Aw Range Research Articles

Estimating cation exchange capacity from hygroscopic water content change

AbstractThe existing models for estimating cation exchange capacity (CEC) from easy‐to‐measure hygroscopic water content (θh) were based on a single water activity (aw) value rather than on the processes that govern soil water vapor adsorption for a distinct aw range. Here, we present a new CEC estimation model based on θh data of 119 soils with different clay mineralogy (i.e., illitic [IL], montmorillonitic [ML], and kaolinitic [KA] samples) and organic carbon (OC) contents for the aw range from 0.23 to 0.57 (Δθ0.23–0.57) and validate its performance. Based on the hypothesis that multilayer adsorption exhibits a higher correlation with CEC than monolayer adsorption and capillary condensation, the aw range from 0.23 to 0.57 was chosen with CEC calculated as CEC = k × Δθ0.23–0.57. The performance of the new model is compared to the Arthur (2017) model and the Torrent (2015) model, which considers a single θh value. The average proportionality coefficient (k) varied with the dominant clay mineralogy of the investigated soils. For soils dominated by 2:1 clay minerals (i.e., IL and ML), the new model showed a good estimation accuracy (Nash‐Sutcliffe model efficiency [E] ≥ 0.85; root mean squared error [RMSE] ≤ 4.18 cmol(+) kg−1). The new model performed better for IL and ML samples than for KA samples, and yielded more accurate CEC estimations than the Arthur model and Torrent model for soils with 2:1 clay minerals. For soil with high OC content (>23.2 g kg−1), the new model slightly underestimated CEC (E = 0.66; RMSE = 5.87).

Novel foodborne source of Xerochrysium xerophilum (Pitt) Pitt, raw peanuts stored at low moisture and high CO2

Peanut seed is ideally stored at low water activity (aw) and low temperature, since common mycotoxigenic fungi cannot survive below 0.8 aw, and the life cycle of seed-infesting insects is interrupted below 15 °C. Hermetic storage bags at 0.77 aw, room temperature, and ≥ 9000 mg/m3 CO2, were tested for their potential use in countries where temperature control is unaffordable. Despite these inhospitable conditions, peanuts displayed an unusual white powder that we identified as the xerophilic fungus Xerochrysium xerophilum (syn. Chrysosporium xerophilum), and sequenced its genome. Previously, only 19 sequences shorter than 1700 nt were available. For six decades, only three food sources of X. xerophilum were known: dry prunes, chocolate, and dry coconut. Here we report peanut as a novel food source of this extremophile and make one isolate available at the USDA culture collection. Given the slow growth of this fungus in culture, we developed a robust PCR to sequence and identify X. xerophilum on contaminated peanut seed without performing DNA extraction. The broad range of oxygen, aw and temperatures that support growth of X. xerophilum will significantly affect the decisions made regarding the conditions to safely store peanuts.

Evaluation of surface structure, water sorption properties, water plasticizing effect, and color stability of dried chrysanthemums

The storage stability of two varieties of chrysanthemum (Chrysanthemum morifolium Ramat. cv. HSGJ and C. morifolium Ramat. cv. JSHJ)was evaluated in this study through an analysis of their water binding behavior. This involved examining the surface structure of the powdered dried chrysanthemums, studying the water sorption characteristics, and quantifying the water plasticizing effect. In comparison to JSHJ, powdered HSGJ had a smaller particle size and more hydrophilic groups on its surface. Additionally, it exhibited significantly higher characteristic values of adsorbed water, excluding the number of adsorbed monolayers (Nam). The sorption of water by the powdered chrysanthemums conformed to a type II isotherm. The Guggenheim–Anderson–deBoer (GAB) model was trustworthy for predicting water sorption within the range of aw from 0.112 to 0.907 and temperatures between 20 and 40 °C. The exothermic interactions between water molecules and the primary sorption sites of the powdered samples became more powerful when the temperature decreased. Powdered chrysanthemums can be preserved at 30 °C if the humidity is below 0.0852 g/g for HSGJ and 0.0766 g/g (d.b) for JSHJ. The color changes of powdered samples were not significantly affected when stored in a glassy state. This study is significant for identifying the drying endpoint, predicting shelf life, and choosing suitable packaging materials for dried chrysanthemums.

