Figure 2-12: The process of swallowing (Thexton, 2001). The food bolus is dark grey.
Swallowing is the transportation of food from the mouth into the stomach, and can be divided into four stages: oral preparatory stage, oral stage, pharyngeal stage, and oesophageal stage (Gleeson, 1999).
The oral preparatory stage involves ingesting and masticating food, the addition of saliva, the formation of the food bolus, and the trapping of the bolus between the tongue and the hard palate (Figure 2-12a). The oral stage begins once the decision to swallow has been made, the tongue traps the bolus against the hard palate, the back of the tongue forms a ‘chute’ and the bolus moves into the oropharynx. In the pharyngeal stage the soft palate is raised to stop the bolus entering the nasal cavity and contractions along the pharyngeal wall and soft pallet allow the bolus to enter the pharynx (Figure 2-12b). The
epiglottis moves to cover the larynx and the hyoid bone and larynx also move upwards to stop the bolus entering the windpipe (Goyal & Mashimo, 2006).
The bolus then moves into the oesophagus in the oesophageal stage by the tongue pressing against the soft palate to close off the oral cavity and the lower pharynx muscles relaxing (Figure 2-12 c & d). The upper pharynx muscles also contract to force food into the oesophagus. The oesophagus relaxes to receive the bolus. Foods then move along the oesophagus via peristaltic contractions of the oesophagus muscles, although liquids can travel by gravity alone in some cases. When food reaches the end of the oesophagus, muscles guarding the entrance of the stomach relax. The same muscles contract once food has entered the stomach to prevent regurgitation (Bourne, 2002; Goyal & Mashimo, 2006).
The operation of the mouth, pharynx and oesophagus are integrated by a neuronal network, where sensory input and feedback are vital in controlling all four phases described (Miller, 1999). One aspect of this sensory control is the state of the food bolus before it is ready for swallowing. The food bolus must meet a certain physical state before it is suitable for swallowing (Hutchings & Lillford, 1988), although the exact criteria to meet this state are unknown.
Several different variables have been presented as critical parameters which initiate when swallowing will take place. Hutchings & Lillford (1988) introduced the dual- threshold model, where the bolus needed to reach a certain particle size and lubrication threshold to trigger swallowing. The model involved three dimensions: ‘Degree of structure’, ‘Degree of lubrication’, and ‘Time’ (Figure 2-13). As an example, this model explains why oyster can be swallowed with almost no chewing (at a large particle size) because of its lubricant properties, where as peanuts require extensive chewing to reduce particle size and for saliva to be incorporated. The importance of lubrication was confirmed by Prinz & Lucas (1995). Brazil nut particles were suspended in yoghurt at varying concentrations and particle size. The number of chews and chewing time increased significantly with increasing concentration of nuts (relative to yoghurt) and particle size.
Figure 2-13: A model describing the requirements for swallowing (Hutchings & Lillford, 1988).
Another model, known as the bolus model, has been proposed by Prinz & Lucas (1997) in an attempt to explain what forces keep the food bolus together. This model is based around the trigger for swallowing being the point at which the net cohesive force of the bolus is at a maximum (Figure 2-14). The net cohesive force is defined as Fv-Fa, the force required to separate the mass of particles (Fv) subtracted by the surface tension force pulling a particle towards the oral mucosa (Fa). The safest time for swallowing is when Fv-Fa is greatest, so that minimal particles are left behind. Fv and Fa can be defined by the following parameters:
t d D Fv 2 4 64 3
πη
= Equation 2-3 Fa = 4πrλ Equation 2-4 Where:D is the size of the bolus, η is the salivary viscosity, d is the average separation between particles, and t is the time span over which separation is attempted. r is the size of the particles, and λis the surface tension of saliva. Some of the superficial receptors in the
tongue are suggested to detect these low attractive forces within the bolus (some times around 0.01 N) (Trulsson & Essick, 1997).
Figure 2-14: The cohesive force, Fv - Fa, plotted against the number of chews taken in the masticatory sequence for raw carrot (closed circles) and brazil nut (open squares) according to the
model presented by Prinz & Lucas (1997). Swallowing is believed to take place at the point of maximum cohesive force (as indicated).
The model by Prinz & Lucas (1997) has been supported by research from Peyron et al. (2009) which investigated the texture of the bolus of breakfast cereals using TPA. Cohesiveness, adhesiveness, and springiness were at a maximum point when the bolus was ready to swallow.
The rheology of ready to swallow boluses of different wheat flakes has also been studied (Loret et al., 2009). Significant differences were found in the storage (G’) and loss (G’’) modulus, and yield stress. No significant differences were found between in the yield strain and moisture content. The authors suggested that bolus moisture content may be a trigger for swallowing.