Sensors and Materials, Vol. 13, No. 3 (2001) 169-177
MYUTokyo
S &M0440

Analysis of Tastes of Amino Acids Using
Surface-Polarity Controlled Sensors
M. J. Ju, K. Hayashi and K. Toko
Graduate School of Information Science and Electrical Engineering,
Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan(Received October 8, 2000; accepted February 5, 2001)

Key words:

interaction, electrode impedance, potential profile, ·taste qualities, transducers

The tastes of amino acids were studied using a method for detecting the change in theinteraction between an electrode surface and amino acids in aqueous solution. The
electrode impedance showed different patterns of potential profiles for amino acids withdifferent taste qualities, while it showed similar patterns for amino acids with similar
tastes. Amino acids may be classified into four groups according to their tastes based on
sensor outputs. The sensor response correlated well with human sensitivities and showed
higher sensitivity than human sensation. This method can provide useful transducers forthe development of sensors to analyze taste qualitatively and quantitatively.

1.

Introduction

The measurement of taste with artificial sensors is an essential labor-saving method for
quality control in the food industry; however, the development of a taste sensor is not easy.
Physical sensors have conventionally dealt with a single input such as light or sound; hence
they are relatively easy to construct. Taste sensors have to deal with a large number of
inputs, i.e., chemicals, and the difficulty increases with the number of inputs. Fabricationof such sensors is very difficult. However, human taste sensation is roughly classified into
only five basic types: sour, salty, bitter, sweet and umami. 0·2l Hence taste sensors would be
sufficient if they could discriminate among the five tastes qualitatively and quantitatively.
One way to realize taste sensors is to utilize materials similar to those in biologicalsystems as transducers. A recently developed chemical sensor utilizes lipid membranes to

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transform taste information into signals that are electric potential changes;C3-5J this sensor isable to measure both the quality and the intensity of the taste. It can be used to detect taste
in a manner similar to the human gustatory sensation so that similar patterns are obtained
for similar taste qualities and a sensitivity similar to that of humans is realized. Thus, thetaste sensor has been applied to many kinds of foodstuffs.<6-10l The taste of foodstuffs canbe examined quantitatively using the taste sensor which provides an objective scale forhuman sensory expressions.
Here we developed a taste sensor by an electrochemical method for controling interface
potentials. Electrode surface polarization was controlled with positive, negative andneutral electric potentials. This sensor can detect neutral substances which are difficult todiscriminate by electrochemical methods. Actually, the lipid membrane sensor cannot
detect neutral substances with high sensitivity. This sensor used the essential detecting
mechanism of the lipid membrane sensor; both polarity of the srnface and interaction withchemicals are very important in sensing the tastes. To a great extent, it was possible todetect odor molecules or environmental pollutants dissolved in water.(11)
From the standpoint of taste sensor development, analysis of tastes of amino acids isvery important because each amino acid elicits a complicated mixed taste. Ninomiya et al.
investigated taste intensity and quality of various amino acids in detail by sensory panel
tests.<12J Amino acids are generally classified into several groups that each correspond to acharacteristic taste. In this study, we applied this sensor to amino acids and evaluated theirtaste according to sensor outputs.

2.

Materials and Methods

2.1 Taste substances and experimental setup

All amino acids used in the experiments were L-type and employed without further
purification. Prior to the experiments all substances were dissolved in water containing 10
mMKCl.
Figure l(a) is a simplified diagram showing the setup of the experimental equipment.
A frequency response analyzer (FRA, NF Electronics Instruments 5020) was used tomeasure the electrode surface impedance. A computer (Apple Macintosh) was connectedto the equipment and controlled the measuring process. A Pt-electrode as a working
electrode, a reference electrode composed of an Ag/AgCl wire with saturatedKCl and acounter electrode of a platinum wire were all connected to a laboratory-built potentiostat.
The working electrode was made by setting the 1 mm Pt wire in an acrylic board witha 5 mm diameter hole. The hole was filled with epoxy resin. The Pt wire and epoxy resin,when dried, were polished using sandpaper and 0.1 µm alumina polishing suspension until
the surface became smooth. The Pt-electrode is the most commonly used metallic solid
electrode. It is resistant to oxidation, but it is not totally inert as was often assumed in earlywork.<13l
All three electrodes were dipped in the test solutions containing the taste substances.
The potential of the Pt-electrode used as the transducer was controlled by the potentiostat.
The sinusoidal current and DC voltage from the FRA were applied to the potentiostat
which controlled the potential of the Pt-electrode, and the frequency response was investi­
gated.

