Phase Ratio Variation Research Articles

Thermodynamic characterization of interactions between environmental contaminants and room temperature ionic liquids using static headspace gas chromatography with vacuum ultraviolet detection

Understanding the thermodynamic interactions between an analyte and the sample phase is of paramount importance when choosing a co-solvent in headspace analysis. A sample phase – gas phase equilibrium partition coefficient (Kp) is used fundamentally to describe the distribution of the analyte between the two phases. Kp determinations by headspace gas chromatography (HS-GC) were acquired by two methods: vapor phase calibration (VPC) and phase ratio variation (PRV). Here, we demonstrated a pressurized – loop headspace system in conjunction with gas chromatography vacuum ultraviolet detection (HS-GC-VUV) to directly calculate the concentration of analytes in the gas phase from room temperature ionic liquids (RTILs) sample phases, using pseudo-absolute quantification (PAQ). PAQ, an attribute of VUV detection, allowed for quick determination of Kp and other thermodynamic properties, such as enthalpy (ΔH) and entropy (ΔS) of the system through the use of van't Hoff plots in the temperature range of 70–110 °C. The Kp determinations by PAQ were comparable to those obtained using the VPC method with percent difference ranging from ≤ 1–33%. Kp determinations were made for analytes (cyclohexane, benzene, octane, toluene, chlorobenzene, ethylbenzene, m-,p-, and o-xylene) at the varying temperatures (70–110 °C) using different RTILs (1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][ESO4]), 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]), and tris(2-hydroxyethyl)methylammonium methylsulfate ([MTEOA][MeOSO3])) and (1-ethyl-3-methylimidazolium bis(trisfluoromethanesulfonyl)imide ([EMIM] [NTF2])). The results from the van't Hoff analysis revealed that [EMIM] cation-based RTILs exhibit strong solute-solvent interactions with analytes that have π- electrons.

Flavor release and stability comparison between nano and conventional emulsion as influenced by saliva

Flavour release and emulsion stability depend on volatile organic compounds' environmental conditions, food microstructure, and physicochemical properties. The effect of pH (3.5 vs 7.0) and saliva addition on stability and flavour release from nano and conventional emulsions was investigated using particle size, charge and Lumisizer measurments. Larger particle sizes were observed at lower pressures and in saliva-containing emulsions. At 1700 bar, nano-emulsions (below 150 nm) were created at pH 3.5 and 7.0 including saliva-containing emulsions. As was clear from the creaming velocity measurements, saliva addition decreased the emulsion stability by reducing particle charges and increased viscosity by more than 50%, especially when prepared at pH 3.5 closer to the isoelectric point of the used emulsifier β-lactoglobulin (pH 5.2). (5.2). Flavour release from emulsions was measured at equilibrium using a phase ratio variation to determine partition coefficients and dynamically using an electronic nose. Partition coefficients of the flavour compounds for most conditions were two to four times lower in emulsions prepared at pH 7.0 than at pH 3.5 and in emulsions without saliva. Emulsions prepared with higher pressures showed stronger flavor release rates, while additional salvia dropped the release rate for ethyl acetate at pH 3.5. The physicochemical properties of flavour compounds, saliva addition and pH of emulsions influenced flavour release more than homogenization pressures. The potential in using nano-emulsions in food applications an be attributed higher stability and enhanced flavor release.

Determination of methanol partition coefficient in octanol/water system by a three-phase ratio variation headspace gas chromatographic method

