Video-endoscopic comparison of swallowing waxy rice mochi and waxy wheat mochi: improvement of a traditional Japanese food that presents a choking hazard

Abstract

Mochi is highly cohesive and adhesive, and easy to choke on. Many of the fatal suffocation accidents with mochi occur in the elderly aged 65 years or older. These circumstances prompted us to investigate a special property of waxy wheat which is similar in texture to waxy rice, but is less cohesive and adhesive. We compared the differences in chewing and swallowing movements associated with eating waxy rice mochi and waxy wheat mochi between healthy adults and healthy elderly. Healthy elderly chewed mochi more and longer than healthy adults. Although there was no difference in the number of chewing cycles or total duration of chewing between the two types of mochi, waxy wheat mochi was easier to chew and left less pharyngeal residue. These findings lead us to suggest that waxy wheat mochi is promising as a food that is easy to swallow and difficult to choke on.

Graphical Abstract

This is the image which observed a chewing state of the Waxy wheat mochi by video-endoscopy. A green coloring agent was added to one set of samples to yield green and white mochi, allowing easy identification by video-endoscopy. Each bolus of waxy wheat and waxy rice mochi consisted of both white and green mochi.

Key words:

• waxy wheat
• swallowing
• video-endoscopy
• suffocation
• mochi

Mochi is a traditional Japanese food and an important festive feature during the New Year’s holiday, especially among the elderly. It is made by steaming waxy rice, then applying external force and mixing to increase its viscosity. It is highly cohesive and adhesive, and easy to choke on.

Among accidental deaths in Japan, choking ranks as the number one cause with more than 4000 cases a year.¹⁾ Among the approximately 800 cases that are attributed to food, mochi ranks the highest at 20%, and the rate is particularly high among elderly aged 65 years and older.²⁾ Approximately, half of the elderly population experience reduced activities of daily living (ADL) and/or swallowing function, and approximately one-fifth have some type of disorder that affects daily living.³⁾ Although they may not be afflicted by a specific disease, aging reduces their swallowing function.⁴⁻⁶⁾ It is, therefore, important to ensure that the elderly can eat food safely in countries with aging populations.

Aging in eastern Asian societies is pronounced. China’s niángāo, Taiwan’s môa-chî, and Korea’s tteok are foods that are all made from rice, as is mochi, and thus present a choking hazard. We investigated how to produce mochi that is difficult to choke on, and focused on the properties of waxy wheat for this purpose.^7,8) Rice normally comprises about 20% amylase and wheat comprises about 30%. However, there is no amylase in waxy wheat starch, and just like waxy rice, waxy wheat consists mostly of amylopectin and thus has a similar texture to waxy rice. Unlike waxy rice, waxy wheat is less cohesive and adhesive, given fusion between the wheat starch and wheat proteins.

We have previously conducted a sensory evaluation study with healthy individuals who ate mochi made from waxy rice (i.e. waxy rice mochi) or waxy wheat (waxy wheat mochi).⁹⁾ Almost all participants indicated that waxy wheat mochi was easier to chew and swallow. We also conducted a video-endoscopic study involving another set of healthy individuals that compared differences in chewing and swallowing waxy rice and waxy wheat mochi. Waxy wheat mochi was sufficiently chewed before being swallowed, but waxy rice mochi was not. However, there has been no detailed study on chewing and swallowing that targets elderly individuals. This present study determined by a sensory evaluation and video-endoscopy whether there are differences in chewing and swallowing waxy rice and waxy wheat mochi in healthy adults and healthy elderly.

Materials and methods

Subjects

The participants were 15 healthy adults aged ≤ 64 years (4 men and 11 women; mean age of 46.3 ± 11.2 years) and 8 healthy elderly aged ≥65 years (1 man and 7 women; mean age of 71.5 ± 2.3 years) who did not have any complaints of eating disorders or dysphagia, or of any disease that would reduce the eating or swallowing function. All participants were dentate and had balanced occlusion. None of the participants used dentures.

