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`
`3001
`
`Electronic Tongues–A Review
`
`Yusuke Tahara and Kiyoshi Toko
`
`Abstract— Sensing technologies for objective evaluation such
`as the discrimination and quantification of tastes have been
`developed since around 1990, before the discovery of
`taste
`receptors. Electronic tongues aim to discriminate and analyze
`foods and beverages and are well known as sensing technologies
`that greatly contribute to quality management. A taste sensor,
`i.e., an electronic tongue with global selectivity, is developed to
`realize a sensor that responds to taste chemical substances and
`can be used to quantify the type of taste focusing on the fact
`that humans discriminate the taste of foods and beverages on
`the tongue with the five basic tastes. In this paper, we focus on
`the taste sensor and describe its sensing principle, its difference
`from general electronic tongues that do not aim to quantify tastes,
`examples of its use, and the recent trend of research of electronic
`tongues.
`Index Terms— Electronic tongue, taste sensor, global selectivity,
`lipid/polymer membrane.
`
`I. INTRODUCTION
`
`T HE SENSE of tastes consists of five basic tastes, i.e.,
`
`sourness, saltiness, umami, bitterness, and sweetness.
`When tasting a food or beverage, humans perceive each type
`of taste on sensory organs called taste buds on the tongue.
`Taste buds are composed of approximately 50-100 cells.
`Research on the mechanism behind the reception of taste sub-
`stances [1], [2] has a short history; Taste-2 receptors (T2Rs),
`bitterness receptors present in taste cells, were discovered in
`2000 [3]–[5] followed by the discovery of sweetness receptors
`(T1R2+T1R3) [6] and umami receptors (T1R1+T1R3) [7].
`Each taste receptor receives multiple chemical substances
`constituting a single taste. Namely, taste receptors exhibit
`semi-selectivity rather than rigid and high selectivity. High
`selectivity means one-to-one correspondence to a particular
`chemical substance. Although the mechanisms behind the
`reception of sourness and saltiness have not yet been com-
`pletely clarified, poly-cystic kidney disease 2-like 1 protein
`(PKD2L) [8], [9] and epithelial sodium channel (ENaC) [10]
`have been identified as the candidate receptors, respectively.
`Taste information perceived by taste buds is transmitted to
`taste nerves as a result of the release of neurotransmitters
`and finally reaches the gustatory area in the brain as a
`central tissue. It has been clarified that sweetness, umami,
`and bitterness receptors are expressed at not only the taste
`
`Manuscript received March 18, 2013; accepted May 3, 2013. Date of
`publication May 14, 2013; date of current version July 10, 2013. This work
`was supported by the Grants-in-Aid for Scientific Research A 23240029 from
`the Japan Society for the Promotion of Science, Japan. The associate editor
`coordinating the review of this paper and approving it for publication was
`Prof. Michiel Vellekoop.
`The authors are with the Graduate School of Information Science and
`Electrical Engineering, Kyushu University, Fukuoka 819-0395, Japan (e-mail:
`tahara@belab.ed.kyushu-u.ac.jp; toko@ed.kyushu-u.ac.jp).
`Color versions of one or more of the figures in this paper are available
`online at http://ieeexplore.ieee.org.
`Digital Object Identifier 10.1109/JSEN.2013.2263125
`
`buds in the tongue but also digestive organs, kidneys, and
`even the brain [11], and the clarification of their physiological
`significance is expected in the future.
`While the above-mentioned research on the molecular and
`cellular biology of taste reception has been carried out,
`sensing technologies for objective evaluation such as the
`discrimination and quantification of tastes have been developed
`since around 1990, prior to the discovery of taste receptors.
`As a background for this, sensory tests, in which experienced
`evaluators called sensory panelists actually taste samples to
`evaluate them, are the main method of evaluating taste in
`the food industry; however, they have some problems such
`as low objectivity and reproducibility as well as the great
`stress imposed on the panelists. To resolve this problem, a
`sensing technology for objectively discriminating and quan-
`tifying the taste of foods, called the electronic tongue, has
`been developed. This was named after the similarity to the
`taste sense of humans. Although the concept of chemical
`sensors is generally to detect a target chemical substance
`specifically at a high sensitivity, the taste receptors of humans
`do not necessarily recognize individual chemical substances.
