Phase Ratio Variation Method Research Articles

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.

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.

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).

Effect of Pulp Reduction and Pasteurization on the Release of Aroma Compounds in Industrial Orange Juice

Release of aroma compounds in orange juice according to pulp content and pasteurization was performed by headspace-solid phase microextraction (HS-SPME), HS-SPME-GC-olfactometry, and by the determination of partition coefficients of aroma compounds between the juice and the headspace. Orange juices with 12% and 6% pulp contents were collected from a processing line before and after pasteurization. HS-SPME revealed that the 6% pulp juice was the most affected by pasteurization, with an increase in the release of several aroma compounds that are generated by heat treatment (that is, alpha- and beta-terpinéol) and a decrease in the release of compounds sensitive to pasteurization (that is, neral and geranial). Principal component analysis of olfactometric data allowed discriminating fresh and pasteurized juices, regardless of the pulp content. The determination of the gas/liquid partition coefficients by the phase ratio variation method was possible for 7 aroma compounds. For the most hydrophobic ones, the release of aroma compounds in fresh juices was higher in the 6% pulp juice than in the 12% pulp juice. However, in pasteurized juices, the difference between their partition coefficients was less marked. Potential interactions between cloud proteins and hydrophobic aroma compounds may explain this phenomenon.

Vapour partition of aroma compounds in strawberry flavoured custard cream and effect of fat content

Gas–matrix partition coefficients ( k) of volatile compounds of a strawberry flavour added to a complex model system (custard dessert, COST action 921 recipe) were determined. The influence of fat content on the volatility of the strawberry aroma compounds was investigated using full fat milk- (FFM) and skimmed milk- (SM) custards. For the purpose of this study, the phase ratio variation (PRV) method was used because of its easy application for a mixture of volatiles without requiring any calibration. From the 15 aroma compounds present in the flavouring mixture, seven were volatile enough to be detected and concentrated enough to allow the calculation of k values. Gas–matrix partition coefficients were compared to gas–water partition coefficients. For some aroma compounds (ethyl hexanoate, ethyl butanoate, and hexanal) a significantly lower k value was observed in FFM-custard than in water, indicating a retention effect caused by the matrix. As expected, the fat content and the hydrophobicity of the volatile compounds were of great importance to explain the differences between partition coefficients in SM custards and in FFM custards: for SM-custard, a higher release in the vapour phase was observed than for FFM-custard; the more hydrophobic the aroma compound, the higher the effect. Moreover, ethyl butanoate and hexanal were less retained in SM-custard than in water. This may be attributed to a salting-out effect due to sucrose. The results of this study indicate the importance of the interactions between volatiles and non-volatile compounds as well as of the chemical and physical properties of the aroma compounds in affecting the composition of the gas phase at equilibrium with a complex matrix. More than that, they provided a basis for the studies developed within COST action 921 with the flavoured custard model system.

Determination of partition coefficient of migrants in food simulants by the PRV method

Partition coefficients of six migrants between four simulated foods (water, 10% ethanol, 3% acetic acid, 95% ethanol) and air were determined by the phase ratio variation (PRV) method using headspace analyses by gas chromatography. The migrants were ethyl acetate, methyl ethyl ketone, propyl alcohol, butyraldehyde, acetaldehyde and acetonitrile. The results showed that migrant absorption by the four food simulants was highly dependent on the physicochemical properties of migrants and foods, such as polarity, solubility and hydrogen-bonding.

Static headspace--gas chromatography: theory and practice

Static headspace--gas chromatography: theory and practice

Processing gas chromatographic data and confidence interval calculation for partition coefficients determined by the phase ratio variation method

The phase ratio variation (PRV) method is widely used for the determination of partition coefficient values (dimensionless Henry's law constants) by headspace gas chromatography. Traditional data processing by linear regression has several drawbacks: potential bias introduced by linearization, absence of quality indicator of the resulting value and, in case of replicate determinations, poor utilisation of the existing measurements leading to unnecessarily large confidence intervals. The paper compares existing PRV data processing methods (linear and nonlinear regression, parametric) and derives confidence intervals for the resulting partition coefficient values. The possibility of using several series of measurements to derive a single partition coefficient value with tighter and more reliable confidence intervals is presented for all three processing methods. The methods are tested on published literature data and new experimental data for 12 volatile organic compounds in water at 25 °C. The nonlinear regression based on several series of measurements appears to be the method of choice.