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2.2 Equivalent circuit of the electrode
Figure l(b) shows an equivalent circuit where redox reactions such as hydrogenproduction did not occur. The equivalent circuit was obtained by considering the rough­ness of the electrode surface. The equivalent circuit consists of the connection in parallelof the electrode resistance (R,) and the electrode capacitance (Cai) of the electrical double
layer on the electrode surface. The term R, is the unusual electrical resistance caused byroughness of the electrode surface, where the resistance depended on frequency f R, = ex/f where a was constant. The term Rs is the solution resistance which is connected in series
to the parallel connection of R, and Ca1• We carried out curve fitting on the frequency
response locus to obtain the resistance and capacitance of the electrode. The term R45 is theestimated value of R, atf = 45 Hz.
2.3 Potential profiles of standard solutions
Figure 2 shows dependence of R45 and Ca1 on the electrode potential of the Pt-electrodein 10 mM KCl as the standard solution. In this experiment, data for taste could not be
obained with the addition of buffer solution instead of KCl. This implies that the taste
sensor could not respond to the tastes of amino acids in buffer solution, which has constant
(a)

(b)

Potentiostat

FRA

Impedance due to roughnessof electrode surface
Rr = alfComputer

Cd!

Fig. 1. Setup for the measurement of (a) the electrode impedance and (b) equivalent circuit. WE:
working electrode, RE : reference electrode, CE : counter electrode.
(a) Electrode resistance (R4s)
1.6
1.4=-----------�

(b) Electrode capacitance (Cd1)
1.8
1.6..-..1.2
�l.O

G;' 1.4

2:, 1.2

� 0.8
Cl:,: 0.6

1.0

PZC
0.4
0.2 -�����_.__.,_....___.____..
-0.6
-0.4
-0.2
0.0
0.2

Electrode potential (V)

0.8
0.6

-0.6

-0.4

-0.2

0.0

0.2

Electrode potential (V)Fig. 2. Potential profiles of the Pt-electrode impedance in 10 mM KCl as the standard solution.

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pH. Both R45 and Cct1 depend on the electrode potentials. The electrode impedance
depended mainly on the adsorption and desorption of hydrogen in the negatively charged
region and oxygen adsorption/desorption in the positive region. We referred to thesedependencies as potential profiles of the electrode impedance. The point of zero charge(pzc) is where the surface charge becomes zero and the surface becomes hydrophobic.
The Pt-electrode was not an electrode suitable only for surface polarization, sincedifferent surface states were present which interacted as expected with different taste
substances. The changes in R45 and Cct1 where taste substances were added were M45 and'1Ccti, respectively. Here we measured the changes in electrode potential and regarded thechanges in the pattern as the potential profile.

2.4 Scanning method
A micro-signal of 0.01Vin amplitude was superimposed on the electrode potential. To
prevent the electrolysis of water, the electrode potential was changed over a scanningpotential ranging from - 0.7 Vto+ 0.3 Vin 0.05 Vsteps. The computer controlled themeasurement of inte1face impedance between the electrode surface and the aqueous taste
solution. The responses of the interface impedance were measured with setting the
response to a 10 mM KCl solution as the origin.

3.

Results

3.1 Responses to amino acids
Figure 3 shows the profiles of electric potential of electrode surface impedance for
glycine (Gly), L-tryptophan (L-Trp), L-glutamic acid (L-Glu) and monosodium glutamate(MSG) which represent the sweet, bitter, sour and umami tastes of the amino acids,
respectively.
The averages and standard deviations are shown for measurements made in triplicate
for each substance. It is clearly seen that the resistance patterns for amino acids aredifferent from each other. The peak around 0.1 Vwas associated with the amino subgroupin the amino acid molecules, and could be seen for all amino acids. Electrode capacitancepatterns did not differ significantly between amino acids used in the measurements.

s(b) Potential profile(� Cdr)(a) Potential profile(11 R4s)

1.2

1.0

F""-;::::====::;---------,i'-' 0.8-0.2

0a - .4
<l
-0.6

� 0.6
0.4
0.2 =-.....___...__.....___...__.....___,
-0.6
-0.4
-0.2
0.0
0.2

Electrode potential(V)Fig. 3.

