Textural Evaluation of Rice Cake by Chewing and Swallowing Measurements on Human Subjects

Biosci. Biotechnol. Biochem., 71 (2), 358–365, 2007

y Kaoru KOHYAMA, Hiroko SAWADA, Miho NONAKA, Chiharu KOBORI, Fumiyo HAYAKAWA, and Tomoko SASAKI

National Food Research Institute, 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan

Received May 17, 2006; Accepted October 24, 2006; Online Publication, February 7, 2007 [doi:10.1271/bbb.60276]

The difficulty in masticating and swallowing rice cake amylose may make rice cake easier to masticate. was quantified. Healthy subjects ate pieces of rice cake A simple compression test using an Instron-type (9 g and 3 g) and a modified product (9 g). We used instrument has been commonly adopted to determine the electromyography to measure the activity of the jaw- firmness of food.2) To evaluate the ease of mastication, closing and -opening muscles during chewing, as well as the guidelines on food for elderly people by the Ministry the suprahyoid muscle activity, laryngeal movement, of Health, Labour and Welfare is commonly used in and sound during swallowing. The smaller the rice cake, Japan.3) With this method, a sample is compressed at the shorter the mastication time, the fewer the number a constant speed of 10 mm/s until 67% of the initial of chews, and the less the jaw-closing muscle activity. A sample thickness is achieved. Food firmness is defined modified rice cake product (9 g) was consumed with less as the maximum stress detected during the test. How- mastication effort than the standard rice cake (9 g) and ever, rice cake is not broken by this test and exhibits less with the same effort as the standard (3 g). Both the mechanical resistance under a compressive strain of sample amount and texture influenced mastication, 67% than that produced by real mastication in which a although neither factor caused a significant difference very large deformation is applied to food. Although rice in swallowing characteristics. These observations sug- cake is difficult to chew, it sometimes shows a lower gest that swallowing was induced when the bolus stress value than the guidelines (5:0 104 N/m2) for properties became suitable for swallowing, as healthy people with chewing difficulty.3) subjects could adjust their mastication technique ac- Texture includes the sensory properties perceptible cording to the food amount and texture. by means of mechanical and tactile receptors from the mechanical, geometrical, and surface properties of Key words: rice cake; texture; mastication; swallowing; food.2,4) The textural properties of food are assessed electromyography mainly during mastication.5) A sensory evaluation expressing the perceived texture and mechanical tests Cooked rice cake (mochi) is difficult to chew and describing physical properties have been used to swallow. The chewing force measured by using a small evaluate food texture. Wilkinson et al.5) have pointed pressure transducer embedded in an artificial molar was out the necessity of analyzing the oral process to connect higher with rice cake than with any other food.1) both methods. Masticatory variables obtained from Numerous elderly people have suffocated while attempt- electromyography (EMG) of the masticatory muscles, ing to eat rice cake. Although cooked rice cake is easy to kinematic and/or force measurements, and sensory deform, it is tough to cut off completely. Standard rice assessment are strongly modified by the food tex- cake is made from glutinous rice containing scarcely any ture.2,6–10) Actual human measurements are superior in amylose, and exhibits a unique texture with extreme numerically expressing a difficulty in mastication, as an adhesiveness and toleration to elongation. Rice cake instrumental method alone cannot assess ease or products that are easy for the elderly to eat have recently difficulty, and quantitative treatment in a sensory been developed by the food industry. Some of those evaluation is sometimes difficult. We also introduced products for the elderly have been modified so that they in this study a swallowing measurement in addition to are softer in texture, are less adhesive, and can be cut EMG during chewing to quantify the whole eating with smaller deformation. Non-glutinous starch contain- properties of rice cake. The mastication analysis using ing some amylose forms a gel with poorer ability to EMGs has been well established,2,6–10) while swallowing adhere and stretch than that of glutinous rice. Substitut- that follows the mastication of solid food has not pre- ing non-glutinous rice flour for glutinous rice or adding viously been analyzed.

