Thermodynamic and kinetic aspects of the transport of small molecules in dispersed systems

Colloids and Surfaces B: Biointerfaces 12 (1998) 57 – 65

Thermodynamic and kinetic aspects of the transport of small molecules in dispersed systems P. Landy a, S. Rogacheva a,c, D. Lorient b, A. Voilley a,* a

Laboratoire de Ge´nie des Proce´de´s Alimentaires et Biotechnologiques, ENSBANA, Uni6ersite´ de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France b Laboratoire de Biochimie, Physico-Chimie et Proprie´te´s Sensorielles des Aliments, ENSBANA, Uni6ersite´ de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France c Higher Institute of Food and Fla6our Industries, Department of Organic Chemistry, 26 Maritza Bl6d., 4000 Plo6di6, Bulgaria Accepted 16 September 1998

Abstract The knowledge of the behaviour of flavour compounds in complex multiphase systems with regard to their structure is of great importance in flavour perception of foods. The thermodynamic and kinetic behaviour of three selected flavour compounds belonging to a homologous series of esters, e.g. ethyl acetate, ethyl butanoate and ethyl hexanoate, were studied in simple and multiphase systems. The liquid system was composed of water (with or without sodium caseinate) and/or a lipid, Miglyol. First, the properties of the solutes were determined by means of their liquid–liquid partition at equilibrium and their diffusion in aqueous or lipid phases. This first step allowed to reveal the impact of sodium caseinate and that of the lipid on their behaviour in liquid phases. The second step consisted in the investigation of the transfer of the flavour compounds through the oil phase with a rotating diffusion cell (RDC) and the knowledge of the physico-chemical characteristics of the solutes. The data obtained with the RDC enabled the calculations of the resistances to the transfer through the aqueous phase (Raq), through the interface (RI) and through the oil (Roil). The transfer of ethyl acetate through the interface was the rate-limiting step, while the transfer of ethyl butanoate and ethyl hexanoate through the oil was limited by the diffusion in the aqueous phase. The effect of sodium caseinate is different for the less hydrophobic compound (ethyl acetate) and the more hydrophobic compounds (ethyl butanoate and ethyl hexanoate); in the presence of sodium caseinate, the Raq value increases for ethyl acetate, while the RI value increases for ethyl butanoate and ethyl hexanoate. Those results show the impact of the nature of the solutes on their transfer through the aqueous layer and the oil – water interface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Emulsions; Flavour; Structure; Transport; Interface

1. Introduction

* Corresponding author. Tel./Fax:+ 33-3-8039-6659; e-mail: [email protected].

The physico-chemical behaviour of small molecules such as flavour compounds in food matrices is one of the most important parameters involved in their activity and sensory perception;

0927-7765/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 6 5 ( 9 8 ) 0 0 0 5 7 - 5

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Table 1 Physico-chemical characteristics of the flavour compounds Flavour compound

Formula

Hydrophobicity log P a

Molar volumeb (cm3 Molar mass (g mol−1) mol−1)

Density at 20°C

Solubility (g l−1)

Ethyl acetate Ethyl butanoate Ethyl hexanoate

C4H8O2 C6H12O2 C8H16O2

0.6 1.7 2.8

108.6 153.0 197.4

0.90 0.88 0.87

86 at 20°C 5.6 at 25°C 0.5 at 25°C

a b

88.1 116.2 144.2

Hydrophobicity expressed by the calculated logarithm of the n-octanol/water partition coefficient [23]. Calculated by Le Bas model [24].

hence, this behaviour can have pronounced effects on the flavour quality of foods which are mostly emulsions, i.e. dispersed systems of oil and aqueous phases. Both thermodynamic and kinetic mechanisms influence the flavor release within emulsions and into the gas phase at each step of food product preparation and consumption. One mechanism is the partitioning of the flavour components between the oil and aqueous phase; this process depends on the affinity of the components with each of the two phases. Because of their emulsifying and stabilizing role in lipid-dispersed systems, proteins are important ingredients and many studies have been carried out on the thermodynamic aspects of the binding between volatiles and proteins in simple aqueous [1–4] and oil–water systems [5 – 7]. Another mechanism is the diffusion of the solutes into the oil or the aqueous phase and the kinetic mass transfer between the different phases through the interface. Recently, some techniques have been developed to allow the measurement of the release rate of small molecules in foods [8,9] and from foods [10,11]. Druaux and Voilley [12] discussed in a review such thermodynamic and kinetic methods and point out the importance of the food composition and structure on the flavour release together with the impact of the interfacial mass transfer. The RDC was developed a few years ago in the field of flavour science to investigate the rate of transfer of solutes from an aqueous phase to another phase through a lipid layer, as a model of double emulsions, along with the resistances to the mass transfer within the aqueous boundary layers, across the oil – water interface and within the oil layer [5]. It is difficult to control the release of solutes unless such techniques allow to investi-

