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A soft machine system (texture analyzer equipped with an artificial tongue) was developed to analyze the occurrence of fractures in soft food gels compressed between the tongue and hard palate. Artificial tongues were newly designed with a modulus similar to that of a somewhat tense human tongue, using transparent urethane gels. A model of soft foods that are easy or difficult to crush with the tongue was prepared using gellan gums at different concentrations. A piece of gellan gum gel was uniaxially compressed on an artificial tongue with the flat probe of a texture analyzer. The food gel and artificial tongue were deformed, leading to the fracturing of food gels alone. During compression-induced fracture, the softer food gels exhibited a lower force, lower displacement, and lower true stress than the harder gels on all artificial tongues. The fracture probability of gellan gels was lower for the combination of harder food gels and softer artificial tongues, as softer materials can deform to a larger extent. Fracture displacement of food gels was increased as the modulus of the artificial tongue decreased; however, the fracture force did not differ significantly. These results suggest that an artificial tongue with elastic modulus values similar to those of the actual human tongue is suitable for determining the fracture behavior of foods during compression between the tongue and hard palate.

The structure of soft foods with similar fracture forces may be destroyed by compression between the tongue and palate or mastication during natural oral processing. Our objective was to identify the mechanical properties determining the oral strategy for the initial size reduction of soft gels. Three gellan gels with an equivalent fracture force and different fracture strains were evaluated by instrumental compression testing and sensory evaluation. In addition, a combination of a gellan gel and a sonagel, which has an apparent elastic modulus similar to that of the human tongue, was subjected to a compression test and ultrasound (US) elastography. The fracture profile of the gellan gel on the sonagel in the instrumental compression test corresponded to the oral strategy. The strain ratio of the gellan gel to the sonagel observed in US elastography had a high linear correlation with the reciprocal of the apparent elastic modulus of the gellan gel. The oral strategy of gels changes from tongue–palate compression to mastication with increasing apparent elastic modulus or decreasing the relative strain against the sonagel. The results reveal that the apparent elastic modulus influences the oral strategy for the natural consumption of gels although a threshold value was not clearly determined in conventional compression testing using hard materials. Practical Applications An appropriate care food that can be consumed using compression between the tongue and hard palate without chewing is increasingly required in an aging society. Instrumental methods are often used for texture characterization of such foods. Probes are usually made of metal or materials that are much stiffer than foods and completely dissimilar to the tongue. Ultrasound (US) elastography can detect the strain ratio between food gels and a sonagel, which is used as an artificial tongue in this study. We suggest that the strain ratio obtained from US elastography can be used to evaluate the deformation behavior of soft foods and a soft material whose mechanical characteristics are similar to those of the human tongue. As the strain ratio was highly correlated with the reciprocal of the apparent elastic modulus measured using conventional compression testing, the modulus was an important parameter for selecting the tongue compression.

The objective of this study was to quantify natural eating behavior of soft solid foods and identify a mechanical property of food that has the strongest influence on mastication in humans. Electromyography (EMG) of both masseter and suprahyoid muscles was performed to quantify the muscle effort required to naturally masticate five gellan gels. The gels were categorized into two groups; those with (1) similar fracture strain and three different fracture loads (F1

I learned about food texture by reading many articles in the Journal of Texture Studies (JTS) and Prof. Bourne's textbook Food Texture and Viscosity, Concept and Measurement. I present here his great contributions as both the Editor-in-Chief of JTS and as the author of a superior textbook in the cultivation of future food texture researchers. Prof. Bourne has provided significant guidance to those in the industry using his textbook to suggest appropriate approaches to measure, evaluate, and alter the textures of agro-products and processed foods. I began my career in food rheology, learned the physiological techniques necessary for measuring mastication, and developed relationships with good sensory scientists along the way. I am happy to offer advice to agriculture and food scientists, researchers, and engineers in the food industry regarding food texture. I would like to be a follower of Prof. Bourne and support young food texture scientists in the 21st century. PRACTICAL APPLICATIONS Food texture studies conducted during the last century are beginning to evolve. Based on previous publications in the Journal of Texture Studies, I believe that several areas can be further examined in this century. First, methods to evaluate the textures of heterogeneous materials, such as fruits and vegetables, require improvements. Second, the monitoring of rapid changes in food texture during oral processing should be considered during the design of food products. Third, a new instrumental method that mimics the movement and breakdown of soft foods by the tongue is being developed. The optimization of food processing to reduce the loss of agro-products and production of high-quality foods both require more attention according to the Sustainable Development Goals adopted by the United Nations in 2015.