`As mentioned above, each of the receptors for the five basic
`tastes simultaneously receives multiple chemical substances,
`showing a semi-selective property. Therefore,
`it
`is practi-
`cally impossible to measure the taste of foods containing
`several hundreds of types of taste substance by chemical
`analysis methods, such as liquid and gas chromatography,
`although they can be used to measure the concentration of
`chemical substances. Moreover, there are interactions between
`different tastes and between taste substances. For example,
`the bitterness of coffee is suppressed by adding sugar and
`a synergetic effect for umami can be obtained by mix-
`ing glutamine acid, an amino acid, and nucleotide-derived
`inosinic acid.
`Toko et al. applied for a patent of their taste sensor in
`1989 and developed a taste sensor equipped with multichannel
`electrodes using a lipid/polymer membrane for the transducer
`[12]. This taste sensor is considered to be an electronic tongue
`with global selectivity [13], [14]. Here, global selectivity is a
`term originally proposed by Toko et al. and is defined as the
`decomposition of the characteristics of a chemical substance
`into those of each type of taste and their quantification, rather
`than the discrimination of individual chemical substances, by
`mimicking the human tongue, on which the taste of foods
`is decomposed into each type of taste by each taste receptor
`[15]–[19]. The taste sensor is commercialized taste sensing
`systems SA 402B and TS-5000Z, which are the world’s first
`commercialized electronic tongue system and are currently
`well known to be able to discriminate and quantify tastes [13],
`[14], [20]–[22]. Meanwhile, the electronic tongue proposed
`in 1995 is defined as a sensor used to analyze solutions
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`using the arrays of nonspecific chemical sensors and pattern
`recognition [23]–[26].
`A commercialized electronic tongue and taste sensor are
`the Astree II e-tongue Sensor (Alpha MOS, France) and the
`SA 402B and TS-5000Z taste sensing systems (Intelligent
`Sensor Technology Inc., Japan), respectively. The Astree II
`e-tongue Sensor is used to discriminate solution samples,
`whereas the both of SA 402B and TS-5000Z taste sensing
`systems are mainly used to quantify the intensity of each
`type of taste identified by the human tongue using a taste
`“scale” [27]–[29]. Currently, these sensing systems are used
`to evaluate the bitterness of pharmaceuticals as well as for the
`quality control of foods and beverages.
`Thus far, many review papers on electronic tongues have
`been published [14]–[23], [25], [26], [29]. In this paper, we
`focus on the taste sensor,
`i.e., an electronic tongue with
`global selectivity, and describe its sensing principle, difference
`from general electronic tongues that do not aim to quan-
`tify tastes, examples of its use, and the recent trend of its
`research.
`
`II. ELECTRONIC TONGUES
`Electronic tongues aim to discriminate and analyze foods
`and beverages and are well known as sensing technologies that
`greatly contribute to quality management. Winquist and Lund-
`ström reported a voltammetric electronic tongue in 1997 [30]
`and then developed a hybrid electronic tongue by combining
`the technologies for measuring potentiometry, voltammetry,
`and conductivity [26], [31]–[33]. Six different types of metallic
`electrode were used for the measurement electrodes in voltam-
`metric measurements to obtain different potential responses,
`and principal component analysis (PCA) was used to analyze
`the obtained data and discriminate foods [26], [34].
`Legin and coworkers applied solid-state crystalline ion-
`selective electrodes based on chalcogenide glass to an elec-
`tronic tongue [35], [36], and presented examples of applying
`their system to the analysis and quality management of foods
`and beverages such as wine [37]–[39] and mineral water by
`PCA and analysis using neural network techniques [23].
`Aissy Inc., Japan, a venture from Keio University, provides
`accurate analysis using its original taste sensors and services
`useful for the development of new products and marketing in
`the food industry [40].
`The features of electronic tongues based on sensor arrays
`are (1) low selectivity and high cross-selectivity instead of
`high selectivity and (2) a capability of statistically analyzing
`the outputs from multiple sensors. Sensing technologies based
`on these features, i.e., low selectivity, high cross-selectivity,
`and statistical analysis, have started to be studied in relation
`to electronic noses [41]–[48] and currently with electronic
`tongues, generating new measurement technologies.