Effect of ultrasound-assisted osmotic dehydration on the drying kinetics, water state, and physicochemical properties of microwave vacuum-dried potato slices

The effects of pre-treatments on the drying characteristics, water state, thermal properties, and bulk shrinkage of potato slices during microwave vacuum drying (MVD) were investigated. The pre-treatment included ultrasound in distilled water (USOD-0%), and ultrasound-assisted osmotic dehydration in a 60% sucrose solution (USOD-60%). Results showed that the drying time of potato slices was reduced and the drying rate was increased when USOD-0% was used as a pretreatment, whereas USOD-60% had a negative effect on the drying rate of the samples. The Weibull model was effective in predicting the water changes in potato slices during the drying process. NMR analysis revealed that the relative content of immovable water (M22) increased initially, then decreased for drying, while the transverse relaxation time (T2) and the relative content of free water (M23) decreased consistently. The DSC results indicated that the glass transition temperature (Tg) had an inverse relationship with the water content of the samples, yet had virtually no influence on the volume shrinkage. The sample volumes decreased linearly with the decrease in water content during the initial drying stages. USOD pre-treatment lessened the volume shrinkage of MVD potato slices. Static gravimetry was used to determine the moisture sorption isotherms of MVD potato slices at 30 °C within the aw range of 0.113–0.923. The GAB model accurately fitted the experimental sorption data, which showed sigmoid shape curves for the MVD samples. When aw values exceeded 0.7, the USOD-60% treatment significantly reduce the water sorption capacity of MVD potato slices, while USOD-0% treatment was observed to increase the hygroscopic properties of MVD samples. Compared with the control, USOD-0% pretreatment significantly increased the monolayer water content (X0), sorption surface area (S0), the thickness of sorbed water multilayer (tm), and density of sorbed water (Ds) values of MVD potato slices, while USOD-60% decreased these values.

Growth and Non-Thermal Inactivation of Staphylococcus aureus in Sliced Dry-Cured Ham in Relation to Water Activity, Packaging Type and Storage Temperature.

Dry-cured ham (DCH) could support the growth of Staphylococcus aureus as a halotolerant bacterium, which may compromise the shelf-stability of the product according to the growth/no growth boundary models and the physicochemical parameters of commercial DCH. In the present study, the behavior of S. aureus is evaluated in sliced DCH with different water activity (aw 0.861-0.925), packaged under air, vacuum, or modified atmosphere (MAP), and stored at different temperatures (2-25 °C) for up to 1 year. The Logistic and the Weibull models were fitted to data to estimate the primary kinetic parameters for the pathogen Log10 increase and Log10 reduction, respectively. Then, polynomial models were developed as secondary models following their integration into the primary Weibull model to obtain a global model for each packaging. Growth was observed for samples with the highest aw stored at 20 and 25 °C in air-packaged DCH. For lower aw, progressive inactivation of S. aureus was observed, being faster at the lowest temperature (15 °C) for air-packaged DCH. In contrast, for vacuum and MAP-packaged DCH, a higher storage temperature resulted in faster inactivation without a significant effect of the product aw. The results of this study clearly indicate that the behavior of S. aureus is highly dependent on factors such as storage temperature, packaging conditions and product aw. The developed models provide a management tool for evaluating the risk associated with DCH and for preventing the development of S. aureus by selecting the most appropriate packaging according to aw range and storage temperature.

Water adsorption properties of microalgae powders: Thermodynamic analysis and structural characteristics