Food properties are perceived through oral receptors during eating. In this review, various oral receptors involved in food oral processing of food are briefly described. Touch and taste are the most important senses in oral perception. Different tactile receptors are used to determine the mechanical properties, size, shape and their temporal/spatial changes of food materials in the oral cavity. Instrumental test results are often inconsistent with sensory-perceived food properties. A multiple-point pressure sensor inserted into the mouth with food can be used to mimic the human tactile systems and to relate instrumental and sensory results. Three evaluation systems of food properties with artificial tongues which were designed in different concepts are presented. Physiological research results are useful to simulate oral processing to design texture, flavor and appearance of diets for people with different eating functions. Practical Applications Food oral processing is a dynamic process in which a mouthful of food is reduced in heterogeneity and converted into a more homogenized swallowable bolus. Human-mimetic sensing technologies have been recently developed to analyze the physical and chemical properties, these complex interactions and drastic changes of foodstuffs during oral processing. As practical applications, two current social topics are discussed in terms of homogenization of foods during oral processing. First, with the increase of elderly population, a need for texture-modified foods has emerged: hard food must be converted into paste-like, where the function of oral processing is partly done by pre-oral processing. Second, WASHOKU, the traditional dietary cultures of the Japanese, was added to UNESCO's Intangible Cultural Heritage list in 2013. Seasoning in the mouth, one of the features of oral processing of WASHOKU, is addressed. To improve food texture and flavor is needed for people with various ages and regions.

The objective of this study was to quantify the mastication effort for cooked rice with varying texture. We conducted electromyography (EMG) from both masseter muscles while normal subjects ate 6 g of cooked rice. Rice samples were prepared using different milling ratios and cooked with different amounts in weight of water. The texture of cooked rice samples was instrumentally analyzed. Hardness was similar between 2% milled and 1.80 times weight of water, 6% and 1.65, and 10% and 1.50; however, adhesiveness was lower in lesser milled samples. EMG results showed that the number of chews, mastication time and muscle activities per chew were higher in the harder rice samples that were milled less and cooked with less water. When the hardness of cooked rice was adjusted, EMG variables were not significantly different. The mastication effort per gram of solid content significantly differed with milling ratios. Practical Application Cooked rice is a staple food for many people. The texture of cooked rice can be easily modified by changing milling ratios and water-to-rice grain ratios. As mastication is the first process of digestion, understanding how milling and cooking conditions influence the texture of cooked rice and mastication behavior is important for palatability and nutrition. EMG variables can provide details on mastication effort, proving to be useful for preparing the best texture of cooked rice. As less milled rice requires more mastication per solid content, it is effective to increase number of chews and to decrease eating rate.

This study aimed to elucidate the effects of water volume on flow properties of bolus (passage time and velocities) at the esophagus orifice using ultrasonic pulsed wave Doppler method. Five healthy males and five females were asked to consume 3, 6, 9 and 12 mL water in a single swallow. An ultrasound probe was placed against each subject's neck to observe the flow properties of bolus. The velocity spectra of the bolus at the esophagus orifice were analyzed. Our results revealed that velocity of bolus was easily affected by volume of water, and the passage time and velocities increased as the increase in volume. Practical Applications Ultrasonography, using a medical ultrasound instrument, is an effective method for observing bolus kinematics during swallowing. The velocity spectra of bolus can be visualized for different volume of water using the pulsed wave Doppler method. When a bolus of food is swallowed, aspiration that the bolus partly enters the trachea instead of the esophagus may occur. Water is known to show a high possibility of aspiration. This study revealed that reducing volume of water decreases the passage time and velocities of bolus at the esophageal entrance. Therefore, this technique can be used for designing the texture of food materials for people who experience aspiration or suffocation by food.

We report on an extrusion test and its correlation with sensory perception. The extrusion test was implemented on a texture analyzer and applied to single-component or composite polysaccharide gels. The extrusion force did not depend strongly on the volumetric flow rate, indicating a predominantly elastic contribution to the force. The extrusion force correlated highly with sensory perception of the cutting effort and its ratio to the elastic modulus correlated highly with perception of the extensibility. A combination of the Young's modulus and the extrusion force correlated well with perception of firmness. The robust correlations depended weakly on the flow rate in extrusion, but were much higher for tapering ratios T ≥ 4.5 than for T = 2.5. Notably, the correlation obtained was higher than that with texture profile analysis parameters. We show the correlation of several empirical and fundamental large deformation tests with texture, and suggest that the higher correlation with extrusion reflects the closer mixed-mode deformation in mastication. Practical Applications The report highlights correlation of parameters from empirical and fundamental rheological tests with sensory evaluation. The forces generated in an empirical extrusion test emerge as appropriate and reliable descriptors of several sensory attributes. The approach outlined is appropriate in general for textural mapping of semisolid and solid foods with an apparent yield stress.

