Water Activity Range Research Articles

The Properties of Aloe Vera Powder using Cassava Maltodextrin as Carrier Agents

The aims of this research is to determine the powder properties of aloe vera powder in different amount of cassava maltodextrin addition. The total soluble solid in aloe vera extract before spray drying was adjusted to the total soluble solid as 20%, 25%, and 30% (w/v) by adding the appropriate amount of cassava maltodextrin as carrier agents. The results showed that all aloe vera powder has the water activity in the range of safe water activity for dried food (0.32-0.38). The highest cassava maltodextrin added produce aloe vera powder with the lowest water content (4.49%). The addition of cassava maltodextrin increased brightness, reduced the water content, decreased the cohesiveness between powder particles and increased the flowability of the aloe vera powder. More of cassava maltodextrin addition caused a decrease in solubility and wettability time on aloe vera powder. The aloe vera powder particle had a round shape and uniform size. The higher cassava maltodextrin addition made a larger size of aloe vera powder particle.

Legume Seeds Treated by High Hydrostatic Pressure: Effect on Functional Properties of Flours

Pigeon pea (PP), cowpea (CU), dolichos bean (DB), and jack bean (JB) are legumes that constitute the daily diet in many countries. Legumes are a good source of proteins, carbohydrates, and minerals. Considering that legumes present potentials to be used as ingredients for food formulation, the study of functional and physicochemical properties of flours obtained from legume seeds treated by high hydrostatic pressure treatment (HHPT) (200, 400, 600 MPa) was conducted. Flours were evaluated for polypeptide composition (SDS-PAGE), fluorescence spectroscopy, color, protein solubility (PS), water-holding capacity (WHC), oil-holding capacity (OHC), emulsion activity (EA), emulsion stability (ES), foaming capacity (FC), foaming stability (FS), and least gelation concentration (LGC). PS of PP, CU, and DB diminished with the increase of pressure and only CU showed an increase of PS (7–40%) at the isoelectric point. WHC of PP, CU, and DB varied with the pressure applied; however, WHC of JB was not modified by HHPT as we observed in lambda max fluorescence-emission values and PS. Only PP showed an increment of OHC at 400 and 600 MPa. EA of PP was not affected by HPPT, while DB and JB showed a decrease. ES of CU, DB, and JB was not affected by HHPT. FC of PP, DB, and JB diminished with the increase of pressure. FS of DB and CU (400 MPa) was improved and continued for 120 min. LGC values and the equilibrium moisture content of flours were not influenced by HHPT, but the last decreased with the increase of temperature. Moisture sorption isotherms of flours fitted adequately to H-H equation covering the practical range of water activity (0.10–0.90) at the three temperatures tested. High pressure processed flours of PP, CU, DB, and JB showed functional properties that could be useful for food formulation.

Environmentally Friendly Films Combining Bacterial Cellulose, Chitosan, and Polyvinyl Alcohol: Effect of Water Activity on Barrier, Mechanical, and Optical Properties.

The interest in developing new materials intended for food packaging based on bacterial cellulose is growing in the recent years. Flexible and transparent films from bacterial cellulose-chitosan-polyvinyl alcohol have shown excellent UV-barrier properties. However, this material interacts with ambient moisture modifying its water activity due to its hydrophilic nature. In this work, a study was carried out to evaluate the changes caused by the water activity. Results showed a plasticizing effect of water molecules increasing the water vapor permeability of the samples from 1.86 × 10-12 to 1.17 × 10-11 g/m·s·Pa, percentage of elongation from 3.25 to 36.55%, and distance to burst from 0.64 to 5.12 mm. The increase of the water activity decreased the Young's modulus and tensile strength. The values of the UV-barrier were maintained at the wide range of water activity. Consequently, water molecules do not affect the UV-barrier properties of the films.