--GlyO.lmM:::tirui.
��-0-MSGlmM
-0.6

-0.4

-0.2

0.0

0.2

Electrode potential (V)Electrode potential profiles for Gly, L-Trp, L-Glu and MSG.

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Consequently, we mainly discuss the taste responses of M45 • Error bars for L-Glu wererelatively large, because the pH of the solution was not precisely controlled in theseexperiments, and this affected the impedance significantly.
As seen in Fig. 4, different potential profiles of the sensor outputs were obtained for thedifferent taste substances sweet, bitter, sour and umami-tasting amino acids, and similar
patterns were obtained for similar taste substances. This implies that the tastes of aminoacids can be measured qualitatively by this method and may be explained by interactions
based on electrode polarity. Therefore, the differences among electrode potential profiles
are due to the changes in the surface potential, which involve ionic concentration changes
according to dissociation and adsorption of taste substances on the electrode surfaces.
The most common conventional electrochemical methods involve oxidation-reduction
processes. In this study, however, the change in the interaction between the electrodesurface and the chemical substances was investigated using the surface polarization inaqueous solution where no redox reaction occurs. Therefore, we obtained different
potential profiles formed according to the various inherent characteristics of chemical
substances.
Moreover, the changes in the interaction between the electrode surface and the chemi­
cal substances can be expected to yield information about taste, because the taste ofchemicals depends mainly on the polarity of chemicals.
(a) Sweetness(b) Bitterness
1.2

1.2
1.0

,.. Qty0.1mM, • Ala 1mM
� Thr 0.3mM, � LyaHCI 0.01mM·- S... 0,3mM, + Gtn 1mM

1.0

0.8

0.8
0.6

--

0.4

g�
�

-0.6

-0.4

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0.0

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0.6

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0.4

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-0.6

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0.0

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(d)Umami(c) Sourness
1.2

0.0

-0.4

-0.2

0.0

0.2

0.0

-0.6

-0.4

Electrode potential (V)Fig. 4. Electrode potential profiles of amino acids with different taste qualities.

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3.2 Principal components analysis of sensor outputs
Data obtained using this taste sensor (surface-polarity controlled sensor) were studiedin greater detail by principal components analysis (PCA). The original data were visual­ized on two- or three-dimensional planes using this analysis, which is very effective in
reducing the dimensional space with minimum loss of informationY4l The contribution
rates to the first (PCI), the second (PC2) and the third (PC3) principal component were
63.6%, 17.2% and 9.6%, respectively. Therefore, useful discussions of PCI and PC2 may
be possible using a scatter diagram. Figure 5 shows the obtained data points plotted in ascatter diagram on the PCl and the PC2 plane.
Sweet amino acids and bitter amino acids were located at negative and positive regions
along the PCl axis, respectively. Moreover, umami tasting amino acids were locatedbetween them. However, sour amino acids were found separately on the PC2 axis. As seenin the figure, amino acids were classified more clearly into four groups according to theirtaste, as expected. This PCA classification of tastes of amino acids is similar to humantaste sensation for the classification of tastes of amino acids. 02J
3.3 Cluster analysis
PCA suggests that tastes of amino acids can be classified into four groups by evaluating
the scatter diagram visually. Cluster analysis is the most commonly used term for the classof procedures that seek to separate component data into groups.< 14J To identify a smaller
number of groups such that elements belonging to a given group are, in some sense, moresimilar to each other than to elements belonging to other groups, we carried out the cluster
analysis illustrated in Fig. 6.

0.8
0.6
,-..,

-.._,
N

0.4

Bitterness

0.2

S-weetness

0.0
-0.2
-0.4
-0.8

-0.6
Fig. 5.

-0.4

-0.2

0.0

0.2

0.4

PCl (63.6%)

Scatter diagram of principal components analysis.