Materials

Waxy rice and waxy wheat were used as previously described.⁷⁾ Water was added to the powdered samples and mixed for 15 min with a Ladies kneader (Taisho Electric Mfg Co.), before treating in a pressure cooker to completely gelatinize the starch and molding into a round shape. Each test sample had a diameter of 23 mm, a thickness of ≤6 mm, and weight of 2.5 g. A green coloring agent was added to one set of the samples to yield green and white mochi, allowing easy identification by video-endoscopy. Each bolus of waxy wheat and waxy rice mochi consisted of both white and green mochi (Fig. 1).

Fig. 1. Test food samples.

Note: (A) Two-colored molded waxy rice mochi. (B) Two-colored molded waxy wheat mochi. Each test food sample had a diameter of 23 mm, thickness of ≤12 mm, and weight of 5 g.

Methods

A FNL-10RBS fiberscope (Pentax) and BS-LL1 LED light source (Pentax) were used for video-endoscopy. Videos taken during the examination were saved to an HXR-MC1 digital video camera (Sony) in mpeg-2 format at a frame rate of 29.97 fps.

Video-endoscopy was performed by a dentist specializing in dysphagia evaluation. With the endoscope inserted nasally to the pharynx, the participants ate sample of each waxy rice and waxy wheat mochi twice randomly. The number of chewing cycles and duration of chewing were recorded, and the chewing and swallowing movements were observed. Chewing was evaluated on the 4-point scale shown in Fig. 2, as described by Sasao et al.¹⁰⁾ with grindability defined as the extent of disintegration of the test sample, aggregation defined as the extent to which the bolus held together as a lump, and mixing as the extent to which the two colors mixed. With respect to swallowing, we evaluated the position of the leading edge of the bolus when the swallowing reflex was triggered, the presence of a pharyngeal residue after swallowing and aspiration. The position of the leading edge of the bolus was evaluated according to the method previously described by Hiiemae et al.¹¹⁾ with some modifications,¹²⁾ and was classified into three different areas: oral cavity or epipharyngeal area (OCE), vallecular area (VAL), and hypopharyngeal area (HYP) (Fig. 3). Based on the FEES clinical evaluation standard,¹³⁾ we quantitatively evaluated the presence of a pharyngeal residue after swallowing, as well as the aspiration and penetration.

Fig. 2. Example of grindability, aggregation, and mixing evaluated by video-endoscopy.

Fig. 3. Positions of the leading edge of the bolus upon the swallowing reflex.

Note: OCE, oral cavity area or epipharyngeal area. VAL, vallecular area. HYP, hypopharyngeal area.

Ethical considerations

This study was approved by the ethics committee of Nihon University School of Dentistry (Study of the influence that food characteristics have on swallowing movement, 2010–2012). All participants received an explanation of the objective and methods of the study and provided written consent for participation.

Statistical analysis

Test videos were analyzed by Premiere Pro 2.0 (Adobe), and results were analyzed by Statistics 17.0 (SPSS). p < 0.05 was considered significant.

Results

The mean number of chewing cycles for waxy rice mochi was 19.1 ± 7.5 for a mean total duration of 15.4 ± 5.4 with the healthy adults, and 37.7 ± 15.9 cycles for 27.3 ± 8.9 with the healthy elderly. The mean number of chewing cycles for waxy wheat mochi was 21.0 ± 9.6 for 15.5 ± 6.7 with the healthy adults, and 38.3 ± 19.7 cycles for 25.4 ± 11.1 with the healthy elderly. While there was no significant difference between the number of chewing cycles and total duration of chewing waxy rice or waxy wheat mochi in both groups, the healthy elderly chewed waxy rice and waxy wheat mochi significantly more times and for significantly longer than the healthy adults (Table 1).

Table 1. Chewing cycles (number and duration).