Moisture adsorption isotherms and thermodynamic properties of dried Chlorella pyrenoidesa and Spirulina platensis powders were investigated by using a static gravimetric at temperatures of 20, 30, and 40 °C and within aw range of 0.112–0.976. Structural characteristics involving morphology, functional group, surface element composition, pore size distribution, and surface area were analyzed to explore the water adsorption mechanism. FTIR and XPS analysis indicated that spirulina powder had a higher proportion of hydrophilic sites to bind water molecules in comparison with chlorella powder. BET surface area for chlorella and spirulina powders were 25.10 and 25.19 m2/g, respectively. Water adsorption data were adjusted using a variety of sorption models, and the GAB equation was the most appropriate for representing the experimental data in the studied aw and temperature range. The monolayer moisture content (Xm) values at 20, 30 and 40 °C were 0.0526, 0.0483, and 0.0456 g/g dry basis for chlorella powders, and 0.0581, 0.0553, and 0.0420 g/g dry basis for spirulina powders, respectively. The net isosteric heat (Qst) and differential entropy (δSdif) for both microalgae powders decreased with an increase in water content. Using the compensation theory, we determined that the processes were governed by the enthalpy and that they were non-spontaneous (δG>0) for the samples. Both microalgae powers were classified as micro and mesoporous materials, and the pore radius (Rp) varied from 1.481 to 23.694 nm for chlorella powders and from 1.193 to 39.392 nm for spirulina powders, respectively.

Physical stability of co-freeze-dried powders made from NaCl and maltodextrins – Impact of NaCl on glass transition temperature, water vapour sorption isotherm and water vapour sorption kinetics

NaCl is commonly added into food powders as a humectant or taste enhancer with little consideration of its impact on the physical properties and stability of powders. In this study, maltodextrins of dextrose equivalence (DE) 5, 20 and 45 and 10%w/w NaCl were first dissolved in water and then freeze-dried to produce powder mixtures. After freeze drying, maltodextrin DE 20-NaCl and maltodextrin DE 45-NaCl mixtures were amorphous based on powder X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy analyses while NaCl partially recrystallized in the maltodextrin DE 5 mixture. Compared to the corresponding pure maltodextrin, freeze-dried maltodextrin DE 5-NaCl blend had a lower glass transition temperature (Tg). Furthermore, the Tg depression effect of NaCl increased with maltodextrin molecular weight. Also, freeze-dried maltodextrin 20/45-NaCl mixtures had greater hygrocapacities than their respective maltodextrins throughout the aw range investigated (0.05–0.50). The impact of molecular mixing via freeze drying versus dry mixing on the sorption behaviour of maltodextrin-NaCl blends was also investigated. Freeze-dried maltodextrin-NaCl absorbed the most moisture (from 20% to 70%RH), followed by dry-mixed blends, which absorbed more moisture than predicted after 60, 50 and 40%RH for maltodextrin DE 5, 20 and 45-NaCl mixtures, respectively. In all, formulating NaCl with maltodextrin caused freeze-dried maltodextrin-NaCl blends to be more temperature- and moisture-sensitive.

Effect of water activity on the mechanical glass transition and dynamical transition of bacteria

The purpose of this study was to clarify the glass-transition behavior of bacteria (Cronobacter sakazakii) as a function of water activity (aw). From the water sorption isotherm (298 K) for C. sakazakii, monolayer water content and monolayer aw were determined to be 0.0724 g/g-dry matter and 0.252, respectively. Mechanical relaxation was investigated at 298 K. In a higher aw range of over 0.529, the degree of mechanical relaxation increased with an increase in aw. From the effect of aw on the degree of mechanical relaxation, the mechanical awc (aw at which mechanical glass transition occurs at 298 K) was determined to be 0.667. Mean-square displacement of atoms in the bacteria was investigated by incoherent elastic neutron scattering. The mean-square displacement increased gradually with an increase in temperature depending on the aw of samples. From the linear fitting, two or three dynamical transition temperatures (low, middle, and high Tds) were determined at each aw. The low-Td values (142–158 K) were almost independent from aw. There was a minor effect of aw on the middle Td (214–234 K) except for the anhydrous sample (261 K). The high Td (252–322 K) largely increased with the decrease in aw. From the aw dependence of the high Td, the dynamical awc was determined to be 0.675, which was almost equivalent to the mechanical awc. The high Td was assumed to be the glass-transition temperature (Tg), and anhydrous Tg was estimated to be 409 K. In addition, molecular relaxation time (τ) of the bacteria was calculated as a function of aw. From the result, it is suggested that the progress of metabolism in the bacterial system requires a lower τ than approximately 6 × 10−5 s.

