We investigated the relationship between orally perceived thickness and calculated shear stress on the tongue for mayonnaise and custard. To this end, the applicability of the models of Kokini et al. (1977), describing the mechanical breakdown in the mouth, have been tested. Within a limited range of shear stresses (mayonnaise <150 Pa; custard <30 Pa), there was a linear relationship between shear stress and thickness, in accordance with the work of Kokini et al. (1977). Beyond this range, the linear relationship breaks down and the thickness levels off with shear stress for both mayonnaise and custard. The relationship over the entire range of shear stresses used in this paper can be satisfactorily described by a semilogarithmic (Fechner) relation. For both types of products, the quality of the thickness prediction by the decreasing-height model and the constant-height model of Kokini et al. (1977) is similar. For most mayonnaises, the contribution of the lateral movement of the tongue to the shear stress in the decreasing-height model of Kokini et al. (1977) is orders of magnitude larger than the contribution of the squeezing or compression movement of the tongue towards the palate. This difference in magnitude is affected by the low value measured for the compression force and by the high values for material consistency K. The values for K are high because yield-stress behavior has been neglected when the flow curves were analyzed. For custard, the models of Kokini et al. (1977) are found to be less adequate. It is proposed that this is because the models ignore interactions with saliva. Several routes to improve the modeling by incorporating viscoelastic behavior were unsuccessful. Elongational stress and yield stress were neglected in all tested models.
- oral perception