Sensors and Materials, Vol. 13, No. 3 (2001) 137-144
MYUTokyo
S &M 0437

Analysis of Saltiness and Bitterness of
Inorganic Salts Using Taste Sensors
Satoru Iiyama, Shu Ezak:i 1 and Kiyoshi Toko2
Department of Biological and Environmental Chemistry, Kyushu School of Engineering,
Kinki University, Iizuka 820-8555, Japan1
Department of Electrical Engineering and Computer Science, Kyushu School of Engineering,
Kinki University, Iizuka 820-8555, Japan
2
Graduate School of Information Science and Electrical Engineering,
Kyushu University, Fukuoka 812-8581, Japan(Received August 22, 2000; accepted March 11, 2001)
Key words:
adsorption

taste sensor, lipid membranes, response pattern, inorganic salts, saltiness, bitterness,

The response of a taste sensor to inorganic salts was investigated. It is known that in aseries of halides of monovalent alkali metals, salts with low molecular weights are salty,
whereas those with high molecular weights are bitter. Salty substances such as NaClchanged the potential of positively charged membranes of the sensor in the same way as
negatively charged ones. On the other hand, bitter substances affected both membranes
differently. For example, inorganic salts which contain iodine ions markedly changed thepotential of positively charged membranes. Large changes in membrane potential were
induced by the adsorption of taste substances to lipid membranes. Hence a bitter taste
seems to be attributable to adsorption.

1.

Introduction

Taste can be measured with a multichannel lipid-membrane sensor, which responds todifferent taste qualities by unique patterns of sensor output signals. < 1-7l The sensor utilizes
lipid membranes as transducers of tastes and a computer as a data analyzer. The transduc­
ers play a role of transforming taste information generated by chemical substances into
electric potential changes. As a result, the sensor can easily distinguish the qualities of thefive basic tastes, namely sour, bitter, sweet, salty and umami, and also some beverages such

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

as beer, coffee and aqueous drinks. The sensor has sensitivity, durability and reproducibil­
ity superior to humans.
All substances with a salty taste are soluble salts composed of positive and negative ionsin the solid state which dissolve into water to produce a solution of these ions. The tastes
of salts are complicated; they change with concentration. Sodium chloride is the only
chemical considered to possess the pure salty taste, although under its threshold concentra­tion it tastes sweet. Other salts display the same phenomenon but yield complex salty tastes
at suprathreshold values. <8lBoth the anion and cation contribute to taste quality and to the stimulating efficiency. In
a series of sodium salts, the quality of taste elicited vaties with the anion. A similar effectcan be noted in a chloride series with different cations. It is known that in a series of halides
of monovalent alkali metals, salts with molecular weights below 110 are predominantly
salty, whereas those with molecular weights over 160 are mainly bitter.<9l
The relationships between the structure of chemical substances and their taste have beenstudied extensively.<1 0- 11 l Bitter substances are generally hydrophobic; hence some of them
are considered to adsorb to hydrophobic parts of lipid membranes.<12l Adsorption of bitter
substances to lipid membranes has been employed to build up quartz-crystal microbal­
ances.<13l The structures of inorganic salts are simple; hence it is easier to examine theirtastes than those of other chemicals. In this study, the responses of a taste sensor to
inorganic salts are compared with sensory tests. The bitter taste of inorganic salts seems to
be attributable to adsorption to lipid membranes.

2.

Materials and Methods

2.1 Chemicals

Commercial products were used without further purification. All of the halides ofmonovalent alkali metals and sodiumpicrate were obtained from Wako, and quininehydrochloride was obtained from Sigma. Inorganic salts and bitter substances weredissolved in 1 mM KCl solution.
The lipids are abbreviated as follows: dioctyl phosphate, C; trioctyl methylammonium
chloride, T; oleyl amine, N; decyl alcohol, DA; oleic acid, OA. Lipid membranes
designated C:T=9:1, C:T=3:7 and C:T=5:5 are mixtures of two lipids for which the ratioshows the molar concentration. In the membranes C:T=3:7, T and N are positively
charged, whereas in C:T=9:l, C and OA are negatively charged. The membrane DA issomewhat negatively charged due to the presence of plasticizer.< 14l The membrane
C:T=5:5 is almost totally neutral.

2.2 Measurements using a taste sensor

The taste sensor used in this study was similar to one previously reported.0- 7l The
detecting electrode of each lipid membrane was made of Ag wire whose surface was platedwith Ag/AgCl and which was embedded in a basal acrylic board 2 mm thick. Another I cm-thick acrylic board with eight cone-shaped holes was fixed to the board. The holes
were filled with 100 mM KCl solution, and the eight membranes were fitted on the board
to cover the holes.