gate the effect of the solute nature and that of the interface on the flavour release. As hydrophobicity has a determinant role in the flavour behaviour in foods, our aim was to study the transport of a homologous series of esters in a dispersed system model and to relate this phenomenon to their physico-chemical properties.

2. Experimental

2.1. Materials The flavour compounds, ethyl acetate, ethyl butanoate and ethyl hexanoate, and the oil, Miglyol, were purchased from International Fragrances and Flavors (IFF, Longvic-les-Dijon, France). Miglyol is a triglyceride of caprylic (60%) and capric acids (40%). The characteristics of the flavour compounds are presented in Table 1. Sodium caseinate was obtained from Unilait (Paris, France). Membrane filters (Sartorius, Palaiseau, France) were made of polytetrafluoroethylene (thickness 30 mm, pore size 0.2 mm, volume porosity of 80%).

2.2. Rotating diffusion cell (RDC) The rotating diffusion cell is designed hydrodynamically in such a way that stationary diffusion layers of known thickness are created on each side of the oil layer. A schematic diagram of the cell is shown in Fig. 1; its interior is made of glass and teflon to avoid the retention of flavour compounds. The oil layer is supported on a porous membrane filter which divides the cell into the inner and outer compartments. The central assem-

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Fig. 1. Schematic diagram of rotating diffusion cell showing (A) membrane filter saturated with oil, (B) rotating assembly linked to a motor, and (C) slotted baffle (teflon hollow cylinder).

bly, i.e. the inner compartment, is rotated by a motor at constant known speed up to 5 s − 1. The stationary baffle, positioned in the central assembly and at :2 mm above the filter, ensures stable diffusion layers. The teflon membrane filter is fixed by a push-fit teflon ring leaving an exposed circular area of 1.5 cm in diameter. A teflon lid is screwed to prevent flavour compound loss by volatilization. Care was taken to ensure that no air bubble remains beneath the filter. Miglyol was pre-saturated with water to ensure conditions similar to those of real food emulsions. The filter was filled with the oil phase under vacuum for saturation of the membrane filter pores. The initial flavour compound concentration contained in the inner compartment was 2.4× 10 − 3 mol l − 1. In experiments with sodium caseinate, both compartments were filled with the aqueous protein solution at 5%; the cell

was left turning for 1 h at 4 s − 1. The flux of flavour compound across the oil layer was measured by periodically sampling with a microsyringe from the solution in the outer compartment through a small hole in the lid fitted with a threaded teflon cap; the analysis was performed by gas chromatography. The gas chromatograph was equipped with a flame ionization detector (Chrompack, CP 9000; Chrompack, Middelburg, The Netherlands) and with a 3 m stainless steel column (inner diameter 2.2 mm) packed with Chromosorb W-AW 100– 200 mesh Carbowax 20 M-10%. The operating parameters of the chromatograph were as follows: injector temperature, 190°C; detector temperature, 200°C; column temperature, 60°C for ethyl acetate, 80°C for ethyl butanoate, 130°C for ethyl hexanoate, N2 flow rate, 1.6 ×10 − 5 m3 min − 1; H2 flow rate, 2.5 × 10 − 5 m3 min − 1; air flow rate, 25×10 − 5 m3 min − 1.

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Table 2 Liquid–liquid partition coefficient of the flavour compounds between Miglyol and an aqueous phase, and their diffusion coefficient in oil or in an aqueous phase at 25°C Flavour compound

Ethyl acetate Ethyl butanoate Ethyl hexanoate

Liquid–liquid partition coefficient (Miglyol/aqueous phase)

Diffusion coefficient (×1010 m2 s−1)

Water

5% Sodium caseinate

Water

5% Sodium caseinate

Miglyol

4.3 (5)* 45.3 (2) 738.5 (10)

4.8 (15) 46.4 (16) 697.5 (3)

11.6a 9.4a 7.9a

3.3b 1.6c 1.4c

1.14d 0.93d 0.80d

a

Reference [25]. Reference [22]. c Calculated from Perkins and Geankoplis model [24]. d Calculated from Wilke and Chang model [24]. * Numbers in parentheses represent variation coefficient (%). b

