Participants
A total of 32 Korean women between 65 and 85 years of age at the Seoul Welfare Center were included in this study. Participant recruitment was conducted between December 2017 and February 2018. The recruitment involved healthy older volunteers. Of these older people, those who were capable of independent activities of daily living and following instructions were included in this study.
All the procedures were conducted at the School of Hanyang University in Korea. Participants were informed about the aims, methods, and safety issues of the test, and all of them provided their written consent before participation in this study. This study was approved by the Ethics Committee of Hanyang University (HYI-17–198-4).
Participants visited Hanyang University once, and demographic questions concerning their sex and age were obtained. Participants provided self-assessed responses concerning their ability of independent living and experience of any dental problems; these responses contributed to the selection process. Subsequently, the older participants were classified into two groups according to their dental status: those with natural dentition (not partial denture wearers, n = 24) and those wearing a full denture (n = 8).
None of them had any experience with EMG or IOPI experimentations; thus, detailed instructions were provided. Participants had to attend a single 1-h session, and each experiment lasted 1 h.
Test food
The test food was provided by Pulmuone (Seoul, Korea). The radishes were washed with water to remove impurities and then cut into uniform cube-shaped pieces (1.5 × 1.5 × 1.5 cm) to avoid cooking-related differences; the thick peel was not used. Subsequently, the cubes were blanched in boiling water for different durations (0, 3, 4.5, 7, 9, or 15 min). Next, the cubes were mixed with salt and other ingredients. The sample Kimchi consisted of 90% radish, 3% onion, 2% red chili pepper powder, 1.5% minced garlic, 0.6% salt, 0.5% anchovy sauce, 1.0% salted shrimp, 0.3% minced ginger, and 1.1% white sugar.
The radish Kimchi (Kkakdugi) was made with raw white radishes. Subsequently, the samples were packed into a jar after thorough mixing to obtain a homogeneous sample.
For statistical analysis, a total of six kinds of test food samples were prepared, including hard radish Kimchi as the control and various types of texture-modified cooked radish Kimchi, which were softened by blanching.
Instrumental texture measurement
Puncture tests were performed using a texture analyzer (TAXT plus, Stable Micro Systems, Surrey, UK) to assess the textural characteristics. In this test, the diced radish cubes (1.5 × 1.5 × 1.5 cm) were tested in the texture analyzer with a 100-N load cell to assess their hardness (in N). For the puncture test, a probe rod (5 mm in diameter) was pressed into each sample at a speed of 100 mm/min, and the peak force was assessed [23]. All texture analyses were performed at room temperature with two replicates.
EMG measurements
EMG was performed to assess masticatory properties, and the participants were asked to sit upright in a chair to eat the test food. After carefully cleaning the skin surface, pairs of surface electrodes (T246H, Bioprotech, Daejeon, Korea) were attached to the skin on the left and right masseter muscles and temporalis muscles, and the electrodes of a 4-channel EMG device (LXM 5308, Laxtha, Daejeon, Korea) were also attached over these muscles. Subsequently, the participants were asked to chew and comfortably swallow food as usual. They were required to have a minimum of two chewing cycles and be free of interruptions, such as talking or spitting out the food. For testing, refrigerated samples (4 °C) were dispensed into a plastic dish with six wells and served after 5 min at room temperature. Each participant was presented with food samples varying in hardness—softest (blanched for 15 min) to hardest (no blanching)—in a plastic dish. The six samples of different hardness levels were presented to participants one at a time in the six-well dishes. Subsequently, the individuals chewed and swallowed all Kimchi samples (levels 6 to 1); they took a brief rest to reduce fatigue between the duplicate measurements. By placing sensors on their faces, muscle activity during jaw opening and closing were measured; thus, chewing patterns could be captured during food consumption.
The EMG recordings were performed from the time of ingestion of food until deglutition of the bolus.
The data were classified based on each test food and were analyzed with scripts developed using TeleScan version 2.99 (Laxtha, Daejeon, Korea). For statistical analysis, the total number of chews, the sum of chewing cycles to finish the test food, and the chewing time were monitored [16]. When participants finished eating and orally processing the stimulus, they raised their hands to indicate the end point of eating.
In this way, the total eating time was self-reported and included multiple swallows and the oral clearance time needed to remove residues or oral coatings from the tongue surface.
The chewing time was defined as the duration of chewing of a test food from ingestion until completion of oral clearance [13].
The electromyographic potentials of the left and right masseter and temporalis muscles were recorded, and the mean EMG amplitude was calculated. The EMG voltages across each electrode were averaged.
Peak-to-peak amplitudes, durations, and muscle activities (time-integrated EMG voltages) were calculated for each muscle according to its actions [11].
IOPI tongue pressure measurements
The participants’ tongue pressure and strength were measured using the IOPI (TPS 100, Cybermedic, Gwangju, Korea). Especially, tongue-palate compression based on IOPI values was assessed.
Participants were asked to sit in the most relaxed posture, with their head upright and eyes focused on a target at the horizontal level. They were required to press an air balloon probe between the tongue and hard palate as hard as they could by closing their mouth to reach peak values. Individuals were then asked to press the balloon probe using the maximum effort for several seconds until the sound signal stopped. Brief rests between the triplicate measurements were offered to reduce fatigue [25].
For statistical analysis, the generated tongue pressure was recorded in kilopascal (kPa), and tongue-palate compression was determined as a function of the maximum isometric tongue pressure (MITP) generation capacity using the IOPI [26]. Moreover, the mean peak values for the three records were defined as the average pressure level (APL).
Preference test and hardness sensitivity
To assess differences in the preference of the food samples, a questionnaire suitable for older was designed. The preference for cooked radish Kimchi with various blanching times was investigated to develop elderly-friendly food. For the preference test, each participant self-reported the choices of their favorite test samples based on overall acceptability.
Then, the hardness sensitivity of the participants was evaluated by asking them to assess food as “hard” or “soft.” Participants also rated the intensity of “hard” and “soft” using a 9-point category scale (1 = extremely soft, 2 = very soft, 3 = soft, 4 = slightly soft, 5 = moderately, 6 = slightly hard, 7 = hard, 8 = very hard, and 9 = extremely hard).
Statistical analysis
SPSS software (IBM SPSS Statistics for Windows, version 20.0; IBM Corp., Armonk, NY, USA) was used for the statistical analyses. A p-value of < 0.05 was considered statistically significant.
One-way analysis of variance (ANOVA) was used to compare EMG activity based on changes in radish Kimchi hardness. Data were analyzed by one-way ANOVA, followed by Bonferroni correction. One-way ANOVA was applied for the analysis of each test food.
Comparisons between the two groups (older-individuals with natural teeth and those with full dentures) were performed using Bonferroni correction. In addition, masseter muscle activities and tongue pressure were compared between the two groups (older individuals with natural teeth and those with full dentures) using t-tests.
Pearson correlations were conducted to explore inter-relationships for masticatory parameters. Thereafter, multiple regression analysis was performed to assess the influences of subject score on masticatory performance.