Effects of texture properties of semi-solid food on the sensory test for pharyngeal swallowing effort in the elderly

Background: Increasing viscosity can reduce the risk of aspiration into the airway, but conversely, very thick solid food may require greater strength and effort. We assumed that semi-solid foods with similar viscosities will behave differently in the oropharynx and there might exist the possibility that properties other than viscosity may have clinical relevance. This study aimed to nd out the texture of semi-solid foods that affects the effort of pharyngeal swallow in the elderly. Methods: Nine kinds of semi-solid foods not requiring mastication were selected for texture prole analysis (TPA), and included whipped cream, mayonnaise, soft tofu, mango pudding, boiled mashed pumpkin, boiled mashed potatoes, boiled mashed sweet potatoes, red bean paste, and peanut butter. Hardness, adhesiveness and cohesiveness of each food were measured three times by using the rheometer. A blinded sensory test using a 9-point hedonic scale was also conducted in eighteen elderly people to investigate how much effort was required to swallow food, and how much of the food remained in the pharynx after swallowing. The correlation between texture and sensory outcome was statistically analyzed. Results: Foods that belonged to the same viscosity category showed different texture values, and the participants also rated different scores respectively. Only adhesiveness among three properties was signicantly correlated with the sensory test. (r= 0.882, p=0.002 for dicult to swallow, r=0.879, p=0.002 for sense of residue). Conclusions: Adhesiveness was the most important property of the semi-solid foods, requiring most efforts in pharyngeal swallow in the elderly. If we select and provide food having low adhesiveness value in the same viscosity category, there might be the to make it easier to swallow in older adults.

Hardness, adhesiveness and cohesiveness of each food were measured three times by using the rheometer. A blinded sensory test using a 9-point hedonic scale was also conducted in eighteen elderly people to investigate how much effort was required to swallow food, and how much of the food remained in the pharynx after swallowing. The correlation between texture and sensory outcome was statistically analyzed.
Results: Foods that belonged to the same viscosity category showed different texture values, and the participants also rated different scores respectively. Only adhesiveness among three properties was signi cantly correlated with the sensory test. (r= 0.882, p=0.002 for di cult to swallow, r=0.879, p=0.002 for sense of residue).
Conclusions: Adhesiveness was the most important property of the semi-solid foods, requiring most efforts in pharyngeal swallow in the elderly. If we select and provide food having low adhesiveness value in the same viscosity category, there might be the possibility to make it easier to swallow in older adults.

Background
Oropharyngeal dysphagia is characterized by di culty in safely transferring a liquid or food bolus from the mouth to the esophagus. It can lead to aspiration, dehydration, malnutrition, pneumonia and even death. [1,2] This condition often occurs in patients with neurologic damage due to stroke or progressive neurologic diseases, including Parkinson's disease [3] or cancers of the head and neck. [4]{Garcia, 2010 #1} Since the incidence of diseases associated with dysphagia increases with age, dysphagia is more common in the elderly. [5] The use of texture-modi ed foods and thickened liquids has become a cornerstone of clinical practice to address dysphagia. [6][7][8] In the case of liquids, it is widely accepted that thin liquids (such as water) pose safety challenges for people with dysphagia because they ow quickly. [6,9] Thickened liquids are frequently recommended with the goal of slowing down their ow to allow more time for airway closure. [6,10] Conversely, very thick liquids and solid food materials may require greater strength in terms of the tongue and pharyngeal propulsive forces. If a person has reduced tongue or reduced pharyngeal muscle strength, there is a risk for residues to remain behind in the recesses of the pharynx after swallowing. [11,12] However, we have experienced the effect of semi-solid foods with similar viscosity on swallowing is not the same. Mostly, it requires a lot of effort, but sometimes it is easy and safe for example, peanut butter vs. soft tofu. Which factors make this difference? We assumed that food texture (hardness, adhesiveness or cohesiveness), other than viscosity, might have clinical relevance. Hardness is the force that is required to compress food between the tongue and palate to a given deformation or to penetration. Adhesiveness is de ned that the work required to remove food that adheres to the mouth (generally the palate) during the normal swallowing process and cohesiveness means the strength of internal bonds making up the body of the food. [13] It was especially thought to affect older adults or dysphagia patients than healthy adults. Therefore, this study was aimed to nd out the texture of semi-solid food affects the effort of pharyngeal swallow in the elderly.

Test foods selection
Nine kinds of semi-solid foods with similar viscosities (fork test [14] grade 1, IDDSI [15] level 4 category and British Dietetic Association [16] Texture C) not requiring mastication were chosen for this experiment. They included whipped cream, mayonnaise, soft tofu, mango pudding, boiled mashed pumpkin, boiled mashed potatoes, boiled mashed sweet potatoes, red bean paste, and peanut butter. They were all commercially available at the market as nished products.
Texture pro le analysis (TPA) We measured the hardness, adhesiveness and cohesiveness of each food three times immediately before performing sensory tests by using a CT3 texture analyzer (AMETEK Brook eld, MA, USA) (central temperature 20±0.2℃, stain 70%, probe diameter 20mm, in ltration meter 10.5mm, test speed 10mm/s).
The texture pro le parameters (Fig 1) were determined as follows: (1) hardness was de ned as the maximum force required for compressing foods and was calculated as the peak force of the rst compression (P1) (2) adhesiveness was calculated as the negative area (B1) for the rst bite, representing the work necessary to pull the compressing plunger away from the sample (3) cohesiveness was calculated as A2/A1 (A1 and A2 represent the integrated energy required for the rst and second compression, respectively).