`
`III. TASTE SENSOR
`The fundamental concepts of the taste sensor and electronic
`tongues are totally different except for the electrical detection
`of sample information [14]–[23], [29], [49]. Electronic tongues
`aim to discriminate and analyze foods and beverages using
`
`Fig. 1. TS-5000Z taste sensing system (Intelligent Sensor Technology, Inc.).
`
`sensor arrays such as ion-selective electrodes with different
`specificity property and statistical analysis such as PCA and
`neural network techniques. On the other hand, the taste sensor
`using a lipid polymer membrane was developed to realize a
`sensor that responds to taste chemical substances and can
`be used to quantify the type of taste focusing on the fact
`that humans discriminate the taste of foods on the tongue on
`the basis of the five basic tastes. It is needless to say that
`samples can be discriminated if the five basic tastes can be
`discriminated and quantified. Sensors for astringency, which
`is perceived from a physical stimulus that affects foods, rather
`than taste substances [50], have also been developed. Fig. 1
`shows the commercially available TS-5000Z taste sensing sys-
`tem. This system has the following four concepts: (1) The taste
`sensing system must respond consistently to the same taste like
`the human tongue (global selectivity). (2) The taste sensor
`threshold must be the same as the human taste threshold.
`(3) There must be a clearly defined unit of information from
`the taste sensing system. (4) The taste sensing system must
`detect interactions between taste substances.
`A lipid/polymer membrane comprising a lipid, polyvinyl
`chloride, and a plasticizer is used for the stage of receiving
`taste substances, the key technology of the taste sensor. The
`thickness of the membrane is about 200 μm, and the mem-
`brane can be used about 3,000 times. The development of taste
`sensor with the lipid/polymer membrane was started before
`the mechanism behind the reception of tastes by humans
`was elucidated. Initially, researchers attempted to realize the
`reception of taste substances by mimicking biological cell
`membranes composed of lipids [12], [17].
`The taste sensor has sensor electrodes (working electrodes)
`to which a lipid/polymer membrane is attached and a ref-
`erence electrode, and measures changes in the membrane
`potential generated when these electrodes are immersed in
`a sample solution. The measurement procedure is as fol-
`lows (Fig. 2). First, the membrane potential for a reference
`solution (30 mM KCl, 0.3 mM tartaric acid), Vr , is mea-
`sured. Next, the membrane potential for a sample solution,
`i.e.,
`Vs,
`is measured. The difference between Vs and Vr ,
`Vs – Vr , is used as a relative value. Then, the membrane
`potential for the reference solution is measured again (Vr ’).
`The difference between Vr ’ and Vr , i.e., Vr ’ – Vr , is defined
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`TABLE I
`CHEMICAL COMPONENTS OF TASTE SENSORS
`
`Taste sensor
`Saltiness
`
`Sourness
`
`Umami
`
`Acidic bitterness
`
`Basic bitterness
`Astringency
`Sweetness
`
`Lipid
`Tetradodecylammonium bromide
`n-Tetradecyl alcohol
`Phosphoric acid di(2-ethylhexyl) ester,
`Oleic acid,
`Trioctylmethylammonium chloride
`Phosphoric acid di(2-ethylhexyl) ester,
`Trioctylmethylammonium chloride
`Phosphoric acid di-n-decyl ester
`
`Tetradodecylammonium bromide
`Tetradodecylammonium bromide
`Tetradodecylammonium bromide
`Trimeritic acid
`
`Plasticizer
`Dioctyl phenylphosphonate
`
`Dioctyl phenylphosphonate
`
`Dioctyl phenylphosphonate
`
`Bis(1-butylpentyl) adipate
`Tributyl O-acetylcitrate
`Dioctyl phenylphosphonate
`2-Nitrophenyl octyl ether
`Dioctyl phenylphosphonate
`
`IV. PRINCIPLE OF TASTE SENSOR
`The commercialized taste sensor,
`i.e.,
`the taste sens-
`ing system (Fig. 1) consists of a working electrode with
`a lipid/polymer membrane used to receive taste substances, a
`handle, and a data processing unit. In the electrode structure,
`a Ag/AgCl electrode, inner solution (3.3 M KCl saturated
`AgCl) is contained in a polyvinyl chloride hollow rod with a
`lipid/polymer membrane attached (Fig. 3). The potential of the
`lipid/polymer membrane changes upon electrostatic interaction
`with taste substances and their physicochemical adsorption
`[15]–[17], [19], [29]. Table 1 shows lipids and plasticizers
`used in the taste sensor.