Moisture sorption isotherm, isosteric heat and adsorption surface area of whole chia seeds

This study aimed to evaluate moisture sorption isotherm of whole chia seed. The equilibrium moisture content (EMC) of seeds were detected by saturated salt solutions which have the water activity (aw) range of 0.2–0.9. The isosteric sorption heat was calculated by the Clausius-Clapeyron equation using three different sorption temperatures (15 °C, 25 °C and 35 °C). The adsorption surface area of seeds was also calculated by monolayer moisture content obtained from BET and GAB equation. The EMC content of seeds had an increasing trend and determined as 18–20 g H2O/100 g solid at the highest aw level. The whole chia seeds became less hygroscopic with the rising sorption temperatures at constant aw. The moisture sorption isotherm was determined as Type II. The monolayer moisture content was determined as 2.39–2.91 g H2O/100 g solid. BET and Peleg were the best-fitted models. The isosteric and net isosteric heat were 77.74 and 34.74 kJ/mol at lowest moisture content, respectively. Additionally, the adsorption surface area changed between 95.31 and 102.72 m2/g.

Development of automated measuring device with magnetic fluid

The article presents theoretical principles of the methods and devices for water activity (aw) measurement. The purpose of the research is the increase of the validity of water activity indicator measurement on the basis of a direct method of material test; the optimization of the structure of an automated measuring device to provide the possibilities for a simultaneous study of several samples (for example, food materials) with various water activity levels. A developed measuring device allows a simultaneous determination of indicators in several samples (8) located circumferentially and includes thermal sensors. The sensors in this device are represented by induction coils. They are installed on both leg pipes (made of nonmagnet materials) of a liquid level manometer. The device contains a special silicon organic oil with the pressure of partial steams within 10-6 Pa (at normal conditions) used as a manometric liquid. While measuring the device is vacuumized and thermostated. The method and device approbation resulted in making calibration charts at various temperatures, approximated with the help of software. The device provides the validity of measurements in water activity range from 0.45 to 1.0.

Predictive ion-electrostatic model of disjoining pressure of synthetic gel structures in coarse-dispersed soil substrates

The mechanism of disjoining pressure controlling the swelling and water absorption of gel structures for soil conditioning is discussed. Its quantitative description is based on the fundamental ion-electrostatic model of soil water sorption isotherms containing the phase variable of gel swelling degree and specific surface area index of the soil substrate. The model adequately reproduces the s-shape of soil water sorption isotherms in a wide range of water activity from 0.05 to 0.999 depending on the working doses of hydrogels in the range of 0.05-0.3 mass%. Reinforcement of gel structures with amphiphilic dispersed particles increases the effect of disjoining pressure and water absorption.

Moisture sorption isotherm and shelf life prediction of complementary food based on amaranth-sorghum grains.

The quantity and quality of complementary foods is one of the contributing factors to childhood under nutrition. Complementary food based on amaranth and sorghum grains was formulated. It was deemed to be superior nutritionally to the commonly used complementary foods in Kenya. Knowledge on storage and packaging is important for a long shelf life of the product to be achieved. Water activity and moisture content are key indicators of the stability of dried foods. To establish the proper storage conditions of the product, moisture sorption isotherms at 25°C, 30°C and 35°C were determined over water activity range of 0.11-0.97 and data fitted with six sorption models. Water vapour permeability of common packaging materials was also determined at 25°C and 35°C and the data was used to predict the product's shelf life. The product exhibited type two sorption isotherm according to Brunner classification an indication of monolayer-multilayer sorption behavior. The critical storage moisture content of the complementary food corresponding to 0.6 water activity a point of microbiological stability was found to be 7.5%. Based on Heiss-Eichner model, a shelf life of 1423days at 25°C can be achieved with product initial moisture content of 5g/100g and packaged in 110µm thick aluminum pouch. The shelf life of the product was shortened two to three fold at 35°C in comparison to storage at 25°C.

