Mathematical Models of Release and Transport of Flavors from Foods in the Mouth to the Olfactory Epithelium Marcus Harrison Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, United Kingdom
A computer simulation describing flavor release from chewing gum in the mouth is presented. The rate limiting step for flavor release is assumed to be the transport of flavor volatiles across the gum-saliva interface, which can be described by the stagnant-layer theory of mass transfer. Saliva-flow, mastication, swallowing and transport of the volatiles from the oral cavity through the airways to the olfactory epithelium have been incorporated into the model. In general, the results show that the rates of flavor release primarily depend on the saliva-gum partition and mass transfer coefficients. Surprisingly, release rates from the gum were found to be independent of the chewing frequency of a particular individual at all times. The simulation also predicts that the presence of mucus lining the airways further influences the time-dependent flavor concentrations arriving at the olfactory epithelium. Flavor is one of the primary factors determining the quality and acceptance of a food product. During consumption, flavors are released from the food matrix into the oral cavity, from where they are transported to the nasal cavity through the actions of eating and breathing. The perception of a volatile depends on the concentration of the compound, threshold levels of the individual, and the duration of exposure. A n individual's perception of a particular food product will therefore primarily depend on the amounts and rates of volatile release from the food matrix; however, the overall perception will also depend on cognitive factors, such as pattern recognition and mood. Sensory panels are often trained to perceive the time-intensity (TI) characteristics of a food during eating . In general though, due to large variations among subjects, it is often difficult to analyse the data produced. It is easier, and more informative, to compare the differences between similar food products for one particular individual. More recently electromyography (EMG) mastication and swallowing patterns used in conjunction with sensory techniques have revealed that the perceived flavor of a food depends greatly on how an individual interacts with it. Although flavor is generally understood to be the combined perception of both non-volatile compounds in the saliva (taste) and volatile compounds transported to the olfactory epithelium (aroma), we consider it, for our purposes, to indicate volatile aroma compounds only. Recent technological developments now permit real-time monitoring of flavor volatiles 1
© 2000 American Chemical Society
180 expired from the nose breath-by-breath during eating using Atmospheric Pressure Chemical Ionisation-Mass Spectroscopy (APCI-MS) . This breath-by-breath time-release profile is assumed to represent the availability of flavor volatiles free to interact with the receptors, which are situated in the olfactory bulb. A recent study comparing simultaneous TI curves and breath-by-breath concentrations for gelatine/pectin gels revealed that individuals could discriminate the qualitative aspects of the sensory release profiles, but not the quantitative aspects . However, by processing the raw time-dependent concentration profiles into smooth time-release (TR) curves, these authors found a simple relationship between the changes in volatile concentration over the eating period and the perceived intensity. Both A P C I - M S and sensory TI studies have lead to a greater understanding of the processes influencing flavor release from food. However, a greater depth of knowledge can be obtained by developing and validating mathematical models of flavor release from foods in the mouth. The ultimate goal of this would be to mathematically predict the effect of varying food composition, food structure, and mastication behaviour on the perceived time-intensity flavor release profile. If successful, it would then be possible to use computer simulations to formulate foods for a desired flavor profile, taking into account individual or group differences in eating behaviour. Unfortunately, very little is understood about the mechanisms controlling flavor release from solid or semi-solid foods in the mouth. This, undoubtedly, is due to the variety and complexity of even the simplest solid foods. Despite these problems, last few years has seen rapid progress in the development of mechanistic physico-chemical models to describe flavor release from foods in the mouth . The objective of this paper is to review these recent developments and, in particular, focus on a computer simulation of aroma release from chewing gum in the mouth, which includes the transportation of flavors from the oral to nasal cavities. It should be noted that this paper is concerned only with the passage of the flavor volatiles from the food matrix to the nasal cavity and ignores the psychological and physiological steps of flavor perception. However, it should be straight forward to incorporate certain aspects of perception, such as threshold concentrations and pattern recognition into future versions. 3
