- Email: [email protected]
Volatile delivery under dynamic gas flow conditions
W.L.P. Bredie and M.A. Petersen (Editors) Flavour Science: Recent Advances and Trends 9 2006 Elsevier B.V. All rights reserved.
417
Volatile delivery under dynamic gas flow conditions Robert S.T. Linforth and Andrew J. Taylor
Division of Food Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leics. LE12 5RD, UK
ABSTRACT Under dynamic gas flow conditions (dynamic headspace or in vivo), the measured volatile concentrations in the headspace are very different to those expected from airwater partition data. This is due to the limited amount of the sample (effectively the immediate surface) which is involved in volatile partitioning, and restricted delivery of molecules from the bulk interior over time. Compounds with high (10 -2) a i r / w a t e r partition coefficients deliver flavour inefficiently compared those with lower ones (10-4), primarily because of the proportion of molecules that have to be transferred from the solution to the gas phase as they try to reach equilibrium. 1. INTRODUCTION The tidal volumes of air passing through the upper airway are large compared to its volume [ 1]. The tidal flow of breath transfers approximately 100 ml of air between the lungs and the atmosphere at velocities in the region of 150 ml/s [2]. Equally, the mouth (often thought of as a separate side chamber of the upper airway) can have over a litre of air pumped through it due to the frequent (100 chews/min) bellows-like action of chewing [2]. Consequently, the environment in which flavour is delivered during food consumption is characterised by high gas flows and is very dynamic. Studies modelling flavour delivery in vitro under dynamic gas flow conditions showed that there were differences between compounds related to their air/water partition coefficients (Kaw) [3]. In addition, food components (e.g. emulsions) that changed the partition coefficient (air/product Kap) of a compound could also influence behaviour under dynamic gas flow conditions [4]. The headspace concentration of compounds with Kap values of | 0 -2 decreased rapidly and substantially during headspace dilution compared to compounds with lower Kap values (10-4). This is because molecules were leaving the surface faster than they could be replaced by diffusion and convection from
418 the bulk phase. The differences between compounds arose because, when the Kap was low, only 0.01% of the molecules had to cross the interface into the gas phase to maintain equilibrium (assuming that the volume of solution actively involved in partitioning at the interface, was equal to the volume of diluting gas), whereas when the Kap w a s high this would require 1% of the molecules present to be transported. These experimental in vitro flavour delivery studies were focused on an unstirred aqueous phase with headspace dilution taking place over several minutes. The purpose of this paper was to determine whether the same differences in flavour delivery are seen in vivo, where time scales are much shorter, gas flows much greater and the liquid phase may undergo some stirring. 2. M A T E R I A L S AND M E T H O D S
Atmospheric Pressure Chemical Ionisation - Mass Spectrometry (APCI-MS) was used for both dynamic breath analyses and headspace measurements. For headspace, 500 ml aliquots of solutions of flavour compounds were prepared, placed in 1 litre flasks and allowed to reach equilibrium. The gas phase was then sampled directly into the APCIMS at 30 ml/min and the signal intensity noted (proportional to the gas phase volatile concentration). Breath flavour measurements involved two panellists consuming 15 ml aliquots of the solutions previously used for headspace analyses. The panellists were instructed to place each solution into their mouths, swallow it, and then exhale via the mouth through a tube connected to the end of the APCI-MS sampling tube (also sampling air at 30 ml/min). Panellists consumed two replicates of each sample. The average signal intensity when breath was sampled into the APCI-MS was divided by that for the headspace samples to show the relative concentrations of the two systems. By using the breath/headspace ratio the values obtained are indicative of the relative differences in the two gas phase concentrations and hence effectively independent of both the solution concentration and Kaw. 3. RESULTS AND DISCUSSION The ratio of volatiles in the breath to the headspace concentration was compound dependent (Figure 1, Table 1). For compounds with low values of Kaw (10 -4) the concentration of the volatiles in the breath was close to that observed in the headspace, around 60%. However, the compounds with high values of K~,w (10 -2) were present at very low concentrations in the breath compared to headspace (about 10%). Overall there was a good linear correlation (R 2 = 0.72) between a compounds behaviour in vivo and its K~,w. These results are consistent with studies of flavour release from emulsions, where the large differences observed between compounds dissolved in emulsions and water during headspace analysis are substantially reduced in vivo [4,5]. A lipophilic compound in water (Kaw - 10-2) will deliver flavour inefficiently in vivo and may only achieve 1% of its headspace concentration. The same compound dissolved in an emulsion will have a far lower headspace concentration (Kaw = 10-4), but, in vivo will deliver flavour more
419 efficiently reaching 50% of its headspace concentration. Hence, two systems that differ by a factor of 100 in their headspace concentration will only differ by a factor of 2 in vivo.