y To whom correspondence should be addressed. Tel: +81-29-838-8031; Fax: +81-29-838-7996; E-mail: kaoruk@affrc.go.jp Mastication Measurement of Rice Cake 359 Masticatory muscles can be classified into those for by a vibration sensor also provide the timing and jaw closing muscles, which elevate the mandible, and number of swallows.26,27) The swallowing monitor those for jaw opening which withdraw it.10) Both these system we used was composed of three pressure sensors muscles alternate to perform rhythmic chewing.10–15) placed on the larynx, a contact microphone attached to The masseter muscle, one of the jaw-closing muscles, the cricoid cartilage, and a pair of surface electrodes for works more when a hard food is chewed. The EMG EMG from the suprahyoid musculature.26,27) A similar amplitude from the masseter muscle therefore becomes system has recently been used to measure the flow greater and the level of muscle activity represents the properties of beer and other alcoholic beverages through degree of chewing difficulty.8–10) In contrast, the supra- the human throat (nodogoshi).29) Previous methods23–29) hyoid musculature, which includes the anterior belly of has been useful to detect the swallowing movement for the digastric, mylohyoid and geniohyoid muscles, liquid or semi-solid foods which can be swallowed early comprises jaw-opening muscles.10,15) When an adhesive without chewing. However, many solid foods involving food is chewed, the EMG activity recorded from the rice cake cannot be swallowed in their initial form, but suprahyoid musculature increases.10–14) The suprahyoid can be swallowed after appropriate chewing by the teeth. musculature also elevates the hyoid at the early stage of No measurements have been reported of natural swal- swallowing.10,15) lowing that follows the mastication sequence. Swallowing characteristics can be simply divided into A small piece of rice cake has been recommended for three stages (oral, pharyngeal, and esophageal).15) The elderly people to prevent suffocation. Although this oral stage is the transfer of the food bolus from the recommendation seems quite reasonable, scientifically mouth to the oropharynx, and is considered the late stage quantified evidence has never been presented. This study of mastication. The pharyngeal stage is the highly was designed to objectively determine the effects of coordinated transport of the bolus away from the sample size and texture on mastication and swallowing oropharynx into the upper esophagus.15) Tongue move- by measurements on human subjects who were actually ments that initiate the act of swallowing require eating different-sized pieces of rice cake. We tested two concomitant contraction of the mylohyoid, geniohyoid, sizes of standard rice cake samples and a modified and digastric muscles. As pharyngeal swallowing be- product produced for elderly people with mastication gins, the anterior portion of the tongue is retracted and and swallowing difficulties. depressed. Retraction of the tongue and its elevation against the hard palate force the bolus into the upper part Materials and Methods of the pharynx. The tongue then moves posteriorly to drive the bolus into the pharynx, while the entire larynx Samples. Rice cake (KirimochiÔ, Sato Food Co., is pulled upward and forward. This action causes the Ltd., Niigata, Japan) as the standard and a modified epiglottis at the back of the tongue to depress and protect product for the elderly (Yawaraka FukumochiÔ, kindly the airway. The food bolus is then directed to either side donated by Kissei Pharmaceutical Co., Ltd., Nagano, of the epiglottis. The cricopharyngeal muscle relaxes Japan) were used. Table 1 presents the components of and the bolus is propelled into the esophagus. In the both samples. The standard type was cut