2.3. Liquid– liquid partition

J=kADc

The liquid – liquid partition of the flavour compounds was studied with an aqueous phase with and without sodium caseinate at 5% (pH 7.0). The liquid–liquid partition coefficient P is, respectively the ratio of the concentration (v/v) of the solute in the liquid and aqueous phase; it was determined at 25°C by measuring at equilibrium the concentration of the solute in the organic and aqueous phases. Each measurement was repeated in triplicate.

where k is the overall permeability coefficient (m s − 1), A is the area of the filter (m2) and Dc is the difference of concentration of the solute between the inner and the outer compartment (mol m − 3). The overall resistance to the mass transfer, 1/k, is defined as:

3. Theoretical background

3.1. Mass transfer in rotating diffusion cell The hydrodynamic properties of a rotating disc are created along the cell surface [13,14]. The thickness of the diffusion layer, Z, on each side of the membrane filter is given by the Levich equation: − 1/2 Z=0.643h 1/6D 1/3 aq v

(1)

where h is the kinematic viscosity of the aqueous phase (m2 s − 1), Daq is the diffusion coefficient of the solute in the aqueous phase (m2 s − 1), v is the rotation speed of the filter (s − 1). The rate of transfer of the flavour compound from the inner to the outer compartment is given by the flux J (mol s − 1).

1 2Z 2 l + + = k Daq aki aDoilP

(2)

(3)

where a is the porosity of the membrane, ki is the rate constant of the solute through the oil–water interface (m s − 1), l is the thickness of the membrane (m) and Doil is the diffusion coefficient of the solute in the oil phase (m2 s − 1). The significance of the three terms of Eq. (3) is as follows: 2Z/Daq (denoted Raq) describes the resistance to diffusion through the two stagnant aqueous diffusion layers of thickness Z which are established at each side of the filter, 2/aki (denoted RI) relates to the resistance due to the solute transfer across the two aqueous phase/oil interfaces, 1/aDoilP (denoted Roil) is the contribution of the diffusion through the lipid in the filter. Eq. (3) is then simplified: 1 =Raq + Roil + RI k

(4)

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Fig. 2. Levich plots: transfer of ethyl acetate (), ethyl butanoate (2), ethyl hexanoate ( ) in water through a layer of Miglyol at 25°C. Each Levich plot is a different experiment. The flavour compound was at a molar concentration of 2.4 ×10 − 3 mol l − 1.

Fig. 3. Levich plots: transfer of ethyl acetate in water (), and in 5% sodium caseinate solution () through a layer of Miglyol at 25°C.

Substitution of Eq. (1) into Eq. (3) allows the Levich plot, i.e. 1/k as a function of v − 1/2, to be made. By extrapolation to infinite speed v, the thickness of the stagnant layers Z tends to zero and the intercept is the sum of the second and third terms, RI + Roil, of Eq. (4). The intercepts were obtained by fitting a line from the experimental points. The independent assessment of Roil allows the interfacial resistance RI to be determined.

3.2. Estimation method for diffusion coefficients of liquids The diffusion coefficients of the flavour compounds in the oil phase, Doil, were estimated by using the Wilke–Chang equation [15], using the molar volume increments as proposed by Le Bas [16] and an association parameter f of 1.0 for lipid solvents (this parameter is equal to 2.6 for water). The equation estimates the diffusion coefficient Di of a solute i in a solvent s.

62

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Fig. 4. Levich plots: transfer of ethyl butanoate in water (2), and in 5% sodium caseinate solution (") through a layer of Miglyol at 25°C.

Di =

7.4× 10 − 12(fMs)1/2T hsV 0.6 i

(5)

where Ms is the molar mass of the solvent (g mol − 1), T is the temperature (K), hs is the solvent viscosity (mPa s − 1) and Vi is the molar volume (cm3 mol − 1) of a solute i. The viscosity of Miglyol (hs =26 mPa s − 1) was determined experimentally on a rotational viscometer Rheomat-30 Contraves at 25°C.

