Sensors and Materials, Vol. 11, No. 8 (1999) 469-477
MYUTokyo
S &M 0389

Detection of the Suppression of Saltinessby Umami Substances Using a Taste Sensor
Tomomi Nagamori, Kiyoshi Toko 1 , Yukiko Kikkawa 1 ,Takehiko Watanabe2 and Kenichi Endou2
Department of Electrical Engineering, Ariake National College ofTechnology
150, Higashihagio-machi, Omuta, Fukuoka 836-8585, Japan
1Department of Electronic Device Engineering, Graduate School of Information Science andElectrical Engineering, Kyushu University
6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan2
Food Business Division of JapanTobacoo Inc.
1437-2, Nishiizumida, Sakai-machi, Sashirria-gun, Ibaraki 306-0947, Japan(Received July 10, 1999; accepted September 22, 1999)
Key words:

saltiness, umami taste, taste sensor, suppression, taste interaction, quantification

A multichannel taste sensor, i.e., an electronic tongue, whose transducer is composedof several kinds of lipid/polymer membranes with different characteristics, can detect
taste in a manner similar to human gustatory sensation. In this study, we attempted todetect the suppression of saltiness by the umami substance monosodium glutamate(MSG) using a multichannel taste sensor. To quantify the saltiness, a principal compo­
nent analysis (PCA) was applied to membrane potential data. The saltiness was expressedon the -r scale, which is the relationship between the salty strength perceived by humans
and the NaCl concentration, using PCA. It was quantitatively shown that the saltiness ofNaCl was weakened by MSG.

1.

Introduction

Taste is comprised of the five basic qualities of sourness, saltiness, sweetness,bitterness and umami. To date, however, the main methods of measurement are sensory
evaluations by humans and conventional chemical analyses. A multichannel taste sensor,
i.e., an electronic tongue, whose transducer is composed of several kinds of lipid/polymer

469

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Sensors and Materials, Vol. 11, No. 8 (1999)

membranes with different characteristics, can detect taste in a manner similar to humangustatory sensation.(l-5J Taste information is transformed into a pattern composed of theelectric signals of membrane potentials of the receptor. The sensor outputs are not related
to the amount of specific molecules present, but rather to the taste quality, because similar
patterns are obtained for substances producing the same taste quality. The taste of several
food items such as coffee, beer, mineral water, vegetables and milk were studied using
this sensor.<4J The taste sensor detects and quantifies interactions among tastes such as
suppression,C6) which appears between sweetness or saltiness and bitterness_c7.sJ
Umami is the fifth independent taste produced by monosodium glutamate (MSG)
which is mainly contained in seaweed and disodium inosinate (IMP) in meat and fish. Itis also known that, for substances with the umami taste, interactions between tastes occur,
such as synergism or suppression. The combination of MSG and IMP enhances theumami taste, while umami substances suppress the intensity of the taste of salty sub­stances.
In this study, we applied the taste sensor to salty substances in combination withumami substances and quantified the suppression of saltiness.

2.

Materials and Methods

2,1 Measurefr!ents using taste sensors

l\4easµrements WE:re performep using the tas��-sensing system SA401 (Anritsu Corp.)�hqwn in Fig. 1. The detecting SyTISOr qonsists Qf S�Vf)n electr0<;le� made of lipid/polymer
membranes (i.e., channels). To d�t.e, several kitjdsoflipids,havebeen used according tothe purposes and objects to be measured. Lipi9s used for preparing membranes in this/ 7 ......_.____ Sensor probe
Lipid/polymer membrane/ Reference electrodeSample

Fig. l. Taste-sensing system (SA401, Anritsu Corp.).

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Sensors and Materials, Vol. 11, No. 8 (1999)

Table 1
Lipids used for the membranes.
Channel1234567Lipid (abbreviation)
Decyl alcohol (DA)
Oleic acid (OA)
Dioctyl phosphate (DOP)
DOP : TOMA= 5 : 5
DOP : TOMA= 3 : 7
Trioctyl methyl ammonium chloride (TOMA)
Oley! amine (OAm)study are listed in Table 1. Each lipid was mixed in a test tube containing polyvinylchloride and plasticizer (dioctyl phenylphosphonate) dissolved in tetrahydrofuran. The
mixture was then dried on a glass plate that was set on a hot plate whose temperature wascontrolled at about 30°C. The lipid/polymer membrane was a transparent and soft film
about 200 µm thick.
Each electrode consisted of an Ag wire whose surface was plated with AgCI with aninternal cavity filled with 3M KCI solution (Fig. 2). The lipid/polymer membranes wereadhered to the opening of the tube. The electric potential difference between the multi­
channel electrode and the reference electrode (Ag/AgCI with 3M KCl, saturated AgCl)was obtained by a high-input-impedance amplifier connected to a computer. Chemicals
freshly dissolved in distilled water with 1 mM KCl were used for the experiments. The
measurements were made three times by a rotation procedure which involved one roundof measurements for all the samples.

