Function Of Water Activity Research Articles

RELATIONSHIP BETWEEN WATER ACTIVITY, DEFORMATION SPEED, AND CRISPNESS CHARACTERIZATION

ABSTRACT Very little is known on the rate dependency of the fracture behavior of crispy products as a function of water activity (Aw). Therefore, the effect of deformation speed on instrumental and sensory crispness was studied as a function of Aw. Deformation speed clearly affects the transition Aw range from crispy to non‐crispy. At low deformation speed, the critical Aw was 0.40 ± 0.02 and at high deformation speed between 0.5 and 0.6. The transition was completed at an Aw = 0.56 ± 0.10 and 0.74 ± 0.00 for the low and high speed, respectively. The transition of sensory crispness at regular biting speed started at Aw = 0.48 and was completed at Aw = 0.75. The deformation speed effect is assumed to be caused by the viscoelastic nature of the material in the transition region causing it to behave locally relatively more viscous at low deformation speed and relatively more elastic at high deformation speed. This behavior depends on the plasticizer content, thus, Aw. For the product used in this study, the instrumental deformation speed that best correlate to sensory data is 10 to 40 mm/s. Sensory tests show that crispy attributes are perceived more intense when biting at higher speed.PRACTICAL APPLICATIONSA good characterization of the water activity (Aw) range over which crispness of a food product is lost is of great practical importance. The present study describes the effect of deformation speed and Aw on crispness characteristics as determined by instrumental and sensory measurements. Crispness characteristics were found to depend not only on Aw but also on deformation speed. The Aw at which the crispy material started to lose it crispness started at lower values of Aw for the low deformation speed. Instrumental data should follow a similar trend as sensory crispness as function of Aw. At high deformation speed, instrumental sound data were more closely related to loss of crispness than force data, mainly caused by problems in the force registration. To get a good correlation of sensory and instrumental data, a correct deformation speed should be selected for this purpose. Sensory tests at different biting/chewing speed were done. Crispness attributes were scored lower when Aw was higher, as well as when decreasing the biting/chewing speed.

Viscoelastic Response of Nafion. Effects of Temperature and Hydration on Tensile Creep

The tensile creep of the acid form of Nafion N1110 was examined under controlled environmental conditions of temperature, 25 ≤ T ≤ 110 °C, and water activity, 0 ≤ aw ≤ 0.95. Water plasticizes Nafion at 25 °C; creep strain after 1 h increases and the elastic modulus decreases with increasing water activity. At temperatures ≥40 °C the elastic modulus of Nafion goes through a maximum as a function of water activity; the elastic modulus of Nafion increases as aw increases from 0 to 0.01 and then decreases with increasing water activity for aw > 0.1. Under dry conditions (aw = 0), Nafion undergoes a transition between 60 and 80 °C where its creep rate increases rapidly and its elastic modulus decreases with increasing temperature. Above this transition temperature a small water activity, aw < 0.1, dramatically reduces the creep rate and increases the elastic modulus of Nafion—at elevated temperature water stiffens Nafion. At intermediate temperatures (40−80 °C) the elastic and viscous components of creep recov...

Kinetics of Acrylamide Formation/Elimination Reactions as Affected by Water Activity

The influence of water activity on the kinetics of acrylamide formation and elimination reaction was investigated using low-moisture equimolar asparagine-glucose model systems, which were heated at temperatures between 120 and 200 degrees C for variable heating times. To determine the water content corresponding to the water activities tested, a sorption moisture isotherm was constructed experimentally. The acrylamide concentrations measured at different water activities could be modeled on the basis of a reaction scheme including not only acrylamide formation and elimination reactions but also an alternative Maillard reaction between both reactants. The corresponding rate constants and activation energies were estimated using nonlinear regression analysis. Whereas the rate constant for acrylamide formation varied only slightly with the initial water activity of the model system, the elimination rate constant showed a clear minimum around a water activity of 0.82. The opposite trend, namely, a maximum at a water activity of 0.82, was found for the Maillard reaction rate constant as a function of water activity, which confirms data from literature. The activation energies for the different reactions changed in a comparable way as the corresponding rate constant with water activity.