		Time					Chewing cycles (times)
		Waxy rice	Waxy wheat	Z score	r	p	Waxy rice	Waxy wheat	Z score	r	p
Healthy adults (n = 15)	Mean	19.1	21.0	.72	.19	.470	15.4	15.5	.03	.01	.977
	SD	7.5	9.6				5.4	6.7
	Max	33	38				25	30
	Min	4	3				5	4
Healthy elderly (n = 8)	Mean	37.7	38.3	.28	.10	.779	27.3	25.4	1.33	.47	.183
	SD	15.9	19.7				8.9	11.1
	Max	75	83				42	43
	Min	20	17				14	13
Z score		3.04	2.29				2.75	2.39
r		.65	.48				.57	.50
p		.001^**	.019^*				.004^**	.016^*

Note: Mann–Whitney U test.

^* p < 0.05.

^** p < 0.01.

The leading edge of the waxy rice mochi bolus upon triggering of the swallowing reflex was in OCE for 9/15 healthy adults and 1/8 healthy elderly, while the leading edge was in VAL for the remaining participants. The leading edge of the waxy wheat mochi bolus was in OCE for 2/15 healthy adults, and in VAL for the remaining participants. None of the participants had the swallowing reflex trigger when the leading edge was in HYP. A comparison of the position of the leading edge revealed that, while waxy wheat mochi was found significantly deeper in the airway than waxy rice mochi in the healthy adults, there was no significant difference in the position of the leading edge of the bolus between waxy rice and waxy wheat mochi in the healthy elderly (Table 2).

Table 2. Position of the leading edge of the bolus.

	Healthy adults waxy rice–waxy wheat^a	Healthy elderly Waxy rice–waxy wheat^a	Adults-elderly waxy rice mochi^b	Adults-elderly waxy wheat mochi^b
<VAL・ >VAL	0.04^*	1	0.07	0.53
<HYP・ >HYP^c	－	－	－	－

^a McNemar’s test.

^b Fisher’s exact test.

^c Comparisons were not performed because swallowing had never occurred when the leading edge of bolus was at HYP.

^* p < 0.05.

The grindability, aggregation, and mixing of waxy rice mochi are shown in Table 3. The average levels of grindability, aggregation, and mixing of waxy rice mochi were 1.83 ± 0.69, 2.50 ± 0.50, and 2.00 ± 0.58 in the healthy adults, and 1.71 ± 0.45, 2.86 ± 0.35, and 1.29 ± 0.45 in the healthy elderly, based on the 4-point scale shown in Fig. 2. The average levels of grindability, aggregation, and mixing of waxy wheat mochi were 2.62 ± 0.49, 2.62 ± 0.49, and 2.62 ± 0.49 in the healthy adults, and 2.88 ± 0.33, 2.88 ± 0.33, and 2.34 ± 0.48 in the healthy elderly. Grindability was significantly greater with waxy wheat mochi than with waxy rice mochi for both the healthy adults and healthy elderly. The aggregation values were high for both types of mochi, no significant difference being apparent between them. The mixing values were also high for both waxy rice and waxy wheat mochi with the healthy adults, there being no significant difference between the two. However, waxy wheat mochi showed significantly higher values for mixing with the healthy elderly.

Table 3. Grindability, aggregation, and mixing.

		Grindability					Aggregation					Mixing
		Waxy rice	Waxy wheat	Z score	r	p	Waxy rice	Waxy wheat	Z score	r	p	Waxy rice	Waxy wheat	Z score	r	p
Healthy adults (n = 6)	mean	1.83	2.62	2.18	.50	.046*	2.50	2.62	.46	.11	.701	2.00	2.62	1.97	.45	.087
	SD	.69	.49				.50	.49				.58	.49
Healthy elderly (n = 8)	mean	1.71	2.88	3.23	.83	.001**	2.86	2.88	.01	.03	.955	1.29	2.34	2.88	.74	.006**
	SD	.45	.33				.35	.33				.45	.48
Z score		.25	1.25				1.34	1.25				1.99	1.05
r		.07	.27				.37	.27				.47	.23
p		.836	.336				.295	.336				.073	.374

Note: Mann–Whitney U test.

^* p < 0.05.

^** p < 0.01.