0.6

0.8

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Sensors and Materials, Vol. 137 No. 3 (2001)
("")

u

(I)

N

(a)

r-· ;: : ;-:;-· · �L-:;f"!. 1 1 · r· -r: :;·r,· 1
(c).

i

{d)

____________(bL____. �········.:: --�...............,........:
:

u
(I)

0

r··'··; ........

•

Fig. 6. Dendrogram of sensor outputs. (a) Bitterness, (b) sourness, (c) umami and (d) sweetness.

Fundamental to the use of any technique for cluster analysis is the computation of ameasure of similarity or distance between the respective objects. Thus, cluster analysis
intends to reduce the information on the whole set of n objects to information about, forexample, h subgroups, where h « n. We selected analysis parameters to achieve compact
clusters. The Ward method0 4l was selected when computing distances between the groups,
and we also evaluated the Euclidean distance between the objects.
As seen in Fig. 6, all of the objects could be correctly separated into four clusters based
upon the taste qualities of amino acids. The sensor output indicated different cluster foramino acids which have different taste qualities, while it indicated the same clusters foramino acids which have similar tastes. This implies that the taste sensor could respond to
the taste itself.

3.4 Sensor threshold comparison with the human sense
The threshold concentrations of the sensor for amino acids were compared with thehuman threshold concentrations for amino acids obtained by Schifmann et al.<15l
Figure 7 (a) shows the relationship between the mean values of the sensor output and thescanning concentration. In this experiment, the threshold value of the sensor for eachamino acid was determined according to the lowest value of the sensor response, which
was the 0.2 kQ level. In the cases of L-Gln, L-Ala and L-Pro, for example, sensor
thresholds were approximately 0.005 mM, 0.04 mM and 0.05 mM respectively.
Figure 7 (b) shows the relationship between the threshold values of the sensor and thehuman values. The solid line indicates sensor threshold values that equal human values.
The dotted line is a visual guide. The sensor was 1000 times more sensitive than the humansense.

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Sensors and Materials, Vol. 13, No. 3 (2001)

1.2
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+L-Gln

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,-CysHCl rg

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10

Humans' threshold values (mM)

100

Fig. 7. Relationship between the threshold values of the sensor and humans. The threshold valuesof (a) the taste sensor, and (b) the sensitivity of the taste sensor.

These results show that the correlation between the sensor output and the thresholdconcentration of humans is linear. It also indicated the usefulness of the taste sensor for thequantitative analysis of human sensory expression. Furthermore, better agreement was
obtained between sensor outputs and data from humans. The information from the sensor
could be utilized to evaluate taste quality and to quantify taste intensity at the same time.

4.

Discussion

Amino acids are generally considered essential for humans. Amino acids that are
required for protein synthesis and cannot be synthesized by the organism must be present in
the diet. Such compounds are referred to as essential amino acids. The side chains ofamino acids are important, and they determine whether the amino acids are acidic, basic or
neutral. They also determine the structures of the amino acids, which result in amino acids
having different characteristics and tastes.
In this study, we have developed a chemical sensor by using the surface polarity control
of an electrode to modify the interaction with and the sensitivity to various chemicals.
Electrochemical impedance was measured at each electrode surface potential to determine
the condition of the surface. This sensor could detect both electrolytes and nonelectrolytes
with high sensitivity.
The potential profiles for substances with different taste qualities were significantly
different, and hence each taste could be easily discriminated with high reproducibility. In
contrast, the taste sensor gave similar potential profiles for substances of the same taste
quality, e.g., sour substances such as L-Glu and L-Asp have similar potential profiles.
Umarni substances such as MSG and L-AspNa also have similar potential profiles. (Fig. 4)
Amino acids which have a sweet taste were located in the same region, and bitter aminoacids belonged to a different region, as expected. (Fig. 5) The taste of amino acids wasclassified into four groups in accordance with the results of both the human sensory test andSensors and Materials, Vol. 13, No. 3 (2001)

177

cluster analysis. Furthermore, threshold values of amino acids detected by the sensor had
good correlation with human values.
We can conclude that this sensor and its associated measurement methods respond totaste. The electrode response was more sensitive than that of humans, and it was confirmedthat this method can provide useful transducers for the development of taste sensors which
can be used for both quantitative and qualitative taste analysis. The taste sensor has goodsensitivity, reproducibility, and a durability higher than the sense of taste of humans.

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