Effect of water activity on fracture and acoustic characteristics of a crust model

A crust model is described that is suited to study crispness of bread crusts as a function of steady state water activity. The study of crispness of this type of products as a function of water activity is complicated since the way a bread crust fractures does not depend on the crust only but also on the properties of the crumb, the curvature of the crust and on the gradient of water activity/content inside the crust. To circumvent these problems a baked crust model was developed. The crust model has similar properties as bread crust in terms of morphology and water adsorption behaviour. The suitability of using the crust model to investigate crispness was validated by studying the effect of water activity on its fracture behaviour and accompanying acoustic emission by using a wedge fracture test. Crispness of the crust model was characterized by a low work of fracture and a high number of sound and force events. When the water activity increased to 0.65 a significant increase of the force required to break the sample was observed, due to moisture toughening. Simultaneously a decrease in the number of force and sound events was found, pointing to a decrease of crispness of the crust model due to water plasticization.

A novel approach for color degradation kinetics of paprika as a function of water activity

The influence of temperature and water activity on color degradation in paprika powders was investigated. The Hunter color parameters ( L, a, b), hue angle ( h), and total color difference (TCD) were used to estimate the color changes of paprika during thermal treatment at different temperatures (60, 80 and 99 °C) and water activities (0.459, 0.582 and 0.703). The changes in L, a, b and TCD values were significantly influenced by the processing time, temperature and water activity. The color changes during processing and storage were described by a first order kinetic model. The temperature dependence of the degradation followed the Arrhenius relation. Samples with a high water activity exhibited a high rate constant of the color degradation. The effect of water activity on activation energy was not significant. A linear relationship between the water activity and rate constant of the color parameters was found. By incorporating the linear relationship k ref a w into the Arrhenius equation, a modified Arrhenius equation was proposed which was used to predict paprika color changes as influenced by temperature and water activity.

Apple Pectic Gel Produced by Dehydration

A novel, flexible sheet-like food formed by the high methoxyl pectin–sugar–acid gelation during drying of apple puree was investigated to characterize drying-related properties. Product volume was reduced by 68% over the process, and this shrinkage was successfully modeled by assuming the volume reduction equal to the volume of water evaporated. The sorption isotherm at 25 °C was determined, and a new expression for the moisture content, W, as a function of water activity, a w, of the type $$W\,{\text{ = }}\,{\text{C}}_{\text{1}} \,{\text{exp}}\,\left( {{\text{C}}_{\text{2}} \,a_w^{{\text{C}}3} } \right)$$ resulted as the most accurate for this J-shaped isotherm. The drying kinetics was studied at 50, 60, and 80 °C in a tray dryer. No constant drying rate period was found, and the drying curve was divided in high- and low-moisture zones. For high moistures, an internal–external mixed control diffusive model coupling mass and heat transfer was applied to obtain a mass transfer Biot number of 2.1. In the low-moisture zone, a diffusive, isothermal drying model for strict internal control was utilized. Diffusivities varied around 1 × 10–9 m2/s for high moistures and were about ten times lower at low moistures, although the activation energies were comparable (15,259 and 16,800 J/mol, respectively). The drying time at 60 °C was 6.67 h. The product scored four points out of five in a sensory evaluation of general acceptability.

Biotite stability in peraluminous granitic melts: Compositional dependence and application to the generation of two-mica granites in the South Bohemian batholith (Bohemian Massif, Czech Republic)

Crystallization experiments have been conducted in the system Na 2O–K 2O–MgO–FeO–Al 2O 3–SiO 2–H 2O (with 4% normative corundum) in order to constrain the stability of biotite as a function of water activity and the Mg# of biotite [Mg/(Mg +Fe total)] in equilibrium with peraluminous granitic melts. The temperature at which biotite breakdown starts is strongly dependent on the Mg# of biotite. At 500 MPa, the temperature of biotite breakdown to form orthopyroxene increases from 750 °C to 830 °C, as the Mg# of biotite increases from 0.4 to 0.5. Considering that the system investigated is relevant for Ca-poor peraluminous biotite-bearing rocks (metapelites), the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two-mica granites found in the South Bohemian batholith (specifically the Eisgarn and Deštná granites). The phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths. We demonstrate that Eisgarn granitic melts may have been generated at temperatures in the range 830–850 °C from melting reactions involving biotite with a Mg# up to 0.5 as a reactant. In contrast, Deštná granitic melts cannot have been generated from dehydration melting reactions involving biotite.

Desorption Isotherms and Isosteric Heat of Three Tunisian Date Cultivars

The sorption isotherm of three Tunisian cultivars, Allig, Kentichi, and Deglet Nour, were obtained at 40, 60 and 80 °C. The static gravimetric method was used, according to the COST 90 recommendations. The overall shape of the curves describing the water content as a function of water activity was typical of sugar-rich materials. A difference between varieties exists for the sorption isotherm, thus for the drying behavior. Data were compared with respect to several isotherm models. It was found that the GAB and the BET equations could satisfactorily represent the sorption data up to about 0.9 relative humidity. Through the BET equation, monolayer moisture content was calculated for the isotherm zone corresponding to monomolecular adsorption. In addition, isosteric heat of sorption was determined from sorption data using the Clausius–Clapeyron equation.