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

139

The electric potential across the membrane was detected by a Ag/AgCl electrode filledwith 100 mM KCl and a reference electrode (TOA, HS205C). The construction of themeasuring system is as follows: Ag/AgCl electrode in 100 mM KCl solution I membrane I
reference electrode in taste solution. The membrane potential was changed by applying
taste substances, and the electric signal from each membrane was converted into a digitalcode using a digital voltmeter (ADVANTEST, R6551) through a high-input impedance
amplifier and a laboratory-built eight-channel scanner, and recorded in a computer (NEC,PC-9801).

3.

Results

3 .1 Response of the sensor to salty and bitter substances
Prior to describing the study of the response of the sensor to halides of alkali metals, let
us outline the fundamental properties of the sensor. Figure 1 shows the responses of eight
membranes of the sensor to salty NaCl. The responses were measured relative to 1 mM
KCl solution. Cations and anions. exist homogeneously due to thermal motion in the
aqueous solution of NaCl. However, negative charges of the membranes tend to attract
cations and repel anions, producing a diffuse electrical double layer. Therefore, thenegative charges of C, C:T=9:1, DA and OA membranes are reduced by the shielding-O-C:T=3:7- • -T-D-C:T=5:5--T--N-150 .______.____..........,____.___�
1 50�--�---�---�-�
� 100
crj
.,

-.-c

-6--C:T=9: 1-•-DA---v-OA

50
QD=--=:iii�..-.�::Q:::'.:::.::!C.::...--=-_J___J
-2
-1
-3
-4

Concentration (log[M])Fig. 1. Response curves of taste sensors to NaCl.

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

effect of sodium ions, thus increasing the membrane potential. On the other hand, thepositive charges of C:T=3:7, T and N membranes are reduced by the shielding effect ofchloride ions, thus de.creasing the membrane potential. Consequently, salty substancessuch as NaCl change the potential of positively charged membranes of the sensor in thesame manner as negatively charged ones.
Quinine hydrochloride is a typical bitter substance, which has a positive charge and ahydrophobic portion. The response of the sensor to it is shown in Fig. 2. It markedly
changed the potential of negatively charged membranes, and its threshold concentration
was 0.001 mM. The large potential change is based on the adsorption onto the hydrophobic
part of the lipid membrane, as quantitatively shown using electrochemical theory.< 15) The
c1- ion has only a shielding effect. The threshold concentration was about 1 mM.
Another bitter substance is sodium picrate. Picric acid is a strong acid, because it has
three electrophilic nitro groups. As shown in Fig. 3, it significantly changed the potentialof the positively charged membranes, such as C:T=3:7, T, C:T=5:5 and N; therefore, its
conjugate base is effective. At concentrations of 0.3 mM and 3 mM, the potential of the
C:T=3:7 membrane changed by approximately 300 mV, which is higher than the theoreti­
cal value of 60 mV.

�
�

50

.....

0

'
''
'
'

-0-C:T=3:7-.6r- T
-D--C:T=5:5

Ji

-T-N

----0'

0_, , o,, '',-50�--�---�--�--�

300�---.-----,----r----.

>
E, 200--+-C-1:r-C:T=9.: 1-a--DA-V-OA'i§_,,1:l

E 100
0

.ts./
-5

_
___ -A __
_

-4

-3

-2

Concentration (log[M])Fig. 2. Response curves of taste sensors to quinine hydrochloride.

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

0> -100,.--..,

3..... -200

s::: -300
0� -400-O-C:T=3:7- •- T-:-O-C:T=5:5

-T--N

-500
100

\_.-'

.....

11)

0
-100

�

---c

-6--C:T=9:1-•-DA--->v-OA-200 '-----'---�---�--�

-6

-5

-4

-3

Concentration (log[M])-2

Fig. 3. Response curves of taste sensors to sodium picrate.

The common profile in Figs. 2 and 3, which show the responses to bitter quininehydrochloride and sodium picrate, may be an imbalanced response between the twocharged membranes. In fact, quinine hydrochloride and sodium picrate mainly changed
the potential of negatively charged membranes and of positively charged membranes,
respectively. This suggests that adsorption rather than the shielding effect plays animportant role in the response of the sensor to bitter substances.

3.2 Response of the sensor to various inorganic salts

To elucidate the relationship between taste qualities and kinds of inorganic salts, theresponse of the taste sensor to several salts was measured. Figure 4 shows the responses ofnegatively charged membranes of the sensor to the chloride series with different cations,
namely LiCl, NaCl, KCl, RbCl and CsCl. The result for Na+ is omitted because it is shownin Fig. 1. The responses of the sensors to monovalent cations were not always the same.
The potential change was large in the following order: Li+ > Na+ > K + > Rb+ > Cs+,
coinciding with the order of decreasing crystal radii.
The responses of the positively charged membranes to the sodium series with different
anions, namely NaF, NaCl, NaBr and Nal, are shown in Fig. 5. The responses of the142

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

> wo

200

5 150

150

L·1+

..... 100

�

100

-•-DA

�OA

00----0--0-a�:l!t::::���

200..... 100

100

5 150

150

50

50

I

-b-C:T=9: 1

50

>200

�

-+-C

o,._.-0----c:1-=�;,r,,,=tii:;;:;..,�...c,ot>-���.o-��;.

