

Modification of the microstructural and physical properties of konjac glucomannan-based films by alkali and sodium carboxymethylcellulose

Edible films were cast from konjac glucomannan (KGM) solutions, with or without added alkali (KOH) and/or sodium carboxymethylcellulose (CMC). Four types of KGM-based films (KGM, KGM–KOH, KGM–CMC and KGM–CMC–KOH) were produced and characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), moisture sorption, water vapour permeability (WVP), and tensile tests. Tensile properties were studied as a function of water activity (a_w) over the range from 0.22 to 0.84. SEM revealed that films, with and without KOH, exhibited cross-sectional lamellar structures running perpendicular and parallel, respectively, to the film surface. Alkali treatment produced films with enhanced crystallinity, lower water-sorptive capacity (WSC) and WVP, and higher tensile properties. These effects were attributed to alkaline deacetylation of KGM molecules which permitted greater intermolecular interactions. The presence of CMC appeared to suppress crystallinity of native KGM films, but enhanced that of deacetylated KGM films. Films incorporating CMC exhibited higher WSC and WVP, but variable tensile properties depending on alkali treatment and a_w. The tensile properties of all KGM-based films were profoundly affected by a_w. Tensile modulus (TM) of all films were antiplasticized as a_w was increased from 0.22 to 0.43, but tensile strength (TS) was generally plasticized by water. Tensile elongation of KGM, KGM–KOH, and KGM–CMC–KOH films was generally much less sensitive to water. However, KGM–CMC films exhibited minimum elongation, attributed to antiplasticization by water, over the intermediate a_w range from 0.43 to 0.69. KGM–CMC–KOH films exhibited the highest TM and TS at any particular a_w.     

On the selection of relevant environmental factors to predict microbial dynamics in solidified media

Several studies have shown that food structure causes slower growth rates and narrower growth boundaries of bacteria compared to laboratory media. In predictive microbiology, both a_w or corresponding solute concentration (mainly NaCl) have been used as a growth influencing factor for kinetic models or growth/no growth interface models. The majority of these models have been based on data generated in liquid broth media with NaCl as the predominant a_w influencing solute. However, in complex food systems, other a_w influencing components might be present, next to NaCl. In this study, the growth rate of Salmonella typhimurium was studied in the growth region and the growth/no growth response was tested in Tryptic Soy Broth at 20 °C at varying gelatin concentration (0, 10, 50 g L⁻¹ gelatin), pH (3.25–5.5) and water activity (a_w) (0.929–0.996). From the viewpoint of water activity, the results suggest that NaCl is the main a_w affecting compound. However, gelatin seemed to have an effect on medium a_w too. Moreover, there is also an interaction effect between NaCl and gelatin. From the microbial viewpoint, the results confirmed that the a_w decreasing effect of gelatin is less harmful to cells than the effect of Na⁺ ions. The unexpected shift of the growth/no growth interface to more severe conditions when going from a liquid medium to a medium with 10 g L⁻¹ gelatin is more pronounced when formulating the models in terms of a_w than in terms of NaCl concentrations. At 50 g L⁻¹ gelatin, the model factored with NaCl concentration shifts to milder conditions (concordant to literature results) while the model with a_w indicates a further shift to more severe conditions, which is due to the water activity lowering effect of gelatin and the interaction between gelatin and NaCl. The results suggest that solute concentration should be used instead of a_w, both for kinetic models in the growth region and for growth/no growth interface models, if the transferability of models to solid foods is to be increased.     

Effects of Trehalose Content and Water Activity on the Fracture Properties of Deep-fried Wheat Flour Particles and Freeze-dried Porous Waxy Corn Starch Solids

Wheat flour-based batter containing 0 to 20 % trehalose was deep-fried, dried and held in various water activity (a_w) conditions. The effects of trehalose content and a_w on oil content, water sorption, isothermal mechanical relaxation, and fracture properties were investigated. For comparison, the fracture properties of freeze-dried porous waxy corn starch solids were also investigated. The 10 % trehalose sample had the lowest oil content, water content, and a_w. A force-reduction value (∆F) of the samples was evaluated as a typical mechanical relaxation parameter. ∆F gradually increased with increasing a_w and sharply increased above a specific a_w presumed to be associated with the glass to rubber transition. Compared to ∆F values among the glassy samples, 10 and 20 % trehalose samples had higher ∆F values (were more rigid) than 0 and 5 % trehalose samples. From the fracture measurements of the glassy samples, the first fracture force increased linearly and the number of fracture peaks decreased linearly with increasing a_w. At each a_w, 10 % trehalose had the lowest first fracture force and the highest the number of fracture peaks. Freeze-dried porous waxy corn starch solids showed similar fracture properties to deep-fried samples. These findings suggest that around 10 % trehalose content is optimal for producing deep-fried foods with a brittle texture.     

