Variation Of Water Activity Research Articles

Role of water activity on sporulation traits and resistance to 915 MHz microwave in the emetic type of Bacillus cereus on rice

The objective of this study is to investigate the influence of water activity on the sporulation of emetic strains of Bacillus cereus and the subsequent susceptibility of sporulated B. cereus to 915 MHz microwave treatment. Water activity levels were manipulated in the sporulation medium by adjusting glycerol concentrations to 0 %, 3 %, 7.5 %, and 10 %, resulting in corresponding water activities of 0.996, 0.981, 0.971, and 0.960, respectively and sporulated at 30℃. These changes in water activity were intended to simulate the variations in water activity that can occur during the cultivation and processing of rice, where B. cereus is commonly found. Sporulation rates increased with higher water activity, achieving over 90 % of total cells after 3, 5, 5, and 6 days of incubation at water activities of 0.996, 0.981, 0.971, and 0.960, respectively. Resistance to microwave treatment increased with higher water activity levels during sporulation. Microwave treatment for 5 min yielded a reduction of 3.03 log CFU/g at a water activity of 0.960 and 1.98 log CFU/g at 0.996 during sporulation. Lower water activity led to higher % dipicolinic acid (DPA) release during microwave treatment, as measured using a spectrometer, indicating greater membrane damage. To investigate the morphology of spores, transmission electron microscopy was used. Spores produced at lower water activity levels were observed to be enveloped by the mother cell, whereas those sporulated at higher water activity levels were distinct from the mother cell. Additionally, spores sporulated at higher water activity levels exhibited higher wet density in Percoll gradient. The levels of germination were also compared after sporulation under different water activity medium using L-alanine. The spores evolved under low water activity exhibited higher germination level compared to those from high water activity conditions. These findings suggested that water activity conditions during sporulation induce variations in morphology, density, germination level, and resistance to 915 MHz microwave. Our experimental results indicated that, even with the same sporulation percentage, there can be differences in the maturation stage, indicating that the level of 915 MHz microwave treatment should be adjusted according to the sporulation conditions of B. cereus.

Oral tobacco-free nicotine products: Quality and safety during storage

Oral non-tobacco nicotine products have gained enormous popularity in recent years. The countries of the Eurasian Economic Union also produce and sell this type of innovative but poorly studied goods. As a result, the safety profile and quality of such products as nicotine poaches require urgent comprehensive research. This study featured the changes in quality of nicotine poaches during storage, i.e. nicotine content, water activity, and microbiological index. The research featured nicotine poaches of several popular brands. The authors used standard research methods; the experiments were performed in the laboratory for chemistry and quality control, Institute of Tobacco, Makhorka, and Tobacco Products, Krasnodar, and at the Department of Bioorganic Chemistry and Technical Microbiology, Kuban State Technological University, Krasnodar. The water activity was 0.8911–0.9502 Aw at the initial stage and remained stable in most samples even after six months of storage. Velo Freeze was the only brand to show significant variations in water activity. The nicotine content was 10.115–12.950 mg/g at the initial stage. Only four samples maintained the initial values after six months of storage. The fluctuations of nicotine content were also mentioned by the manufacturer. The microbiological profile remained stable during the six months of storage and met the requirements for similar products, i.e., chewing gum and unglazed caramel. The project needs further research because the qualitative characteristics of nicotine poaches provided rather unambiguous results. Our study will help develop state standards for oral nicotine products. The results obtained will be used to formulate proposals to the organizations responsible for the future Technical Regulations of the Eurasian Economic Union for nicotine products.

Enhancing nutritional value and health benefits of gluten-free confectionery products: innovative pastilles and marshmallows.

