8 Food processing The main purpose of mastication is to break down food into smaller particles that bind to each other through saliva, forming a food bolus ready for swallowing and digestion.1,2 Mastication is a learned automatic complex process involving the interaction of hard and soft tissues in order to grind a food bolus prior to swallowing.3 In the mouth, food undergoes several steps: First, the food is transported from the front teeth to the molars (stage I transport). Here the food is analyzed through taste, retronasal olfaction and oral receptors of the somatosensory system. In the second step, the food is converted into a food bolus by means of the teeth and with the aid of saliva and lingual and facial muscles. In the third step, the chewed food is transported backwards to the oropharyngeal surface of the tongue (stage II transport). Finally, the upper esophageal sphincter is opened. The chewing movement takes place as food conversion and backward transport occur almost simultaneously, and food moves across the tongue surface.4 Through receptors in the oral cavity and nose, changes in food are sensed during chewing, which also leads to the perception of taste, smell, and texture of the food.5 Chewing also initiates various digestive and metabolic activities, both in the mouth and through reflexes in the cephalic phase (the secretion of gastric juices before food enters the stomach, due to the taste, sight, smell and/or thought of food), which ensure that digestion starts.4 Mastication involves several nerves, muscles, and connective tissue structures.6 The m. masseter, m. temporalis, m. pterygoidus medialis and m. pterygoidus lateralis are considered the main muscles of the masticatory system.7 These muscles work in a coordinated way with other muscle groups of the face during chewing, such as the infrahyoidal, suprahyoidal, palatal and temporomandibular muscles.8 During chewing, food hardness influences the masticatory force, activity and amplitude of mandibular (lower jaw) movements.2 Chewing is characterized by a relatively rhythmic movement of the chewing muscles.9 The cooperation between these jaw-opening and closing muscles is elicited by a central pattern generator in the brainstem.1,2 This pattern generator receives signals from the mouth and motor cortex, which are converted into a rhythmic chewing movement.9 Chewing can be unilateral, bilateral or alternating bilaterally. The preferred side in most cases depends on the number of occlusal units (the surface of a tooth that meets the surface of its opposing tooth in occlusion), and where they are positioned. When occlusion is the same on both sides, there is often a preference for alternating two-sided chewing; changing chewing sides regularly.7 Several factors influence the ability to chew efficiently, such as the maximum bite force, maximum mouth opening, sensory function of the tongue, tongue force, the number of occlusal units, and saliva flow. The maximum bite force depends on the muscle volume, the
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