Water activity change at elevated temperatures and thermal resistance of Salmonella in all purpose wheat flour and peanut butter

Water activity (a_w) is a major factor affecting pathogen heat resistance in low-moisture foods. However, there is a lack of data for a_w at elevated temperatures that occur during actual thermal processing conditions, and its influence on thermal tolerance of pathogens. The objective of this study was to gain an in-depth understanding of the relationship between temperature-induced changes in a_w and thermal resistance of Salmonella in all purpose flour and peanut butter at elevated temperatures (80 ^oC). Equilibrium water sorption isotherms (water content vs. water activity) for all purpose wheat flour and peanut butter over the range of 20 to 80 °C were generated using a vapor sorption analyzer and a newly developed thermal cell. The thermal resistance (D₈₀-values) of Salmonella in all purpose wheat flour and peanut butter with initial a_w of 0.45 (measured at room temperature, ~ 20 °C) was determined via isothermal treatment of small (< 1 g) samples. When increasing sample temperature from 20 to 80 °C in sealed cells, the a_w of all purpose flour increased from 0.45 to 0.80, but the a_w of peanut butter decreased from 0.45 to 0.04. The corresponding estimated D₈₀-values of Salmonella in all purpose flour and peanut butter with 20 ^oC a_w of 0.45 were 6.9 ± 0.7 min and 17.0 ± 0.9 min, respectively. The significantly (P < 0.05) higher D₈₀-value of Salmonella in peanut butter than in all purpose flour may be partially attributed to the reduced a_w in peanut butter in comparison to the increased a_w in all purpose flour at 80 °C. The improved understanding of temperature-induced changes in a_w of low-moisture products of different composition provides a new insight into seemly unpredictable results, when using heat treatments to control Salmonella in such food systems.     

An accelerated method for adjustment of equilibrium moisture content of foods

The long equilibration period required to attain a desired water activity by traditional methods can lead to microbiological and physico-chemical changes in the food under study. To mitigate these difficulties, a system has been developed for adjusting the relative humidity of food samples to the desired levels in less than an hour. Food samples are exposed to either a saturated or a near zero water vapour pressure thereby greatly decreasing the time required for equilibration. This is mainly due to the increased mass transfer driving force. Evacuation, along with agitation of the head-space, is found to enhance the rate of moisture change. By using the sorption curves prepared using this technique, the time required for a particular food to reach a certain moisture (or a_w) level can then be predicted. The moisture and a_w levels predicted were found to be in good agreement with experiment.     

Modeling incidence of lipid and sodium chloride contents on sorption curves of gelatin in the high humidity range

Water mobility and availability are predominant factors in controlling organoleptic and biological quality in food. To characterize and quantify the role of protein macromolecular mesh, sodium chloride and fat on the water retention in the high humidity range, sorption isotherms were measured at 20 °C. A specific method was used which provides rapid equilibrium and many experimental points in comparison with the standard saturated salt solution method. Experiments were carried out on gelatin gels to which either sodium chloride (0-45% w/w on an anhydrous gelatin basis) or vegetal hydrogenated fat (0-50% w/w on total sample mass) was added. Desorption isotherms of 40 points each were measured for each sample for a_w values in a range from 0.60 to 0.99. Results precisely show the specific behavior of water in a protein-sodium chloride mix for a_w values between 0.65 and 0.75. Water activity was then predicted from sample composition. Good agreement with measurements was obtained (1) assuming no incidence of lipids on water-holding capacity and (2) adapting a Ross-type model considering crystallization and introducing an interaction factor related to sodium chloride content.     