The research focuses on enhancing the nutritional value and potential health benefits of gluten-free confectionery products, developing innovative pastilles and marshmallows enriched with medicinal herb extracts, probiotics, and bioactive compounds from natural sources. Physicochemical properties, including water activity, texture, and color, are assessed to evaluate the quality of the final products. Moreover, in vitro digestibility of the confectionery products is also investigated, with a focus on the release of bioactive compounds such as total phenolic compounds (TPC) and total anthocyanin (TAC) during simulated gastrointestinal digestion. Results indicate that the addition of specific ingredients to pastille samples does not lead to variations in water activity (~0.44), preserving the original properties, quality, and stability of the food. In contrast, the incorporation of additives in marshmallow products significantly increases water activity (p ≤ 0.05), attributed to their moisture-retaining effect. In general, our findings reveal that texture properties and color parameters are significantly affected by different formulations (p ≤ 0.05) for both confectionery products. Notably, the use of fruit and berries puree, along with the incorporation of additives, improves the functionality of confectionary products in terms of consumer acceptance (harder pastilles and softer marshmallow) and product quality. Furthermore, the study reveals that bioactive compounds are released and become more bioaccessible during digestion, particularly in the intestinal phase, with a maximum release exceeding 97% of TPC and TAC for both pastille and marshmallow samples. These findings pave the way for the development of a new category of gluten-free confectionery products, enriched with functional ingredients that offer potential health benefits, aligning with consumer preferences for natural, functional, and health-conscious treats. This research contributes to the evolving the landscape of functional confectionery products and underscores their potential as immune-boosting and naturally based food options.

Dry pasteurization of paprika (Capsicum annuum L.) by short time intensive microwave-infrared radiation: Inactivation of Salmonella Typhimurium and Aspergillus flavus considering quality degradation kinetics

The inactivation of S. Typhimurium and A. flavus along with quality degradation kinetics was studied during combined microwave-infrared (MW-IR) heating of paprika. The spatial changes in the distribution of temperature and variation in water activity (aw) of the paprika samples resulted in a 7.389 log reduction in S. Typhimurium, and 6.182 log reduction in A. flavus. During heating, the deterioration of red pigments was more pronounced compared to that of the yellow pigments. The alteration of color was observed to be due to the increase in a large number of brown pigments. The inhibition of DPPH radicals accelerated with an increase in the power level of MW-IR radiation; the inhibition rate increased from 0.0859 to 0.1485 s−1. Also, the pungency of dried paprika was found to increase due to moisture reduction, inactivation of peroxidase, and the short-duration of heating.

Storage stability of freeze-dried arazá (Eugenia stipitata Mc Vaugh) powders. Implications of carrier type and glass transition

Arazá, a flavorsome fruit rich in antioxidants (phenolics/carotenoids/flavonoids/ascorbic acid) may be appropriate for preparing powdered-functional foods and beverages. Variations in water-activity (aw) and matrix physical state (glassy/rubbery) can induce undesirable changes in dehydrated products color antioxidant composition/activity reducing shelf-life. Solving this problem requires a thorough analysis of the aw/physical state effects on the arazá powder physicochemical properties. Objectives were to: (a) analyse aw, glass-transition temperature (Tg) and carrier-type (maltodextrin-DE10/Arabic-gum) effects on freeze-dried arazá color, antioxidant composition/activity; (b) Determine storage-time influence on antioxidant content/activity/polyphenol composition considering the best performing aw values previously determined. Critical aw values calculated from Tg/aw interrelationship overestimated the aw limit corresponding to samples color/antioxidant composition/activity maximum stability, therefore, Tg was not a reliable indicator of the properties stabilities. Antioxidant content/activity storage behavior (20 °C/aw = 0.11 (best performing aw optimized across all properties)) were satisfactorily predicted by the Weibull equation. Shelf-lives, calculated considering 90% properties retention as the acceptability limit, were 34 (arazá/maltodextrin) and 50 (arazá/Arabic-gum) days. Polyphenolic profile included 6 phenolic acids, one coumaric acid derivative, 4 flavonoids and one ellagitannin. Using a mix of maltodextrin-DE10/Arabic-gum might improve antioxidant content/activity stability obtained with the single carriers and extend their shelf-life.