A pharyngeal residue was present only with waxy rice mochi. It was found in a small amount in the healthy adults (1/15) and in the healthy elderly (4/8). There was only a significant difference between the presence of a pharyngeal residue with waxy rice and waxy wheat mochi in the healthy elderly (Table 4). None of the participants showed signs of aspiration or penetration.

Table 4. Pharyngeal residue.

	Waxy rice mochi		Waxy wheat mochi		Z score	r	p
	Present	Absent	Present	Absent
Healthy adults	1	14	0	15	1.00	.27	.317
Healthy elderly	4	4	0	8	2.00	.71	.046^*
Z score	2.35		0
r	.49		.00
p	.101		1.000

Note: Mann–Whitney U test.

^* p < 0.05.

Discussion

Mochi as a food product

Similar to wheat and corn, rice is a member of the Poaceae family, and is the staple food in Asian countries such as Japan and Africa. It is often added as an accompaniment to main dishes in Europe and America, but is also used in such main dishes as paella and risotto. It is a food product eaten throughout the world, and even served as a dessert in south-eastern Asia. Waxy rice is cultivated in Japan, the Korean peninsula, China, the Philippines, Thailand, Laos, Indonesia, India, Vietnam, and Myanmar; and in Laos in particular, it is a staple product. Mochi not only serves as an event food in Japan, the process of making mochi itself is an annual event and religious ritual, and it has been an important food product since the Tumulus period (sixth century).

Mochi is made by first washing waxy rice with water and then soaking it in water. Once the water has been drained, it is steamed to make steamed rice. This steamed rice is then kneaded until it gelatinizes. This process endows mochi with its unique cohesive and adhesive properties. However, these properties contribute to choking. When considered by age, those who do not eat mocha, as assessed by a questionnaire survey, were 7.2% in their 20s, 6.0% in their 30s, 2.4% in their 60s, 1.8% in their 70s, and 2.3% in their 80s. This suggests that most people do not avoid eating mochi.¹⁴⁾ However, the large number of reports of aspiration resulting from mochi^2,15−18) highlight it as a dangerous food product for the elderly. It is important for this reason to provide the elderly with mochi that is easy to chew and difficult to choke on.

Mastication

When comparing healthy adults with healthy elderly, there was no difference in the number of chewing cycles or total duration of chewing between waxy rice and waxy wheat mochi. In other words, there was no difference in the chewing action for both types of mochi. However, the findings of greater grindability and mixing of waxy wheat mochi by video-endoscopy suggest that it is easy to grind and can be mixed in a short amount of time. Moreover, given that the adhesiveness of waxy rice mochi decreases due to the increased water content in the bolus contributed by water and saliva,¹⁹⁾ it is likely that saliva would work more rapidly on waxy wheat mochi which is easier to grind. The number of chewing cycles and total duration of chewing was significantly greater in the healthy elderly than in the healthy adults. This may reflect decrease in the saliva content,²⁰⁾ tongue action, and chewing function.²¹⁾

Swallowing movement

When a liquid is swallowed all at once, a bolus is formed in the oral cavity, and a swallowing reflex is triggered before the bolus reaches the pharynx. However, when eating solids, the bolus mixes with saliva during chewing and is actively delivered to the oropharynx by the tongue, followed by swallowing. This is called stage 2 transport, and is a movement that occurs with high probability during chewing.²²⁻²⁵⁾ Since the main cause of aspiration is a delayed swallowing reflex,²⁶⁾ triggering the swallowing reflex early in stage 2 transport may present a method to prevent aspiration and choking.

With respect to the bolus position, there was a significant difference between waxy rice and waxy wheat mochi in the number of healthy adults who had the swallowing reflex trigger when the bolus was in OCE (9/15 for waxy rice mochi and 2/15 for waxy wheat mochi). In contrast, no significant difference was apparent in the healthy elderly (1/8 for waxy rice mochi).

