**4.2 Mouthfeel and texture perception**

Ingestion of food is also guided by mouthfeel and texture perceptions as well as appreciation of the masticated bolus. Oral processing is a complex progression of neuromuscular interactions which prepares and assess food prior to swallowing. As food is being broken down and imbibed with saliva, it is in contact with the oral sensory receptors and an integrated sensory evaluation of tastes, volatile compounds and texture will occur [141, 142].

Electromyography (EMG) has been used to assess electric activity of the main masticatory muscles in healthy participants. Coupled to chewing cycle counts, tongue mobility, tongue pressure, salivation or videofluoroscopy studies, the mechanistic involved pre- and peri-swallowing can be assessed [143–148]. In a study looking at gels and sols, muscle activity required until swallowing appeared to be well correlated with the required force deployed for large compression deformation which occurs at the initiation of mastication [145]. Tournier and colleagues observed large inter-participant variation both for salivation and oral processing after 5 healthy participants masticated 4 breads of different textures and compositions [146]. Mori and colleagues studied the laterality of the posterior tongue movement in 20 young and health adults by [147]. Gummy jelly, sponge cake and mashed potatoes were assessed as representative of gradient mastication intensity requirements at the initiation, middle and end of mastication stages. For the purpose of the study, the participants were asked to masticate only on the right side or only on the left side. They measured a more intense EMG activity on the side of mastication for the gummy jelly and documented that tongue activity was affected by the food texture. Finally, Matsuo and colleagues investigated the masticatory function of 22 young participants an 32 community-dwelling older individuals of either normal or oral hypofunction. For their investigation, the test samples were a control meatloaf, meatloaf containing lotus root, control chicken ball or chicken ball with almonds slivers (10 g per bite). The documented mean EMG amplitudes and integrated EMG activity were lower for control samples which were softer test foods. Oral functions, including dental condition, appeared to decline with age and would have influenced the assessed mastication capacity [148].

The characterization of the biomechanics of bolus preparation and its impact on swallowing capacity is still being investigated [149–151] and remains limited in application for daily clinical investigations or association to diversified menu items around the globe. Few foods have been assessed. However, the variability in bolus process are highlighted and the impact could be important.

Individual physiognomy, quality of dentition and capacity to prepare foods for deglutition are only one part of the complex equation. The foods and liquids consumed bring their share of complexity. The lubrication of the bolus is also dependent on salivation. Intra-oral assessment of bolus texture and correlation to sensory perceptions are challenging.
