Chapter 11
Physiochemical Models of Flavor Release from Foods Flavor Release Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/26/15. For personal use only.
Kris B. de Roos Givaudan Roure Flavors, Ltd., CH-8600 Dübendorf, Switzerland
A n overview is given of the physico-chemical models that have been developed to describe flavor release from foods during perception by sniff and by mouth. Three different basic models of flavor release can be distinguished which differ mainly in the way diffusion takes place. Comparison of theoretical predictions with experimental data shows that molecular diffusion is the major transport mechanism during flavor perception by sniff (release from static product phase). On the other hand, flavor release in the mouth (release from dynamic product phase) is primarily controlled by the rate of surface renewal as affected by eddy diffusion. During the last few years good progress has been made in the development of mathematical models that address specific problems that are associated with flavor release in the mouth.
A flavoring applied to different products generates often totally different odor and taste sensations. Due to the differences in sensory response to the same flavoring in different media, flavorists have to spend much time on modifying and optimizing flavorings for new target applications. In view of the large number of flavor ingredient and product types, this is a tedious and time-consuming job. Therefore, a better understanding and predictability of the changes in sensory response that result from differences in flavor release is of high practical significance. For that reason much work has been done during the last years to relate flavor compound concentrations in a product to perceived strength and character. An overview will be given of the physico-chemical models that have been developed for predicting flavor release from foods. Special attention will be paid to the proportions released at a given time because they determine the character of the perceived flavor. To duplicate the flavor of a desirable product in another product, it is not only important to achieve the same flavor impact from the individual flavor constituents but also to achieve their impact at the right time. This means that the physico-chemical models of flavor release must be able to predict maximum flavor intensity (I ), the time to reach maximum flavor intensity (t ) and the duration of the flavor release. max
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max
© 2000 American Chemical Society
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Principal Symbols 2
A Surface area of the interface (m ) C Concentration (g/L) D Diffusion coefficient (m /s) / Volume fraction (L/L) J Mass flux, g/cm s k Mass transfer coefficient (m/s) k„ Overall mass transfer coefficient (m/s) M Mass (g) η Number of extraction steps in non-equilibrium model of flavor release Ρ Partition coefficient t Time (s) ν Air flow rate (L/s) V Volume (L) V* Volume of one phase that is in equilibrium with volume of other phase (L) δ Effective thickness of the stagnant layer (m) Subscripts a, L, ρ, o and prefer to gas, liquid product, oil and water phase, respectively Superscripts / refers to interface
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2
2
Factors Affecting Flavor Release and Perception To be perceptible, aroma compounds must be released into the headspace in the mouth and transferred to the nose, where aroma perception takes place. In contrast with aroma compounds, taste compounds must be present in the saliva because taste compounds, such as salt and sugar, are perceived in the mouth. During consumption of solid foods, the volatile flavor compounds are generally first released into saliva before they are released into headspace and transferred to the nose. This means that interfacial mass transfer from both solid to liquid and liquid to air phase determines the flavor release from solid products. This review will focus on aroma, in particular, on aroma release during perception by sniff (smell) and by mouth ("taste"), because the release under these conditions is of highest practical significance. The factors that affect the flavor release from foods are phase partitioning and mass transport. Flavor release from product to vapor phase will only take place if the phase equilibria are disturbed. So, non-equilibrium is the driving force for mass transport. The vapor phase is in equilibrium with the product phase if there is no effective mass transport at the product-air interface. Under these conditions, the concentrations in these phases obey the following relation: P =C /C œ
G
(eq 1)
P
where P is the gas-product partition coefficient and C and C are the concentrations of the flavor compound in air and product, respectively. GP
G
P
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The rate at which equilibrium can be achieved is determined by the mass transfer coefficient. The mass transfer coefficient k is a measure for the velocity at which the solute diffuses through the phase. Consequently, 1/k stands for the resistance to mass transfer.
Phase Partitioning V a p o r pressures in the product medium can be influenced by many factors, such as: a.
Temperature Temperature has a strong effect on the gas-product partition coefficient. Correction for the effect of temperature on phase partitioning and flavor release has not received much attention, so far. The reason could be that the effect of temperature on phase partitioning is complex (7, 2).
b.
Composition of the Aqueous Phase Certain molecules, such as alcohol (3, 4), salt (2) and sugars (5, 6) influence vapor pressures of aroma compounds through their effect on the solvent properties of the aqueous phase. There is no special binding between the flavor molecules and the dissolved compounds. Depending on the nature of the change in solvent properties all flavor compounds are affected to a lower or higher extent. In practice, this means that in the physico-chemical models of flavor release, the gas-water partition coefficient P w to be replaced by the gasliquid partition coefficient P . n
a
s
G
GL
a
Flavor Binding / Complex Formation In this case there is a specific interaction between dissolved molecules and the aroma compounds. It is important to discriminate here between dissolved, bound and total flavor concentration. Only the free dissolved molecules exert a vapor pressure. In liquid foods, the exchange of flavor molecules between the bound and unbound state is often very fast compared to the transport o f flavor across the water-gas interface (7). This means that the transport across the water-gas interface is the rate-determining step and that the gas-water partition coefficient in the release equations can simply be substituted by the gas-liquid partition coefficient
d.
Acid-Base Equilibria This is a special case of flavor binding. The ionic form of the flavor compound is here the bound form.
e.
Phase Partitioning between Aqueous and Lipid Phase The air-to-product partition coefficient of an emulsion can be easily calculated from the volume fractions f, and/,, o f oil and water and the oil-gas and water-gas partition coefficients P and P (8-JO): 0G
WG
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Ρ
-c
1
IC =
(eq 2)
If the flavor compounds i n the aqueous phase are also participating in the equilibria c and d mentioned above, the calculation of gas-product partition coefficients becomes more complex (77). In practice, this means that there is no longer a linear increase or decrease of the vapor pressure with carbon chain length as in water or oil (Figure 1). 12 10 ο ο ο