Kinetic of induced honey crystallization and related evolution of structural and physical properties

Induced crystallization is carried out by adding fine crystals to the liquid honey in order to increase the rate of the process and to obtain a uniform and stable product. The aim of this research was to describe the kinetic of crystallization of honey and the evolution of its physical properties on the basis of different fructose/glucose ratio. To three honey samples selected on the basis of increasing fructose/glucose ratios, 5% of fine crystals have been added, before storage at 14 °C until complete crystallization. During storage, kinetic of crystallization were determined by differential scanning calorimetry and microstructure by a polarising microscope. Moreover, variations of water activity, colour and texture parameters were evaluated during storage.The Avrami equation was found to well describe the crystallization kinetic, although the relation of the Avrami parameters with the nucleation and crystal growth is not entirely clear.The composition of honey was found to influence not only the rate of crystallization, but also the qualitative parameters of sample texture and colour, leading to more pronounced changes during honey storage as the amount of crystallized glucose increased.

Effect of water activity on lipid oxidation and nonenzymatic browning in Brazil nut flour

Brazil nut flour (BNF), as a byproduct from Brazil nut water-soluble extract production, has high contents of protein and linoleic acid. The effect of minor variations in water activity (aw) on lipid oxidation and nonenzymatic browning reactions (NBR) rates in BNF has been studied. For that, the tendency of radical formation detected directly by electron spin resonance spectroscopy, formation of lipid hydroperoxides and browning index were measured in samples with initial aw of 0.101 and 0.196 throughout 18 days of storage at 60 °C. The tendency of radical formation and the formation of lipid hydroperoxides sharply increased and linearly correlated with browning index for BNF with initial aw of 0.196, but not for BNF with initial aw of 0.101. Furthermore, volatile aroma compounds (VACs) including those from the scission of lipid hydroperoxides and those from the breakage of brown polymeric compounds formed during NBR were determined by HS-SPME–GC–MS analysis and the latter were tentatively identified only in BNF with initial aw of 0.196. Hence, there is indication that the lower the initial aw, the greater the storage stability of BNF. Moreover, lipid oxidation products may have contributed to NBR for BNF with initial aw of 0.196.

Effect of long term chilled (up to 5 weeks) then frozen (up to 12 months) storage at two different sub-zero holding temperatures on beef: 1. Meat quality and microbial loads

Beef loins (LL) stored under different chilled-then-frozen storage combinations (up to 5 and 52weeks, respectively) and two frozen holding temperatures were evaluated for microbial load and meat quality parameters. We found holding temperature effects to be negligible, which suggest −12°C could deliver comparable quality LL to −18°C across these same storage periods. Meat quality parameters varied significantly, but when compared to existing consumer thresholds these may not be perceptible, colour being the exception which proved unacceptable, earlier into retail display when either chilled and subsequent frozen storage periods were increased. There was insufficient detection of key spoilage microbes to allow for statistical analysis, potentially due to the hygienic and commercially representative LL source, although variation in water activity, glycogen content, pH and other moisture parameters conducive to microbial proliferation were influenced by chilled-then-frozen storage. These outcomes could be applied to defining storage thresholds that assure beef quality within export networks, leveraging market access, and improving product management.

The effect of fluid composition, salinity, and acidity on subcritical crack growth in calcite crystals