Subjects
Eighteen healthy elderly volunteers (two males and sixteen females), with an average age of 72.5±6.9 years (range, 65-83), were recruited from the community through poster advertisements. They did not have any history of stroke or other brain dysfunction, head and neck cancer or anatomic alterations in that area, speech or swallowing di culties. Additionally, they were not taking any medication that could affect their swallowing function. This study was approved by the University Hospital Institutional Review Board and informed consent was obtained from every participant.

Test procedure
All participants were invited to the test room next to the TPA room and received a 10 minute educational presentation of the sensory test before they participated in the experiments. Immediately after TPA, foods were provided in a random order and blinded manner, and they were asked to swallow 5 grams of each test food at once without mastication. They rated each score about; 1) how much effort was required to swallow food, and 2) how much of the food remained in the pharynx after swallowing using a 9-point hedonic scale (1 = very less, 9 = very much).

Statistical analysis
Statistical analyses were performed by using SPSS version 12.0 (SPSS, Inc., Chicago, IL, USA). The average value of hardness, adhesiveness and cohesiveness of each test food was obtained. Moreover, the average score of 18 elderly people for two sensory tests was calculated. The Pearson correlation coe cient was used to evaluate the correlation between TPA and the sensory tests. The signi cance level was set at p<0.05.

Results
The average values of hardness, adhesiveness and cohesiveness for each food were as shown in Table  1. Foods that belonged to the same viscosity category showed different texture values. Whipped cream had low values of hardness and adhesiveness, and red bean paste showed high values in both. Boiled mashed pumpkin, potato and sweet potato were not high in hardness but in adhesiveness. Cohesiveness did not show distinctive features between foods. The scores of sensory tests were as shown in Table 1. The participants rated different scores respectively. Whipped cream was the easiest to swallow and mango pudding had the least feeling of sticking to the throat. Peanut butter was the hardest to swallow and had greatest sticky feeling.
In the correlation test between TPA and sensory tests, only adhesiveness among three properties was signi cantly correlated with both sensory tests. (r= 0.882, p=0.002 for di cult to swallow, r=0.879, p=0.002 for sense of residue). (Fig.2)

Discussion
It was found that among the three texture properties, adhesiveness was associated with the sensation of swallowing di culty in the elderly. Adhesiveness is de ned as the work necessary to overcome the attractive forces between the surface of the food and that of other materials with which the food comes into contact and calculated by measuring the negative area for the rst bite, representing the work necessary to pull compressing probe away from food. [13] This de nition might explain why adhesiveness is related to the di culty of swallowing and sense of residue in the elderly.
To date, studies related to dysphagia diet have been conducted in terms of viscosity, [6,10] and accordingly, classi cation of dysphagia diet has been mostly based on viscosity. [15,17] Although viscosity is important, at the highest viscosity level (solid foods), further classi cation is primarily related to chewing (minced, grinded or soft etc.) and less related to pharyngeal swallowing. We assumed that other texture properties besides viscosity might be associated with pharyngeal swallowing di culties in the elderly. We also found out the relevant possibility of adhesiveness.
Only one study mentioned the relation between food texture and dysphagia. [18] They investigated the association between the texture of semi-solid foods and beroptic endoscopic swallowing study ndings in post-stroke dysphagic patients and suggested signi cant relation with cohesiveness according to residue deposition and gumminess according to aspiration. However, they had limitation about the use the different viscous foods restricted to stroke dysphagic patients and there was also a lack of explanation about the reason of their relations.
Motor function related to swallowing becomes increasingly dampened with age. Age-related anatomical changes in swallowing function include a smaller cross-sectional area of masticatory muscles (masseter and medial pterygoid), increased lingual atrophy and fatty in ltration and decreased lingual muscle ber diameter. [19] Age-related diminishment in strength, mobility and endurance is also evident in the tongue, and pharyngeal muscle. [20,21] For this reason, food texture affects swallowing in older people more.
Major limitation of this study is that sensory test is not su cient to evaluate the effect of food texture properties on the swallowing performance. It might be a bit controversial to compare the TPA results with the sensorial test performed which is based on subjections. It would be better if more accurate functional data of swallowing physiology such as video uoroscopic swallowing study were added. We have tried to reduce the likelihood of confounding factors. We only used foods that were commercially available at the market as nished products. We performed sensory tests immediately after the TPA test. Food was provided in a random and blinded order, and the participants swallowed food without chewing.
Nevertheless, the problems of food homogeneity, mixing with saliva and temperature changes can make another limitation of this study.

Conclusions
Adhesiveness was the most important property of semi-solid food requiring most efforts in pharyngeal swallow in the elderly. If we select and provide the food having low adhesiveness value in the same viscosity category, there might be a possibility to make it easier to swallow in older adults.

Declarations
Ethics approval and consent to participate