into pieces esophagus, the bolus is carried toward the stomach by of 20 20 15 mm (9 g) and 20 20 5 mm (3 g). gravity and peristalsis. Therefore, the esophageal stage Yawaraka FukumochiÔ was provided in cylindrical is controlled reflexively and cannot be changed con- bars of 25 mm diameter and was cut into pieces 15 mm sciously. thick (9 g). Each piece was cooked for 75 s in boiling Videofluorography is commonly used to monitor the 6,9,16,17) whole swallowing movement. Ultrasonic tech- Table 1. Composition and Instrumentally Measured Texture of the niques have also recently been utilized mainly for Rice Cake Samples observing the oral stage.18–21) Manometers or other Yawaraka pressure sensors inserted into the esophagus can detect Composition Standard the swallowing characteristics during the esophageal FukumochiÔ stage.22) Those techniques, however, are difficult to Energy (kcal/100 g) 231 161 apply in food science because they require large and Water (g/100 g) 42.3 60 Protein (g/100 g) 4.2 2.6 expensive equipment and/or medical techniques; more- Lipid (g/100 g) 0.8 0.6 over, the first method employing x-rays is invasive. Carbohydrate (g/100 g) 51.8 36.3 We describe here non-invasive, safe, and inexpensive Two-bite texture profile analysisa p sensors for monitoring the swallowing characteristics of humans. Previous research has used a small pressure Firmness (kPa) 210 790 2 Energy for first bite (kJ/m3) 62:9 2:252:8 1:1 sensor attached over the thyroid cartilage to monitor Adhesiveness (kJ/m3) 17:1 0:517:9 0:7 ns the movement of the larynx during the swallowing of Cohesiveness 0:660 0:009 0:523 0:001 liquid.23–27) Another technique to detect swallowing aMean and standard error values of 6 replicates. activity of the submental or suprahyoid muscles uses ns, not significant; (p < 0:01)or(p < 0:001), differed significantly surface electrodes.24–28) Swallowing sounds picked up from standard by t-test. 360 K. KOHYAMA et al. water, and then cooled in a covered plastic Petri dish for small amount of water. Signals were output through an 15 min at room temperature (22 C). MEG-6108 amplifier with three AP-610J units (Nihon Kohden, Tokyo, Japan), and converted to kPa units after Instrumental measurements. A two-bite test of each calibration with known loads applied by a rheometer sample (9 g) was performed by using a food rheometer (RE-33005 as already described). (RE-33005, Yamaden, Tokyo, Japan) with a cylindrical A contact microphone (JM-0116, Ono-Sokki, Tokyo, plunger (5 mm diameter).14) The speed was 1.0 mm/s, Japan) was attached to the skin surface with double- and the final sample deformation was set at 80% of the sided adhesive tape beside the cricoid cartilage.26,27) The original height. Six measurements were taken for each swallowing sound was detected by the microphone sample. The firmness determined by the first peak height connected to an amplifier (SP-2200, Ono-Sokki; 0 dB in the load-time curve, the area under the first com- Gain). pressive peak, adhesiveness (the work required to pull the probe from the sample), and cohesiveness (the ratio Data analysis. The seven signals (three EMGs, three of the second compressive peak to the first) were pressures, and sound) were stored in a WindowsÒ calculated.2,14) Firmness is expressed as the stress after computer by using the MP150 program (Biopac the load value had been divided by the plunger cross- Systems, Goleta, CA, USA) at 1000 Hz for further sectional area, and the work values (first peak area and analysis with wave-analysis software (AcqKnowledgeÒ, adhesiveness) were determined per unit volume. ver. 3.5.7, Biopac Systems).14) The signals from both masseter muscles appeared Subjects. Eleven volunteers (mean age of 26.7 years), almost simultaneously as the subjects closed their jaws who were free