This study aimed to elucidate the effects of food bolus volume on tongue movement at the initiation of swallowing using ultrasonography. Four healthy women were asked to consume 3- and 6-mL spoon-served different food samples (hard gel, soft gel, sol and water) in one swallow each and in their natural manner of eating. An ultrasound probe was positioned under each person's chin to produce coronal images of the tongue. The up-down movement of the dorsal surface of the tongue was observed in the M-mode. The grooving depth for 6 mL of gels and water was greater than that for 3 mL. The duration of grooving for 6 mL of gels was longer than that for 3 mL. The velocity of descending and ascending grooving was not significantly different between the volumes for each food sample, and the velocity of descending grooving was greater than that of ascending grooving. Practical Applications Ultrasonography, using a medical ultrasound instrument, is an effective method for observing tongue movement during the oral phase of swallowing. The M-mode can visualize the kinematics of the dorsal surface of the tongue and bolus passing from the tongue to the pharynx at the initiation of swallowing. Different kinematics of tongue movement can be observed for different volumes of food bolus, mechanical properties and individuals. This technique can be used for designing the texture of food materials for the elderly and infants who experience aspiration or suffocation by food.

The cervical auscultation method was applied to investigate sounds generated while swallowing various foods with different consistencies, including water, yogurt, and konjac jelly. Dynamic viscoelasticity, viscosity, and compression or penetration force of yogurt and konjac jelly were measured. The yogurt was fractured by the tongue while swallowing, whereas konjac jelly was not. Significant differences were detected in the durations of the swallowing sound component, generated by food bolus flow through the pharynx. The root mean square amplitude of the component that was attributed to the water flow was significantly higher than that for the flow of other food samples. No significant differences were detected in the duration of sound components appearing before and after the bolus flow or in the total duration among the food samples. Scattered or heterogeneous food bolus flow appeared to significantly reflect the duration of swallowing sounds, generated by the food flow. PRACTICAL APPLICATIONS Nursing-care foods are in much demand in the expanding elderly society. Ease in swallowing is an important factor for such foods and it is strongly correlated to food texture. Our results indicated significant differences in the duration and amplitude of swallowing sounds, generated by food bolus flow through the pharynx. Because the swallowing sounds depend on the food texture, monitoring such sounds can aid in designing and evaluating the foods to ease swallowing. The method used in this study is noninvasive and cost effective and is therefore practical for use in the food industry. The dependence of the swallowing sounds on human subjects can be analyzed if the data from a greater number of subjects are gathered.

A total of 445 Japanese texture terms were classified. First, terms with almost the same meaning represented by different Chinese characters, and onomatopoeic words derived from the same sound were classified into same-term groups for a total of 271 term groups. Next, eight experts in texture studies categorized the 271 term groups into the three texture attributes proposed by the International Standard Organization (ISO 11036): mechanical attributes, geometrical attributes and other attributes. Term groups were compared within each of the three attributes, and their similarities were assessed by 96 texture scientists/technologists. The data were analyzed by multidimensional scaling and cluster analysis. The classification in this study showed some similarities with the classification of texture terms in other languages. However, the results also revealed some characteristics that were unique to Japanese texture vocabulary, such as a wide variety of terms concerned with stickiness and elasticity. Practical Applications Developing a comprehensive list of texture terms in every language is necessary for comparing terms between languages, as well as for sensory evaluation within each language. The data obtained in this study can be applied to preliminary lexicons of descriptive sensory evaluation of texture. Classification of texture terms helps sensory evaluation researchers to define descriptors and to train panelists. The data also provide useful clues for worldwide understanding of Japanese texture terms.