Impact of Water Activity, Temperature, and Physical State on the Storage Stability of Lactobacillus paracasei ssp. paracasei Freeze-Dried in a Lactose Matrix

The aim of this study was to determine whether the combined effect of water activity and temperature on inactivation rates of freeze-dried microorganisms in a lactose matrix could be explained in terms of the glass transition theory. The stabilized glass transition temperature, Tg, of the freeze-dried products was determined by differential scanning calorimetry at two different temperatures, T (20 and 37 degrees C), and different water activities (0.07-0.48). This information served as a basis for defining conditions of T and water activity, which led to storage of the bacteria in the glassy (T < Tg) and nonglassy (T > Tg) states. The rates of inactivation of the dry microorganisms subjected to different storage conditions were determined by plate counts and could be described by first-order kinetics. Rates were analyzed as a function of water activity, storage temperature, and the difference between Tg and T. Inactivation below Tg was low; however, Tg could not be regarded as an absolute threshold of bacteria stability during storage. When the cells were stored in the nonglassy state (T > Tg), inactivation proceeded faster, however, not as rapid as suggested by the temperature dependence of the viscosity above the glass transition temperature. Furthermore, the first-order rate constant, k, was dependent on the storage temperature per se rather than on the temperature difference between the glass transition temperature and the storage temperature (T - Tg).

Modeling the Effect of Water Activity and Storage Temperature on Chemical Stability of Coffee Brews

This work was addressed to study the chemical stability of coffee brew derivatives as a function of water activity (aw) and storage temperature. To this purpose, coffee brew was freeze-dried, equilibrated at increasing aw values, and stored for up to 10 months at different temperatures from -30 to 60 degrees C. The chemical stability of the samples was assessed by measuring H3O+ formation during storage. Independently of storage temperature, the rate of H3O+ formation was considerably low only when aw was reduced below 0.5 (94% w/w). Beyond this critical boundary, the rate increased, reaching a maximum value at ca. 0.8 aw (78% w/w). Further hydration up to the aw of the freshly prepared beverage significantly increased chemical stability. It was suggested that mechanisms other than lactones' hydrolysis, probably related to nonenzymatic browning pathways, could contribute to the observed increase in acidity during coffee staling. The temperature dependence of H3O+ formation was well-described by the Arrhenius equation in the entire aw range considered. However, aw affected the apparent activation energy and frequency factor. These effects were described by simple equations that were used to set up a modified Arrhenius equation. This model was validated by comparing experimental values, not used to generate the model, with those estimated by the model itself. The model allowed efficient prediction of the chemical stability of coffee derivatives on the basis of only the aw value and storage temperature.

Relative Stability of Selenites and Selenates in Feed Premixes as a Function of Water Activity

Sodium selenite is more hygroscopic than sodium selenate. It is, therefore, more likely to dissolve when dispersed in feeds of relatively high water activity. When dissolved, it may form selenious acid and disperse as a vapor. This is easily demonstrated by mounting a filter paper wetted with a reagent such as ascorbic acid over the subject feed, but not in contact with it. The paper turns brown as elemental selenium is formed from reduction of the vapor. Analysis of the paper ensures that the brown is indeed selenium. Though premixes are generally low enough in moisture content to ensure stability of the selenites, this is not true of many feeds. The water activities of a number of feeds, feed premixes, and feed ingredients have been determined instrumentally and compared to those of saturated solutions of sodium selenite and sodium selenate. There is no question that the selenite often dissolves with the potential to react and, in so doing, loses its nutritional function.