from functional mastication problems and and freely changed chewing sides during mastication; required no dental treatment, participated in the masti- therefore, the masseter EMG data from the left and right cation and swallowing measurements. Large inter-sub- sides were averaged. From these masseter EMGs, i) the ject variation was reported and we intended to test the number of chewing strokes, ii) mastication time, and iii) mouthful size effect.14,30,31) To reduce individual dif- mean chewing cycle time were derived.13,14) For each ferences in masticatory variables caused by the size of chewing cycle, iv) the amplitude or the maximum each subject’s mouth, we selected females aged 20 to 42 voltage, v) burst duration, and vi) muscle activity years with a height of 152 to 166 cm and a body mass estimated as the time-integral of the EMG voltages index of 18 to 23. They gave their informed consent were read. They were averaged for all the chewing prior to the experiment. cycles.14) The muscle activities were summed for total muscle activity, and we defined the value as the Masticatory EMG and swallowing measurements. mastication effort for both the jaw-closing and jaw- Three rice cake samples with two replicates were ran- opening muscles.14,31) domly served to the subjects. Each sample was cooked During the swallowing periods, nine characteristic individually 15 to 16 min before consumption. The sub- time points were read from the suprahyoid musculature jects took a sample into the mouth with chopsticks by activity, pressure curves, and swallowing sound.26,27) themselves, and started to eat it in a normal way Figure 1 indicates, via p1 and p9, the starting and end according to instruction from an experimenter. They points of suprahyoid EMG activity, p4 and p5, the peaks freely rinsed their mouths with water before or between for the upper pressure sensor, p3 and p6, those for the the trials. The recording session lasted for about 30 min. middle sensor, p2, the starting time to decrease the The EMG activities were recorded from both the left pressure of the lower sensor, p7, the end time to increase and right masseter muscles, and the suprahyoid muscu- the pressure of the lower sensor, and p8, the starting lature with bipolar surface electrodes (NE-155A, Nihon time of the swallowing sound. The period between pi Kohden, Tokyo, Japan) and an MEG-6108 amplifier and pj is labeled tij. For example, the duration of the with three AB-610J units (Nihon Kohden, Tokyo, suprahyoid musculature was calculated as p9-p1; there- Japan), as previously reported.13,14) All the subjects fore, we labeled it t19. We used seven parameters (t12, were right-handed, so a ground electrode was placed on t23, t34, t67, t28, t19, and t27). We also picked up the the left wrist. amplitude and muscle activity of the suprahyoid The larynx movement during swallowing was moni- musculature, amplitude of the sound, and pressure tored by an array of pressure sensors.26,27) Three small differences at the first peak for the upper and middle pressure sensors (PS-2KA, Kyowa Dengyo, Tokyo, sensors. Japan) with a diameter of 6 mm were arranged in a row 8 mm apart on polyethylene foam (30 27 5 mm Statistics. Statistical analyses were performed with thickness). A belt (400 27 mm width) was used to SPSS (ver. 14.0J for WindowsÒ) software (SPSS, put the sensors around the subject’s neck with the center Chicago, IL, USA) with the statistical significance set of the lowest sensor placed on the thyroid cartilage of at p < 0:05. A t-test was conducted on the mechanical each subject.26,27) The position and tightness of each parameters, and the Wilcoxon signed-rank test was sensor were determined while the subject swallowed a applied on human recording variables. Mastication Measurement of Rice Cake 361