2.2 Materials
The samples were NaCl and MSG as salty and umami tastes, respectively, which wereof the highest analytical grade (Kanto Chemical Co., Inc). For studying the suppression ofsaltiness by the umami taste, 1 mM and 10 mM MSG, plus NaCl at different concentra­
tions, were measured.
Fresh 1 mM KCI solution was used as a reference sample and also as a rinsing fluid forthe electrodes after every measurement. The response electric potential of the sample wasmeasured ·as the difference between the potentials of the sample and 1 mM KCI solution as
the reference. Each measuring time was set to 10 s, and the rinsing of electrodes after eachmeasurement was repeated 15 times for about 60 s. Each sample was measured threetimes.
2.3 Sensory evaluation
Sensory evaluations of samples were performed three times on different days; 15, 17
and 15 panelists tasted the NaCl solution and its combination with MSG. Pair tests werecarried out for three terms: one was selected from two samples (pure NaCl solutions andthose consisting of NaCl and MSG) in terms of taste such as "having a very strong taste,""having a salty taste" and "having an umami taste."472

Sensors and Materials, Vol. 11, No. 8 (1999)

Ag/AgCl____ 3MKCI+ sat. AgCII 00 mMKC!+ agar..!_....�!-__ Lipid/polymer
membraneWorking electrode

Reference electrodeFig. 2. Working and reference electrodes.

3.

Results and Discussion

Figure 3 shows the response patterns for NaCl at different concentrations and illus­
trates the systematic change of the response potentials according to the NaCl concentra­tion. Standard deviations are shown by error bars. As the NaCl concentration increased,
the response potentials of the DA (ch. 1), OA (ch. 2) and DOP (ch. 3) membranes
increased, whereas those of the 3: 7 (ch. 5), TOMA (ch. 6) and OAm (ch. 7) membranes
decreased. The increase in chs. 1-3 occurs because Na+ cations have an electric screening
effect on the negatively charged membranes. The decrease in chs. 5-7 is brought about by
c1- anions.
Figure 4 shows the response patterns for 1 mM and 10 mM MSG. The response
potential of the 5: 5 membrane (ch. 4) is negative; this is characteristic of substances withan umami taste.<9J Similar response patterns were obtained for other umami substancessuch as disodium inosinate (IMP), disodium guanylate (GMP), disodium succinate
hexahydrate and sodium aspartate monohydrate (data not shown). The response potentialfor 1 mM MSG is very small, because the maximum is about 10 mV at most.
Figure 5 shows the response patterns for the mixed solutions, which contain 1 mM
MSG and NaCl at three different concentrations of 30, 100 and 300 mM. The response
patterns for the mixed solutions clearly shift downward in chs. 1-3 due to the presence of
1 mM MSG, while they shift upward in chs. 6 and 7. This change can be considered toreflect the decrease in NaCl concentration compared to Fig. 3, i.e., a suppression of thesaltiness by MSG.
A principal component analysis (PCA) was applied to these data to confirm thisexpectation and quantify saltiness according to a previous report.<6l This analysis is amultivariate analysis that reduces the dimensional space without losing information. Datapoints are expressed in seven-dimensional space in this case because there are seven­channel outputs and then become expressed in.lower-dimensional space.

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Sensors and Materials, Vol. 11, No. 8 (1999)

NaCl concentration
O1mM

• 3mM
® 10mM•

100mM

Iii 300mM
J;. 1000mM

D 30mMcii�it.)·;::t5Q)
Q)
Q)
Cl)
C:g_-100&
Cl)

DAOA

DOP

5:5

3:7

TOMA OAm

ChannelFig. 3. Response patterns for NaCl.

1100

MSG concentrationcii�
2O

1 mM

J;.

10mM

0a.
t.)
·;::t5Q)
Q)
Q)
Cl)
C:g_&Cl)

-100
DAOA

DOP

5:5

3:7

TOMA OAm

ChannelFig. 4.

Response patterns for MSG.

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Sensors and Materials, Vol. 11, No. 8 (1999)

I NaCl alone

1-030 mMNaCl•

100mMNaCI® 300mMNaCI0Q)
en
C:0C.
en

� -100

INaCl+MSGI-···
D30mMNaCl + 1mM MSG• 100mMNaCl+ 1mM MSG
l!I 300 mMNaCl + 1mM MSG

DAOA

DOP

5:5

3:7

TOMA OAm

ChannelFig. 5. Response patterns for the NaCl solutions and mixed solutions of NaCl and MSG.