Moisture sorption characteristics of Mohanthal, a chickpea flour (besan) basedtraditional dairy product

Moisture sorption characteristics of Mohanthal, a traditional dairy product from Gujarat were investigated at temperature of 5–45 °C and water activities (aw) of 0.11–0.97. The sorption isotherms obtained were sigmoid in nature. Nine different mathematical models were found to effectively describe the moisture sorption data on the basis of regression analyses and goodness of fit. Each model was statistically evaluated by means of percent root mean square and coefficient of determination. The GAB model gave the best fit in the entire range of aw. Temperature dependence of the GAB constants and good fit were determined. Using Caurie’s model, the properties of sorbed water such as monolayer moisture content, number of adsorbed monolayers, density of sorbed water, bound water content and surface area of adsorption which decreased with an increase in temperature, were calculated. The monolayer moisture content ranged from 1.82 to 6.25 g/100 g solids in Mohanthal.

Organic manure and lime change water vapour sorption of a red soil by altering water repellency and specific surface area

AbstractManure and lime as fertiliser and conditioner are widely used on red soils to improve soil fertility and to alleviate soil acidity. Investigating the effects of these compounds on soil interfacial properties is very important for understanding some physical, chemical, and biological processes that are often overlooked. Using a long‐term field experiment (2002–2018) on a red loamy clay soil, we explored how the soil water vapour sorption isotherm, soil organic carbon (SOC), pH, cation exchange capacity (CEC), specific surface area (SSA), and water repellency (WR) interacted and were affected by low‐rate pig manure (LM, 150 kg N ha−1 year−1), high‐rate pig manure (HM, 600 kg N ha−1 year−1), and high‐rate pig manure with lime (HML). The HML only increased soil pH relative to the HM. The SSA determined with N2 (SSAN2) showed a significant negative correlation with SOC. Both SSA determined with H2O (SSAH2O) and contact angle (CA) were significantly positively correlated with SOC. Manure amendments increased soil water content change (WCC) in the water activity (aw) range of 0.05–0.2 (WCC0.05–0.2) by 6.3–30.2%, but decreased WCC0.2–0.6 and WCC0.6–0.93 by 5.1–12.5% and 2.8–7.0%, respectively. Local hysteresis became more pronounced with high manure or high manure with lime amendment in the intermediate aw range (0.45–0.75). The observed changes of water vapour sorption were attributed to the alteration of SOC affecting SSA and WR.HIGHLIGHTS Manure effects on soil water vapour sorption depended on sorption stage and direction. Manure with lime treatment did not affect soil interfacial properties relative to manure treatment. Increasing manure application increased local hysteresis at the intermediate aw range (0.45–0.75). Soil water vapour sorption behaviour was linked to SOC, SSA, and WR.

Models of the water activity effect on microbial growth rate and initiation.

The role water activity, aw, plays in microbial growth by itself or in conjunction with other factors, notably temperature and pH, has been described mathematically by different algebraic models obtained by fitting experimental growth rate vs. aw relationships. Many of these models have one, two, or all three cardinal parameters, namely the minimal, optimal, and maximal aw, in their formulation. Although they all have good fit as judged by statistical criteria, their different mathematical structures have different ramifications concerning the threshold aw for growth initiation, and the growth pattern around and beyond the optimal aw level where it exists. The focus of this review is on the biological implications of the different growth rate vs. aw models inferred exclusively from their mathematical properties, leaving out any statistical fit considerations. It also describes a recently proposed single-parameter model of monotonic or the monotonic part of experimental growth rate vs. aw curves, which can be combined with a decay term to produce a general conceptual model of peaked and monotonic microbial growth rate vs. aw relationships over the entire aw range. KEY POINTS: • Traditional and new growth rate vs. aw models are presented and their implications compared. • Analogy between aw and the temperature or pH effect on microbial growth rate is reassessed. • Cardinal parameters alone do not establish a unique growth rate vs. aw relationship.