`The composition of the membrane is designed considering
`the charges on the membrane surface and hydrophobicity on
`the basis of physicochemical properties of substances with
`each basic taste; for example, an electrical potential change
`for bitterness is induced when bitter substances are adsorbed
`onto the membrane owing to the electrostatic and hydrophobic
`interactions of their charges with the membrane, and a poten-
`tial change for sourness is induced when protons bind to the
`membrane [29], [51]. A bitterness sensor, i.e., sensor electrode
`to measure bitterness, has a membrane with a lower content
`of charged lipids to increase hydrophobicity. In contrast, a
`saltiness sensor, i.e., sensor electrode to measure saltiness, has
`a membrane with a higher content of charged lipids to increase
`hydrophilicity and easily induce the electrostatic interaction
`with ions. In addition, the content of lipids is selected from
`the optimal range and an appropriate plasticizer is adopted
`so that marked changes in the membrane potential can be
`obtained by adding a small amount of taste substances. Fig. 4
`shows schematics of the membranes in saltiness and bitterness
`sensors used for the evaluation of foods. NaCl and iso-α acid,
`which is well known as the bitterness component of beer, are
`shown as examples of salty and bitter substances, respectively.
`A larger amount of lipids is included in for the saltiness
`sensor (Fig. 4(a)) than the bitterness sensor (Fig. 4(b)).
`Here, iso-α acid is present in the lipid/polymer membrane,
`which will be explained with the results of measuring the
`amount of adsorbed taste substance.
`The electrode with a lipid/polymer membrane immersed
`into a sample solution containing taste substances can be used
`
`Fig. 2. Measurement procedure of taste sensing.
`
`Fig. 3. Sensor electrode.
`
`as the change in membrane potential caused by adsorption
`(CPA). Finally, the membrane is rinsed with a sensor rinsing
`solution (30 vol% EtOH, 100 mM HCl or 30 vol% EtOH,
`10 mM KOH and 100 mM KCl). Here, the lipid/polymer
`membranes of sensor electrodes to measure bitterness and
`astringency also respond to taste substances other than bitter
`and astringent substances, respectively, shown as relative value
`(Vs–Vr ). On the other hand, CPA value (Vr ’–Vr ) of these
`membranes can selectively respond to bitter and astringent
`substances, respectively, because bitter or astringent sub-
`stances are adsorbed onto the lipid/polymer membrane of the
`sensor electrodes. [29].
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`Fig. 5. Membrane electric potential in a negatively charged membrane.
`
`(a)
`
`(b)
`
`(a) Schematic illustration of lipid/polymer membranes for salty
`Fig. 4.
`substances and (b) acidic bitter substances.
`
`to determine the intensity of the taste by detecting changes in
`the potential of the lipid/polymer membrane and by evaluating
`the difference between the membrane potential of the sample
`solution and that of the reference solution. Fig. 5 shows the
`response mechanism of a negatively charged membrane. When
`the concentrations of the inner and outer solutions across
`the membrane are different, the membrane potential is the
`difference between the potential of the inner solution and
`that of the outer solution. The membrane potential comprises
`a surface potential generated at
`the interface between the
`membrane surface and the solution and a diffusion potential
`in the membrane. Regarding the surface potential, a diffuse
`electrical double layer is formed in the solution layer near the
`membrane surface. When the lipid/polymer membrane comes
`into contact with an electrolyte solution, it is charged as a
`result of the ionization of dissociative groups of the lipid in the
`membrane surface and the adsorption of ions. For negatively
`charged lipid/polymer membranes, cations are attracted to the
`vicinity of the membrane surface owing to the electrostatic
`interaction, whereas anions move away from the membrane
`surface. Thus, a diffuse electrical double layer is formed by
`the negative charges and cations on the membrane surface. The
`diffusion potential in the membrane is a potential difference
`caused by the difference between the mobility of cations and
`that of anions in the membrane.
