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Modelling food breakdown and bolus formation during mastication : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Bioprocess Engineering at Massey University, Palmerston North, New Zealand
Mastication is a complex process that transforms food into a bolus which can be swallowed
safely. However, it can be simplified within an engineering context, where the mouth is the
equipment carrying out a unit operation to convert ingested food (raw material) into a safe-to-swallow
bolus as the process output. Two questions emerge from this observation (i), ‘What
processes transform the food from its initial state into a bolus?’ and (ii), ‘How do humans
assess when the food is ready to initiate swallowing?’ This research examines these questions
and a mathematical model is developed which can track the bolus properties during
A range of common foods with contrasting textures were examined in an observational study
to investigate the rate processes of mastication. A wide range of breakdown pathways and
textures were observed, from the brittle fracture of carrots to the work softening of a fibrous
beef steak. However, despite differences in their structure and properties, breakdown is
dominated by a small number of rate processes. Size reduction and work softening occur at
occlusion and account for majority of the structural breakdown. Absorption, dissolution and
melting are generally more subtle. These are facilitated by mixing, which is the circulation,
gathering, folding and placement of food on the occlusal plane.
Mechanical sensory testing (MST) occurs simultaneously to breakdown as the food is
manipulated around the mouth during occlusion and between chews. During occlusion this
provides gross information about the toughness and hardness, whereas between chews the
tongue-palate interactions provide more detailed information about yield and flow. These
MST’s assess the properties of volume, adhesion, bolus deformation, particle deformation and
particle size. Adhesion refers to the binding forces between the food particles and the oral
surfaces. Bolus deformation is important for boluses that do not contain individual particles. If
they do then swallowability is constrained by their size and individual deformability.
In order for safe swallowing, thresholds for each of the MST properties must be met. These
were justified using a hazard and operability study of swallowing, on the premise that
attempting to swallow a bolus which does not meet the threshold property could result in
aspiration or choking. As mastication proceeds, the properties assessed by the MST’s are
evaluated against the required threshold properties and contribute to the decision making
process of whether to swallow or continue chewing.
From this analysis, this work proposes a universal conceptual model of mastication that
combines the rate processes, the MSTs and a decision making model. The conceptual model
was then described mathematically. It is universal, that is, it is not specific to any food type,
because solid foods follow similar physical breakdown paths where occlusion is of primary
importance followed by the incorporation of saliva and the other rate processes. Model
parameters require in-vivo experiments about the breakdown dynamics of specific foods.
Subject variability was avoided by using single subject studies for three separate foods; brown
rice, a sweetened gelatine gel and peanuts embedded in a food matrix. Each case study
explored a limited number of rate processes and food properties. Bolus properties predicted
by the model were compared to the experimental data. The output of the model, including
particle size distribution and moisture content, closely matched the data during mastication
and at swallow point using input parameters fitted from the single subject experiments.
This work provides a platform for further research into mastication modelling. It is
recommended the mathematical model be expanded to mechanistically describe the mixing
and work softening of non-particulate food boluses. Additional experimental work would
achieve a better understanding of the heat transfer in the mouth which would improve the
models ability to handle heat sensitive foods.
The model developed here has the potential to aid future food design where a particular
breakdown pathway is desired and will reduce the number of time intensive in-vivo