It is unlikely that these differences in flavour release are due to dynamic gas flow during swallowing. Air does not pass between the lungs and the atmosphere during swallowing and is effectively static [2], molecules pass from the solution into the gas phase as it is swallowed and are exhaled in a 'plug' that is representative of the extent of equilibration.
75 o
~
50
9
9
25
0 1' -.0
!
-2.0
-3.0
-4.0
-5.0
Log Kaw Figure 1. Relationship between Log Kaw and the breath/equilibrium headspace ratio. Reprinted with permission from Linforth et al., J. Agric. Food Chem., 50, 1111-1117. Copyright 2002 American Chemical Society. The fraction of the solution (and hence active volume) participating in volatile delivery could be the limiting factor. When small volumes of solutions of low Kaw compounds are allowed to reach equilibrium with large volumes of air (e.g. 1:100), the final concentration of the gas phase will be reasonably close to that achieved with equal volumes of the two phases. This is because the proportion of molecules required per volume of gas is low. In contrast, high Kaw compounds have to transfer a larger proportion of the molecules present (per volume of air) to reach equilibrium. This can deplete the amount of these compounds in the solution, resulting in a low final concentration in both phases. The fraction of the solution delivering volatile compounds to the air may be limiting if there is limited movement of molecules from the bulk of the sample to the surface. The breath volatile concentration when consuming volatile compounds dissolved in water, or water thickened with hydrocolloids was found to be identical [6]. This would occur if the hydrocolloid did not significantly affect the transfer of bulk elements of the sample to the interface, implying limited movement for water itself.
420 4. C O N C L U S I O N S It would appear that the restricted movement of volatile compounds from the bulk phase to the interface can be limiting for in-vivo flavour release (compared with static headspace analysis). This is particularly the case when achieving equilibrium requires the movement of a large proportion of the molecules from the liquid to the gas phase. Table 1. Ratio (percent) between exhaled breath and headspace (HS) volatile concentration for a range of compounds. Each value is the mean of 4 replicates. Compound 1-Butanol Ethanol Propan-2-ol Pyrazine Dimethyl pyrazine 3-Hexenol Furfuryl acetate Diethyl methylpyrazine 2-Methylbutanol Butanone Carvone Hexanol Diacetyl Methyl acetate Benzaldehyde 2-Hexenal Guaiacol Carvone L inalool Terpineol 2-Pentanone 1,8-Cineole Menthol Isobutyl methoxypyrazine Meth~,l salic~late
Breath/HS (%) 70 66 65 60 51 49 48 47 45 39 39 38 37 33 30 29 29 28 28 28 28 26 24 23 23
Compound Ethyl acetate Octanol Octanone Acetaldehyde Menthone 2-Pentanone Isoamyl acetate 2-Methylbutanal Hexanal Ethyl butyrate Hexanal Butanal 2-Methylbutanal Ethyl hexanoate Methyl propanal Citronellal Decanone Octanal Decano I Ethyl octanoate Decanal Methyl furan Limonene Cymene Pinene
Breath/HS (%) 22 22 20 19 19 15 15 15 14 13 13 13 9.9 8.6 7.8 7.6 6.3 5.4 5.2 2.6 2.1 1.1 0.68 0.52 0.17
References
1. M. Datum, J. Vent, M. Schmidt, P. Theissen, H.E. Eckel, J. Lotsch and T. Hummel, Chem. Senses, 27 (2002) 831. 2. M. Hodgson, R.S.T. Linforth and A.J. Taylor, J. Agric. Food Chem., 51 (2003) 5052. 3. M. Marin, I. Baek and A.J.Taylor, J. Agric. Food Chem., 47 (1999) 4750. 4. K. Doyen, M. Carey, R.S.T. Linforth, M. Marin and A.J. Taylor, J. Agric. Food Chem., 49 (2001) 804. 5. D.D. Roberts, P. Pollien, N. Antille, C. Lindinger and C. Yeretzian, J. Agric. Food Chem., 51 (2003) 3636. 6. D.J. Cook, T.A. Hollowood, R.S.T. Linforth and A.J. Taylor, Chem. Senses, 28 (2003) 11.
