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The particle size distribution of solid foods after human mastication : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Auckland, New Zealand
Bolus formation is a complex process for which two theories are generally accepted, both theories describe how food changes through the mastication process which results in specific properties of the bolus being detected in the mouth to initiate swallowing. This research aimed to identify how food type, portion size (2 g and 4 g) and subjects affects the fate of ingested food solids and their particle size distributions, and bolus moisture content at the swallow point. Then the dynamics of bolus formation up to and past the point of natural swallowing were investigated by the use of a single subject to identify key trends.
Trials involved up to five processed foods; subjects were asked to chew portions of food and expectorate the bolus at the point they felt ready to swallow, or to expectorate the bolus at a specific number of chewing cycles. The solids loss from the bolus and moisture content of the bolus was determined. Particle size distribution (PSD) was measured for the expectorated bolus, and the debris (solids rinsed from the mouth after the bolus).
The food type had the greatest influence on the bolus moisture content, loss of solids from the bolus and PSD of the bolus and debris fractions. Solids are lost from the bolus progressively from the first chew cycle. PSD differed significantly between the bolus and debris fractions, and the PSDs were characteristic for each food type. The rate of change in PSD appears to plateau near the swallow point for some foods, whilst moisture addition continues to increase up to and past the point of swallowing. The bolus moisture content at the swallow point was approximately 50%, despite the differences in chewing strategy between subjects. Saliva does not appear to be added at a constant rate due to no significant effect of portion size.
The results from these studies indicate that bolus does not have to meet specific particle size criteria to achieve a safe swallow, and that particles circulate in multiple compartments during mastication. Results suggest a defined moisture content is required for a safe swallow.