The Wavelength‐Dependent Complex Refractive Index of Hygroscopic Aerosol Particles and Other Aqueous Media: An Effective Oscillator Model

AbstractWe present an effective oscillator model for the wavelength‐dependent complex refractive index of weakly absorbing aqueous solutions. The model is derived using the causal connection between the real and imaginary parts of the refractive index that is described through the Kramers‐Kronig relation. Using cavity‐enhanced Raman spectroscopy, we measure both the real and imaginary parts of the refractive index of several aqueous solutions whose optical properties are relevant to seawater, aqueous sea‐salt particles, and aqueous organic aerosol. It is demonstrated that the effective oscillator model accurately describes both the real and imaginary parts of the refractive index of aqueous solutions across a wide range of water activities and optical wavelengths. Finally, through a comparison with measurements, we verify that mixing rule calculations utilizing oscillator parameters from solutions containing a single solute and water can be used to predict the optical properties of aqueous solutions containing multiple solutes.

FREEZE DRYING AND MOISTURE ADSORPTION KINETICS OF KEFIR POWDER

Freeze drying and moisture adsorption behaviour of kefir powder were investigated by fitting the experimental drying data to ten thin layer drying and sorption isotherm models. Moisture adsorption isotherms of kefir powder were determined at 5-35°C and within the range of 0.11–0.88 water activity. By statistical comparison of the values, Midilli et al. was found to be the best model describing the freeze drying behaviour. The GAB and Oswin equations gave the closest fit to the adsorption data over the tested range of temperatures and water activities. Additionally, adsorption isotherms data were used to determine the thermodynamic properties such as isosteric heat, sorption entropy and Gibbs free energy. The enthalpy entropy compensation was valid for the adsorption process and showed that the process was controlled by the enthalpy. Gibbs free energy was negative at all of the tested temperatures, which indicates that moisture adsorption of kefir powder was a spontaneous process.

English

Moisture sorption isotherms of Pleurotus ostreatus were determined at three different temperatures (10, 30 and 40) and relative humidity (6 to 88%), using standard static gravimetric method. The samples were used to determine the equilibrium moisture content and the data fitted to the Guggenheim-Anderson-de Boer (GAB) equation to describe the moisture adsorption characteristics within the water activity used for this study. Model parameters c, k and Mo were derived. The GAB monolayer moisture values (Mo) were 0.179, 0.163 and 0.123 at 10, 30 and 40°C respectively. The c and k value decreased from 0.729 to 0.613 at 10°C and 19.455 to 18.551 at 30°C, and increased to 0.712 and 24.737 when the temperature was further increased to 40°C. The mean relative percent modulus (%E) were 1.318, 5.393 and 3.224% at 10, 30 and 40°C respectively. The r2 values obtained varied between 0.980 and 0.997, indicating strong correlations between the experimental and predicted data; whereas the error values ranged from 0.007 to 0.017, showing very good fitness between the experimental moisture sorption and the modeled sorption. Thus, GAB model could be used to predict the moisture sorption behavior of P. ostreatus mushroom at the range of water activity used in this study. Key words: Pleurotus ostreatus, mushroom, Guggenheim-Anderson-de Boer (GAB) model, adsorption, isotherm, monolayer.

Continuously Distributed Glass Transition of Maca (Lepidium meyenii Walpers) Powder and Impact on Caking Properties

Water sorption isotherm, glass transition temperature (Tg), and caking properties of maca (Lepidium meyenii Walpers) powder were investigated. A broad endothermic shift reflecting continuously distributed glass transition was detected by differential scanning calorimetry, which prompted the evaluation of onset Tg and offset Tg. From the onset and offset Tg curves and water sorption isotherm, critical water activity (water activity at glass transition temperature of 25 °C) was determined to be 0.258 for the onset Tg and 0.480 for the offset Tg. The critical water activity values were validated by isothermal mechanical relaxation measurement. Caking properties of maca powder were compared with maltodextrin. The degree of caking and cake hardness for maltodextrin increased at a water activity just above critical water activity for onset Tg. Maca powder, on the other hand, showed a small degree of caking in the water activity range of 0.225 to 0.576, and the degree of caking drastically increased at a higher water activity. The hardness of maca cakes increased gradually at water activity values greater than 0.432. Since maca powder showed a continuously distributed glass transition, the molecular mobility required for caking was considered to have been provided incrementally by the increase in water activity above critical water activity for the onset Tg.