417
Volatile delivery under dynamic gas flow conditions Robert S.T. Linforth and Andrew J. Taylor
Division of Food Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leics. LE12 5RD, UK
ABSTRACT Under dynamic gas flow conditions (dynamic headspace or in vivo), the measured volatile concentrations in the headspace are very different to those expected from airwater partition data. This is due to the limited amount of the sample (effectively the immediate surface) which is involved in volatile partitioning, and restricted delivery of molecules from the bulk interior over time. Compounds with high (10 -2) a i r / w a t e r partition coefficients deliver flavour inefficiently compared those with lower ones (10-4), primarily because of the proportion of molecules that have to be transferred from the solution to the gas phase as they try to reach equilibrium. 1. INTRODUCTION The tidal volumes of air passing through the upper airway are large compared to its volume [ 1]. The tidal flow of breath transfers approximately 100 ml of air between the lungs and the atmosphere at velocities in the region of 150 ml/s [2]. Equally, the mouth (often thought of as a separate side chamber of the upper airway) can have over a litre of air pumped through it due to the frequent (100 chews/min) bellows-like action of chewing [2]. Consequently, the environment in which flavour is delivered during food consumption is characterised by high gas flows and is very dynamic. Studies modelling flavour delivery in vitro under dynamic gas flow conditions showed that there were differences between compounds related to their air/water partition coefficients (Kaw) [3]. In addition, food components (e.g. emulsions) that changed the partition coefficient (air/product Kap) of a compound could also influence behaviour under dynamic gas flow conditions [4]. The headspace concentration of compounds with Kap values of | 0 -2 decreased rapidly and substantially during headspace dilution compared to compounds with lower Kap values (10-4). This is because molecules were leaving the surface faster than they could be replaced by diffusion and convection from
418 the bulk phase. The differences between compounds arose because, when the Kap was low, only 0.01% of the molecules had to cross the interface into the gas phase to maintain equilibrium (assuming that the volume of solution actively involved in partitioning at the interface, was equal to the volume of diluting gas), whereas when the Kap w a s high this would require 1% of the molecules present to be transported. These experimental in vitro flavour delivery studies were focused on an unstirred aqueous phase with headspace dilution taking place over several minutes. The purpose of this paper was to determine whether the same differences in flavour delivery are seen in vivo, where time scales are much shorter, gas flows much greater and the liquid phase may undergo some stirring. 2. M A T E R I A L S AND M E T H O D S
Atmospheric Pressure Chemical Ionisation - Mass Spectrometry (APCI-MS) was used for both dynamic breath analyses and headspace measurements. For headspace, 500 ml aliquots of solutions of flavour compounds were prepared, placed in 1 litre flasks and allowed to reach equilibrium. The gas phase was then sampled directly into the APCIMS at 30 ml/min and the signal intensity noted (proportional to the gas phase volatile concentration). Breath flavour measurements involved two panellists consuming 15 ml aliquots of the solutions previously used for headspace analyses. The panellists were instructed to place each solution into their mouths, swallow it, and then exhale via the mouth through a tube connected to the end of the APCI-MS sampling tube (also sampling air at 30 ml/min). Panellists consumed two replicates of each sample. The average signal intensity when breath was sampled into the APCI-MS was divided by that for the headspace samples to show the relative concentrations of the two systems. By using the breath/headspace ratio the values obtained are indicative of the relative differences in the two gas phase concentrations and hence effectively independent of both the solution concentration and Kaw. 3. RESULTS AND DISCUSSION The ratio of volatiles in the breath to the headspace concentration was compound dependent (Figure 1, Table 1). For compounds with low values of Kaw (10 -4) the concentration of the volatiles in the breath was close to that observed in the headspace, around 60%. However, the compounds with high values of K~,w (10 -2) were present at very low concentrations in the breath compared to headspace (about 10%). Overall there was a good linear correlation (R 2 = 0.72) between a compounds behaviour in vivo and its K~,w. These results are consistent with studies of flavour release from emulsions, where the large differences observed between compounds dissolved in emulsions and water during headspace analysis are substantially reduced in vivo [4,5]. A lipophilic compound in water (Kaw - 10-2) will deliver flavour inefficiently in vivo and may only achieve 1% of its headspace concentration. The same compound dissolved in an emulsion will have a far lower headspace concentration (Kaw = 10-4), but, in vivo will deliver flavour more
419 efficiently reaching 50% of its headspace concentration. Hence, two systems that differ by a factor of 100 in their headspace concentration will only differ by a factor of 2 in vivo.