Determining the methanol partition coefficient (Kow) in octanol/water system is important to assess the risk of its impact to humans and the environment. In this paper, we report a novel method for the determination of the Kow of methanol with headspace gas chromatography (HS-GC), using a three-phase equilibrium-based phase ratio variation approach. Three headspace vials with different phase ratios [(1) 0.2mL of water plus 3mL of octanol, (2) 1mL of water plus 3mL of octanol and (3) 1mL of water plus 10mL of octanol] were used in the experiment. After pre-standing at 25 °C for > 10h and equilibrating in the headspace oven at a temperature of interest for additional 25min, the methanol signals of the vapor phase (i.e., the headspace) in these vials were measured by HS-GC, from which the Kow of methanol can be calculated based on the established equations. The results showed that the method has good precision (within 0.06 log unit in standard deviation) and acceptable accuracy (with method error within 0.5 log unit). A correlation between the Kow of methanol and temperature extrapolated the Kow values in a temperature range of interest. The present method represents a simple and in-situ approach for the determination of the Kow of methanol and is also suitable to be applied to other volatile compounds.

Optimal experimental designs for estimating Henry's law constants via the method of phase ratio variation

When measuring Henry's law constants (kH) using the phase ratio variation (PRV) method via headspace gas chromatography (GC), the value of kH of the compound under investigation is calculated from the ratio of the slope to the intercept of a linear regression of the inverse GC response versus the ratio of gas to liquid volumes of a series of vials drawn from the same parent solution. Thus, an experimenter collects measurements consisting of the independent variable (the gas/liquid volume ratio) and dependent variable (the GC−1 peak area). A review of the literature found that the common design is a simple uniform spacing of liquid volumes. We present an optimal experimental design which estimates kH with minimum error and provides multiple means for building confidence intervals for such estimates. We illustrate performance improvements of our design with an example measuring the kH for Naphthalene in aqueous solution as well as simulations on previous studies. Our designs are most applicable after a trial run defines the linear GC response and the linear phase ratio to the GC−1 region (where the PRV method is suitable) after which a practitioner can collect measurements in bulk. The designs can be easily computed using our open source software optDesignSlopeInt, an R package on CRAN.

Determination of Aroma Compound Partition Coefficients in Aqueous, Polysaccharide, and Dairy Matrices Using the Phase Ratio Variation Method: A Review and Modeling Approach.

The partition of aroma compounds between a matrix and a gas phase describes an individual compound's specific affinity toward the matrix constituents affecting orthonasal sensory perception. The static headspace phase ratio variation (PRV) method has been increasingly applied by various authors to determine the equilibrium partition coefficient K in aqueous, polysaccharide, and dairy matrices. However, reported partition coefficients are difficult to relate and compare due to different experimental conditions, e.g., aroma compound selection, matrix composition, equilibration temperature. Due to its specific advantages, the PRV method is supposed to find more frequent application in the future, this Review aims to summarize, evaluate, compare, and relate the currently available data on PRV-determined partition coefficients. This process was designed to specify the potentials and the limitations as well as the consistency of the PRV method, and to identify open fields of research in aroma compound partitioning in food-related, especially dairy matrices.

An approach to adapt aroma in fat-free yoghurt systems: Modelling and transfer to pilot scale

Aroma retention and flavour perception of dairy products largely depend on the fat content of their respective varieties. When the milk fat content of model milk solutions was increased from 2% to 12%, a linear increase in the matrix-air partition coefficient KMG, as determined by the phase ratio variation (PRV) method, was observed for six aroma compounds in the log P range of +1.8 to +4.5, namely ethyl butanoate, ethyl-2-methylbutanoate, (Z)-3-hexenyl acetate, ethyl hexanoate and limonene. (Z)-3-hexenol, the least hydrophobic compound (log P = +1.6), was slightly less retarded at increased fat content. With the exception of limonene, it was observed that the higher the log P, i.e., the hydrophobicity of the aroma compound, the more pronounced was the linear increase of KMG and hence retention. The successful scale-up from basic laboratory research into pilot plant for the aroma compound retention in fat-free yoghurt is presented.

Phase ratio variation approach for the study of partitioning behavior of volatile organic compounds in polymer sample bags: Nalophan case study.