t19

Time

p1 p9

t27

t12 t23 t34 t67

p5 p2 p3 p4 p6 p7 Lower

Upper

Pressure sensor output EMG signal Middle

t28

p8 Microphone output

Fig. 1. Schematic Drawing of Swallowing Data. From the top: EMG activity from the suprahyoid musculature; pressure output from the upper, middle, and lower sensors; and swallowing sound from a contact microphone. Characteristic points (p1–p9) for the swallowing parameters are given.

the curves cross corresponded to 54% of the initial 5 height. Table 1 presents the results of the texture profile 4 analysis. The samples differed significantly in all textural parameters except for their adhesiveness. 3 We adopted a compressive strain of 80%, which was 2 greater than that in previous studies using the two-bite texture analysis for rice cakes (50%32) and 67%12)); 1 however, neither sample was torn until 80% strain. The Load (N) firmness was equal to the mechanical stress at 80%. 0 Considering that the deformation applied by a human 01020304050 –1 bite would exceed the normal deformation used in the mechanical tests, the chewing force required for the –2 standard rice cake was far higher than that for the Time (s) modified product. Mechanical stress at higher strain is likely to be more important in determining a textural Fig. 2. Texture Profile Curves for the Rice Cake Samples. assessment than that at lower strain; therefore, the stress Each initial sample was 15 mm thick. It was cooked for 75 s and cooled for 15 min at room temperature. The sample was bitten twice under a high compressive strain accurately expresses the by a plunger of 5 mm diameter at a constant speed of 1 mm/s. The mechanical properties of food in chewing. Starch gels black line represents the standard rice cake, and the grey line made from glutinous starch have exhibited similar represents Yawaraka FukumochiÔ for elderly people. mechanical characteristics, regardless of species. Wheat noodles made from glutinous varieties displayed lower Results and Discussion mechanical stress than those from normal varieties up to a deformation of 50%, and exhibited greater stress at a Mechanical properties of rice cake samples strain of 80% or more.33) Standard rice cake made from Figure 2 dipicts the two-bite curves for both samples. glutinous rice not containing amylose was easy to As the plunger speed was constant (1 mm/s), the time deform to 50% of its initial thickness, but difficult to cut axis could be interpreted as the deformation within each off completely. The modified product made from non- direction and bite. Yawaraka FukumochiÔ was firmer glutinous rice reduced the stress at large deformation, than standard rice cake at the early stage of compression thereby decreasing the chewing difficulty. (i.e. small deformation), both the curves crossed, and We also noted the curve shape in the minus-load then the opposite was observed when the compressive region. Although the adhesiveness values (i.e. upper deformation became larger. On average, the point where area of the curve and below 0 in load) were similar and 362 K. KOHYAMA et al.

Cycle time Muscle activity (area) 1mV Duration

RM EMG signal Amplitude SH

24 68 Time from beginning chewing (s)

Fig. 3. Typical EMG Signals during the Mastication of a Rice Cake. The sample was 9 g of the standard rice cake. From the top: left and right masseter muscles (LM, RM) and suprahyoid musculature (SH). Five chewing strokes in the early stage of mastication. A subject took a piece of rice cake and kept it in the mouth. The subject then started chewing the sample at time zero. Both sides of masseters and suprahyoid alternately worked to close and open the mandible.

Table 2. Results of Mastication Measurements for the Rice Cakes

EMG parameter Standard (9 g) Standard (3 g) Yawaraka FukumochiÔ (9 g) Number of chewing strokes 49:0 3:4 32.8 2.9 33.1 3.5 Mastication time (s) 37:4 3:4 24.3 3.1 23.0 2.8 Chewing cycle (s) 0:754 0:039 0.717 ns 0.046 0.677 0.042 Masseter muscles Amplitude (mV) 1:144 0:167 0.983 0.139 0.896 0.148 Duration (s) 0:232 0:013 0.214 ns 0.015 0.215 0.012 Muscle activity (mV s) 0:0297 0:0050 0.0247 0.0039 0.0231 0.0036 Total muscle activity (mV s) 1:475 0:292 0.836 0.172 0.774 0.149 Suprahyoid musculature Amplitude (mV) 0:561 0:048 0.535 ns 0.061 0.512 0.056 Duration (s) 0:436 0:053 0.451 ns 0.036 0.409 ns 0.032 Muscle activity (mV s) 0:0453 0:0071 0.0445 ns 0.0099 0.0398 ns 0.0083 Total muscle activity (mV s) 2:137 0:401 1.502 0.414 1.358 0.344