4. Results and discussion

4.1. Physico-chemical characteristics of the fla6our compounds at 25 °C The characteristics of the flavour compounds in the aqueous and oil phase were investigated by the determination of the liquid – liquid partition at equilibrium and that of their diffusivity in aqueous phase and oil (Table 2). As the hydrophobicity of the ethyl esters increases, their liquid–liquid partition coefficient between water and Miglyol, i.e. their affinity for Miglyol, increases. In the presence of sodium caseinate, the liquid– liquid partition coefficients of the flavour compounds were not significantly different from that without protein. From measurements of vaporliquid equilibrium, Landy et al. [3] obtained no retention for ethyl acetate by sodium caseinate at

5% in aqueous solutions, and respective retentions of 10 and 61% for ethyl butanoate and ethyl hexanoate in comparison to water. Hence, for ethyl acetate, no variation of the liquid–liquid partition coefficient could be due to the absence of interactions between sodium caseinate in aqueous solution at 5%; however in the case of ethyl butanoate and ethyl hexanoate, the effect of sodium caseinate was not revealed by liquid–liquid partition because of their hydrophobicity and their strong retention by Miglyol. Buttery et al. [17], Jouenne [18], Espinosa [19] showed that the presence of vegetable oil at a low content in water could significantly affect the vapor-liquid partition coefficients of alcohols, aldehydes, ketones and esters. Hence, the effect of the lipid and protein content can be easily shown from volatility measurements when these non-volatile constituents are studied separately; conversely the influence of proteins on the thermodynamic properties of volatiles can be difficult to determine in a lipid-containing system. At vapor-liquid equilibrium, Landy et al. [7] did not report any significant variation of the volatility of ethyl esters whatever the degree of dispersion of systems containing 20% of oil and an aqueous phase with or without sodium caseinate at 0.5%. For those reasons, the relative effect of each constituent of a food product has to be investigated as long as one of the constituents is not dominating the effect of the other constituents. It is necessary in order to

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Fig. 5. Levich plots: transfer of ethyl hexanoate in water ( ), and in 5% sodium caseinate solution ( ) through a layer of Miglyol at 25°C.

reveal the effect of the protein adsorbed at the water–oil interface, or present in the aqueous phase, in comparison with that of the oil phase [20]. A decrease in the diffusion coefficient of the esters in aqueous sodium caseinate solutions was observed in comparison to water (Table 2). Several studies indicated that the volatile diffusivity decreased with the molar volume of the solute and could be highly affected by the presence of a second solute, e.g. proteins or polysaccharides [21,22]. Both thermodynamic and kinetic properties of small molecules in simple and biphasic systems were then used to investigate the molecular transport through a lipid layer in order to reveal the role of protein in the release rate of solutes and to discuss the contribution of those properties to this transport.

4.2. Mass transfer at the lipid– water interface The Levich plots, 1/k as a function of v − 1/2, for the transfer of the series of ethyl esters in the absence of sodium caseinate are shown in Fig. 2. The individual plots for ethyl acetate, ethyl butanoate and ethyl hexanoate with or without sodium caseinate are, respectively presented in Figs. 3–5. Table 3 gives the values of the intercepts, i.e. the sum of Roil and RI, together with the Roil and RI values. The overall resistance (l/k) was increasing in the following order: ethyl acetate,

ethyl hexanoate and ethyl butanoate, whether or not sodium caseinate is present. This effect is due for ethyl acetate to its polarity, i.e. to its low affinity for the oil, but the obtained order is not that of the increasing hydrophobicity for ethyl butanoate and ethyl hexanoate. The 1/k results cannot be explained by the Roil values which decrease as the solute is more hydrophobic. However, the resistances to the transfer through the liquid interface (RI) controls the transfer of the solutes across the lipid layer at infinite speed, i.e. when the aqueous stagnant layer tends to zero; e.g. for ethyl acetate, ethyl hexanoate and ethyl butanoate in water, the increasing RI values are, respectively 1.021×105, 0.472× 105 and 0.095× 105 m − 1 s. The overall resistance corresponding to ethyl acetate is of the same order with or without sodium caseinate, which means that the protein has no effect on the transfer of this ethyl ester through the interface and the oil layer (Fig. 3; Table 3). In contrast, the resistance to the transfer through the interface increases from 0.095 to 0.286× 105 m − 1 s for ethyl butanoate, and from 0.472 to 1.018×105 m − 1 s for ethyl hexanoate (Figs. 4 and 5; Table 3). Fig. 3 corresponding to the transfer of ethyl acetate shows a difference of slopes of the Levich plots with or without sodium caseinate, whereas the slopes are similar for ethyl butanoate and ethyl hexanoate (Figs. 4 and 5). So for the three studied compounds in the presence

