Chapter 6: Relationships between oral processing behaviour and expectorated
6.3.5. Moisture content of expectorated bolus
The moisture content (MC) of the original food samples and expectorated food boluses are shown in Table 6-6. The MC of most expectorated boluses increased significantly after oral processing except for tomato juice. The moisture content of tomato juice (MC = 0.94) does not increase when it is ready to swallow (the increase in MC = 0.00%), because of the high initial MC along with a viscosity and stretch-ability which are smaller than saliva. This indicates that the expectorated moisture content of very high MC foods will not change or increase because of the addition of saliva during oral processing. The MCs of higher MC food samples (cheese tub, sour cream, chocolate mousse, and Greek yoghurt, MC > 0.60) do not change much (the MC increase is from
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0.2% to 9.2%). The expectorated MC of medium MC food samples (plum jam and condensed milk, 0.1 < MC < 0.60) increased significantly after oral processing (the MC increase is from 43% to 60%). The expectorated MC of low MC samples (Nutella and peanut butter, MC = 0.02) increased dramatically before swallowing (the MC increase is from 900% to 1000%). This corresponds to previous research which showed that greater amounts of saliva are produced for dry or tough food than for moist or soft food (Anderson, Hector, & Linden, 1985; Bilt, 2009; Mioche, Bourdiol, & Monier, 2003). This research shows that moisture content is an important parameter for ready-to- swallow boluses.
Moisture content appears to reach a certain level before swallowing, which may differ for liquid, semi-solid and soft-solid foods. Loret et al. (2011) found that as cereal boluses had similar water content (50%) before swallowing for different cereals, it might be an important marker for swallowing. However, limited research has been done on liquid, semi-solid and soft-solid foods. The data (Table 6-6) also shows that there is much more residual food left in the mouth for low MC foods than high MC foods.
Table 6-6. Moisture content (MC) of 9 original foods and expectorated bolus (mean ± SD)
Food Samples
Original food Expectorated food bolus
MC IMB (gwater/gsample) MC DMB (gwater/gdrymass) MC IMB (gwater/gsample) MC DMB (gwater/gdrymass) Lost solid weight (g) Added moisture (g) MC increase (%) Cheese tube 0.65±0.00 1.82±0.03 0.71±0.04 2.58±0.56 0.44±0.13 0.76±0.56 9.2% Chocolate mousse 0.73±0.00 2.66±0.00 0.76±0.01 3.14±0.19 0.20±0.07 0.48±0.19 4.1% Condensed milk 0.25±0.00 0.34±0.00 0.40±0.08 0.70±0.25 1.00±0.35 0.36±0.25 60% Greek yoghurt 0.84±0.00 5.43±0.08 0.86±0.01 6.40±0.48 0.09±0.04 0.97±0.48 0.23% Nutella 0.02±0.00 0.02±0.00 0.22±0.14 0.32±0.25 1.91±0.52 0.30±0.25 1000% Peanut butter 0.02±0.00 0.02±0.00 0.20±0.13 0.29±0.24 1.29±0.63 0.27±0.24 900% Plum jam 0.30±0.00 0.43±0.01 0.43±0.07 0.77±0.22 0.62±0.27 0.34±0.22 43% Sour cream 0.71±0.00 2.39±0.00 0.75±0.02 2.99±0.30 0.22±0.08 0.60±0.30 5.6% Tomato juice 0.94±0.00 14.61±0.03 0.94±0.00 16.67±0.80 0.05±0.01 2.06±0.80 0%
MCIMB means initial food sample mass based moisture content, MCDMB means dry mass of food sample or bolus based moisture content. The expectorated bolus data is the average values for 8 subjects.
Paired T-tests show that the moisture content (MC = water content / sample weight) is significantly different between foods (p < 0.05), while there is no significant difference
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in the MC of expectorated food boluses between subjects (p = 0.615). A linear
regression analysis showed that the expectorated bolus MC (g water/g sample) is positively
correlated to the original MC (g water/g sample) (S = 0.009, R-Sq = 0.99, expectorated MC
= 0.193 + 0.794 × original MC). These results indicate that the MC of expectorated bolus is affected by the MC of the original food sample more than oral processing behaviour.
Figure 6-6. The expectorated bolus moisture content increased with the original food moisture content.
The black line is the regression trend line. Left: moisture content is based on initial food sample mass (MCIMB), right: moisture content is based on dry mass of food sample and bolus (MCDMB).
The linear regression analysis shows the expectorated bolus moisture content (MC exp)
is inversely proportionate to the stretch-ability of the original food at 20ȗC (S = 0.127,
R-Sq (adj) = 0.79, MC exp = 0.903 - 0.005 × F max) and 37ȗC (S = 0.130; R-Sq (adj) =
0.78, MC exp = 0.803 - 0.006 × F max) (Figure 6-6). Similarly, stretch-ability of the
expectorated bolus is inversely proportionate to the original food sample moisture
content (MC ori, S = 18.01, R-Sq (adj) = 0.74, expectorated F max = 79.4 - 88.0 × MC ori).
Figure 6-7. Expectorated food bolus moisture content vs. stretch-ability of original food measured at
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One-way ANOVA analysis shows added water was significant different during consumption of sticky and non-sticky food samples (p<0.05). The sticky (high stretch- ability) food samples stimulated less saliva secretion (e.g. Nutella and peanut butter) during oral processing, because the expectorated bolus MC is lower than the less sticky food samples. Low moisture content foods become stickier during oral processing due to the addition of saliva and its mixing with the foods.