Moisture sorption isotherm of cross-linked wheat gluten and epoxydized oil using GAB and BET

The moisture sorption isotherm of wheat gluten/epoxydized sesame or sunflower oil resin was determined at 25, 40, and 55°C. The resin was prepared by cross-linking epoxydized oil and wheat gluten using 1, 2, and 3% zinc chloride as the catalyst. The experiment was carried out over 0.1–0.9 water activity ( a w) range using gravimetric sorption analyzer (Q 2000, TA Instruments, New Castle, PA, USA). The resin isotherms were found to be type III shape where the equilibrium moisture content (EMC) was higher at lower temperatures. The EMC of gluten epoxy resin was dependent on the degree of cross-linking because more cross-linking decreased EMC. The Guggenheim Anderson-de Boer (GAB) parameters support the theory of the free volume as it relates to monolayer absorption. This could be attributed to the decrease in the number of water binding sites due to the development of dense areas during cross-linking and increase in the free volume. The GAB and Brunauer–Emmett–Teller were found to be suitable for predicting the water sorption isotherm for gluten protein resin because it provided low root mean square error. The heat of sorption based on the Clausius–Clapeyron equation (qst) increased with decrease in moisture content.

Influence of Abiotic Factors Temperature and Water Content on Bacterial 2-Chlorophenol Biodegradation in Soils

Halogenated compounds are environmental pollutants toxic to humans and wildlife. Certain microorganisms degrade these halogenated compounds. However, little is known about the potential of microorganisms in bioremediation under extreme conditions, specifically in arid and semi-arid soils frequently exposed to high temperatures and desiccation periods. Arid and semi-arid environments and deserts make up vast areas of Earth’s landmass. To investigate the degradation of 2-chlorophenol (2-CP) in soils as a function of temperature and water availability, three bacterial species were tested, two soil mesophiles of the genus Rhodococcus, R. opacus and R. erythropolis, and a soil thermophilic isolate, Parageobacillus thermoglucosidasius. Degradation trials in soil samples with these species were performed over a range of water activity from 1 to 0.4. At their optimum growth temperature, R. opacus showed maximum 2-CP degradation at water activity 0.9 sharply decreasing when lowering water activity. Nevertheless, the Parageobacillus isolate (optimum growth temperature 60oC) showed maximum 2-CP degradation rates at water activity 0.5 which represented highly desiccating conditions. Parageobacillus degradation of 2-CP was very low at water activity above 0.9. Thus, biodegradation of 2-CP in soils is possible even under arid conditions although different microbial species might be involved in this task depending on the interactions of abiotic factors and the diversity of microbial communities in soils. These results contribute to understand the potential biodegradation of specific halogenated compounds in the environment which is of great relevance to comprehend the fate of halogenated pollutants (i.e., 2-CP) in deserts, arid and semi-arid soils.

ADSORPTION ISOTHERMS OF KIWIFRUIT DRIED BY DIFFERENT DRYING METHODS

The aim of this study was to determine the moisture adsorption isotherms of kiwifruit dried by hot air, vacuum and freeze drying methods. Equilibrium moisture contents (EMC) of dried kiwifruits were obtained by using the standard gravimetric method at two different temperatures (25 and 45°C) within a range of water activities from 0.112 to 0.936. The adsorption isotherms of dried kiwifruits decreased with increasing temperature and exhibited type III behavior. Six different isotherm equations (GAB (Guggenheim-Anderson-de Boer), BET (Brunauer-Emmett-Teller), Oswin, Henderson, Halsey and Peleg) were employed to describe the experimental adsorption isotherm data. Adsorption data obtained at both 25 and 45°C for vacuum dried samples and data obtained at 45°C for hot air and freeze dried samples were best represented by Peleg equation. For the samples dried by hot air and freeze drying, GAB equation gave the best description of the experimental data obtained at 25°C.

