Moisture sorption and the applicability of the Brunauer-Emmet-Teller (BET) equation for blanched and unblanched mushrooms

Moisture sorption isotherms of blanched and unblanched mushrooms over 0.11–0.75 a_w were determined at 27°C and 37°C by using the static gravimetric method. Adsorption and desorption behaviors of blanched and unblanched mushrooms were compared. The unblanched material adsorbed more water than that of the blanched. In desorption isotherms, the equilibrium moisture contents of the unblanched material were found to be higher than those of the blanched throughout the entire a_w range. The BET equation was tested to fit the experimental moisture sorption data over the 0.11–0.43 a_w, 0.11–0.55 a_w, 0.11–0.64 a_w, and 0.11–0.75 a_w. Nonlinear regression analysis was used for the determination of the parameters in the equation. The quality of the fit of the BET model over each a_w range was judged from the value of the relative percent root mean square (% RMS). The moisture sorption behavior over 0.11–0.43 a_w of mushrooms has indicated that the BET equation is applicable generally up to 0.43 a_w. Monolayer moisture contents and C constants in the BET equation obtained for each a_w interval were reported. The water activities corresponding to the monolayer values have been determined and discussed related to mushroom storage. The net isosteric heats of adsorption and desorption were estimated from equilibrium sorption data, using the integrated form of the Clausius-Clapeyron equation. The heats of adsorption/desorption decreased with increase in moisture content and approached to a constant value. It was concluded that moisture adsorption/desorption occurred by physical mechanisms at high moisture contents, but at low moisture contents, besides physical adsorption, chemisorption was also observed.     

Predicting Water Activity from 0.30 to 0.95 of a Multicomponent Food Formulation

The objective was to develop an equation to calculate the a_w of a mixture of known composition at a given moisture content. A simple equation was derived by combining the Smith isotherm with the Lang-Steinberg mass balance: log (1–a_w) = [MW–Σ(a_iw_i)]/Σ(b_iw_i) where M is moisture content of the mixture, W is total dry weight of the mixture, a_i and b_i are Smith isotherm parameters for the individual ingredient, and w_i is the dry weight of the ingredient. For comparison of experimental and calculated a_w values, an experimental value was used to calculate M from the mixture isotherm and this M was used to calculate a_w, from the model. Four binary and one ternary mixtures of two macromolecules, two solutes and one complex ingredient were included. Twenty-one comparisons between calculated and experimental a_w, over a_w 0.30–0.95 resulted in a maximum error of only 1.86% and a mean error of -0.25%, showing excellent agreement. Only one constraint was found; when the mixture contains i solute (sugar or salt) and the a_w, is below the a_w of saturation for that solute, that solute is deleted from the summations in the Model.     

Water activity and temperature effects on mycotoxin production by Alternaria alternata on a synthetic tomato medium

Alternaria spp. have been reported to be the most frequent fungal species invading tomatoes. Certain species, in particular the most common one, A. alternata, are capable of producing several mycotoxins in infected plants and in agricultural commodities. Alternariol (AOH), alternariol monomethyl ether (AME), and tenuazonic acid (TA) are some of the main Alternaria mycotoxins that can be found as contaminants of food. The objective of this study was to determine the effect of water activity (a_w, 0.904, 0.922, 0.954, and 0.982) and temperature (6, 15, 21 and 35 °C) on mycotoxin production on a synthetic tomato medium of a cocktail inoculum of five strains of A. alternata isolated from tomato fruits affected by Blackmould. The optimum AOH production occurred at 0.954 a_w after 28 days of incubation at 21 °C. A temperature of 21 °C was the most favourable for AOH synthesis at all a_w levels. The maximum concentration of AME was determined at 0.954 a_w and 35 °C. The optimum conditions for TA accumulation were 0.982 a_w and 21 °C. At the 0.904 a_w no growth or germination was registered at 6 °C and 15 °C over the whole incubation period. At 21 °C and 35 °C growth occurred slowly but none of the toxins were detected at this a_w level. In general, high a_w levels were favourable for mycotoxin production. None of the other toxins was detected at quantifiable levels at 6 °C after the whole incubation period. A storage temperature of 6 °C or below could be considered as safe for tomato fruits and high moisture tomato products (a_w > 0.95), in relation with Alternaria toxins. The results obtained here could be extrapolated to evaluate the risk of spoilage in tomato fruits and tomato products caused by this pathogen.     

Malt Concentrates and their Mixtures with Sweetened Condensed Milk: Moisture Sorption Isotherms

Moisture sorption isotherms of malt extract concentrates with and without sweetened condensed milk were determined using a gravimetric procedure. Isotherms were type I, typical of high sugar foods. Desorption curves above the absorption showed hysteresis. Monolayer moisture contents of samples varied from 0.0558 to 0.0878 g water/g solid and multilayer moisture 0.660 to 1.093 g water/g solid. Smith plots for concentrates with and without sweetened condensed milk showed biphasic behavior with breaks at water activities in the range 0.500–0.660. Malt concentrates and sweetened condensed milk sorbed independently below 0.55 a_w and interacted above 0.66 a_w.     

