A Non-invasive Measurement of Tongue Surface Temperature

35 Oral temperature, tongue specifically, is a key factor affecting oral sensation and 36 perception of food flavour and texture. It is therefore very important to know how the 37 tongue temperature is affected by food consumption. Unfortunately, traditional 38 methods such as clinical thermometers and thermocouples for oral temperature 39 measurement are not most applicable during food oral consumption due to its invasive 40 nature and interference with food. In this study, infrared thermal (IRT) imager was 41 investigated for its feasibility for the measurement of tongue surface temperature. The 42 IRT technique was firstly calibrated using a digital thermometer (DT). The technique 43 was then used to measure tongue surface temperature after tongue was stimulated by 44 (1) water rinsing at different temperatures (0-45 ℃ ); and (2) treated with capsaicin 45 solutions (5, 10, and 20 ppm). For both cases, tongue surface temperature showed 46 significant changes as a result of the physical and chemical stimulation. Results 47 confirm that IRT is feasible for tongue temperature measurement and could be a 48 useful supporting tool in future for the study of food oral processing and sensory 49 perception. 50 51 52


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Temperature is no doubt a very important factor for food oral processing and its 62 sensory perception because of two known reasons. Firstly, temperature affects 63 material properties of food and therefore its oral behaviour. For example, increasing 64 temperature will normally reduce the viscosity of a fluid food or hardness/firmness of but an enhanced burning sensation at a higher serving temperature (60°C) ( surface. Conventional methods are also not applicable when the temperature of 105 materials is not in equilibrium (such as during food oral processing). Moreover, these 106 probes will interfere with food movement during oral processing and cause 107 uncomfortable feelings. The infrared thermal (IRT) imager has been developed as a 108 fast, non-contact and non-invasive technology for surface temperature measurement, 109 including body surface (Ring, 2007). By using IRT, temperature can be easily  Tongue surface temperature can be affected by two very different food stimulation: the temperature of the food (physical stimulation) or the chemical compounds of food capsaicin made no significant impact on the orthonasal aroma delivery, but it caused a 139 significant reduction in retronasal aroma release and enhanced saliva production (date 140 to be published separately Missouri, USA) was prepared by dissolving 1 g in 100 mL of 95% food grade alcohol.

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The final stimulating solutions were diluted from the stock, which consisted of 5, 10 172 or 20 ppm of capsaicin. About 1 mL food-grade alcohol was dissolved in 500 mL 173 drinkable water as the control solution (0 ppm). These four solutions were kept in 174 34 °C (the normal tongue temperature at rest) water bath prior to experiments.    All tests were performed in a laboratory designated specifically for human studies.

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The laboratory entry was so designed to prevent any external disturbance and 208 interference. Room temperature (20 ± 1°C) was controlled by air conditioning and the 209 humidity (50 ± 10 %) was controlled by a humidifier. Proper and stable lighting was 210 maintained by fluorescent lighting and no direct ventilation was allowed in the lab.

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During tests, a jaw-shaped metal support was placed in front of the subject so that the 212 subject could sit in a relaxed position with jaw comfortably resting on a sponge 213 padding. A visible mark was made on the sponge so that subjects can place the jaw 214 precisely on the spot. IRT was placed ahead of the support and the lens was set with a 215 distance of 0.20 m in the direction perpendicular to tongue surface. Therefore, an 216 angle of 40 -45° between lens and sponge padding was maintained (Fig. 2). The 217 focus was manually adjusted to ensure high quality imaging. All participants were postgraduate students recruited from the campus and consents 223 were obtained before performing tests. All subjects were non-smokers, not suffering 224 from any illness or discomfort and were not on long-term medication. Prior to 225 temperature measurements, subjects were asked to refrain from intense exercise, 226 caffeine and alcohol for at least 1 h and were asked to relax on a comfortable chair in    Ten subjects (age: 24.6 ± 1.6 yrs, F = 5, M = 5, BMI: 21.3 ± 2.2 kg/m 2 ) were involved 263 in thermal treatments of tongue on four consecutive days. On the first day, ice-cold bottled water was applied and the rest of bottled water (at different temperatures) was performed one-by-one on following days in an increasing order of temperature.

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Therefore all subjects went through all four temperatures within the four consecutive 267 days. All subjects orally took in a mouthful thermally pre-equilibrated bottled water 268 without swallowing and held the liquid in the mouth for 10 sec before expectoration.

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With a stopwatch for timing, this process was repeated for 3 to 5 min until a total of 270 550 mL of the water was used up. Then, subjects were immediately asked to place jaw 271 to the designated position shown in Figure 2 and stretch out their tongues for 60 sec.

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Thermal images were taken every 15 sec by the investigator.   Integrating the information of all temperature measurements by IRT from ten 318 subjects, it was found that the mean temperature of tongue surface was 34.14°C    Above results show that water rinsing can indeed alter the temperature at tongue 376 surface. We tend to believe that the range of temperature change was so large and 377 significant that its effects on tongue's sensory functionality (e.g the perception of 378 tastes, discrimination of texture, and etc) cannot be ignored.      Even though IRT offers accurate and reliable temperature measurement, the technique 438 was unable to give continuous imaging. Images can only be taken at an interval of at 439 least 15 sec. Also, images were taken manually by an operator, which will inevitably 440 involve some small variations in shooting time (and then the temperature). A video 441 recording IRT will be needed for continuous monitoring of temperature change.  It should also be mentioned that this study did not perform a screening to volunteers 452 for their sensitivity to capsaicin. Participated subjects were students coming from 453 different provinces or regions in China, with possibly very different food culture and 454 very different previous food exposure. It could be reasonable to assume that regional 455 and diet differences of subjects might give difference responses to capsaicin, but this 456 has not been taken into consideration in the analysis.     humidity of the laboratory is kept within a comfortable limit. The lens of IRT is positioned as 608 perpendicularly to tongue surface to minimize geometrical errors. The distance and angle of lens is 609 fixed so that approximately the same number of pixels is covered each time during experiments. 610 The sketch was produce to illustration purpose and was not to the proportion. 611 612  The temperature and humidity of the laboratory is kept within a comfortable limit. The lens of IRT is positioned as perpendicularly to tongue surface to minimize geometrical errors. The distance and angle of lens is fixed so that approximately the same number of pixels is covered each time during experiments. The sketch was produce to illustration purpose and was not to the proportion.     Temperature variation was given in negative values, which was the measured temperature compared to that measured immediately after capsaicin treatment.