Mean and standard error values of 11 subjects. ns, not significant; (p < 0:05)or(p < 0:01), differed significantly from Standard (9 g) by Wilcoxon signed-ranks test. the minimum loads occurred during the first tensile texture-modified rice cake (Yawaraka FukumochiÔ). stage, the plunger separated earlier in Yawaraka When the sample amount was decreased to one third FukumochiÔ than in the standard rice cake, which (from 9 g to 3 g), the total muscle activities were de- adhered for a longer time (Fig. 2). This result suggests creased by 44% for the masseters and by 30% for the that the modified product was easier to remove from the suprahyoid musculature. The activity for the 3 g samples oral organs. was evidently far larger than a third of that for 9 g. Eating with a reduced mouthful increased the mastica- Mastication parameters tion effort per unit food weight, as has been reported We evaluated the mastication difficulties of rice cake previously.14,31) The smaller size could effectively samples by electromyography (Fig. 3). When the size decrease the chewing effort per piece; however, the of the rice cake piece was decreased from 9 g to 3 g, larger piece and fewer mouthfuls reduced the total the number of chewing strokes, mastication time, and muscle activity per sample weight. amplitude of the masseter muscles became significantly The total muscle activities were reduced by 48% smaller (Table 2). The result was significantly decreased for the masseter muscles and 37% for the suprahyoid total mastication activity for both the jaw-closing and musculature with Yawaraka FukumochiÔ, compared jaw-opening muscles. The chewing-cycle time, duration with those for the standard rice cake of the same of contraction expressed as the duration of the masseter quantity. This modified product had an alternative or muscles, and amplitude of the jaw-opening muscles greater effect compared with the 3 g rice cake in were similar for both pieces. Those parameters, all reducing the mastication effort without decreasing the influenced by texture, were significantly smaller in the mouthful amount. Mastication Measurement of Rice Cake 363

1 Swallowing parameters We also measured swallowing by following the 0.5 chewing sequence from the suprahyoid EMG activity, larynx movement, and swallowing sound. Figure 4 presents an example of the recorded data. Before 0 swallowing, the electrodes on the suprahyoid musculature, the upper two pressure sensors, and the micro- SH EMG(mV) -0.5 phone detected no signal; but the lowest pressure sensor exhibited positive pressure because it was pressed by -1 the mound of the thyroid cartilage. When swallowing 12 Upper started, the suprahyoid musculature began to work, 10 Middle perhaps to move the bolus from the oral cavity to the Lower 8 oropharynx with the tongue. The output of the lower 6 pressure sensor then decreased, followed by the peak of 4 the middle sensor, and then that of the upper one. This 2 sequence corresponded to the elevation of the larynx. Pressure (kPa) 0 After a short signal of the swallowing sound had been -2 observed, the EMG activities diminished, followed by 1 the second peaks from the upper and middle pressure 0.8 0.6 sensors; ﬁnally, the pressure of the lower sensor 0.4 increased again to the initial level. This observation 0.2 was similar to previous ﬁndings for male subjects 0 26,27) -0.2 swallowing rice gruel without chewing. The output -0.4 patterns of the EMG and the early part (<1 s) of the -0.6 lower pressure sensor were similar to those of submental

Swallowing sound (V) Swallowing -0.8 -1 EMG and laryngeal movement detected by a different 25) 0 0.5 1 1.5 2 type of sensor while the subject was drinking liquid. Time (s) Our observation method detected different swallowing patterns while the subject was drinking a beverage from Fig. 4. Typical Swallowing Recordings for a Rice Cake. a beer mug, when the larynx moved up and down From top to bottom: EMG activity from suprahyoid muscles (SH); repeatedly.29) Such continuous drinking has also been pressure output from the upper, middle, and lower sensors; and observed by EMG measurement.28) swallowing sound. The subject swallowed a bolus of the standard rice cake (9 g) after freely chewing. Table 3 presents the results for 11 subjects. No significant differences were caused by piece size or texture. However, the swallowing sound and peak pressures for the upper and middle sensors tended to The substituted product contained more water (60%) be greater for both 9 g samples than for the 3 g standard than the standard rice cake (42%) (Table 1). Katsuta et sample. al. have commented that a greater amount of water was As measured by the instrumental methods (Fig. 2 required for making rice starch gel (dango) from non- and Table 1), the two samples differed significantly, glutinious starch flour than from glutinious flour.34) but the difference in texture decreased during mastica- Substitution by non-glutinious rice increased the water tion.10,13,14) The bolus texture sometimes differs for level, and the energy per weight of product became various types of food.35) The two rice cake samples used low (Table 1). However, the mastication effort per unit in this study were close in texture; therefore, the bolus energy (i.e. based on a similar solid content) as properties would become similar at the beginning of previously discussed for cooked rice14) was still lower swallowing, since chewing modified the initial texture, in the modified product. as shown for cooked rice samples.13,14) We did not detect In Japanese daily life, the rice cake is sometimes any significant difference between the two sample sizes substituted for cooked rice as a staple food with meals; of rice cake in any swallowing variables. This result may sometimes it is served as a snack between meals. If the have been due to the small size of the boli. Neither the rice cake is assumed to be essential as a staple food that 9 g nor 3 g sample was difficult to swallow after enough must be eaten in a large amount with each meal for chews by healthy subjects. Previous reports on rice gruel energy intake, textural modification of rice cake would have also stated that similar measured periods were not help provide enough energy without greatly reducing the influenced by the properties of boli after elevation of the mouthful size. If rice cake is considered a non-essential larynx began, as swallowing is a reflex response.26,27) confectionery or snack, the nutritional value may be less Another possibility is that it may be derived from the important; thus, serving it in small pieces may be present trend without any statistical significance of the appropriate for the elderly. swallowing sound and larynx movement upward of the 364 K. KOHYAMA et al. Table 3. Results of Swallowing Measurements for the Rice Cakes