Shelf‐life estimation of packaged dried foods as affected by choice of moisture sorption isotherm models

The moisture sorption isotherms (MSIs) of Oswin, Halsey, GAB, and linear models were compared for their ability to predict the shelf life of typical permeable packages of four dried foods (onion flakes, rice crackers, cassava-flour-based baked product, and dried tea) whose isotherm data are given in the literature. Parameters of MSI equations were used to mathematically simulate aw change of the packaged foods. The simulation model was applied to predict the times to reach the critical aw values under storage at 30 or 37°C and 65% or 85% RH. The GAB equation was best able to fit the moisture sorption data with the highest coefficient of determination (>.99) and the lowest root mean square error (≤0.01). The linear model consistently estimated the shelf life in close agreement with the GAB equation. The Oswin and Halsey equations differed with each food in their ability to estimate the shelf life accurately. Practical applications Shelf-life determination is a critical issue for packaged dried foods where the moisture sorption isotherm selected may influence its accuracy. While a convenient and simple way of estimating shelf life is required, the validity and limitations of the applied method should be examined and determined. The general versatility of the GAB equation and the tolerable validity of the linear sorption model over the low aw range were confirmed, which is useful for practical shelf-life determination of packaged dried foods.

Ecophysiology of Fusarium chaquense a Novel Type A Trichothecene Producer Species Isolated from Natural Grasses.

Fusarium chaquense, a recently formally described novel species, has been identified as an T-2 toxin (T-2), HT-2 toxin (HT-2) and other toxins producer in natural grasses (Poaceae) from Argentina. The major objective of this study was to describe the effect of water activity (aW, 0.995, 0.98, 0.95, 0.93 and 0.91), temperature (15, 25 and 30 °C) and incubation time (5, 15 and 25 days) on growth and to evaluate the production of T-2, HT-2 toxins and beauvericin (BEA) by two F. chaquense strains in a grass-based media. The results showed a wide range of conditions for F. chaquense growth and mycotoxin production. Both strains had a maximum growth rate at the highest aW (0.995) and 25 °C. Regarding mycotoxin production, more T-2 than the other analysed mycotoxins were produced by the two strains. T-2 production was favoured at 0.995 aW and 30 °C, while HT-2 production at 0.98–0.95 aW and 15 °C. The maximum levels of BEA were produced at 0.995 aW and 25–30 °C. Two-dimensional profiles of aW by temperature interactions were obtained from these data in order to identify areas where conditions indicate a significant risk of mycotoxins accumulation on grass. For its versatility on growth and mycotoxin production in a wide range of aW and temperatures, F. chaquense would have an adaptive advantage over other Fusarium species, and this would explain its high frequency of isolation in natural grasses grown up in the Chaco wetlands.

Effects of maltodextrin on physicochemical properties of freeze-dried avocado powder

The effect of maltodextrin on the moisture sorption isotherm, glass transition temperature (Tg), and degree of caking of freeze-dried avocado samples at room temperature (25°C) was investigated. The incorporation of maltodextrin reduced the water sorption capacity of the powder due to its less hygroscopic nature. Parameters derived from the Guggenheim, Anderson, and de Boer (GAB) model describing the properties of absorbed water are discussed. The water absorption isotherm possessed the characteristic sigmoid-shaped type II isotherm curves and the model gave the best fit over the whole range of aw tested. The differential scanning calorimetric method was used to measure the Tg of freeze-dried avocado samples. Increasing the water content decreased the Tg, and Tg was increased with increasing maltodextrin content. Increased maltodextrin content to solid material in the freeze-dried sample was associated with less sensitivity to caking as evidenced by Tg values. In addition, increased maltodextrin content in the powders caused brighter, less yellowish, and more greenish coloration and protected color change including browning index. The antioxidant capacity was significantly decreased with increasing maltodextrin content. Thus, the effect of maltodextrin concentration on physicochemical properties was a promising way to preserve the physical property and chemical compounds in freezedried avocado powder.