Sorption of volatile organic compounds on the inner surface of polymer sampling bags leads to important underestimations of the real headspace concentration. Introducing a wide range of volatiles in a two-phase system containing Nalophan revealed that recoveries decreased down to 57 % in a period of 22 h. In this work, a phase ratio variation approach is investigated to quantify the degree of scalping, and thus enabling to compensate for sorption phenomena. This method requires limited measurements, without the need for time-consuming calibrations. By spiking identical amounts of volatiles in three two-phase systems, each having unique polymer volume/mass ratios β (β 1 = 0, β 2 = 80.9, β 3 = 161.9), individual partitioning coefficients could be experimentally determined for a wide range of compounds. Additionally, a correlation was found between these partitioning coefficients and the liquid molar volume for a number of aliphatic, aromatic, and oxygenated compounds.

Determination of solid–liquid partition coefficient of volatile compounds by solid phase ratio variation based headspace analysis

A novel method has been developed for the determination of the solid–liquid partition coefficient of volatile analytes using headspace gas chromatography (HS–GC). The method is based on the headspace measurement for the three-phase equilibrium in sample vials containing different amounts of solid sample. Theory is presented that allows calculation of partition coefficient of the analyte from the HS–GC data obtained at the condition of solid phase ratio variation (SPRV). Analysis of errors demonstrates that this solid phase ratio variation method generates results with reasonable precision for all volatile compounds, much better than the liquid-focused (i.e., liquid phase ratio variation) method reported early.The solid phase ratio variation method was used to determine the solid–liquid partition coefficient of toluene in a water–polystyrene system, using polymer particles with average size ranging from 0.4 to 2.1μm. The results show that the partition coefficient of toluene in the given system is proportional to the specific surface area of the polymer particles, as expected.In summary, the method is a simple and practical tool for determining the solid–liquid partition coefficient of volatile analytes.

Effect of konjac glucomannan addition on aroma release in gels containing potato starch

The present study aimed to measure the retention of aroma compounds (ethyl acetate, ethyl hexanoate and carvacrol) in dispersions based on konjac glucomannan and/or potato starch, and to highlight the influence of konjac glucomannan on the mechanisms involved in aroma retention. Publications on the effect of konjac glucomannan on aroma release are scarce. Konjac glucomannan is a polysaccharide used as a food additive for its viscous and emulsifying properties. Retention of aroma compounds in dispersions was calculated from partition coefficients which were measured using the phase ratio variation method. This method, consisting of analyses of the headspace at equilibrium, enables the determination of the partition coefficient of volatile compounds in a gas/liquid system without external or internal calibration. The three aroma compounds chosen for this study behave differently toward amylose. Prior to the release study, the complexing behavior of carvacrol with starch, hitherto unknown, was investigated by X-ray diffraction: V6III amylose complexes were formed with carvacrol. Our results showed no specific interaction between ethyl hexanoate and potato starch or konjac glucomannan. Ethyl acetate retention seemed to be due to trapping in the complex network of polysaccharides and to the density of this network. Retention of carvacrol was influenced by the nature of polysaccharides present in the dispersion, and was mainly governed by specific interaction with starch. Additionally, the addition of konjac glucomannan to potato starch dispersions decreased the retention of volatile compounds complexing starch, but had little effect on the retention of the other aroma compounds.

Comparison of volatile release in hydrocolloid model systems containing original and regio selectively carboxylated β-glucans

The release of 13 volatile compounds in hydrocolloid model systems containing original or regio selectively carboxylated β-glucan at different pHs was analyzed using static headspace gas chromatographic analysis. There was significant difference in the release of most compounds, except for butyl propionate, 2-propanol, and 2,3-butanedione, in both the original and carboxylated β-glucan hydrocolloid model systems. However, the effects of pH in the hydrocolloid systems were not significant for most compounds studied, except for butyl propionate, ethyl caproate, 3-methyl-1-butanol, 2,3-butanedione. In particular, the release of 2,3-butanedione was considerably decreased at pH 10 compared to pH 4 and 7. In addition, the partition coefficient of each ethyl ester compound was measured to investigate the release of aroma compounds using the phase ratio variation (PRV) method. The difference in the partition coefficients of short and medium chain-ethyl esters between the reference solution and the original/carboxylated β-glucan solutions was not significant. In contrast, the release of relatively long chain-ethyl esters (ethyl caprylate and ethyl nonanoate) from both the original and carboxylated cellulose solutions was decreased compared to that in reference solution.