EMG parameter Standard (9 g) Standard (3 g) Yawaraka FukumochiÔ (9 g) Suprahyoid musculature Amplitude (mV) 0:573 0:068 0.584 ns 0.083 0.579 ns 0.084 Duration, t19 (s) 0:810 0:087 0.880 ns 0.093 0.765 ns 0.084 Muscle activity (mV s) 0:0640 0:0112 0.0730 ns 0.0142 0.0569 ns 0.0149 Swallowing sound (V) 0:546 0:208 0.331 ns 0.104 0.500 ns 0.170 Peak pressure (kPa) Upper sensor 13:03 3:38 10.74 ns 3.76 13.12 ns 3.36 Middle sensor 9:22 2:52 7.28 ns 2.81 9.87 ns 2.36 Swallowing period (s) t12 0:190 0:039 0.317 ns 0.093 0.202 ns 0.037 t23 0:376 0:051 0.346 ns 0.047 0.336 ns 0.040 t34 0:055 0:025 0.047 ns 0.027 0.075 ns 0.024 t67 0:344 0:051 0.310 ns 0.070 0.251 ns 0.047 t28 0:394 0:057 0.350 ns 0.052 0.357 ns 0.033 t27 0:992 0:063 0.964 ns 0.073 0.924 ns 0.082

Mean and standard error values of 11 subjects. No significant differences between 3 g and 9 g standard samples, and between Yawaraka FukumochiÔ and standard (each of 9 g). thyroid cartilage; a greater bolus may induce a greater observed in the swallowing parameters. Swallowing was swallowing sound and stronger larynx movement. likely to have been induced when the bolus properties The relationship between the bolus nature and became suitable for swallowing as healthy subjects swallowing characteristics of healthy humans has not could adjust their mastication method to the food yet been clarified. Modified rice gruel for dysphagic amount and texture. patients exhibited smaller swallowing EMG activities and shorter duration of the suprahyoid masculature Acknowledgments with a faster rise of the larynx at the beginning of swallowing than with standard gruel, since it was easier This work was supported in part by the Program for to swallow.27) As the subjects swallowed rice gruel Promotion of Basic Research Activities for Innovative without chewing, a clear textural effect was observed. Biosciences (PROBRAIN) and by the program for Normal subjects displayed greater amplitude and dura- Development of Evaluation and Management Methods tion of submental EMGs when they swallowed a larger for Supply of Safe, Reliable and Functional Food and volume of water; however, the differences between 3 ml Farm Produce from the Ministry of Agriculture, Forestry and 10 ml were small.25) In contrast, videofluoroscopic and Fisheries of Japan. 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