Exploring the performance of logistic regression model types on growth/no growth data of Listeria monocytogenes

Several model types have already been developed to describe the boundary between growth and no growth conditions. In this article two types were thoroughly studied and compared, namely (i) the ordinary (linear) logistic regression model, i.e., with a polynomial on the right-hand side of the model equation (type I) and (ii) the (nonlinear) logistic regression model derived from a square root-type kinetic model (type II). The examination was carried out on the basis of the data described in Vermeulen et al. [Vermeulen, A., Gysemans, K.P.M., Bernaerts, K., Geeraerd, A.H., Van Impe, J.F., Debevere, J., Devlieghere, F., 2006-this issue. Influence of pH, water activity and acetic acid concentration on Listeria monocytogenes at 7 °C: data collection for the development of a growth/no growth model. International Journal of Food Microbiology. doi:10.1016/j.ijfoodmicro.2006.09.023.]. These data sets consist of growth/no growth data for Listeria monocytogenes as a function of water activity (0.960–0.990), pH (5.0–6.0) and acetic acid percentage (0–0.8% (w/w)), both for a monoculture and a mixed strain culture. Numerous replicates, namely twenty, were performed at closely spaced conditions. In this way detailed information was obtained about the position of the interface and the transition zone between growth and no growth. The main questions investigated were (i) which model type performs best on the monoculture and the mixed strain data, (ii) are there differences between the growth/no growth interfaces of monocultures and mixed strain cultures, (iii) which parameter estimation approach works best for the type II models, and (iv) how sensitive is the performance of these models to the values of their nonlinear-appearing parameters. The results showed that both type I and II models performed well on the monoculture data with respect to goodness-of-fit and predictive power. The type I models were, however, more sensitive to anomalous data points. The situation was different for the mixed strain culture. In that case, the type II models could not describe the curvature in the growth/no growth interface which was reversed to the typical curvatures found for monocultures. This unusual curvature may originate from the fact that (i) an interface of a mixed strain culture can result from the superposition of the interfaces of the individual strains, or that (ii) only a narrow range of the growth/no growth interface was studied (the local trend can be different from the trend over a wider range). It was also observed that the best type II models were obtained with the flexible nonlinear logistic regression, although reasonably good models were obtained with the less flexible linear logistic regression with the nonlinear-appearing parameters fixed at experimentally determined values. Finally, it was found that for some of the nonlinear-appearing parameters, deviations from their experimentally determined values did not influence the model fit. This was probably caused by the fact that only a limited part of the growth/no growth interface was studied.

Effect of water activity on carotenoid degradation in dehydrated carrots

The effect of water activity on the stability of carotenoids in dehydrated carrots was studied. Freeze-dried carrots from blanched and unblanched batches were placed in air-tight glass jars containing saturated salt solutions with water activity ranging from 0.052 to 0.75, at 40 °C. The equilibrium moisture, water activity, α- and β-carotene, and lutein contents were analysed at different storage times. The Guggenheim–Anderson–de Boer (GAB) equation was applied to model experimental data for moisture content as a function of water activity, and to calculate the monolayer water activity, where the oxidative reactions are expected to be at minimum. Estimated monolayer water activity was 0.33 (confidential limits at 95%: 0.26 and 0.38). α- and β-carotene, and lutein degradation followed pseudo-first-order kinetics in all dehydrated carrots, with rate constants ranging from 0.031 to 0.374 days −1. Similar rate constants were found between α- and β-carotene, whereas lutein degraded faster. In both blanched and unblanched batches the rate of carotenoid degradation was at a minimum over the water activity range 0.31–0.54. Blanching resulted in a higher initial carotenoid content, but it accelerated carotenoid decrease during storage of dehydrated carrots.

Equilibrium Sorption Isotherms and Isosteric Heat of Rose Hip Fruits (Rosa Eglanteria)

For sorptional data to be useful in simulation and design purposes, they must be represented by equations valid in the conditions usually found in industrial practice. In this regard sorptional models that include the influence of temperature on desorption and sorption equilibrium values are most valuable. In this article, water desorption and sorption isotherms of rose hip fruits (Rosa Eglanteria) were experimentally determined by the gravimetric method, and from these the isosteric heat of sorption was calculated. According to the ANOVA test carried out for this fruits, no significant differences were found between experimental desorption and adsorption isotherms. Seven models were tested to mathematically represent the moisture content as a function of water activity (aw) in the aw range of 0.11 to 0.85 and temperatures of 20, 40, and 60°C, for further use in process simulation. The five-parameter GAB model was most accurate, with an MRE of −2,9 % and R2 = 0.989. The values obtained for the isosteric heat of sorption were fitted with a previous published equation, with an MRE% = −0.05. The isosteric heat of sorption derived from the GAB five parameters equation, for the corresponding monolayer moisture content, only differed by 1.25% with the calculated in this paper.

Phase relations of a F-enriched peraluminous granite: an experimental study of the Kymi topaz granite stock, southern Finland

Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl2Si2O8 (plagioclase) + 2[AlF3]melt = CaF2 (fluorite) + 2Al2SiO4F2 (topaz). The obtained partition coefficient for F between biotite and glass D(F)Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite.