AbstractChemically activated processes of subcritical cracking in calcite control the time‐dependent strength of this mineral, which is a major constituent of the Earth's brittle upper crust. Here experimental data on subcritical crack growth are acquired with a double torsion apparatus to characterize the influence of fluid pH (range 5–7.5) and ionic strength and species (Na2SO4, NaCl, MgSO4, and MgCl2) on the propagation of microcracks in calcite single crystals. The effect of different ions on crack healing has also been investigated by decreasing the load on the crack for durations up to 30 min and allowing it to relax and close. All solutions were saturated with CaCO3. The crack velocities reached during the experiments are in the range 10−9–10−2 m/s and cover the range of subcritical to close to dynamic rupture propagation velocities. Results show that for calcite saturated solutions, the energy necessary to fracture calcite is independent of pH. As a consequence, the effects of fluid salinity, measured through its ionic strength, or the variation of water activity have stronger effects on subcritical crack propagation in calcite than pH. Consequently, when considering the geological sequestration of CO2 into carbonate reservoirs, the decrease of pH within the range of 5–7.5 due to CO2 dissolution into water should not significantly alter the rate of fracturing of calcite. Increase in salinity caused by drying may lead to further reduction in cracking and consequently a decrease in brittle creep. The healing of cracks is found to vary with the specific ions present.

A new analysis of Mars "Special Regions": findings of the second MEPAG Special Regions Science Analysis Group (SR-SAG2).

A committee of the Mars Exploration Program Analysis Group (MEPAG) has reviewed and updated the description of Special Regions on Mars as places where terrestrial organisms might replicate (per the COSPAR Planetary Protection Policy). This review and update was conducted by an international team (SR-SAG2) drawn from both the biological science and Mars exploration communities, focused on understanding when and where Special Regions could occur. The study applied recently available data about martian environments and about terrestrial organisms, building on a previous analysis of Mars Special Regions (2006) undertaken by a similar team. Since then, a new body of highly relevant information has been generated from the Mars Reconnaissance Orbiter (launched in 2005) and Phoenix (2007) and data from Mars Express and the twin Mars Exploration Rovers (all 2003). Results have also been gleaned from the Mars Science Laboratory (launched in 2011). In addition to Mars data, there is a considerable body of new data regarding the known environmental limits to life on Earth-including the potential for terrestrial microbial life to survive and replicate under martian environmental conditions. The SR-SAG2 analysis has included an examination of new Mars models relevant to natural environmental variation in water activity and temperature; a review and reconsideration of the current parameters used to define Special Regions; and updated maps and descriptions of the martian environments recommended for treatment as "Uncertain" or "Special" as natural features or those potentially formed by the influence of future landed spacecraft. Significant changes in our knowledge of the capabilities of terrestrial organisms and the existence of possibly habitable martian environments have led to a new appreciation of where Mars Special Regions may be identified and protected. The SR-SAG also considered the impact of Special Regions on potential future human missions to Mars, both as locations of potential resources and as places that should not be inadvertently contaminated by human activity.

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn2O3) to hausmannite (Mn3O4) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn2O3 and Mn3O4) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings.

Molecular mechanism of deactivation of C. antarctica lipase B by methanol

The catalytic activity of Candida antarctica lipase B upon alcoholysis of a constant concentration of 15.2% vinyl acetate (vol/vol) and varying concentrations of methanol (0.7–60%) in toluene was determined experimentally by measuring the initial reaction velocity. The molecular mechanism of the deactivation of the enzyme by methanol was investigated by fitting the experimental data to a kinetic model and by molecular dynamics simulations of C. antarctica lipase B in toluene–methanol–water mixtures.The highest catalytic activity (280U/mg) was observed at methanol concentrations as low as 0.7% methanol (vol/vol), followed by a sharp decrease at higher methanol concentrations. For methanol concentrations above 10% (vol/vol), catalytic activity was at 30% of the maximum activity. A variation of water activity in the range 0.02–0.09 had only minor effects. These experimental observations are described by a simple kinetic model using three assumptions: (1) a ping-pong bi–bi mechanism of the enzyme, (2) competitive inhibition by the substrate methanol, and (3) by describing enzyme kinetics by the thermodynamic activities of the substrates rather than by their concentrations. Two equilibrium constants of methanol (KM,MeOH=0.05 and Ki,MeOH=0.23) were derived by modeling methanol binding to the substrate binding site of the lipase in molecular dynamics simulations of protein-solvent systems at atomic resolution.Thus, the sharp maximum of catalytic activity of C. antarctica lipase B at 0.7% methanol is a direct consequence of the fact that methanol–toluene mixtures are far from ideal. Understanding the thermodynamics of solvent mixtures is prerequisite to a quantitative model of enzymatic activity in organic solvents.