General Theoretical Model Once a food is ingested it is rapidly coated by a thin-film of saliva. It can therefore be assumed that flavor, on release from a solid food, must first pass through the saliva phase before partitioning into the headspace of the oral cavity. Of course, for the case of semi-solid foods, such as sauces, flavor is already in the liquid phase and therefore can be released into the headspace directly. It can therefore be assumed that the passage of flavors from the food product to the headspace is a three phase problem involving the food, saliva, and gas phases. It is unlikely that simple diffusion of flavor molecules in the bulk phases of the food or saliva can determine the rates of release in the mouth as mastication removes diffusion gradients and generates fresh interfaces. Therefore, it can further be assumed that mass transfer of flavor compounds across the macroscopic interfaces is the rate limiting step for release. Based on these assumptions, Hills and Harrison proposed a general framework of flavor release in the mouth . Within this framework, mathematical models were developed and validated showing that the rate-limiting step for release was the transfer of flavor mass across the solid-saliva and saliva-gas interfaces for solid and semi-solid ' foods. The mechanisms governing mass transfer across the solid-saliva and liquid-gas interfaces depend greatly on the nature of the food matrix. A number of simple release 6
181 mechanisms have been established with the aid of mathematical models. B y far the simplest method to consider is the dissolution of a sugar matrix, such as a boiled sweet . In this case, the driving force for release across the interface is the non-equilibrium concentration difference of sucrose between the food product and saliva, which can be described by the stagnant layer theory of interfacial mass transfer. A s the matrix dissolves, all flavors are simultaneously released into the surrounding saliva, from where they partition into the headspace of the oral cavity. Another example of simultaneous release is from gelled sweets, where the surface of the gel matrix melts instead of dissolving . The rate at which the phase transition proceeds depends on the bulk melting temperature of the gel, which itself is dependent on the matrix composition. For soft gels possessing melting points below the mouth temperature, the driving force for release is the rate at which heat can diffuse into the gel matrix and initiate melting. For harder gels with melting points above mouth temperature, the diffusion of sucrose from the surface of the gel into the adjacent saliva phase is the rate-limiting step for release, because this lowers the melting temperature of the surface layer. These predictions have been recently observed by time-intensity sensory studies . A very different mechanism of release is extraction of volatiles from the food matrix during mastication. DeRoos and Wolswinkle proposed a non-equilibrium partitioning model to describe the extraction of flavor compounds from chewing gum into the surrounding saliva . These authors found that flavor release from model systems was predicted with high precision, whereas adjustments to the basic equations had to be made in order for the theory to comply with release conditions in the mouth. 6
Once in the aqueous phase the concentration of flavor will be diluted by saliva flowing into the oral cavity. Partitioning of volatiles from the liquid into the gas phase is further complicated by the presence of other food ingredients in the saliva. For example, volatiles residing in the lipid phase of an emulsion will slowly partition into the aqueous phase as saliva flows into the mouth . Also, proteins and polysaccharides are known to bind both reversibly and irreversibly to volatiles thus reducing the free flavor available for release " In addition, the presence of these macromolecules will effect the overall viscosity of the saliva further influencing the rates of release ' " . Saliva viscosity will also influence the residual thickness coating the inside of the oral cavity long after the majority of the food has been s w a l l o w e d , thus p o t e n t i a l l y i n f l u e n c i n g the aftertaste. A l l of these models focus only on foods that either remain whole or are already liquefied. In reality, most foods breakdown via fragmentation during the mastication process. This dramatically increases the surface area of the food allowing a greater proportion of the flavor to be released from the food matrix into the surrounding saliva. For a realistic simulation of flavor release to be developed, the time-dependence of the surface area needsto be calculated as the fragmentation process proceeds. Furthermore, the effect of saliva flow into the oral cavity , swallowing , and partitioning of flavors into the gas phase all have to be incorporated into the model. Despite these difficulties, Harrison et al. made a first attempt at developing a computer simulation of flavor release from solids as they fragment in the mouth . However, validating such a model is a challenging task, partially due to first, the complexity of the eating process and second, the difficulty in quantifying volatile concentrations in the oral cavity. One way of validating the model is to monitor the volatiles in the breath stream with mass spectrometry. This requires that the models previously discussed be extended to include the transport of the flavors from the mouth to olfactory epithelium (figure 1). A n early attempt at modelling this phenomenon was tackled by introducing a constant gas1112
flow through the headspace . However, this model is not a good representation of the physical reality as flavors are not transported to the nasal cavity at a constant rate. First, movements of the mouth, such as chewing, regularly pump flavors from the oral cavity into the breath stream; and second, absorption of flavor molecules by the mucus lining in the throat and airways influences the extent and rate at which flavors are transferred to the olfactory epithelium. In the following section, we present a new mathematical model to describe flavor release from chewing gum in the mouth and the subsequent transport of these volatiles from the oral cavity to the olfactory epithelium.