Effect of water activity and temperature on growth of Alternaria alternata on a synthetic tomato medium

Alternaria alternata is a toxigenic fungus, predominantly responsible for Blackmould of ripe tomato fruits, a disease frequently causing substantial losses of tomatoes, especially those used for canning. The objective of this study was to determine the effect of water activity (a_w, 0.904, 0.922, 0.954, 0.982) and temperature (6, 15, 21 and 35 °C) on germination and radial growth rate on a synthetic tomato medium of a cocktail inoculum of five strains of A. alternata isolated from tomato fruits affected by Blackmould. The shortest germination time (1.5 days) was observed at 0.982 a_w, both at 21 °C and 35 °C. The germination time increased with a reduction on a_w. The fastest growth rate was registered at 0.982 a_w and 21 °C (8.31 mm/day). Growth rates were higher when a_w increased. No growth or germination was observed at the lowest a_w level evaluated (0.904) after 100 days of incubation at 6 °C and 15 °C. A temperature of 6 °C caused a significant reduction in growth rates, even at the optimum a_w level. The knowledge on the ecophysiology of the fungus in this substrate is necessary to elaborate future strategies to prevent its development and evaluate the consumer health risk.     

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

The effectiveness of the filters available for the Novasina Hygrometer for measurement of water activity (a_w) 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 a_w measurement; however, neither of the filters studied allowed measurement of a_w in the pet food containing propylene glycol.     

Relationship Between Water Activity and Freezing Point Depression of Food Systems

A simplified freezing point depression (FPD) equation was derived for calculating water activity (a_w) of food systems. The a_w values as calculated by FPD data agreed with literature data for a variety of foods to within ± 0.01 a_w units. The FPD equation was found particularly useful for calculating a_w values of frozen foods at temperatures between 273.1.5–233.15 °K (0 to ?40°C).     

Estimating Water Activity in Systems Containing Multiple Solutes Based on Solute Properties

A water activity (a_w) model for multicomponent systems expressed as a linear first order sum of molalities and requiring only data from binary solutions was tested. The model was developed based on a linear relationship between a_w and molality for each solute. Water activity predictions were validated and compared with Ross’equation using a_w data reported of different complex electrolyte and nonelectrolyte solutions. The equation proved to have a good predictive application, was extremely simple and exhibited comparable accuracy to Ross’equation in the mixtures considered.     

Predicting Water Activity in Solutions of Mixed Solutes

The Ross equation is a multiplicative procedure for estimating the water activity (a_w) of aqueous unprocessed food preparations that contain only solutes, only food polymers, or mixtures of solutes and polymers. Using accurate a_w values of single solute solutions in the Ross equation resulted in prediction accuracies to ± 0.01 a_w for 118 of 120 multiple-solute solutions. The exceptions at ±0.02 a_w occurred within a range of concentrated solutions containing NaCl, KCl, and CaCl₂.     

Water Activity and Freezing Point Depression of Aqueous Solutions and Liquid Foods

Water activity (a_w) of binary aqueous solutions and liquid foods (skim milk, coffee beverages, peach and tomato juices) at different total solids concentration (by wt).was evaluated, measuring freezing point depression (FPD). The calculated values differed from those obtained by electric hygrometer by less than 0.01 a, units. Water activity was also evaluated in model systems containing ethanol, for which a_w determination by conventional electric hygrometer is not possible.     

Sorption Behavior of Mechanically Mixed and Freeze-Dried NaCl/Casein Mixtures

Adsorption isotherms of mechanically mixed and freeze-dried mixtures of NaCl/casein at solute percentages of 1, 5, 10, 15, and 20 (wet basis) were obtained over the a_w range 0.23–0.927 at 20°C. Experimental isotherm values were compared to those calculated by a mass balance equation. The mechanically mixed samples sorbed additively as predicted by the mass balance equation below 0.755 a_w, whereas the freeze-dried mixtures sorbed more water than predicted (positive interaction) below 0.755 a_w, Both types of mixtures sorbed less water than predicted by the mass balance equation (negative interaction) above the saturation a_w of the NaCl, 0.755.     

Water Activity Determination with the Proximity Equilibration Cell

The objective was to develop a new method for water activity (a_w) 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 ? a_w). 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 a_w range from 0.40–0.98, and (4) with minor replication can bring the standard deviation of the mean to less than ± 0.01.     