Molecular dynamics simulation of the A-DNA to B-DNA transition in aqueous RbCl solution

Unrestrained molecular dynamics (MD) simulations have been carried out to characterize the stability of DNA conformations and the dynamics of A-DNA→B-DNA conformational transitions in aqueous RbCl solutions. The PARM99 force field in the AMBER8 package was used to investigate the effect of RbCl concentration on the dynamics of the A→B conformational transition in the DNA duplex d(CGCGAATTCGCG)2. Canonical A- and B-form DNA were assumed for the initial conformation and the final conformation had a length per complete turn that matched the canonical B-DNA. The DNA structure was monitored for 3.0 ns and the distances between the C5′ atoms were obtained from the simulations. It was found that all of the double stranded DNA strands of A-DNA converged to the structure of B-form DNA within 1.0 ns during the unrestrained MD simulations. In addition, increasing the RbCl concentration in aqueous solution hindered the A→B conformational transition and the transition in aqueous RbCl solution was faster than that in aqueous NaCl solution for the same electrolyte strength. The effects of the types and concentrations of counterions on the dynamics of the A→B conformational transition can be understood in terms of the variation in water activity and the number of accumulated counterions in the major grooves of A-DNA. The rubidium ion distributions around both fixed A-DNA and B-DNA were obtained using the restrained MD simulations to help explain the effect of RbCl concentration on the dynamics of the A→B conformational transition.

Effect of water activity on anthocyanin degradation and browning kinetics at high temperatures (100–140 °C)

The effect of water activity on anthocyanin degradation and non-enzymatic browning (NEB) indices was determined in a blackberry juice system heated to high temperature. Blackberry juice samples mixed with quartz sand at three different water activities (aw of 0.34, 0.76 and 0.95) and having the same concentration of anthocyanins (51±4mg100g−1 WB) were heated in a hermetically sealed reactor under non-isothermal conditions (100 to 140°C). Reaction kinetics were well-represented by first-order reaction models. A non-isothermal method was used for the identification of kinetic parameters, and the confidence interval for each parameter was determined via Monte-Carlo simulations. The estimated parameters revealed an increase in the anthocyanin degradation reaction rate constant (kref) from 0.9 to 3.5×10−3s−1 as the water activity decreased from 0.99 to 0.34. The corresponding activation energies (Ea) for anthocyanins and NEB index were modified to a lesser extent by variations in water activity. Hence, the reduction of water activity had a negative effect on anthocyanin stability at high temperatures.

Amorphous Materials: Properties, structure, and Durability: Oxidation state of iron in hydrous phono-tephritic melts

The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H2O-CO2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa. The oxygen fugacity (fO₂) varied from NNO-2.9 to NNO+2.6 in log fO₂, relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the Fe2+/ΣFe ratio of milligram-sized samples within ±0.03 (2σ). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The Fe2+/ΣFe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on Fe2+/ΣFe at given fO₂ was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower Fe2+/ΣFe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher K2O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in FeOT (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log fO₂ and Fe3+/Fe2+ for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as H2O or K2O, on the redox state of Fe in silicate melts.

Freeze-drying of acerola ( Malpighia glabra L.)

Several quality parameters, such as water activity ( a w), glass transition temperature ( T g), Vitamin C content, shrinkage and rehydration capacity were investigated for the freeze-drying of acerola fruits. The variation of temperature with time at different positions of the samples was measured during the freezing of samples, performed prior to the freeze-drying. Drying kinetic curves were obtained for different types of samples. The extent of shrinkage after freeze-drying was investigated and related to the glass transition temperature, T g. The variation of water activity ( a w) along the drying and sorption isotherms was obtained for different freezing techniques. It was observed that freeze-dried acerola fruits can be easily reconstituted, and important nutritional parameters are well preserved after the process. They may be considered as a good source of Vitamin C, whose content was best preserved for freeze-drying of fruits at an intermediary stage of ripening.

