Oral processing

Nearly all solid or semi-solid foods are subjected to the process of chewing in the oral cavity. The food particles are sufficiently broken up by each type of tooth, with the aid of the tongue and cheeks directing and keeping the food between molars for chewing (Chen, 2009; van Vliet et al., 2009; Xu & Bronlund, 2010). The forces applied on the teeth vary with different types of foods which are being chewed. The force applied to single tooth is also different from the total force between all the contacting teeth during chewing. On foods like biscuits, carrots and cooked meats, forces range from 70 to 150 N on any single tooth (Anderson, 1956), while forces on all the containing teeth range from 190 to 260 N (Gibbs et al., 1981).

From a physiological point of view, there are two main roles of chewing: to ensure fragmentation of food into particles small enough to be properly lubricated by saliva and form a cohesive bolus for swallowing (Hutchings & Lillford, 1988; Prinz & Lucas, 1995) and to have an enhanced release of flavor and aroma from food structure (van der Bilt, 2012). The result of chewing is mainly determined by oral factors (i.e. dental factors, jaw muscle activity, bite force, masticatory performance, saliva and swallowing of food) and the characteristics of the food (van der Bilt, 2012). The former is about the individuality of human beings which leads to variations of chewing. For example, the size of oral cavity varies dramatically from person to person (Chen, 2009); the large variation of biting force has been observed between different races, gender and individuals in the same ethnic groups (Paphangkorakit & Osborn, 1997; Bourne, 2002). The latter is the key factor influencing how a food is orally processed and sensually perceived (Jalabert-Malbos et al., 2007). Mastication is an early step in the process of size reduction to small molecules that can be absorbed into the bloodstream. It usually

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reduces particle size by two or three orders of magnitude before passing to the stomach (Bourne, 2004).

Texture is the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing, touch and kinesthetics. This definition conveys four important concepts as follows: 1) texture is a sensory property and, thus, only the individual can perceive and describe it, 2) it is a multi-parameter attribute, 3) it derives from the structure of the food (molecular, microscopic or macroscopic), 4) it is detected by several senses, the most important ones being the senses of touch and pressure (Szczesniak, 1963; Bourne, 2002;

Szczesniak, 2002). Though texture is sensory perceptions of human beings, it is finally determined by food structure (i.e. physical properties). Therefore, mechanical and tactile elements are often simultaneously investigated by various instruments and then

correlation is established between sensory terms and mechanical/tactile properties. These rheological or mechanical assays may be classified as either empirical or

fundamental. The benefit of using fundamental rheological methods (e.g. dynamic test, failure test, etc.) to evaluate the mechanical elements of texture is that they are linked to theories that explain molecular and microstructural mechanisms of texture. It is possible that mechanical properties measured up to and at fracture predict changes occurring during oral processing (Foegeding, 2007; Foegeding & Drake, 2007). In addition to these fundamental rheological methods, there is a very important empirical instrumental testing to imitate the first two bites of chewing which is called Texture Profile Analysis (TPA) (Szczesniak et al., 1963; Bourne, 1978). This procedure, as shown in Fig. 2-11, gives six or more different texture notes.

Fracturability is defined as the force at a significant break early in the first bite. The maximum force of the first bite is defined as hardness. The ratio of the positive

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force areas under the first and second compressions (A2/A1) is defined as cohesiveness. The negative force area during the first decompression is adhesiveness. The distance the piece of food recovers its height between the first compression and the beginning of the second compression is defined as springiness. These parameters correlates highly with sensory ratings (Szczesniak et al., 1963; Kim et al., 1996; Drake et al., 1999; Breuil & Meullenet, 2001; Szczesniak, 2002; Di Monaco et al., 2008).

Figure 2-11 A generalized texture profile analysis curve obtained from the instron universal testing machine (Bourne, 2002).

The influence of food characteristics on the chewing process has been

extensively studied. It is believed that the food structure is a key factor influencing the chewing behaviour of various foods (Chen, 2009). In general, a hard food would require more chewing cycles. For example, a mouthful of apple has shown to take 7 chewing

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cycles to form the first bolus to swallow, while it has shown to take 16 and 19 chewing cycles to form a bolus to swallow a mouthful banana and cookie respectively (Hiiemae et al., 1996). Chen (2009) replotted the number of chewing cycles against the yield stress of different foods obtained from Engelen (2005) finding a perfect linear

relationship between these two parameters, which strongly supports it. Jalabert-Malbos et al. (2007) also found harder foods such as carrots, peanuts and coconut required more chewing cycles than softer foods like egg white, gherkins and mushrooms. A clear relationship between food hardness and jaw muscle activity has been found in many studies. A harder food results in increased jaw muscle activity and longer burst duration of the muscle activity, regardless of food (Agrawal et al., 1998; Peyron et al., 2002; Foster et al., 2006; van der Bilt et al., 2007). Furthermore, the masticatory force was observed to increase from 100 to 150 N when the hardness of silicone rubber increased by a factor of 2 (Kohyama et al., 2004). In addition, the food characteristics also influence the jaw movement (Horio & Kawamura, 1989; Peyron et al., 2002).

Food type has a great effect on the degree of fragmentation of food. Jalabert- malbos et al. (2007) studied the fragmentation of ten natural foods upon chewing. The authors found that carrots had the highest number of chewing cycles, but the median particle size of bolus (D50) was large. Peanuts and coconut had the similar chewing

cycles, but the D50 of peanuts was half of that of coconut. Egg white had the smallest

chewing cycles, but its D50 was not the lowest. These differences in degree of food

fragmentation are mainly attributed to the food structure in these products. Peyron et al. (2004) drew similar conclusions when studying 6 natural foods (peanut, almond,

pistachio, carrot, radish and cauliflower).