Age-related changes in functional adaptation to bolus characteristics during chewing

We investigated how the physical properties of food affect chewing kinematics, and examined the effects of aging on behavior. Twenty young and 21 older volunteers were asked to freely chew rice crackers and spit the food out at their average chewing duration. We compared chewing characteristics among the conditions (age × rice cracker), and examined the area of rectified masseter and suprahyoid electromyographic bursts per chewing cycle. We also evaluated temporal changes in those values. In addition, the bolus properties at the end of chewing were compared. The harder/larger the rice cracker, the longer the chewing duration and the greater the number of chewing cycles. These values also increased with age. Chewing cycle time was not affected by age and rice cracker properties, except in one condition: for the rice cracker with the lowest hardness and density, the chewing cycle time was longer than for the other rice crackers. Chewing cycle time decreased at the middle stage of chewing, followed by an increase, with increasing suprahyoid electromyographic activity at the late stage. The physical properties of the bolus at the end of chewing did not differ between age groups, and depended on the initial properties. The water absorption rate of the bolus was significantly greater for the rice cracker with the lowest hardness and density compared with other rice crackers. The results demonstrated that not only hardness, but also other factors, such as density, significantly affected chewing behaviors.