Effects of Maltodextrins on the Kinetics of Lycopene and Chlorogenic Acid Degradation in Dried Tomato.

Maltodextrins (MD) are frequently used as processing aids in tomato drying. The aim of this study was to investigate the effect of the addition of MD on the stability of lycopene and chlorogenic acid, which are the main lipophilic and hydrophilic antioxidants in processed tomato, respectively. Tomato powder added with 10% MD (dextrose equivalents, DE 12) and a control tomato powder were stored in the water activity (aw) range 0.17–0.56, for 180 d at 30 °C. At the aw level of 0.17, which was below the monolayer moisture content (Mo), chlorogenic acid was stable, while lycopene content decreased faster in tomato added with MD than in control tomato, probably due to a decrease in matrix hydrophilicity and greater oxygen diffusion in the oil phase. Maximum stability occurred in both tomato powders at aw of 0.3, that was in close proximity to Mo (first-order rate constant for lycopene, k = 7.0 × 10−3 d−1 in tomato added with MD). At high aw levels, MD increased the rate of lycopene degradation with respect to the control, possibly by hampering its regeneration by chlorogenic acid, which conversely was found to be more stable than in the control tomato.

Moisture sorption characteristics of ready-to-eat snack food enriched with purslane leaves.

Green leafy vegetables are important part of our regular diet that provides us with essential nutrients necessary to support human life and health. Since these are produced in surplus and have limited shelf-life, preservation of vegetables can help to prevent wastage and increase nutrient availability in the diet during off-season. Purslane leaves, rich in micronutrients were dried using tray dryer at 60 ± 2°C for 7h. and incorporated at 20% level in khakhra. Organoleptic and physico-chemical properties of the product were evaluated. Further, the sorption properties of khakhra, were studied at 27 ± 2°C over a range of water activity of 0.10-0.92. Several models were chosen to fit sorption behavior, but the Peleg model showed the best fit. The present study in fact provides a platform to understand the rationalities behind the physico-chemical changes in the product, which, in turn, helps in predicting the best suitable packaging material for storage in addition to the stability of the product.

Aflatoxin B1 (AFB1) production by Aspergillus flavus and Aspergillus parasiticus on ground Nyjer seeds: The effect of water activity and temperature

Nyjer oil seed cake supports high levels of aflatoxin B1 (AFB1) production. AFB1 is a secondary metabolite of Aspergillus flavus and A. parasiticus, classified as a Class 1A carcinogen. The aim of this study was to determine the effects of temperature (20, 27, and 35 °C) and water activity (0.82, 0.86, 0.90, 0.94, and 0.98 aw) on fungal growth and AFB1 production of A. flavus and A. parasiticus on ground Nyjer seeds over a 30-day incubation period.Linear regression models indicated that both fungal growth and AFB1 production were significantly influenced by water activity of Nyjer seeds and incubation temperature. The two fungi did not grow on Nyjer seeds at 0.82 aw at the three incubation temperatures. The most favorable growth conditions for both fungi were 0.90–0.98 aw at 27 °C or 0.90–0.94 aw at 35 °C. The optimum temperature for AFB1 production was 27 °C for both A. flavus and A. parasiticus (with regression coefficients of 6.01 and 9.11, respectively). Both fungi were likely to produce high levels of AFB1 at 0.90 aw (with regression coefficients of 3.56 for A. flavus and 7.17 for A. parasiticus). Aspergillus flavus only produced AFB1 on seeds with 0.90–0.98 aw at 27 °C (in the range of 203–282 μg/kg) and on seeds with 0.90 aw at 35 °C (212 μg/kg). No detectable AFB1 was produced by this fungus in any other culture conditions that were studied.Aspergillus parasiticus, in contrast, was able to produce AFB1 under all of the growth conditions. At 20 °C, this fungus produced the highest level of AFB1 (212 μg/kg) at high water activity (0.98 aw). At 27 °C, A. parasiticus produced high levels of AFB1 (in the range of 209–265 μg/kg) at a wide range of water activities (0.86–0.98 aw). In the entire study, the highest AFB1 concertation for A. parasiticus was detected on seeds incubated at high temperature (35 °C) and low water activity (0.86 aw). The findings of this study could help optimize the storage conditions of Nyjer oil seeds to reduce aflatoxin contamination.