Study of interactions between aroma compounds and acacia gum using headspace measurements

This study deals with retention measurements of aroma compounds in acacia gum aqueous solutions. In that purpose, six volatile molecules differing in their physicochemical properties were analysed: ethyl butyrate, ethyl hexanoate, ethyl octanoate, cumene, 1-octanol and linalool. Their gas/liquid partition coefficients were measured by headspace gas chromatography using the phase ratio variation method from acacia gum solutions. Three acacia gum samples exhibiting different emulsifying properties were studied for concentrations ranging from 0 to 20 wt%. These measurements made possible to evaluate the interactions between aroma compounds and acacia gums through the calculation of retention. A correlation between retention and lipophilicity was observed, thus proving that acacia gum interacts with aroma compounds via hydrophobic bonding. Significant interactions occur for volatile molecules that have a pronounced lipophilic character (logP > 3.6). The effect of gum concentration was also studied. No linear relationship was evidenced with retention and a significant effect was observed for concentration higher than 5 wt%. This work also establishes that the emulsifying properties of acacia gum can be evaluated by retention measurements for lipophilic aroma compounds (logP > 3.6).

Proposed alternative phase ratio variation method for the calculation of liquid–vapour partition coefficients of volatiles

The phase ratio variation PRV method is a classical way to determine the partition coefficients of volatile compounds between their solution and vapour phases in a variety of circumstances. However, some results obtained by this method can be disappointing. A new PRV equation in which the initial liquid-phase solute concentration is replaced by the liquid-phase solute concentration at equilibrium is proposed. This proposed PRV equation is a second-order polynomial equation. To thoroughly examine the possible modes of calculation, noisy dummy data were generated using both the classical, first-order PRV model (PRV1) and the proposed, second-order model (PRV2). Thus, pseudo-data obtained from simulations were compared to published experimental data. We observed that the second-order model, PRV2, produces a lower variability, allowing improved K precision. Moreover, the obtained K(PRV2) values are very close to those obtained by classical equilibrium headspace analysis (EHSA). The PRV2 model we propose responds to the demand for a simple, reliable method and is a useful alternative for the calculation of liquid–vapour partition coefficients.

Solubility and hydrolysis of HCFC‐22 (CHF2Cl): Upward revision of rate constants for aqueous reactions of CHF2Cl with OH− at elevated temperature

AbstractHenry's law constants of CHF2Cl in water at temperature T in K, KH(T) in M atm−1, were determined to be ln(KH(T))=−(11.1±1.5)+((2290±500)/T) at 313–363 K by means of a phase ratio variation headspace method. The temperature‐dependent rate constants for aqueous reactions of CHF2Cl with OH−, k(T) in M−1 s−1, were also determined to be 3.7×1013exp(−(11, 200/T)) at 313–353 K, by considering the gas–water equilibrium, the aqueous reaction at room temperature, and liquid‐phase diffusion control. The liquid‐phase diffusion control was approximated with a one‐dimensional diffusion first‐order irreversible chemical reaction model. The k(T) value we determined is 10 times (at 353 K) or 3 times (at 313 K) as large as the value reported (R. C. Downing, Fluorocarbon Refrigerants Handbook, Prentice Hall: Englewood Cliffs, NJ, 1988). This upward revision of k(T) indicates that the removal efficiency of CHF2Cl directly through the hydrolysis (CHF2Cl + OH−) is higher than previously expected at temperatures, such as 353 K, relevant to wet flue gas cleaning systems for ozone‐destruction substance‐destruction facilities. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 639–647, 2011

