Determining the initiation of chewing using lower jaw movements

EMA was used to detect chewing activity by monitoring lower jaw movements as rhythmic lower jaw movements are the main characteristic of chewing activity. Section

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3.2.2 showed that the EMA sensor coil on the lower incisor can track the trajectory of the lower jaw, so the EMA is a direct and reliable method to track lower jaw movement during oral processing of various foods (Birkholz, Kroeger, & Neuschaefer-Rube, 2011; Henriques & van Lieshout, 2013; Hertrich & Ackermann, 2000; Kaburagi & Honda, 1996; Mooshammer, Hoole, & Geumann, 2006; Neto Henriques & van Lieshout, 2013; Rong, Loucks, Kim, & Hasegawa-Johnson, 2012; Tabain, 2003; Terband et al., 2009).

The EMA displacement - time plots are shown in Figures 5-5 to 5-9. Chewing activity is noticeable in some of these figures as shown by a white arrow. Generally, these results showed three types of lower jaw movement during oral processing: 1) no lower jaw movement (Figure 5-5 and 5-6) ; 2) had lower jaw movement, but the movement was irregular and weak in frequency and amplitude (Figure 5-7 and 5-8) ; 3) had rhythmic lower jaw movement with greater frequency and certain amplitude (Figure 5- 9). These three types of lower jaw movement were considered to potentially correspond to three food types: liquid, semi-solid and soft-solid. However, there are no clear boundaries between liquid and semi-solid, semi-solid and soft-solid foods (Figure 5-4). Hard-solid food definitely requires chewing behaviour before swallowing; therefore hard-solids were not included in this study. However, it is unclear at what point foods do require mastication. This could be somewhere in the semi-solids to soft-solids range. This is the focus of this section.

No lower jaw movement means the amplitude of the tongue and lower jaw movements are less than 3mm in any dimension except for swallowing movements; the number of movement cycles (peaks) is less than 2, and the movement frequency is less than 1 per

second(Figure 5-5 and 5-6). The irregular tongue and lower jaw movement means the

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uneven (Figure 5-7 and 5-8). Chewing activity is defined as regular lower jaw

movement cycles, the frequency is about 0.9s-1, and the amplitude is greater than 6 mm

(Figure 5-9). The displacement-time plots show that most food samples do not have rhythmic lower jaw or tongue movement. According to data and plots, both subjects chewed peanut butter, Nutella, and Philadelphia cream cheese during oral processing. Three food samples were unique in this experiment. They were condensed milk, sour cream and plum jam (Figure 5-7 and 5-8). Subjects had lower amplitudes of jaw movements during consumption of these foods. These movements were not rhythmic, but the frequency and the amplitude were close to what would be observed during mastication. Other food samples were classified into the no lower jaw movement group or the rhythmic lower jaw movement group.

Figure 5-5. The tongue and lower jaw movements during oral processing of bottled water in X and Z axes.

There are no tongue and lower jaw movements between food ingestion and swallowing. The black arrow points to the swallowing point, the red arrow points to the end of food ingestion.

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Figure 5-6. The tongue and lower jaw movements during oral processing of chocolate mousse in X and Z

axes. There are no tongue and lower jaw movements between food ingestion and swallowing. The black arrow points to the swallowing point, the red arrow points to the end of food ingestion.

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Figure 5-7. The tongue and lower jaw movement during oral processing of condensed milk in X and Z

axes. The lower jaw moves in low frequency (< 1/s), and the amplitude of the tongue and lower jaw is weak (< 6mm) and uneven. The black arrow points to the swallowing point, the red arrow points to the end of food ingestion.

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Figure 5-8. The tongue and lower jaw movement during oral processing of plum jam in X and Z axes. The lower jaw moves in low frequency (< 1/s), and the amplitude of the tongue and lower jaw is weak (< 6mm) and uneven. The black arrow points to the swallowing point, the red arrow points to the end of food ingestion. The white arrow points to lower jaw movement indicative of chewing activity.

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Figure 5-9. The tongue and lower jaw movement during oral processing of Philadelphia cream cheese in X and Z axes. There are regular and greater lower jaw movement cycles (0.9s-1) and tongue movements (> 6mm) between food ingestion and swallowing. The black arrow points to the swallowing point, the red arrow points to the end of food ingestion. The white arrow points to lower jaw movement indicative of chewing activity.

Based on the results shown in Figures 5-5 to 5-9, these foods probably all sit on the overlap area of semi-solid and soft-solid foods (Figure 5-4), as these foods are considered to have similar rheological and mechanical properties (refer to Chapter 4). The boundary in terms of rheological and mechanical properties required to initiate chewing is not considered to be a clear line as some non-food factors are also likely to

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be important, such as individual differences (e.g. age, gender and oral condition), individual preference and personal experience.

From the foods tested in this study using EMA the boundary for initiation of chewing was determined to be condensed milk, sour cream and plum jam. The next step was to determine whether the result would be the same using EMG to observe chewing activity.