The challenge of mastication: preparing a bolus suitable for deglutition.

The main function of mastication is to transform a solid food into a bolus that can be swallowed safely. The bolus characteristics such as particles size or cohesiveness, are continuously sensed during mastication and they are important in initiating deglutition. This study examined the following question: What is the condition of the bolus just before swallowing? Ten subjects with normal dentition aged 37.5 +/- 3.7 years were asked to chew without swallowing six different foods (three nuts and three vegetables) while the number of cycles and the duration of the sequence were recorded. The particle size distribution shown by the expectorated food bolus just before swallowing was examined by image analysis. The results showed that, for a given food, the sizes of the bolus particles just before swallowing were comparable in all subjects. However, the number of cycles and duration of the sequence varied between subjects. Taken together these data strongly suggest that the granularity of the bolus before swallowing has to reach a predetermined state which is obtained by using an individual chewing strategy. This suggests that the bolus structure reflects a key factor for homeostasis and explains the large interindividual variability of the mastication physiologic parameters.