`
`Concentration dependence of six types of electrode on the five
`Fig. 6.
`basic tastes and the astringent taste. MSG is the abbreviation of monosodium
`glutamate.
`
`V. APPLICATION OF THE TASTE SENSOR
`
`A. Measurement of Basic Tastes
`
`the
`Similar to the above-mentioned electronic tongues,
`initial
`taste sensor discriminated and quantified tastes by
`statistically analyzing the PCA values and other parameters
`using the responses from multiple sensor electrodes with a
`lipid/polymer membrane [12], [15]–[17]. However, researchers
`succeeded in expressing the intensity of each type of taste
`directly from the response of the electrodes by improving the
`selectivity and sensitivity of the sensor electrodes with respect
`to each taste, i.e., realizing a global selectivity. Specifically,
`when the change in the membrane potential of a sample
`solution (even unknown) is smaller than that of the refer-
`ence solution, the intensity of the taste is low. In contrast,
`the larger the change, the higher the intensity of the taste.
`Fig. 6 shows responses of sensor electrodes used in the
`commercially available taste sensing system. The threshold
`for tastes identified by humans is low for signals of toxic and
`rotten substances, i.e., bitterness and sourness (increasing in
`this order), and is highest for sweet substances, the energy
`source for humans. Following these biological properties, the
`threshold and sensitivity of each sensor electrode are adjusted
`in the taste sensing system. Unlike electronic tongues, the taste
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`Fig. 7.
`Relative values and CPA values of the acidic bitterness sensor
`response to each taste substance (n = 5): saltiness; 300 mM KCl, 0.3 mM
`tartaric acid, sourness; 30 mM KCl, 3mM tartaric acid, umami; 10 mM MSG,
`basic bitterness; 0.1 mM quinine-HCl, acidic bitterness; 0.01 vol% iso-α acid,
`astringency; 0.05 wt% tannic acid, sweetness; 1 M sucrose. Umami, basic
`bitterness, acidic bitterness, astringency and sweetness samples include 30 mM
`KCl and 0.3 mM tartaric acid.
`
`sensor can convert the measured values into sensory values by
`simple linear regression or multiple regression analysis using
`two sensor outputs without using any complicated statistical
`methods such as pattern recognition, and can provide a taste
`scale [29].
`
`B. Global Selectivity
`Fig. 7 shows the relative and CPA values obtained from
`an acidic bitterness sensor, i.e., sensor electrode for acidic
`bitterness (Table 1) for samples with basic tastes. The relative
`value for the acidic bitter substance is –100 mV, whereas
`those for the salty and umami substances are approximately
`–40 mV. In contrast, the CPA value is –67 mV for the acidic
`bitter substance but is nearly zero for other taste substances.
`Namely,
`the CPA value of the acidic bitterness sensor is
`highly selective to acidic bitter substances. Fig. 8 shows the
`measurement results obtained from a basic bitterness sensor,
`i.e., sensor electrode for basic bitterness. Four bitter substances
`and other taste substances [29] were tested. From the CPA
`values, the basic bitterness sensor responds to all bitter sub-
`stances but does not respond to other taste substances. These
`results support the fact that the basic bitterness sensor has
`global selectivity. In addition, the CPA value highly correlates
`with the results of sensory evaluation, as shown in Fig. 8.
`As mentioned above, the basic bitterness sensor conforms
`to the concept of the taste sensing system described in
`Section 3.
`
`C. Application to Foods and Beverages
`As electronic tongues are used in the quality control of foods
`and beverages, the taste sensing system has been similarly
`applied to not only quality control but also services for
`
`(a)
`
`(b)
`
`Sensor performances of the basic bitterness sensor [29]. (a) CPA
`Fig. 8.