Multilayer emulsions as delivery systems for controlled release of volatile compounds using pH and salt triggers

Multilayer oil-in-water (M-O/W) emulsions were compared to primary oil-in-water (P-O/W) emulsions as carriers for volatile organic compounds (VOCs) under various environmental conditions (pH and salt). The M-O/W emulsion consisted of soy oil coated with β-lactoglobulin (βLG) and pectin layers. The release of VOCs with different physiochemical properties from aqueous solutions and emulsion systems was measured using static and dynamic headspace methods. The partition coefficients ( K) calculated by the phase ratio variation (PRV) method, showed different volatile release profiles between the emulsion types. An increase in VOC release was found for the unstable P-O/W emulsion at pH 5, whereas M-O/W emulsions were stable at the same pH and retained the hydrophobic VOCs. Hydrophobic interactions and hydrogen bonds with the secondary dense layer of pectin may be responsible for the improved retention. Increasing pH and ionic strength acts as a VOC release trigger to detach the pectin from the interface. The release rates from initial dynamic curves support the results under equilibrium conditions. The results of this study demonstrate the capability of using M-O/W emulsions for controlled release of VOCs, as well as an alternative system to create stable emulsions with similar VOC release profiles.

The dynamics of aroma compound transfer properties in cheeses during simulated eating conditions

In vivo aroma release during solid food consumption is a complex phenomenon that depends on food structure and composition, as well as on oral processing (combination of mastication and incorporation of saliva into the food product). The objective of this study was to understand and to predict the physico-chemical properties of aroma compounds through the dynamics of flavor release during in-mouth oral processing of food before bolus swallowing. Within this context, the evolution of two aroma compounds during bolus formation was explored by studying the two main properties that account for mass transfer: air/bolus partition and mass transfer coefficients. Four types of industrial cheese products (varying in fat and firmness) flavored with ethyl propanoate and 2-nonanone were chosen. Each matrix was mixed with various amounts of artificial saliva to mimic boluses at different stages of mastication. The air/bolus partition coefficient was determined by the static phase ratio variation method (PRV), while the mass transfer coefficient was obtained by non-linear regression from dynamic headspace experiments. Results showed that there is a dilution effect on the air/bolus partition coefficient and both a dilution and a product effect (firmness) on the mass transfer coefficient of ethyl propanoate in the bolus. These results were also validated with 2-nonanone for the low-fat cheeses.

Inherent and process-induced hydrophobicity influences aroma retention in fat-free dairy matrices

It was shown that both inherent and process-induced hydrophobicity can influence the aroma retention in fat-free dairy matrices. The extent and direction to which these parameters take effect depends on the composition of the dairy matrix as well as on the hydrophobicity (log P) of the aroma compounds. The phase ratio variation (PRV) method was used to determine the matrix–air partition coefficient K MG. For the hydrophobic aroma compound limonene, K MG increased when the ratio of casein protein to whey protein was increased in model milk, while the less hydrophobic ethyl hexanoate and the hydrophilic diacetyl remained unaffected. This was attributed to the hydrophobic character of the casein micelle. Upon acidification with glucono-δ-lactone, K MG of diacetyl decreased significantly while K MG of ethyl hexanoate and limonene slightly increased. This was attributed to the predominance of hydrophobic interactions in acidified dairy matrices, which were analysed via bonds analysis using destabilising buffer solutions.

