Involved mechanisms

The state of organisation of the matrix, caused by macromolecules, results in a more or less dense entanglement, or a three-dimensional network, established between a more or less significant number, of macromolecule chains. These two organisations, although very different from the rheological point of view, can trap either, a continuous phase, which contains the compounds of flavour. The presence of the macromolecules limits the exchange between the various phases: liquid or solid, and vapour.

From a dynamic point of view, the transfer of the compounds is conditioned by the renewal in volatile compounds at the food-air interface, itself related to the migration of volatile compounds within the product. In a viscous system, friction strength can be estimated between the macromolecules and the aroma compounds. In a gelled system, this limitation of the exchanges was characterised by a restriction on the diffusion of aroma compounds within the food matrix. The presence of a three-dimensional network can generate the reduction in the diffusivity of volatile compounds, slowing down their migration to the matrix-air interface (Voilley et al. 1998, Rega et al. 2002, Lubbers and Guichard 2003). For these physical phenomena, it is necessary to add the possibility of molecular interactions between the aroma compound and the macromolecule, which also generates a limitation in the exchanges between phases, until the trapping of part of the volatile compound in the product.

Several mathematical models were worked out to account for the release of flavour compounds from viscous, or gelled matrices. The model of Harrison and Hills (1997) integrates the effect of interaction between components, viscosity of the system and the mass transfer coefficient of aroma compound. Thus, the mass transfer coefficient is inversely proportional to the square root of the viscosity of the solution. This model applied by Bakker et al. (1998) significantly fitted the release of the diacetyl from solutions of gelatine in stirred condition. However, the models adapted from the Harrison's model, supposed a homogeneity of the phases and, thus a system under agitation. This condition is not obtained in the gelled system. In this case the diffusivity of aroma compounds within the matrix have often been discussed. However, difficulties were noted in obtaining diffusivity coefficients of small volatile solutes as aroma. The importance of the parameter 'diffusivity of the volatile compounds' in the establishment of a mathematical model for flavour release from gelled systems was partly called into question by Juteau et al. (2004). The reduction in the time of residence of volatile at the air-gel interface would allow a better explanation on the decrease in the mass transfer coefficients in the studied gel. However, the determination of the apparent kinetic orders of the release in different samples tended also to confirm the presence of a gradient of concentration between the surface layer and the bulk of the matrices.

To summarise, Table 15.3 presents a digest of knowledge on the effect of the proteins and polysaccharides, in increased concentrations, on flavour release.

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