Food Breakdown during Chewing: Investigating the Link between Bolus Structure, Saliva and Texture Perception

Abstract

Texture plays a signi cant role in determining the acceptance and appreciation of a food product. It is perceived during chewing | a process which transforms food into a cohesive mass (bolus) that can be safely swallowed. This involves progressive changes to food structure and mechanical properties which, through the stimulation of oral receptors, sends signals to the brain that are interpreted as sensory texture. This thesis aims to gain a deeper understanding of how food breakdown during chewing relates to texture perception, which is essential in designing food structures to enhance sensory appeal or meet special needs. Short-dough biscuits, Haloumi cheese and dark chocolate were chosen to represent different solid food structures. Material characterisation techniques were applied to analyse the structure, moisture content, rheology and tribology of boluses, chewed by a single individual or multiple subjects. The results reveal that the structural transformations involved in oral processing are food-specific. Comminution dominates bolus structure during early chewing, corresponding to the perception of "hard" (biscuit and chocolate) or "soft" (cheese). The particle size produced after the first few chews determines whether attributes such as "crumbly" and "dry" (biscuit), "lumpy" (cheese and chocolate), or \smooth" and "sticky"(chocolate) are perceived mid-chewing. The simultaneous addition of saliva during chewing causes fractured particles to agglomerate, leading to the formation of a single mass and the perception of "sticky", "smooth" and "soft" by point of swallow. Saliva also confers flowability onto the food bolus. Depending on its physical interactions with food, saliva performs multiple functions within the bolus. In addition to agglomeration, saliva softens the biscuit matrix through sugar dissolution and starch digestion, while it causes melted chocolate to phase invert from an oil-continuous material to a water-continuous emulsion. A comparison of boluses produced by multiple subjects indicates that saliva quantity has a bigger influence on bolus rheology than the rheology of saliva itself. However, when compared to Newtonian substitutes, saliva makes a large contribution to lubrication such that expectorated chocolate boluses are significantly more lubricating than simulated boluses.