To gain insight into the effect of oral processes on perception, we defined a set of five specific oral manipulations and investigated their effects on the perception of low and high fat versions of two semi-solid foodstuffs, vanilla custard desserts and mayonnaises. Behavior modifications ranged from simply placing the stimulus on the tip of the tongue to vigorously moving it around in the mouth. Sensory ratings for mouth-feel and flavor attributes were made 5 s after placing the stimulus in the mouth, and after-feel attributes were rated immediately after swallowing. Most attributes showed a similar pattern, with lowest attribute ratings where the tongue’s movement was restricted and gradually increasing ratings with increasing complexity of the tongue movements. An individual’s normal oral processing behavior typically resulted in the most intense sensations of flavor and mouth-feel. Residence time for all mouth-feel attributes, except prickling, was determined by the time required for tongue movements. The exact tongue movements required for sensations appeared to be related to food groups and individual foods, rather than to specific mouth-feel attributes.
Once placed in the mouth, food is subjected to a complex series of manipulations by the tongue, teeth, lips and cheeks, during which it is converted into a form suitable for swallowing. Sensory assessments of food begin prior to ingestion when the food is seen, handled and smelled (Sharma et al., 1973), and continue in the mouth to assess taste and texture during oral processing and swallowing. Some sensations persist after swallowing and are termed after-feel.
Sensations are gathered during each phase of oral food processing. Initial perceptions occurring at low shear rates include attributes related to touch (e.g. perceived homogeneity based on the presence, size, and shape of particles), and those requiring only small deformations (elasticity, stickiness to the palate, and viscous behavior) (Rosenthal, 1999). Next, the food structure is broken down during the first chews (solid food) and the food mixed with saliva to form a coherent bolus (solid and semi-solid food) (Prinz and Lucas, 1997). During this phase, attributes related to the physical deformation and breakdown properties (e.g. hardness, softness, brittleness, plasticity, crispness, and sponginess) are detected (Sherman, 1969; Agrawal et al., 1997). As more saliva is added to the bolus, attributes are perceived that relate to the physical structure (e.g. smoothness, lumpiness, and pastiness), consistency (e.g. creaminess and wateriness), and adhesion to the palate (e.g. stickiness).
Hutchings and Lillford (1988) modeled the breakdown over time of various foods during oral processing. Key elements of their model are the mechanical breakdown of food structure and the degree of lubrication effected by the saliva and by the moisture and fat in the food. Lubrication was also the proposed underlying mechanism for a sub-set of the mouth- and after-feel attributes generated by a quantitative descriptive (QDA) panel for vanilla custard desserts (de Wijk et al., 2002). The other sub-set consisted of the attributes melting and thickness, which are probably related to the viscosity of food and its reduction through chemical and mechanical breakdown.