Rheological properties and moisture content
A regression analysis of rheological properties of nine samples shows that the viscosity
and shear stress of original food samples have negative linear regressions with MC exp at
20ȗC (S = 0.142, R-sq (adj) = 0.74; MC exp = 0.929 - 0.093 × viscosity; MC exp = 0.929 -
0.002 × shear stress). The original food moisture content (MC ori) has a strong positive
linear regression with MC exp.
Figure 6-8. Moisture content of expectorated bolus vs. viscosity plot at 20ÛC. Left: moisture content is besed on initial food sample mass (MCIMB), right: moisture content is based on dry mass of food sample and bolus (MCDMB).
These results indicate that MC exp correlates with food original rheological property,
especially stretch-ability and viscosity. Viscoelasticity only relates to MC exp at 37ȗC.
Saliva flow rate affects MC exp, but the effect is less than the original food rheological
property. The pH value has little effect on MC exp.
6.3.6. pH value
The pH of food samples ranged from 3.18 ± 0.01 to 6.49 ± 0.03 (Table 6-7), which
indicates that they are acidic. Previous research has shown that sour food increases SFR more than monosodium glutamate, salt and sugar (Neyraud, Heinzerling, Bult, Mesmin,
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& Dransfield, 2009; Spielman, 1990), so it was expected that there would be a correlation between pH and MC of the expectorated bolus. pH is not the only factor to affect SFR. Oral mechanical movement, food MC, individual preference and texture perception also impact SFR (Engelen et al., 2007; van Vliet et al., 2009; Ito et al., 2001; Loret, et al., 2011).
Linear regression analysis showed that there were no strong correlations between pH
and the added moisture in the oral cavity (S = 0.337, R2 (adj) = 0.07, added moisture =
1.13 - 0.13 × pH value). The main reason was all samples had different initial food moisture contents, so the MC of expectorated bolus will be different regardless of pH. Individual SFR after food stimulation, oral processing behaviour, food pH and individual food preference are considered to influence the MC of expectorated bolus. This analysis indicates that the pH value is not the main influencing factor on the stimulated SFR of tested foods. No significant linear correlation was found between the
pH value and oral processing behaviour (both ORT and muscular activities) (both R2 <
0.10).
Table 6-7. pH value of nine samples (mean ± SD)
Samples Nutella Peanut butter
Cheese tub Sour
cream
Plum jam Condensed
milk Chocolate mousse Greek yoghurt Tomato juice pH 6.12±0.01 5.86±0.01 4.66±0.00 4.49±0.01 3.18±0.01 6.36±0.01 6.49±0.03 3.98±0.01 4.16±0.01 SFRfood 0.90±0.70 0.78±1.27 2.91±2.35 5.25±3.27 2.70±1.86 2.96±2.25 5.27±2.96 12.07±9.37 46.12±40.06
This section shows that pH value of tested food is not the dominant factor for the expectorated bolus moisture content. The original food moisture content has more effect on the expectorated bolus moisture content. The tested foods include semi-liquid, semi- solid and soft-solid foods and different levels of stickiness too. Different types of foods require different oral processing behaviours, which influence saliva secretion. The various oral residence times and tongue - jaw movements also influence saliva flow during food oral processing. Compared to the original food moisture content, oral residence time and oral movements, the pH has much less effect on the MC of the expectorated bolus.
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6.4. Conclusions
The results of measurements and analysis on selected food samples indicated that saliva secretion depends more on the physiological mechanism of individuals rather than a stimulus in the oral cavity. Food properties and mechanical movement also influence saliva secretion. Food properties and personal preference dictate the extent of food
bolus mixing with saliva. The stretch-ability of the original food measured at 37 ÛC has
the potential to be a predictor of the stretch-ability of expectorated bolus (refer to Section 6.3.3). In addition, stickier foods require longer ORT and more oral work.
The muscle activity data showed that submental (swallowing) muscles use similar forces to process and swallow food bolus during consumption of different foods, while masseter (chewing) muscles use different force to process food according to food properties. The submental muscle conducted significantly more work than masseter muscle during food oral processing. Submental muscle activities have stronger correlations with ORT and the stretch-ability of expectorated food bolus than the masseter muscle. The property of the ready-to-swallow bolus is affected by the muscle activities during oral processing. The properties of the initial food influence muscle activities during oral processing which in turn both influence the stretch-ability of the ready-to-swallow bolus.
Increases in moisture content (MC) of expectorated boluses are associated with the MC
of the original food sample. MC of the ready-to-swallow bolus (MC exp) appears to
reach a certain level (0.20 ± 0.13 < MCIMB < 0.94 ± 0.0, where MCIMB means the initial
food sample mass based moisture content), which may differ for liquid, semi-solid and
soft-solid foods. MC exp is affected by MC ori and rheological properties of the original
food sample more than oral processing behaviour of subjects. The saliva flow rate of subject during consumption of food sample and pH value of food sample has less or
little effect on MC exp.
In brief, food sample and oral movement in the oral cavity stimulate more saliva secretion to facilitate the bolus formation. The muscle activities and oral residence time
determine the stretch-ability of the ready-to-swallow bolus with the MC ori contributing
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The relationships between food properties and oral processing behaviour, the individual physiological characters and food oral processing and the original food and expectorated food have been investigated in chapters 5 and 6. However, what drives the oral movement in the oral cavity and how it might occur during the oral processing have not been explained. This will be investigated in Chapter 7.
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