Physicochemical and functional properties of soy protein isolate as a function of water activity and storage

The effect of storage temperature on the physicochemical characteristics, solubility and gelling properties of soy protein isolate (SPI) with different water activities ( a w) was investigated. SPI with a w of 0.19 (SPI-0.19) was placed in environments with relative humidity (RH) of 33% and 74%. After reaching equilibrium, in 20 days, the samples were named SPI-0.33 and SPI-0.74. For SPI-0.74, modifications in the protein characteristics started during the equilibrium period, with a decrease in the solubility and alterations in both the electrophoretic profile of the soluble proteins and in the gelling characteristics. During the 180-day storage period, SPI-0.19 and SPI-0.33 showed similar behaviours: decrease in protein solubility and alteration in hardness, cohesiveness and microstructure of the gels. These alterations were more pronounced during storage at 45 °C than at 25 °C, and in SPI-0.33 more than in SPI-0.19. Results suggest that storage conditions – temperature and RH – affect the functional properties of the proteins and the use of the isolates as a functional ingredient.

Crispy banana obtained by the combination of a high temperature and short time drying stage and a drying process

The effect of the high temperature and short time (HTST) drying stage was combined with an air drying process to produce crispness in bananas. The fruit was dehydrated in an air drier for five minutes at 70°C and then immediately set at a HTST stage (130, 140, 150°C and 9, 12, 15 minutes) and then at 70°C until water activity (a w) was around 0.300. Crispness was evaluated as a function of water activity, using sensory and texture analyses. Drying kinetics was evaluated using the empirical Lewis model. Crispy banana was obtained at 140°C-12min and 150°C-15min in the HTST stage, with a w = 0.345 and a w = 0.363, respectively. Analysis of the k parameter (Lewis model) suggests that the initial moisture content of the samples effects this parameter, overcoming the HTST effect. Results showed a relationship between sensory crispness, instrumental texture and the HTST stage.

Influence of water activity and temperature on conidial germination and mycelial growth of ochratoxigenic isolates ofAspergillus ochraceuson grape juice synthetic medium. Predictive models

AbstractThe first stages in the development ofAspergillus ochraceus, an ochratoxin A‐producing fungus that infects grapes and may grow on them, have been studied on a synthetic nutrient medium similar to grape in composition. Spore germination and mycelial growth have been tested over a water activity (aw) and temperature range which could approximate to the real conditions of fungal development on grapes. Optimal germination and growth were observed at 30 °C for all three isolates tested. Maximal germination rates were detected at 0.96–0.99awat 20 °C, while at 10 and 30 °C the germination rates were significantly higher at 0.99aw. Although this abiotic factor (aw) had no significant influence on mycelial growth, growth rates obtained at 0.98awwere slight higher than those at otherawlevels. Predictive models for the lag phase before spore germination as a function of water activity and temperature have been obtained by polynomial multiple linear regression, and the resulting response surface models have been plotted. Copyright © 2005 Society of Chemical Industry

Texture parameters of dry-cured ham m. biceps femoris samples dried at different levels as a function of water activity and water content

Instrumental texture parameters of m. biceps femoris (BF) samples from six commercial dry-cured hams, dried to a different degree, were related to water activity and water content. Samples were carved into cubes and Texture Profile Analysis (compression 50%, 10 mm high samples), water activity ( a w 25 °C) and water content analyses were performed. A negative non-linear relationship between hardness and water content and water activity was observed. From this relationship, critical X and a w values, below which there is a dramatic increase in hardness, can be found (around 0.55 kg H 2O/kg dry matter and around 0.70, respectively). Cohesiveness and springiness showed a positive linear relationship with water content and water activity.

Thermodynamics of the Solubility of Water in 1-Hexanol, 1-Octanol, 1-Decanol, and Cyclohexanol

The solubility of water in 1-hexanol, 1-octanol, 1-decanol, and cyclohexanol was determined as a function of water activity by the isopiestic method at 298.2 K. The solubility of water in the alcohol was expressed by a Setchenov type of equation and the correlation coefficients were related to the virial coefficients of the McMillan-Mayer theory of solution. From the solubility data both the activities and the osmotic coefficients of the alcohols were calculated. The Henry’s law constants for the solubility of water in the alcohols are given. They depend linearly on the Gibbs energy of hydration. The excess Gibbs energy of mixing of water and alcohols is positive as a consequence of the strong intermolecular interactions of the two pure components of the mixture.