Water weakening of chalk— Mechanical effects of water–glycol mixtures

Chalk petroleum reservoirs are characterized as fragmentary parts of calcite skeletons produced by plankton algae, known as coccolithophorids. As water injection into chalk reservoirs is an important method for improved oil recovery, chalk–water interactions have received considerable attention during the decades due to reservoir compaction. In the present study the mechanical properties of high porosity outcrop chalk (> 40%) were studied with mixtures of water and glycol. The objective was to test the hypothesis that the activity of water is the key parameter in water-weakening of chalk. This hypothesis is based on experiments with brine-saturated chalk of different salinities. The fact that water and glycol are fully miscible fluids allows variation of water activity from unit to zero. Four types of tests were used: (1) triaxial tests at low confining pressure to assure shear failure conditions, (2) triaxial tests at higher confining pressure to assure a stress path passing through the end-cap, (3) hydrostatic tests to determine the hydrostatic yield stress and (4) “Brazilian” tests. The tests showed increasing strength with decreasing water activity. Compared with the results from brines, there was good agreement, confirming the hypothesis. Previously results using water–methanol mixtures resulted in the same conclusion. A water weakening mechanism is proposed which involves an additional pressure exerted on the grains by attraction of water molecules to the chalk surface. This adsorption pressure acts like an increase in pore pressure, decreasing the cohesion of the chalk. Evaluation of different mechanisms in chalk–water interactions, indicate that pressure solution and adsorption pressure may be the two most important ones.

Kinetics of hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in binary water–cosolvent mixtures; the role of solvent activity and solute–solute interactions

Rate constants are reported for the pH-independent hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in aqueous solution as a function of the concentration of added cyanomethane (acetonitrile), polyethylene glycol (PEG 400) and tetrahydrofuran (THF). The concentration of water was varied between ca. 25 and 55.5 M. It was found that the variation in water activity yields only a minor contribution to the observed variation in rate constants. Interestingly, for both cyanomethane and PEG 400 log(k) varies approximately linearly with the molar concentration of water. Medium effects in highly aqueous solutions ([H(2)O] > 50 M) of ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methyl-2-propanol have also been determined. Unexpectedly, in this concentration range the alcohols induce significantly smaller effects per unit volume than cyanomethane. The present results are discussed in terms of pairwise interaction parameters. Isobaric activation parameters have been determined and reveal remarkable differences in the nature of the induced medium effects.

Influence of water activity on the ice-nucleating activity of Pseudomonas syringae

Pseudomonas syringae is known as a biological ice-nucleating agent. The bacterium has the unusual property of increasing the temperature at which water freezes by a few degrees. However, the ice-nucleating activity (INA) always remains lower for in vitro cultivated cells, than for cells grown in planta. We examined the effects of the hydrophobic environment and of water availability, on the in vitro growth and INA of P. syringae. The hydrophobic environment was modified by addition of fatty acids, vegetable oils or silicone oil to the culture medium. Addition of olive oil (1%), or traces of silicone oil in the culture medium had a positive effect upon the expression of INA. Variations in water activity from 0.990 to 0.988 by addition of sugar beet fibres or sodium chloride in the culture medium were followed by an increase in INA. This study suggested that control of the medium's water activity must be considered as an important parameter for optimization of INA in P. syringae.

Water activity does not influence the enantioselectivity of Lipase PS and lipoprotein lipase in organic solvents

The activity and enantioselectivity of Lipase PS from Pseudomonas cepacia and lipoprotein lipase from Pseudomonas sp. were investigated in organic solvents preequilibrated to water activities ranging from <0.1 to 0.53, using as a model reaction the transesterification between (±)-sulcatol and vinyl acetate. Variations of water activity markedly influenced the transesterification rate but did not modify the enantioselectivity of the two enzymes.