Static headspace analysis of volatile compounds released from β-lactoglobulin-stabilized emulsions determined by the phase ratio variation method

The release of volatile organic compounds (VOCs) from oil-in-water emulsions stabilized by β-lactoglobulin (βLG) was studied under equilibrium conditions. The effects of lipid type and mass fraction were investigated using soy oil (SO) and anhydrous milk fat (AMF). The release was compared using systems of a buffered aqueous solution, pure oil and emulsions. Gas–matrix partition coefficients (K) for five volatile compounds: 1-propanol, ethyl butyrate, heptanal, octanol and 2-decanone, were determined by static headspace gas chromatography (GC). The compounds were chosen based on their polarity, vapour pressure and functional groups. Two indirect methods to measure K were used: phase ratio variation (PRV) and phase ratio calibration (PRC). These two methods were found to be simple and accurate alternative to measure K without using external calibration. The VOC release depended mainly on the physiochemical properties and affinity of the compounds to the matrix. Higher retention in oil and emulsion systems than in aqueous solution was observed for all VOCs except the more hydrophilic 1-propanol. Ethyl butyrate showed the highest K value for emulsions whereas 2-decanone had the lowest. There was a significant decrease in K values for emulsions with higher lipid content. When comparing lipid type, emulsions made with the more saturated AMF, compared to SO, resulted in lower K values for the more hydrophobic compounds. The role of βLG on VOC release was compared with Tween 20, both as emulsifiers. A significant decrease in the headspace concentration was seen above the aqueous solution for heptanal, octanol and 2 decanone. This may be attributed to the hydrophobic cavity site of the protein and covalent bonds with the aldehyde. In emulsion systems, the presence of the emulsifier conformation at the interface had an effect on the affinity with intermediate hydrophobic VOCs. This study provides an important understanding of how VOC release can be controlled using βLG stabilized emulsions in food systems.

Relationship between the emulsifying properties of Acacia gums and the retention and diffusion of aroma compounds

Five samples of Acacia senegal gum were selected because of their different emulsifying properties. Acacia gum samples were first characterized by various analytical methods including intrinsic viscosity and arabino-galactan protein amount as determined by gel permeation chromatography. In a second step, stability of emulsions containing orange essential oil and Acacia gums was evaluated by measurement of turbidity. In addition, flavour release of two different hydrophobic aroma compounds from Acacia gum aqueous solutions was studied. Retention was measured using the phase ratio variation method under equilibrium whereas diffusion was assessed by DOSY NMR. This study established the relationship between the emulsifying ability of Acacia gum and the behaviour of aroma compounds in the corresponding solution. Better emulsifying ability of Acacia gum sample appears strongly correlated to an increase in flavour retention and a decrease in molecular mobility as the result of mostly hydrophobic interactions.

Thermodynamics of partitioning of allyl isothiocyanate in oil/air, oil/water, and octanol/water systems

The thermodynamics of partitioning of allyl isothiocyanate (AITC) in oil/air, oil/water, and octanol/water systems were investigated using the van’t Hoff approach with olive and canola oils as model food oils. The partition coefficients were determined by phase ratio variation method at temperatures between 4 and 50 °C. AITC showed much stronger partitioning into oil or octanol phase than into air or water phase because of its strong hydrophobic nature. As temperature increased, the oil/air, oil/water, and octanol/water partition equilibria were shifted toward air, oil, and octanol phases, respectively. These partitioning behaviors are attributed that the air-to-oil transfer of AITC is an entropically unfavorable, enthalpy-driven spontaneous process, whereas both water-to-oil and water-to-octanol transfers are energetically unfavorable, entropy-driven spontaneous processes. Canola oil was found to be more effective as a solvent for AITC than olive oil in oil/air system but less effective in oil/water system.

Determination of liquid–gas partition coefficients of BuA and MMA by headspace-gas chromatography utilizing the phase ratio variation method

The phase variation method is described and applied in order to determine liquid/gas partition coefficients of monomers in aqueous solutions as well as the effect of the presence of one monomer to the partition coefficient of the other. The method is based on the relationship between reciprocal peak area and the phase ratio in the vial containing the sample solution. It involves regression analysis of headspace gas chromatography measurements of a number of sample vials with a variation of phase ratios. The method is applied in the case of the monomers butyl acrylate and methyl methacrylate. Experimental data for the partition coefficient of the two acrylates are modeled with the Statistical-Associating-Fluid Theory (SAFT).