An in vitro evaluation system of food texture was developed using artificial tongue and a conventional uniaxial compression apparatus to mimic the tongue-palate compression. Deformation behavior of agar gels on artificial tongues from silicone rubber with three levels of elastic modulus was observed during compression by a nondeformable plate. The results were compared with the oral strategy for size reduction from sensory evaluation by human subjects. Agar gels fractured upon compression when the strain of the gels was larger than that of artificial tongue, whereas they did not when the strain of the gels was equivalent to or smaller than that of the artificial tongue. When apparent elastic modulus of artificial tongue was approximately 5.5 × 104 Pa, fracture profile of the gels corresponded well to human tests. Results suggest that the oral strategy for size reduction might be determined by sensing the relative strain of the gels to the tongue during compression to a somewhat small strain. Practical Applications In an aged society, there is an increasing demand for properly texturized foods for safe consumption by those who have mastication or chewing difficulty. Food of soft texture is often preferred by these people because of easy tongue-palate compression and no need of the teeth mastication. An in vitro evaluation system of food texture developed in the present study provides a simple method for food manufactures in assessing textural acceptability of products without using human subjects. This evaluation system can lead to the innovation of food product design for textural diversity and thus improved quality of life for these disadvantaged populations.

To determine an effective method to evaluate the texture of cucumbers, human-bite measurement using a multiple-point sheet sensor and instrumental tests using wedge probes with various angles and a cylindrical probe were conducted. To detect small differences in texture, cross-sectional slices taken from the middle part of a cucumber fruit were served at 4 or 22C. The 60°-wedge probe that could completely divide samples like human incisors performed best compared with sharper wedges or cylindrical probe that partly broke the tissue. The human-bite force for cucumber slices of 4C was significantly lower than that of 22C. Breaking force and energy with the wedges of 60 and 30° in the compression test and those for flesh puncture test demonstrated a similar temperature effect. A faster compression speed close to the human-bite speed (20 mm/s) was necessary to detect the significant temperature effects. Practical Applications The texture of cucumber was analyzed more precisely using a wedge probe with an angle of 60° inserted into sectional slices of the samples similar to the action of human front teeth that completely cut off the section. A cylindrical probe could insert only part of the cucumber tissues and a blade or a sharper wedge probe partially broke the slices to the depth which the probe reached. Faster test speed (>10 mm/s) close to the average speed of human bite is preferred to detect temperature effects on fracture force. If there are some differences in breaking properties of cucumbers, the proposed conditions of compression test with a 60° wedge at 20 mm/s will detect delicate differences as those due to serving temperature were small.

To relate to mechanical and sensory properties of soft gels, electromyography (EMG) of the suprahyoid musculature during palatal reduction (i.e., compression between the tongue and the hard palate) followed by normal swallowing was investigated. EMG was recorded using healthy adults as subjects to monitor the activities of the masseter muscle and the suprahyoid musculature during oral processing. Mechanical properties of gels were examined by compression tests at various strains and deformation rates. The duration of oral processing was prolonged and the EMG activity of the suprahyoid musculature increased with increasing concentration of gelling agents. The EMG activity of the suprahyoid musculature correlated well with the compression load of gels at extremely large strains (e.g., 90% strain) and with sensory perceived hardness. The EMG activity of the suprahyoid musculature is an effective parameter in analyzing human oral processing of soft gels and is deducible objectively by compression load at large strains. PRACTICAL APPLICATIONS Texture design of nursing-care foods is important challenges in the food industry, as there is an increasing demand with increasing number of people with difficulties in mastication and/or swallowing in this aged society. The electromyography (EMG) activity of the suprahyoid musculature can be used as an objective parameter to analyze eating behavior of soft gels that are broken down through palatal reduction without chewing before swallowing. Surface EMG provides food manufactures with a strategy for the texture design of nursing-care foods because soft gels are the base of these food products.

The effects of food thickness on human bite force were investigated using a multiple-point sheet sensor and electromyography. The total force and contact area were directly measured in real time using the sensor, and the average stress detected between the teeth and a food sample (active stress) was derived from the ratio of the force to the contact area. Raw carrots and fish gels with different thicknesses were bitten between the upper and lower incisors or molars by healthy women. For hard and brittle carrots, the peak force, contact area and active stress at the fracture point were greater in thicker samples when biting with incisors. For soft and tough gels, the peak force and contact area were increased as thickness increased; however, the active stress was similar in incisor bites. These values did not differ significantly with thickness in the molar bites of both samples. Although the force and contact area were greater for molar bites than incisor ones, the active stresses at fracture point were independent of tooth position. The sample thicknesses influenced the bite force of humans and the stress applied to the sample, and the effects differed with sample texture.

A spoon sensor to measure lip pressure when slurping liquid food was developed. Several sheet-type sensors of 3 mm diameter were aligned along the back-center line of the bowl of a spoon, at intervals of 5 mm and the lip pressure was evaluated while average adults ingested five samples of semi-liquid foods with varying viscosities. Samples with greater viscosity required correspondingly greater lip pressure and maximum force, as well as a greater impulse or time integral of the force detected by a single sensor. The sum of the force, the impulse, duration and velocity using pressure data from a sequence of four sensors were calculated. Thicker samples required greater force and impulse, and longer duration, but lower velocity. The test subjects slurped semi-liquid foods differently, adapting to viscosity. Mechanoreceptors in the lips may play an important role in the discrimination of food viscosity.