Glass transitions for freeze-dried and air-dried tomato

Glass transition temperatures of freeze-dried tomato conditioned at various water activities at 25 °C were determined by differential scanning calorimetry (DSC). Air-dried tomato with and without osmotic pre-treatment in sucrose/NaCl solutions was also analyzed. Thermograms corresponding to the low water activity domain (0.11 ⩽ a_w ⩽ 0.75) revealed the existence of two glass transitions, which were attributed to separated phases formed by sugars and water and other natural macromolecules present in the vegetable. Both transitions were plasticized by water and experimental data could be well correlated by the Gordon-Taylor equation in the low-temperature domain, and by the Kwei model in the high-temperature domain. For higher water activities, the low-temperature glass transition curve exhibited a discontinuity, with suddenly increased glass transition temperatures approaching a constant value that corresponds to the T_g of the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through the melting enthalpy dependence on the moisture content.     

A New Model for Describing the Water Sorption Isotherm of Foods

This work investigated the applicability in the food area of a new sorption equation recently developed by Ferro Fontán et al. (1982). The equation may be written, 1n (γ/a_w) =α (m)^-r, where, γ, α and r are parameters to be determined, m is the moisture content, and a_w is water activity. It was found that the three parameter equation is able to describe the water sorption isotherm of 18 different foods in an extensive range of a_w (up to about 0.95) with only 2–4% error (average) in the predicted moisture contents. Foods examined comprised, among other, oilseeds, starchy foods, and proteins.     

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.     

GLASS TRANSITION TEMPERATURE (Tg) AND WATER ACTIVITY (aw) OF DEHYDRATED APPLE PRODUCTS

The onset glass transition temperature (T_go) of Granny Smith apple products was evaluated as a function of drying conditions (air drying and freeze-drying) and moisture and water activity (a_w) levels. T_go was determined in the 0–0.75 a_w range, and it was found to have a maximum value at a_w= 0 (T_go= 4.5 ± 1.6C) for freeze-dried apple and a minimum value at a_w= 0.75 (T_go= -81.4 ± 0.1C) for freeze-dried apple juice. A linear relationship between T_go and a_w was obtained in all cases. Theoretical T_go values at a_w= 0 were calculated using these equations, and were within the range of 3.0C (freeze-dried apple) and 10.5C (apple dehydrated at 30C). When Gordon and Taylor's linear model was applied to the experimental data of all the apple products, K values within the range 2.4 (freeze-dried apple juice) and 3.6 (apple dehydrated at 60C), and the theoretical T_g of solids values at a_w= 0 between the range -16.3C (freeze-dried apple juice) and -1.9C (apple dehydrated at 60C) were obtained. No effect of the type of drying on the value of T_g was detected.     

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.     

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.     

Inclusion of the variability of model parameters on shelf-life estimations for low and intermediate moisture vegetables

Shelf-life is the time period during which products retain market-acceptable quality while meeting legal and safety requirements. Deterministic models yield single value estimations of shelf-life typically based on average or worst-case values for input parameters. In deterministic calculations, considering the input parameter variability can be challenging. In this study, a Monte Carlo procedure and the G.A.B. model for moisture sorption isotherms were used to predict shelf-life frequency distributions for intermediate moisture (IM) tomato slices, and low moisture (LM) onion flakes and sliced green beans. End of shelf-life for IM tomato slices (initial a_w = 0.8) was assumed to occur for a 10% moisture loss, and when a_w changed from 0.25 to 0.4 for LM onion flakes and LM sliced green beans. The estimated shelf-life for tomato slices, LM onion flakes, and LM sliced green beans based on the deterministic approach was 243, 86, and 79 days, respectively. The Monte Carlo procedures yielded shelf-life frequency distributions with values ranging 181–366, 76–95, and 71–90 days, respectively. Products would fail before the deterministic shelf-life value with an unacceptably high probability of 51.6, 48.6, and 53.0%, respectively. If 5% is an acceptable probability that the actual shelf-life is shorter than specified, the estimated values would be 211, 81, and 73 days, respectively. X_m and K were the most influential G.A.B parameters on the shelf-life of the three products. The package area, product amount, and water vapor transmission rate were high contributors and had the expected effect on shelf-life as demonstrated by deterministic estimations.     

Sorption Behavior of Mechanically Mixed and Freeze-Dried Sucrose/Casein Mixtures

Interactions between sucrose and casein were investigated by adsorption isotherms. Mechanically mixed and freeze-dried sucrose/casein mixtures at sucrose percentages of 1, 5, 10, 15 and 20% (wet basis) were compared to calculated isotherms using the mass balance equation, over the a_w range 0.23–0.93 at 20°C. The mechanical mixtures did not exhibit solute/polymer interactions. The freeze-dried systems showed positive interaction (i.e., sorbed more water than calculated) for 1 and 5% sucrose at all a_w values. For the 10, 15 and 20% sucrose mixtures, positive interaction continued below 0.86 a_w; however, above 0.86 a_w, no interaction was apparent. That is, experimental sorption values equaled calculated values.