Development of a device and method for the time-course estimation of low water fluxes and mean surface water activity of food products during ripening and storage

Accurate measurement of water activity (a_w) is an important goal for the food industry because a_w is a key parameter in microbial growth, biological reaction rates and physical properties. An experimental device was setup using air-product water balance to non-destructively estimate the time-course of mean a_w at the food product surface under well-controlled airflow conditions. The device is especially suited for studying the ripening of cheeses and fermented meat products, where water fluxes exchanged between products and air are very low. The validation tests performed with a_w-known model products showed that water fluxes of 10^?7 kg s^?1 can be estimated with an accuracy better than 2% over very short periods of time, and that surface a_w can be estimated with an absolute uncertainty of less than 0.01 a_w units. A handful of cheese ripening trials illustrate the potential of the method, highlighting the effects of a low air velocity and high air RH on the water exchanges occurring at a cheese surface, thus demonstrating the strong surface sensitivity to external air conditions.     

Effect of Temperature on the Moisture Sorption Isotherms and Water Activity Shift of Two Dehydrated Foods

The water activity (a_w) of eight salt solutions was determined at three temperatures (25, 30, 45°C) using a pressure transducer-vapor pressure manometer. The a_ws of the salts showed a decrease with increasing temperature, which was explained with the help of a thermodynamic equation. This is opposite to the increase in a_w with increase in temperature for foods. Moisture sorption data for fish flour and cornmeal were obtained at 25–65°C. The Guggenheim-Anderson-deBoer model was evaluated and shown to be comparable to the Brunauer-Emmett-Teller model for prediction of the monolayer. Product was equilibrated at different a_ws at 25°C then subsequently shifted to 30°C and 45°C in a sealed chamber. The resultant a, change, measured on the Kaymont-Rotronics, was predictable from the isotherm at each temperature using the Clausius Clapeyron relationship.     

Influence of water activity and temperature on growth and mycotoxin production by Alternaria alternata on irradiated soya beans

The aim of this study was to determine the effects of water activity (a_w) (0.99-0.90), temperature (15, 25 and 30 °C) and their interactions on growth and alternariol (AOH) and alternariol monomethyl ether (AME) production by Alternaria alternata on irradiated soya beans. Maximum growth rates were obtained at 0.980 a_w and 25 °C. Minimum a_w level for growth was dependent on temperature. Both strains were able to grow at the lowest a_w assayed (0.90). Maximum amount of AOH was produced at 0.98 a_w but at different temperatures, 15 and 25 °C, for the strains RC 21 and RC 39 respectively. Maximum AME production was obtained at 0.98 a_w and 30 °C for both strains. The concentration range of both toxins varied considerably depending on a_w and temperature interactions. The two metabolites were produced over the temperature range 15 to 30 °C and a_w range 0.99 to 0.96. The limiting a_w for detectable mycotoxin production is slightly greater than that for growth. Two-dimensional profiles of a_w × temperature were developed from these data to identify areas where conditions indicate a significant risk from AOH and AME accumulation on soya bean. Knowledge of AOH and AME production under marginal or sub-optimal temperature and a_w conditions for growth can be important since improper storage conditions accompanied by elevated temperature and moisture content in the grain can favour further mycotoxin production and lead to reduction in grain quality. This could present a hazard if the grain is used for human consumption or animal feedstuff.     

The growth and aflatoxin production of Aspergillus flavus strains on a cheese model system are influenced by physicochemical factors

Traditional cheeses may be contaminated by aflatoxin-producing Aspergillus flavus during the ripening process, which has not been sufficiently taken into account. The objectives of this study were to evaluate the influence of water activity (a_w), pH, and temperature on the lag phases, growth, and aflatoxin production of 3 A. flavus strains (CQ7, CQ8, and CG103) on a cheese-based medium. The results showed that the behavior of A. flavus strains was influenced by pH, a_w, and temperature conditions. The CQ7 strain showed the maximum growth at pH 5.5, 0.99 a_w, and 25°C, whereas for CQ8 and CQ103 strains, no differences were obtained at pH 5.5 and 6.0. In general, low pH, a_w, and temperature values increased the latency times and decreased the growth rate and colony diameter, although a_w and temperature were the most limiting factors. Maximum aflatoxin production on the cheese-based medium occurred at pH 5.0, 0.95 a_w, and 25 or 30°C, depending on the strain. This study shows the effect of pH, a_w, and temperature factors on growth and aflatoxin production of 3 aflatoxigenic A. flavus strains on a cheese-based medium. The findings may help to design control strategies during the cheesemaking process and storage, to prevent and avoid aflatoxin contamination by aflatoxigenic molds.     

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.