Drying kinetics and sorption behavior of two varieties banana (sagor and sabri) of Bangladesh

The study was concerned with determining the drying kinetics and sorption behavior of two varieties ripe banana of Bangladesh namely, Sagor and Sabri. The fresh ripe banana collected from the local market were sliced into three different thicknesses (4 mm, 6 mm and 8 mm) and dried at 45°C, 55°C and 65°C in a cabinet dryer. The result showed that the drying rate increases with the increase in temperature and decreases with the increase in slice thickness. The sorption properties of dried banana were studied over a wide range of water activity (0.11-0.93). The BET and GAB models were fitted to the sorption data. It was found that both varieties gave sigmoid (type II) shape isotherm and GAB equation gave the much higher value of monolayer moisture content compared to BET equation. The chemical compositions of fresh and dried banana were determined. The dried products gave substantially higher solid content as well as protein, ash and carbohydrate.
 SAARC J. Agri., 16(2): 181-193 (2018)

Measurement of effective diffusion coefficients in dairy powders by confocal microscopy and sorption kinetic profiles

Abstract A new method for the visualisation and determination of local diffusion coefficients in dairy powders is described based on real-time visualisation of penetration of fluorescent dyes into individual particles of spray-dried dairy powders including skim milk powder, milk protein concentrate and whey protein isolate. The rehydration process was controlled by adding polyethylene glycol (PEG) as a viscosity modifier to the aqueous phase in ratios of 1:0, 1:1, 1:3 and 1:4 aqueous rhodamine to PEG, respectively. Real-time effective diffusivity values were obtained from analysis of confocal laser scanning microscope images. Particle size was measured optically. Results indicated that for all dairy powders, rehydration rates were highly dependent on particle size. Effective diffusivity increased linearly with increasing particle size and average effective diffusivity of the liquid phase was calculated for all particle size distributions using this dependence. The Guggenheim-Anderson-de Boer (GAB) water sorption relationship was used to model water sorption isotherms over a broad range of water activities. Vapour phase systems had significantly higher effective diffusivity than liquid phase systems. The results obtained by this new method is broadly in agreement with previously published works, suggesting this new method may be used to measure the hydration of individual powder particles.

The effect of encapsulants on the heat of sorption in vacuum-dried cajá powder

Equilibrium moisture data from the powdered pulp of Spondias mombin, L., both with and without additives - 58% maltodextrin (MD) or 58% gum arabic (GA) - were determined at 20, 30, 40 and 50 °C, using the static gravimetric method for a water activity range of 0.06 to 0.90. The resulting isotherms were sigmoidal and typical Type III, with the Guggenheim-Anderson-de-Boer model (GAB) adjusted to the experimental data for equilibrium moisture vs water activity. The addition of additives was shown to affect the isotherms in such a way that, for the same water activity, samples of CP+GA and CP+MD showed a lower equilibrium moisture content and were not affected by the variations in temperature. The isosteric heat of sorption for the caja powder with additives was greater (less negative) than that of the caja powder with no additives, suggesting that polar sites in the product without the addition of GA or MD are more active. An empirical exponential relationship can describe the dependence of the heat of sorption as a function of the moisture content of the material.