`values of response to six taste substances. (b) Relationship between results
`of CPA values of the basic bitter substances and human sensory score. The
`bitter substance materials: 0.1 mM of hydrochloride salts. Data are expressed
`as mean ± SD (n = 4). All samples include 30 mM KCl and 0.3 mM tartaric
`acid as a supporting electrolyte.
`
`consumers and marketing in the food industry. Moreover,
`methods of determining the expiration date of foods using the
`taste sensing system have been developed [29]. In practice, the
`tastes of various foods and beverages, including black tea [52],
`green tea [53], milk [54], Prosciutto ham [29], rice [55], pork
`[56], table salt [57], and ginseng [58], have been quantified
`using the taste sensing system. Fig. 9 shows a taste map where
`the intensities of taste for beer in various countries are mapped.
`In the map, the ordinate represents the bitterness intensity and
`the abscissa represents the sourness intensity, providing the
`visualized information of taste as well as the discrimination
`of products.
`The taste sensing system has also been examined for use
`in the selection of feed appropriate for the growth of local
`chickens with the aim of reducing the breeding cost [59].
`The feed in the reference shows greater responses to umami
`and koku than other types of feed. Here, koku is also called
`kokumi in academic fields and is generally known as rich
`taste, thick taste, or good body. Kokumi, or koku, substances
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`Fig. 9. Taste map of beer.
`
`were discovered by Ueda et al. [60] and its receptor has
`also been discovered recently [61]. Koku substances add
`thickness, mouthfulness, and continuity to the taste of foods.
`A typical koku substance is glutathione (γ -L-glutamyl-L-
`cysteinylglycine). The taste sensing system can be used to
`quantify koku by measuring the CPA value for umami. In
`actuality, the effect of kokumi flavor in noodle soup base
`has been demonstrated and quantified using the taste sensing
`system [29], [62].
`The above applications of the taste sensing system can
`be used to provide taste information (type and intensity of
`taste) to consumers and as a marketing tool
`in the food
`industry as well as to compare own products with others and
`determine consumers’ preference. In other words, the taste
`sensing system can be used not only for quality management
`based on the discrimination and analysis of foods, which is the
`aim of electronic tongues, but also to add taste information to
`products as an added value. It is a device that can indicate
`consumers’ preference of foods.
`Moreover, arbitrary tastes can easily be created by uti-
`lizing the database obtained from measurements using the
`taste sensing system. On the basis of this concept, coffee
`provided by Japan Airlines is designed using the taste sensing
`system. Manually making coffee with a desired taste will be a
`time-consuming trial-and-error task. The taste sensing system
`enables us to accurately create a desired taste in a short period
`of time.
`The taste sensing system can be used to detect the inter-
`action between taste substances, as described in Section 5.6
`in detail. It is known that cooking oil generally makes the
`
`taste of foods milder. The changes in taste when cooking oil
`was added to solutions with various tastes such as sourness,
`bitterness, and astringency were measured [29], [62]. The
`results revealed that the responses to bitterness and astringency
`markedly decreased. In contrast, the responses to other tastes
`including umami and saltiness remained unchanged. These
`results indicate that cooking oil suppresses bitterness and
`astringency, which are relatively stimulating and sustained, to
`make the taste of foods milder.
`
`D. Sweetness Sensor
`The development of sweetness sensor, i.e., sensor electrode
`for sweetness was behind that of other taste sensor elec-
`trode. This is mainly because sweet substances are nonelec-
`trolytes, i.e., substances without charges, and the potential
`of lipid/polymer membranes is hardly changed by sweet
`substances. Although Brix meters may be used as an alter-
`native method of measuring sweetness, they perform indirect
`measurements in which the refraction index of solutions is
`measured. Therefore, measurement results greatly depend on
`the composition of solutions, and it is difficult to accurately
`measure the intensity of sweetness. In actuality, general solu-
`tions with a high viscosity also show a high refraction index
`(i.e., they respond to Brix meters). Toyota et al. succeeded in
`detecting sweet substances, such as sucrose, glucose, fructose,
`and raffinose, by modifying lipid/polymer membranes using
`substances that electrostatically interact with sweet substances
`(sweet-responsive substances, SRSs) in advance. They found
`a clue to resolving the problem related to sweetness sensors,
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`and their developed sweetness sensors have already been
`practically used [63], [64]. For realizing highly functional
`sweetness sensors, it is also an urgent task to develop methods
`of quantifying high-intensity sweeteners, such as aspartame,
`acesulfame-K, and saccharin, which are popularly used in
`sweet foods and beverages because of their low calorie
`content.