It is unlikely that these differences in flavour release are due to dynamic gas flow during swallowing. Air does not pass between the lungs and the atmosphere during swallowing and is effectively static [2], molecules pass from the solution into the gas phase as it is swallowed and are exhaled in a 'plug' that is representative of the extent of equilibration.
75 o
~
50
9
9
25
0 1' -.0
!
-2.0
-3.0
-4.0
-5.0
Log Kaw Figure 1. Relationship between Log Kaw and the breath/equilibrium headspace ratio. Reprinted with permission from Linforth et al., J. Agric. Food Chem., 50, 1111-1117. Copyright 2002 American Chemical Society. The fraction of the solution (and hence active volume) participating in volatile delivery could be the limiting factor. When small volumes of solutions of low Kaw compounds are allowed to reach equilibrium with large volumes of air (e.g. 1:100), the final concentration of the gas phase will be reasonably close to that achieved with equal volumes of the two phases. This is because the proportion of molecules required per volume of gas is low. In contrast, high Kaw compounds have to transfer a larger proportion of the molecules present (per volume of air) to reach equilibrium. This can deplete the amount of these compounds in the solution, resulting in a low final concentration in both phases. The fraction of the solution delivering volatile compounds to the air may be limiting if there is limited movement of molecules from the bulk of the sample to the surface. The breath volatile concentration when consuming volatile compounds dissolved in water, or water thickened with hydrocolloids was found to be identical [6]. This would occur if the hydrocolloid did not significantly affect the transfer of bulk elements of the sample to the interface, implying limited movement for water itself.
420 4. C O N C L U S I O N S It would appear that the restricted movement of volatile compounds from the bulk phase to the interface can be limiting for in-vivo flavour release (compared with static headspace analysis). This is particularly the case when achieving equilibrium requires the movement of a large proportion of the molecules from the liquid to the gas phase. Table 1. Ratio (percent) between exhaled breath and headspace (HS) volatile concentration for a range of compounds. Each value is the mean of 4 replicates. Compound 1-Butanol Ethanol Propan-2-ol Pyrazine Dimethyl pyrazine 3-Hexenol Furfuryl acetate Diethyl methylpyrazine 2-Methylbutanol Butanone Carvone Hexanol Diacetyl Methyl acetate Benzaldehyde 2-Hexenal Guaiacol Carvone L inalool Terpineol 2-Pentanone 1,8-Cineole Menthol Isobutyl methoxypyrazine Meth~,l salic~late
Breath/HS (%) 70 66 65 60 51 49 48 47 45 39 39 38 37 33 30 29 29 28 28 28 28 26 24 23 23
Compound Ethyl acetate Octanol Octanone Acetaldehyde Menthone 2-Pentanone Isoamyl acetate 2-Methylbutanal Hexanal Ethyl butyrate Hexanal Butanal 2-Methylbutanal Ethyl hexanoate Methyl propanal Citronellal Decanone Octanal Decano I Ethyl octanoate Decanal Methyl furan Limonene Cymene Pinene
Breath/HS (%) 22 22 20 19 19 15 15 15 14 13 13 13 9.9 8.6 7.8 7.6 6.3 5.4 5.2 2.6 2.1 1.1 0.68 0.52 0.17
References
1. M. Datum, J. Vent, M. Schmidt, P. Theissen, H.E. Eckel, J. Lotsch and T. Hummel, Chem. Senses, 27 (2002) 831. 2. M. Hodgson, R.S.T. Linforth and A.J. Taylor, J. Agric. Food Chem., 51 (2003) 5052. 3. M. Marin, I. Baek and A.J.Taylor, J. Agric. Food Chem., 47 (1999) 4750. 4. K. Doyen, M. Carey, R.S.T. Linforth, M. Marin and A.J. Taylor, J. Agric. Food Chem., 49 (2001) 804. 5. D.D. Roberts, P. Pollien, N. Antille, C. Lindinger and C. Yeretzian, J. Agric. Food Chem., 51 (2003) 3636. 6. D.J. Cook, T.A. Hollowood, R.S.T. Linforth and A.J. Taylor, Chem. Senses, 28 (2003) 11.