Relationship between Physical Properties of a Food Bolus and Initiation of Swallowing

To clarify the relationship between the physical properties of a food bolus and the initiation of swallowing, we measured the texture of food boluses collected from the oral cavity at three different stages of mastication: M stage, the halfway stage of mastication; L stage, immediately prior to swallowing; and +20% stage, the number of chewing strokes 20% beyond a point at which swallowing would usually occur. Eleven healthy adult participants masticated highly adhesive rice cakes (RC), hard and brittle peanuts (P) and hard biscuits (HB). The texture of the bolus was measured by texture profile analysis. Hardness of the bolus decreased significantly from the M to L stage during mastication of all three types of food. This finding suggests that the decrease in hardness of a bolus may be a necessary factor for the initiation of swallowing, although it is not a sufficient factor by itself for the initiation of swallowing. For both the P and HB boluses, adhesiveness had a maximum value at the L stage immediately prior to swallowing, and cohesiveness increased significantly from the M to L stage. In contrast, adhesiveness of the RC bolus decreased significantly (p<0.001) from the M to L stage, with cohesiveness remaining unchanged. These findings suggest that the initiation of swallowing during mastication of hard and brittle food commences once a clump bolus formed, while swallowing of highly adhesive food is triggered when adhesiveness of the bolus is sufficiently reduced to meet the swallowing threshold.