`
`E. Measurement of Amino Acids
`Amino acids are the basic building blocks of proteins essen-
`tial to human lives and important components in foods. Some
`amino acids have two types of taste, for example, sweetness
`and bitterness. Methods of evaluating the taste of amino acids
`were examined [65]. The relative values were measured using
`taste sensor for L-alanine (L-Ala) as the sweet amino acid,
`L-tryptophan (L-Trp), L-leucine (L-Leu), and L-isoleucine
`(L-Ile) as the bitter amino acids, and L-methionine (L-Met)
`as the amino acid with a composite taste. The results were
`compared with those of sensory evaluation. The correlation
`coefficient between the relative value obtained from the sweet-
`ness sensor for L-Ala and the value obtained by the sensory
`evaluation was 0.97, and between the relative values obtained
`from the bitterness sensor for L-Trp, L-Leu, and L-Ile and
`the values obtained by the sensory evaluation was also 0.97,
`indicating that the intensity of taste of each amino acid can
`be measured using the taste sensors.
`Moreover, the sensory evaluation revealed that 300 mM
`L-Met with a composite taste of bitterness and sweetness
`has bitterness corresponding to that of 30 mM L-Trp and
`sweetness corresponding to that of 300 mM L-Ala [65]. The
`estimated intensities of bitterness and sweetness obtained from
`the sensors for 300 mM L-Met corresponded to the bitterness
`of 10–30 mM L-Trp and the sweetness of 100–300 mM L-Ala,
`respectively. These results indicate that the estimated values
`obtained from the taste sensor and the results obtained by
`the sensory evaluation are in good agreement. Therefore, for
`L-Met with both sweetness and bitterness, i.e., amino acids
`with a composite taste, the intensities of coexisting tastes can
`be estimated using the bitterness and sweetness sensors. It is
`found that the taste sensor can be used to quantify the taste
`of amino acids.
`
`F. Application to Pharmaceuticals
`The bitterness of not only foods but also pharmaceuticals
`has been successfully quantified using taste sensor, which
`has now been practically used to evaluate the bitterness of
`pharmaceuticals [49], [66]–[71]. Most pharmaceuticals have
`strong bitterness, and enhancing the medication compliance
`by patients is an important task for pharmaceutical manufac-
`turers. The taste of a sample prepared by mixing bitterness-
`masking materials used to suppress bitterness, i.e., sucrose,
`α-cyclodextrin, BMI-40 (Kao Corporation, Japan), with a
`bitter substance, in this case quinine chloride, was measured
`using taste sensor. The results highly correlated with the
`results obtained from sensory evaluation by sensory panelists,
`indicating the applicability of taste sensor to the detection
`
`of the effect on suppressing the bitterness of pharmaceuti-
`cals [29], [71]. Orally disintegrating tablets (ODTs) attracted
`much attention approximately 20 years after the start of their
`research and development and 10 years after commercial-
`ization. In particular,
`the use of ODTs has been rapidly
`promoted since 2000 and has been becoming the mainstream
`of oral medication. Harada et al. evaluated the bitterness of a
`carrageenan-containing propiverine hydrochloride ODT [66].
`For ODTs containing pectin, agar, or λ-, ι-, or κ-carrageenan,
`the intensity of bitterness at the complete disintegration was
`measured using taste sensor and compared with the results of
`sensory evaluation by panelists, showing a strong correlation
`with a high correlation coefficient of R = 0.907. Moreover,
`propiverine hydrochloride eluted from these ODTs was sam-
`pled for different elution times and the intensity of bitterness
`was measured, demonstrating that the time dependence of
`the change in bitterness intensity can be evaluated using
`taste sensor.
`A group led by Uekama and Arima reported the suppression
`of bitterness for various drugs using β-