Determination and modelling moisture sorption isotherms of dehydrated butterfly-pea (Clitoria ternatea L.) flower powder

Butterfly-pea flower (Clitoria ternatea L.) is one of the edible flowers that is widely processed into dried flowers or powder form. Processed butterfly-pea flower is mostly used as a food colorant or flavonoid and anthocyanin-rich teas which are naturally present in the flower. Some polyphenolic acid, i.e., gallic acid, protocratic acid, and chlorogenic acid are also contained in this flower. During the storage period, color degradation occurs which will reduce the quality of the powder. Therefore, a study on the determination of moisture sorption isotherm from butterfly-pea powder is necessary information to maintain the quality of this product for a longer storage period. The objectives of this research are to evaluate the behavior of moisture sorption isotherm, construct its mathematical modelling and analyze the color changes at different temperatures and aw. 60 mesh of butterfly-pea powder with 28% initial moisture content (db) was stored at aw 0.3 – 0.9 (in a saturated salt solution containing MgCl2, K2CO3, NaC1, KC1, and BaCl2) at 30, 40, and 50°C by static gravimetric method. The results showed that based on the Brunauer classification, the behavior of moisture sorption isotherm of dehydrated butterfly-pea powder is in accordance with the Type II-sigmoid curve, while the Peleg model is the best model in predicting the moisture sorption isotherm. Recommended storage conditions for butterfly-pea flower powder are at 30°C with equilibrium moisture content at 23-30% (db). During storage powder color turns darker over the entire aw range with a color index of L* (lightness, 9.97); a* (redness, 2.33); b* (yellowness, -5.56).

Microencapsulation of Renealmia alpinia (Rottb.) Maas pulp pigment and antioxidant compounds by spray-drying and its incorporation in yogurt.

Renealmia alpinia (Rottb.) Maas pulp was processed by spray drying using Maltodextrin (MDX), and Gum Arabic (GA), and the mixture of both encapsulating agents (MDX-GA). Yield, moisture, water activity (a w ), apparent and bulk densities, size and morphology of capsules, color, and antioxidant potential (antioxidant activity, total carotenoids, and phenolic compounds) were analyzed. The encapsulates were incorporated as pigments in yogurt and the stability of antioxidant compounds (1, 7, 14, 21, and 28days of storage) and the sensory properties were evaluated. The yields of all formulations (MDX, GA, MDX-GA) were around 17.86% with low moisture and a w range values (2.62-3.29% and 0.276-0.309, respectively). The microcapsules presented multiples particle sizes (0.67-27.13µm) with irregular and smooth surfaces. Furthermore, these capsulates preserved yellow color and the retention of carotenoids was significantly higher with MDX (34.12mg/100g of powder), while the phenolic compounds and antioxidant activity increased with GA (474.17mg GAE/100g and 552.63mg TE/100g of powder, respectively). The main compounds β-carotene and gallic acid were identified and quantified in positive and negative mode respectively using LC-MS/MS. Finally, the addition of the encapsulated pigments to yogurt allowed to obtain a yellow coloration and the yogurt added with MDX-GA presented the best formulation with not significant changes in antioxidant activity and acceptable sensory attributes up 28days of storage.

Water and temperature relations of Fusarium langsethiae strains and modelling of growth and T-2 and HT-2 mycotoxin production on oat-based matrices

In the UK and Northern Europe, ripening oats can become contaminated with T-2 and HT-2 mycotoxins, produced mainly by Fusarium langsethiae. There are indicative levels related to the maximum limits for oat grain for these toxins. The objectives of this study were to examine the effect of interacting conditions of temperature (10–30 °C) and water activity (aw, 0.995–0.90) on (a) lag times prior to growth, (b) growth and (c) T-2 and HT-2 toxins by two strains of F. langsethiae isolated from oats in the UK and compare this with the type strain (Fl201059) which has been genomically sequenced, and (d) develop (and validated with published data) a probabilistic models for impacts of temperature × aw on growth and toxin production.All three strains had an optimum aw range and temperature of 0.995–0.98 and 25 °C for growth. For T-2 + HT-2 production these were 0.995 aw and 20 °C. Overall, the type strain produced higher amounts of T-2 + HT-2 with a HT-2/T-2 ratio of up to 76. Using this study data sets and those from the literature, probabilistic models were developed and validated for growth and T-2 + HT-2 toxin production in relation to temperature × aw conditions. These models, when applied in stored oats, will be beneficial in determining the conditions on the relative level of risk of contamination with these two toxins in the context of the EU indicative maximum levels.