The largest element of data contained in the original data is transformed into the firstprincipal component (PCl ), and the second and third largest items are transformed into
the second and third principal components (PC2 and PC3), respectively; in this way, weextract information from the original data in order of its importance.
Figure 6 shows the relationship between PCl , which was obtained by applying thismethod to Fig. 3, and the NaCl concentration. The contribution rates, the relative
magnitude of transformed information, of the original data to PC l, PC2 and PC 3 were
98.9%, 0.7% and 0.4%, respectively. This indicates that PCl characterizes the patterns in
Fig. 3 and we can discuss these data using only PCl.
The PCl is expressed by the formula(1)

where a; is the factor loading, v; the response potential of channel i, and v'; the response
potential of channel i averaged over the measured samples.
In Fig. 6, a straight line is drawn using the least-squares method. The value of PCl
increases in proportion to the NaCl concentration logarithmically. This relation agrees
with the r scale, which is the reported relationship between salty strength ( r) and NaCl
concentration: 0 oi
T0 = 2.8 3log(</)0/0.056),
(2)

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Sensors and Materials, Vol. 11, No. 8 (1999)

100

0 ---------·-----·-----,---. ·----------------·

t-100

I

NaCl concentration (mM)

Fig. 6. Relationship between PCl and NaCl concentration.

where </Jn is the NaCl concentration in g/100 cm3 • The r scale has also been prepared forother taste qualities ·such as sweetness, sourness and bitterness with different values of thecoefficient. It can represent the subjective strength of a taste in solution. The least­squares line in Fig. 6 is given by
PCl = 81.26 logC- 121.1,(3)

with C denoting the molar concentration; hence we can obtain the relationship between rnand PCl as
rn = 0.0348 x PCl + 1.44.

(4)

If the PC 1 value i� calculated, we can estimate the strength of saltiness expressed on
the r scale. The strength of saltiness can then be obtained from the response potentials ofsamples which contain MSG using eq. (4). The result is shown in Table 2 as a decrease insaltiness.
We observe a decrease in the strength of saltiness by adding 1 mM MSG to the NaClsolutions. The salty strength is decreased from 1.40 to 0.19 in a 30 mM NaCl solution by
the addition of 1 mM MSG.
This result implies the satisfactory detection of the suppression of saltiness by MSG. A
similar suppression was found for other mixtures with 10 mM MSG.
Table 3 summarizes the sensory tests in humans for a 100 mM NaCl solution and its

476Sensors and Materials, Vol. 11, No. 8 (1999)

Table 2Estimation of salty strength for NaCl alone and its mixture with MSG expressed on the r scale.
NaCl concentration (rnM)30100300

NaCl alone
1.40
2.88
4.23

Mixed solution (NaCl+ 1 rnM MSG)
0.19
1.64
2.85

Table 3Results of sensory evaluations for 100 mMNaCl solution and its mixtures.
Intensity
Salty
13
NaCl alone'
32+5 mMMSG
12*****Paired t-test
29NaCl alone'
158+ lOmMMSG**
Paired t-test
10NaCl alone'
0515+20 mMMSG*
Paired t-test<•lSensory evaluations were perfom1ed on different days.
*p < 0.001**p < 0.01***p < 0.05

Umami taste*215**

14*

mixtures with MSG. The suppression of saltiness by MSG appeared at the lowest MSG
concentration. In fact, 13 persons stated that 100 mM NaCl alone was more salty, while
only two persons reported a more salty taste for 100 mM NaCl+ 5 mM MSG. Increasing
MSG concentrations in the mixed solutions made it difficult to distinguish between two
samples with respect to saltiness.
It was reported by sensory evaluations that a suppression of saltiness by umami
substances was not observed, whereas a decrease in saltiness can also be found at low
MSG concentrations. <11J This observation relating to low MSG concentrations does notcontradict the results herein.
In this study, it was shown that the suppression of the saltiness of Na Cl by MSG could
be quantified using a multichannel taste sensor. The suppression of saltiness by MSG wasobserved even for mixtures at a high ratio such as 300 mM NaCl to 1 mM MSG. The
results of sensory tests in humans agreed with this result because the suppression effectclearly appeared at low MSG concentrations. This appears to suggest that the taste sensor
is so sensitive that it is possible to detect the suppression of saltiness.
The same measurements and procedures described here were carried out on a bitter­
tasting drug. (6l As a result, the decrease in the bitter taste of the drug with increasing
sucrose concentration was quantified. The taste sensor can provide a novel, automatedmethod to measurJ the strength of the bitterness of a drug in place of sensory evaluation.
In addition, st\idies of the interaction of tastes using this sensor, which utilizes lipid
membranes, will contribute to the clarification of receptors in the human gustatory
system.

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477

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