In a study on healthy adults, we have previously reported that, compared to waxy wheat mochi, the leading edge of the bolus of waxy rice mochi was found in OCE when the swallowing reflex was triggered, and that reflex was triggered sooner with waxy rice mochi.⁹⁾ Similar results were found with healthy adults in the present study, which is likely to reflect the fact that the cohesiveness and adhesiveness of waxy rice mochi allows it to travel from the oral cavity to the pharynx in a lump. Moreover, although there was no difference in the swallowing movement between waxy rice and waxy wheat mochi among the healthy elderly, there was a difference among the healthy adults. Stage 2 transport is normally induced with solids. However, stage 2 transport was not induced in the healthy adults when eating waxy rice mochi, whereas it was in the healthy elderly. Healthy adults might be able to change how to eat waxy rice mochi and waxy wheat mochi, but healthy elderly might not be able to do this. This difference may explain why many elderly choke on waxy rice mochi.

The swallowing reflex in this study occurred for almost all the healthy elderly when the bolus was in VAL. This is consistent with reports that, with aging, sensations in the oral cavity and pharynx decline, resulting in an increased threshold for the swallowing reflex.²⁷⁻²⁹⁾ Thus, the bolus enters the pharynx before the swallowing reflex is triggered for elderly people, leading to aspiration and choking.

While there were no cases of a pharyngeal residue with waxy wheat mochi, one healthy adult and 4 healthy elderly had a small amount of pharyngeal residue with waxy rice mochi. Although a significant difference was only apparent among the healthy elderly, this nonetheless suggests that the high cohesiveness and adhesiveness of waxy rice mochi made it easier to leave a pharyngeal residue.

Significance of the video-endoscopic evaluation

We visually observed the grindability, aggregation, and mixing before swallowing, as described by Sasao et al.¹⁰⁾ A bolus can be observed prior to swallowing by video-endoscopy or by a video-fluoroscopic examination of swallowing. While the latter allows one to observe the entire process from chewing to swallowing, the need to mix a contrasting agent with food changes the physical properties of the food sample. In contrast, video-endoscopy allows the test food itself to be used as the food sample, and is thus suitable for observing the grindability and mixing of the bolus. We used video-endoscopy to observe the chewing of waxy rice and waxy wheat mochi for this reason. However, while there have been reports on the video-endoscopic evaluation of liquids and solids,^10,13,30) very little has been reported on the use of this method with Eastern foods and popular regional foods, believe that video-endoscopy will allow for new discoveries in future studies of the chewing and swallowing of food products considered to be dangerous, or of products with unique physical properties.

Waxy wheat mochi as substitute for mochi

We performed in previous studies^31,32) a sensory evaluation of waxy rice mochi and waxy wheat mochi with 20 healthy adults and 34 healthy elderly people. There was no difference in taste. Moreover, the low adhesiveness and cohesiveness of waxy wheat mochi was evaluated as being tasty and easy to swallow. Waxy wheat mochi was therefore considered to be a safe food without sacrificing the taste of mochi.

Study limitations

None of the participants showed signs of aspiration or penetration in this study. This may reflect the possibility that the adult and elderly participants were all healthy, and could swallow even when the shape of the bolus was not optimal. Although we found that waxy wheat mochi was safer than waxy rice mochi for the elderly who did not show any signs of marked functional decline, we cannot draw the same conclusion for the elderly with a declined eating or swallowing function or for patients with eating disorders or dysphagia. Future studies will need to assess the specific characteristics of patients for whom waxy wheat mochi can be safely eaten.

Conclusion

Our findings suggest that waxy wheat mochi was easier to chew and less likely to leave a pharyngeal residue than waxy rice mochi. Given that people in East Asian countries tend to favor cohesive foods, it is easy to envisage that the risk of choking will rise with the aging of populations.

Mochi made from waxy wheat was less cohesive than that made from waxy rice, was easier to eat, and will have likely appeal to the elderly who do not have markedly declined physical functions. Waxy wheat mochi holds promise as a food product that is easy to swallow and difficult to choke on.

Funding

This work was supported by a research grant from the Graduate School of Dentistry at Nihon University.