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Table 3 Values of the intercept 1/k together with the resistances to mass transfer through the lipid phase (Roil) and the lipid-aqueous phase interface (RI) for the flavour compounds dissolved in water or a 5% sodium caseinate aqueous solution Solute

Sodium caseinate (% m m−1)

1/k×10−5 (m−1 s)

Roil×10−5 (m−1 s)*

RI×10−5 (m−1 s)

Ethyl acetate

0 5

1.764 1.829

0.743 0.743

1.021 1.086

Ethyl butanoate

0 5

0.185 0.375

0.090 0.090

0.095 0.286

Ethyl hexanoate

0 5

0.478 1.024

0.007 0.007

0.472 1.018

* Roil was calculated from the mean of the liquid–liquid partition coefficients of the solutes in the aqueous phase with and without sodium caseinate (Table 2).

of sodium caseinate, the viscosity of the solution and the diffusivity are the influencing factors for ethyl acetate because of the change in slopes; but for ethyl butanoate and ethyl hexanoate, the transfer through the interface, depending on the rate constant ki, plays an important role. The comparison of the relative percentages of the resistances to the mass transfer calculated at 0.875 s1/2 allows to estimate directly the rate-limiting steps of the solute transfer (Table 4). For ethyl acetate, one of the rate-limiting steps is the transfer through the oil – water interface in water (38%), whereas Raq represents the limiting step in the presence of sodium caseinate (59%); for the other solutes, the rate-limiting step is the transfer through the aqueous stagnant layers, representing more than 50% of the resistances, whether sodium caseinate is added or not. From Table 4, it is observed that the presence of sodium caseinate

induced a decrease in the resistance through the aqueous layer (Raq) for ethyl acetate and an increase in the interfacial resistance (RI) for ethyl butanoate and ethyl hexanoate. Consequently, the rate-limiting step represents the diffusion in the aqueous phase for the three compounds in the presence of sodium caseinate, and for ethyl butanoate and ethyl hexanoate in water, while this step corresponds to the interfacial transfer for ethyl acetate in water. Harvey et al. [5] reported that the transfer of ethyl acetate and ethyl butanoate through a layer of tributyrin was limited by the diffusion through the aqueous layers, while for 2,5-dimethylpyrazine (log P = 0.2) the interfacial transfer was rate-limiting. The different rate-limiting step for the transfer of ethyl acetate through tributyrin or Miglyol is explained by the higher value of its partition coefficient P between tributyrin and water (P= 6) than be-

Table 4 Relative percentages of each of the resistances, Raq, Roil and RI, to the overall mass transfer of the flavour compounds through a Miglyol layera Solute

Sodium caseinate (%, m m−1)

Raq (%)

Roil (%)

RI (%)

Ethyl acetate

0 5

35.0 59.2

27.4 16.6

37.6 24.2

Ethyl butanoate

0 5

86.2 78.0

6.7 5.3

7.1 16.7

Ethyl haxanoate

0 5

74.4 58.8

0.4 0.3

25.2 40.9

a

Those percentages were calculated by considering that at v−1/2 =0.87 s1/2, 1/k corresponded to 100% of the resistances.

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tween Miglyol and water (P = 4). This partitioning process also confirms that the rate-limiting step for 2,5-dimethylpyrazine through tributyrin is the interfacial transfer (P =2). Hence, the more the compound has an affinity for the lipid phase, the more the transfer of the compound is limited by the transfer through the stagnant aqueous layers. An apparent effect of the interfacial resistance due to an adsorbed layer of sodium caseinate was also observed for the esters (very low for ethyl acetate) and the pyrazine. This study allows us to establish which step of the solute transfer contributes to its limitation and to reveal that the presence of proteins can affect differently the resistances as a function of the solute nature.

5. Conclusion The rotating diffusion cell method enabled to carry out a fundamental study on the transfer of solutes from aqueous phases to oil, and from oil to aqueous phases by the determination of resistances to the solute mass transfer and the estimation of the rate limiting step of the transfer. As such, this study represents a contribution to a better understanding of the flavour release in dispersed systems, it should be completed however by measurements of vapor-emulsion transfer in order to get an overall view of the kinetic distribution during eating as a function of the structure of the interface. The data obtained such as the release rates and the resistances to the mass transfer can be ultimately included into a mathematical model describing the interphase transport of solutes from an oil droplet to an aqueous phase, and from the latter to the gas phase.

Acknowledgements S. Rogacheva wishes to thank the French Ministe`re des Affaires Etrange`res for financial support.

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