The chewing patterns measured with jaw kinematics and electromyography (EMG) of ten young adults and ten healthy elderly subjects chewing six food products (rice, beef, cheese, crispy bread, apple, and peanut) were compared. The chewing sequence was divided into five periods by number of chewing cycles, each corresponding to 20% of the entire chewing sequence. Elderly subjects exhibited lower EMG amplitudes and longer jaw-closing duration than younger subjects, but the maximal vertical and lateral displacements of the jaw were not significantly different at any period of mastication. EMG amplitudes, muscle activities during the jaw-closing phase, duration of contraction, and vertical jaw movements decreased in the mastication process. Muscle activities during occlusion and inter-burst duration increased in the later period. Food properties modified EMG activities and jaw-kinematics more significantly in the earlier stage of mastication, but the food effects continued until the latest stage. Generally, the effects of the mastication period and the food type on the masticatory variables except maximal EMG amplitude were similar for both ages. The amplitude decreased significantly while mastication proceeded for young subjects but stayed unchanged for elderly. This suggests that the elderly found it more difficult to adapt their chewing force to changing the food texture during in mouth degradation.

The effects of food texture on the chewing patterns of elderly and young people, masticatory recordings using electromyography (EMG) were carried out. Fourteen French adults (mean 29.4 years) and 23 elderly (mean 67.7 years) participated. Six food products (rice, beef, cheese, crispy bread, apple and peanuts) were tested. The chewing pattern of elderly subjects was characterized by a significant increase of number of chews and chewing duration for all foods except rice. Whatever the food type, muscle activity per chew (mean amplitude × burst duration) was lower for elderly than for young subjects. Single chews appeared less effective for food reduction in elderly than in young subjects. This can be partly compensated for by increasing the chewing duration and number of chews. No significant difference was found between both groups of subject for the total amount of EMG activity required prior to swallow whatever the food chewed.

Electromyograms (EMGs) of mastication of kelp snack were compared with the physical properties measured by a tensile test and sensory assessment. The number of chewing strokes, masticatory time, total muscle activities and total duration for whole mastication agreed with the breaking force in the tensile test and with sensory difficulty of mastication. Amplitude, EMG activity, and duration per chew also showed the same tendency. In the masticatory cycle, significant differences among the samples were not observed. As mastication proceeded, the amplitude and EMG activity decreased, and the EMG-duration shortened. The EMG activity, amplitude, and masticatory rhythm changed significantly, and sample differences decreased from the middle to the late stage of mastication. Just before swallowing, the chewing cycle was lengthened, and no significant difference was observed among samples at the last chew.

The texture of six varieties of cooked rice with different amylose contents was measured with a Texturometer. Masticatory behavior of the rice was evaluated using electromyography (EMG) of masticatory muscles. Differences in mastication patterns among the seven subjects examined were greater than between rice varieties. Masticatory behavior was more related to the adhesiveness and stickiness of rice as measured using a Texturometer than to the hardness. The number of chews and masticatory time, total duration of mastication and total muscle activities were highly correlated with each other, and were greater in cooked rice with a high amylose content, which showed low adhesiveness and stickiness values using a Texturometer. Cooked rice with a high amylose content was masticated with high masseter muscle activities. The ratio of jaw-opening muscle activity to the preceding jaw-closing muscle activity was low in high-amylose varieties, which corresponds to the ratio of stickiness to hardness using a Texturometer. Between-sample differences in mastication parameters were more apparent in the early stage of mastication, and no significant difference was observed in the late stage.

A multiple-point sheet sensor was used to observe the distribution of surface pressure during the biting of crackers adjusted to various water activities. The textural characteristics were clearly reflected in the three-dimensional displays so obtained. Textural differences between samples were most evident in the first 0.3 s. The magnitude, position and duration of biting pressure varied among subjects. For all subjects, crisp crackers with low water activities showed a large pressure distribution with respect to both time and position. Samples with high water activities displayed a higher maximum force on the area under the incisors than samples with low water activities, and required more work per incision. These results are opposite to the rheometric data measured with a single-point sensor. Using this method, we could directly display textural differences due to the mechanical and geometrical properties of food, and could evaluate their changes in the oral cavity during biting.