Concentration (log[M])Concentration (log[M])

-4

-3

-2

-1

-4

-3

-1

-2

Fig. 4. Response curves of taste sensors to various monovalent cations.

�o�--1-111-m-+e:::1!:1!H!�

]-100

� -200-0-C:T=3:7-4--T-D--C:T=5:5-200

p.. -300'---'---'---'----'--� -300'---'---'--�---'---'--'
0

�-100

�

-y- N

-100

Br-

-� -200-200

p..-300'----'----'--....,__-�-� -300'--�-�--'----'--�
�

�

4

�

�

-1

Concentration (log[M])

�

�

4

�

�

-1

Concentration (log[M])Fig. 5. Response curves of taste sensors to various monovalent anions.

sensors to monovalent anions also differed depending on the species. The change in themembrane potential increased in the following order: 1-> Be> Cl-> F .- In contrast to
cations, this is the same as the order of increasing crystal radii.

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

143

3.3 Adsorption of inorganic salts to lipid membranes

Anions of high atomic weights such as Br and 1- markedly changed the potential of
positively charged membranes; therefore, they may adsorb to hydrophobic membranes. If
the ions adsorb to the membranes, their after-effect will be detected when the membranes
are washed with 1 mM KCl solution.
Table 1 summarizes the potentials of positively charged membranes which were dipped
into 100 mM NaF, NaCl, NaBr and Nal, and the potentials of the membranes which weretransferred into 1 mM KCI. If these ions have only a shielding effect, the potential values
in 1 mM KCl should be zero, because the data were obtained relative to 1 mM KCI. But asshown in the table, the effect of pretreatment with inorganic salts remains even in 1 mM
KCl solution, indicating the occurrence of adsorption.

4. · Discussion
Eight lipid membranes of the taste sensor are roughly divided into negatively charged
membranes and positively charged ones. Salty NaCl affected both membranes equally by
the shielding effect (Fig. 1). Contrary to this, negatively charged membranes unilaterally
responded to bitter quinine hydrochloride. In the same manner, positively charged
membranes markedly responded to bitter sodium picrate. The large potential changes
exceed the theoretical value of 60 mV; hence this effect may be attributable to adsorption.
The responses of the two types of membranes to bitter substances are not balanced becauseof adsorption. Moreover, two types of bitter substances must exist: one which mainly
affects negatively charged membranes; the other, positively charged ones.
It is known that in a series of halides of monovalent alkali metals, salts with low
molecular weights are salty, whereas those with high molecular weights are bitter.<9l
Therefore, the responses of the taste sensor to various inorganic salts were investigated. It
was revealed that the responses to cations are large and occur in the order Li+> Na+> K+>
Rb+> Cs+ , whereas those to anions are large and occur in the order 1 >- Br> Cl-> F- (Figs.
4 and 5). Combinations of cations with high atomic weights, such as Rb+ and Cs+, andanions with high atomic weights, such as Br and 1 ,- may primarily affect the positively
Table 1After-effect ofvarious inorganic salts on positively charged membranes (unit, m V).
t�l00mMNaF
1mMKCllOOmMNaCl
1mMKCll00mMNaBr
1mMKCllO0mMNaI
1mMKC_l

C:T=3:7

T

C:T=5:5

N

-1.6
2.6
-89.3
-5.6
-148.9-21.8-221.9
-82.4

-12.2
4.1
-93.9
-11.7
-161.1-34.3-263.6
-59.4

-31.1
2.5
-10.2
-9.2
-25.7
5.4
-94.4
-18.5

-58.2
2.6
-82.5
-13.1
-140.9-23.7-219.1-98.4

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

charged membranes of the sensor, which resembles the effect of sodium picrate. This
result explains why inorganic salts with high molecular weights are bitter.
The order of responses of the sensors to cations and anions were opposite with respect
to the crystal radii. Electronegativity seems to explain the order of responses best.
Electronegativity values for anions are as follows: r - 2.5, Be 2.8, CI- 3.0, F - 4.0. <1 6l The
value fo r a positively charged membrane is unknown; the value of Na+ 0.9 could besubstituted tentatively for that of the membrane. Differences in electronegativity between
cations and anions in Nal, NaBr, NaCl and NaF are 1.6, 1.9, 2.1 and 3.1, respectively. The
difference in Nal is the smallest; hence, the interaction between Na+ as the positively
charged membrane and 1- must be stronger than the others.
The taste sensor was developed to evaluate the tastes of foods objectively. Moreover, it
was very useful in revealing that the taste of astringency consists of both chemical andphysical stimulations, whereas the sense has been recognized as a physical one, namely atactile sensation. <3J The further application of the sensor to the study of taste is expected.

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