Evaluation of storage stability of dried powdered coriander, parsley and celery leaves based on the moisture sorption isotherms and glass transition temperature

The aim of study was to evaluate the storage stability of microwave-vacuum dried (MWVD) and hot air convective dried (HACD) powdered coriander, parsley and celery leaves based on their moisture sorption isotherms and glass transition temperatures (Tg). Dried coriander, parsley and celery leaves were powdered and kept at a storage temperature (TS) of 20 °C for 10 weeks with a water activity ranging from 0 to 0.813 until equilibrium was reached. The values of coefficient of determination (R2 > 0.99) and percentage global relative square error of the approximation (4.9<GRSE<11.3%) indicate that Guggenheim-Anderson-de Boer (GAB), Peleg, Lewicki, Curie and Oswin models well described the relationship between water activity (aw) and the equilibrium moisture content (EMC) of dried materials. The variations in Tg vs. aw or EMC were adequately predicted by linear, square, polynomial, Khalloufi, Gordon-Tylor and Jenckel-Heusch models (R2 > 0.81; 2.95<GRSE<43.5%). For the optimum range of aw (0.2<aw < critical aw), dried powdered coriander, parsley and celery leaves showed the highest stability at EMC from 0.028 to 0.061, 0.022 to 0.074 and 0.027–0.064 g H2O/g DM, respectively. The optimum TS of dried powdered coriander, parsley and celery leaves was from 57.8 to 20.0, 61.3 to 20.0, and 69.8 to 20.0 °C, respectively.

Effects of glass transition and hydration on the biological stability of dry yeast.

The purpose of this study was to determine the effects of glass transition and hydration on the storage stability of baker's dry yeast (Saccharomyces cerevisiae). The glass transition temperature (Tg ) of the yeast decreased with increase in water activity (aw ), and aw at which glass transition occurs at 25 °C was determined as the critical aw (awc ). From mechanical relaxation measurements at 25 °C, the yeast exhibited a large mechanical relaxation above the awc , and the degree of mechanical relaxation increased gradually with increasing aw . This behavior corresponded to a gradual increase in molecular mobility with increasing aw in the rubbery liquid state. Freezable water was observed from aw ≥0.810, and the proportion of freezable water increased with increasing aw . Examination of the effect of aw on the residual biological activity of yeast samples stored at 25 °C for 30 days revealed maximum residual biological activity at aw =0.225 to 0.432. In the lower aw range, the residual biological activity decreased because of oxidation of lipids. In the higher aw range, the residual biological activity decreased gradually with increasing aw . The yeast samples maintained a relatively high residual biological activity, because they could maintain relatively low molecular mobility even in the rubbery liquid state, as suggested by their mechanical relaxation behavior. At aw ≥0.809, residual activity decreased to a negligible value. This could be explained by the appearance of secondary hydrate water (freezable water). Hydrate water protects yeast cells from lipid oxidation but reduces the Tg . As a result, the yeast cells are stabilized maximally only at the awc . PRACTICAL APPLICATION: Although the growth rate of yeast cells becomes negligible below a certain aw , the biological activity of dry yeast decreases gradually during storage. The fact that dry yeast can be maximally stabilized at the awc is practically useful as a criterion for controlling storage stability. In addition, it was found that a remarkable reduction in the molecular mobility, which is otherwise ordinarily increased due to the glass-to-rubber transition, is prevented in yeast. It is possible that the crystallization of amorphous sugar can be prevented by yeast extract. The suggested effect is expected to result in enhanced quality of carbohydrate-based foods.

Evaluation of solid state of rice flours produced by different milling processes using ATR-FTIR spectroscopy.

This study evaluated the influence of the milling process on solid state of rice flours according to water activity using ATR-FTIR. A band at 1740 cm-1 attributed to the C=O stretching of lipids was detected for crystalline samples, and it disappeared at a high aw range. The CH band at 2930 cm-1 of crystalline samples gradually shifted to a higher wavenumber with aw. This band of the α-formed and wet-milled samples shifted to higher wavenumbers above 0.8aw. A band due to OH stretching mode in the 3500-3000 cm-1 region did not shift with aw. The result obtained from IR spectra suggests that the parameter K calculated by Guggenheim-Anderson-de Boar model reflected not only the interaction between water molecules but also the changes of state in solids. Consequently, the results from this study provide insights about the adsorption properties of nonideal solids such as rice flour.