Rasgos de una configuración epistemológica

Oral processing is controlled by the central nervous system. It is a physiological and physical process (Chen, 2009). Physiological factors are those relating to the subject, e.g., by age, gender, dental status, while physical factors explain the variation related to the properties of the food (Chen, 2009 and Woda et al., 2006a). Four stages of oral processing can be described that track the food experience: the first bite, comminution and lubrication, bolus formation and swallowing (Hutchings and Lillford 1998; Prinz and Lucas 1997, Prinz and Lucas 1997, van der Bilt et al. 2006, Engelen et al. 2005).

Our metabolic rate demands mastication of food to acquire energy and essential nutrients, comminution increases the surface area exposure of the food particles to the digestive enzymes in the gut, thus providing energy at a higher rate (Prinz and Lucas 1997; Lucas, et al. 2002).

The oral operations occurring between the first bit and the terminal swallow involve a series of decision steps to ensure masticatory operations are co-ordinated and proceed in the right order (Chen, 2009). Lucas et al. (2002) produced a conceptual model (Figure 2-1) with decision boxes to describe different oral operations including, grip, first bite, fracture, size reduction, transportation and swallowing. The decision boxes do not describe decisions made by the central nervous system but are a simple analytical way of describing a general sequence of events. In between each event is some sort of transport whether gathering and placing the

2-10 food in the occlusal plane or transport towards the pharynx. This model does not address the sensory input necessary to make the decisions at the steps.

Figure 2-1: A conceptual model depicting food oral processing as a sequence of events. Decision boxes are shown as diamonds while process boxes are rectangular (from Lucas et al., 2002).

Hiiemae (2004) proposed a conceptual model of feeding that involves a series of sequential stages (Figure 2-2). The process model asks three questions and the progress of food from ingestion to swallowing is regulated by sensory input from the orofacial complex. The first question is whether the food is suitable to eat; the second is whether the material is suitable for swallowing. The last gate is a threshold which has time and volume components and asks if there is enough food to swallow. If there is not enough food it implies a below volume threshold, however the bolus will get swallowed after some time regardless.

Crack Comminution Sculpture Fracture Grip Yes PS reduction? Transport Yes Intended final particle shape? Yes No No No Dental Functions

2-11 Figure 2-2: The process model of feeding (from Hiiemae 2004).

Stage 1 transport is the act of moving the food from the front teeth after the first bite to the molars for size reduction. During this transport stage, properties of the food such as taste, surface texture are detected. If the food is perceived to be toxic it is spat out at this stage. After the first transport stage processing begins where particle size reduction occurs, and food is mixed with saliva to ready it for bolus formation. Once the food particles are processed sufficiently second stage transportation moves them selectively to the back of the oral cavity to form a bolus. Hiiemae’s (2004) model is supported by sirognathographs of 3D jaw movements (Figure 2-3). The different sequences of the mastication cycle can be identified by the changes in jaw movement during swallowing and mouth clearance. Stage 1 transportation is easily distinguishable but the second stage of transport is less evident. It has been postulated that stage 2 transportation occurs simultaneously with the chewing process as more than one bolus is prepared from the one mouthful. Multiple swallows occur for most

2-12 feeding sequences. With solid food, some portion of the food may become ‘swallowable’ before others and be selectively moved to the oropharynx. As mastication continues more food is transported to the oropharynx and ultimately swallowing is completed (Hiiemae, 2004).

Figure 2-3: Sirognathograph record of a human subject eating an unpeeled apple. The arrows highlight the different jaw movement profile during swallowing (from Hiiemae, 2004).

Accounting for multiple swallows in a mathematical model of bolus formation is an important consideration. The mass or volume of food consumed to produce Figure 2-3 is not mentioned, but the volume of the initial bite would be an important contributing factor to the number and also the mass of the multiple swallows. There is obviously a size threshold for a safe to swallow bolus, which will be governed by an individual’s oral physiology and possibly the properties of the food as they contribute to forming a bolus. The composition (solid or liquid) of the intermediate swallows will depend on the food being chewed. In studies on the mastication of meat, the loss of solids during mastication was low, suggesting minimal partial swallows or primarily liquid swallows (Mioche et al., 2002a). Subjects chewing on gelatine sweets used several partial swallows before mastication was completed, the swallows consisted of saliva and dissolved gel and are required to remove excess liquid (Sprunt and Smith., 2002).

The conceptual models presented by Lucas et al. (2002) and Hiiemae (2004) have strong similarities although the later appears more complex and provides a more in-depth explanation of what occurs in each of the stages. The models address the movement of the food and the action of chewing but (apart from comminution) do not address the mechanisms or other physical changes to the food that transform it into a safe to swallow bolus.

2-13 It is the aim of this work to extend these conceptual models by critically examining the food experience during mastication and to develop a mechanistic framework necessary to create a quantitative model of food mastication.

2.3.1 The Mouth Process Model

A major step towards a mechanistic framework was made by Hutchings and Lillford (1988) who developed a three dimensional conceptual model to show the dynamic process of food breakdown and texture perception. The model is descriptive rather than quantitative (Figure 2-4). It demonstrates the effect of ‘Time’ in the mouth and how this affects the ‘Degree of Structure’ and the ‘Degree of Lubrication’. Food follows a breakdown path and in order to be swallowed safely must cross a threshold of structure and lubrication. Although the model was intended for modelling textural change it helps to visualise the rate processes that need to be considered before a mathematical approach to food breakdown can be taken.

Figure 2-4: The mouth process model. A food may be swallowed when the ‘degree of structure’ has been reduced below the plane ABCD and its ‘degree of lubrication’ has crossed the plane EFGH. Key:(1) Tender juicy steak (2) tough dry meat (3) dry sponge cake (4) Oyster (5) liquids and semisolids.

From Hutchings and Lillford (1988).

The degree of structure refers to the physical properties of the food which change during mastication. The degree of lubrication is based on the food bolus requiring a specific degree of lubrication to be safe to swallow. The axes in Figure 2-4 have no defined units. Hutchings and Lillford (1988) stated that to assign one physical method to define the ‘Degree of Structure’ or ‘Degree of Lubrication’ is unreasonable considering the many factors that contribute to the

2-14 process. This approach agrees with current research which finds that the threshold for a safe to swallow bolus consists of particle size and lubrication (Peyron et al., 2004; Jalabert-Malbos et al., 2007). While it remains a challenge to quantify these, it nevertheless provides a useful insight into the oral processes occurring during mastication and helps one consider the rate processes and mechanisms of food breakdown.