Swallow thresholds in human mastication.

Brazil-nut particles were broken down and classified by sieves into four classes of particle size. Samples of these classes were suspended in plain yoghurt in varying concentrations and presented to nine humans for chewing. Both the number of chews made before swallowing and the time needed to swallow increased significantly with particle size and concentration but the chewing frequency (number of chews/time) decreased. The rate of change of the chewing frequency, averaged over the chewing sequence (chewing frequency/time), was calculated for each food input and termed the swallowing index. This index did not differ significantly for concentrations above 20%, but increased sharply at lower concentrations. These results were interpreted in terms of a model in which food is swallowed only when particles are both small enough and sufficiently lubricated. For our food mixture, the lubrication threshold was satisfied by a 20% concentration and the particle-size threshold was 1.4 mm. Chews made with concentrations lower than this and containing smaller particle sizes were few in number and slow, reflecting the need to detect particle size with the oral mucosa.