Sensors and Materials, Vol. 11, No. 8 (1999) 447-456

MYUTokyo
S &M0387

Detection of Mutual Interaction between
Taste Substances by Impedance Measurements of
Lipid/Polymer Membranes
Shu Ezaki and Hideyuki Kunihiro
Department of Electrical and Computer Engineering, Kinki University
Iizuka, Fukuoka 820-8555, Japan(Received June 29, 1999; accepted October 14, 1999)
Key words: taste sensor, lipid/polymer membrane, synergistic effect, impedance measurement,umami sub,stance

Response differences between the membrane potential and the membrane impedanceof lipid/polymer membranes to aqueous solutions of mixed taste substances were investi­
gated. Thin membranes formed by dip-coating were useq to measure the impedance,
whereas thick films formed by the same process in a multichannel taste sensor were used
to measure the electric potential. The changes in memprane impedance were significantly
accelerated by the combination of two umami substances in a manner similar to thesynergistic effect in the human sensation, whereas such acceleration did not occur in thechanges in potential.

1.

Introduction

Until recently, a chemical sensor for human sensations could not be realized, because· it was difficult to test all the various substances that affect human· chemical sensations.
Taste- and odor-sensing systems with global selectivity, wl:\ich is the opposite of specificselectivity in conventional sensors, have been recently developed using biomimetic
technology. C1-3l

447

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

Lipids, which are major components of a biological membranes, have been identified
for their sensitivity to taste substances_C4l A multichannel taste sensor with lipid/polymer
membranes has been developed. cz.5J It responds to basic taste substances with global
selectivity in a manner similar to the human sense.
The taste sensor is fundamentally based on the measurement of the electric potential
difference across a membrane just as taste cells cause a membrane potential change as thefirst step in gustatory sensation. However, the sensor is less sensitive to hydrophobic or
nonelectrolytic substances than the human sense.
For the purpose of detecting taste substances including nonelectrolytes, some mea­
surement methods other than potentiometry were proposed for taste sensing using a lipid/
polymer membrane.<6-9l The characteristics of the interaction between the lipid/polymer
membrane and taste substances were investigated using impedance measurements,<6>
surface plasmon resonance,(?) quartz oscillation<8l and semiconductor technology.<9l These
attempts yielded information different from that obtained by potentiometry.
The basic response characteristics of membrane impedance when taste substanceswere added one at a time were reported. C6l The membrane impedance of the lipid/polymer
membrane effectively responded to nonelectrolytic taste substances. There are, however,numerous taste substances in actual foods, and substances in combination can affecthuman chemical sensations.
In this study, the responses of membrane impedance to mixed taste substances incombination were measured and compared with those of the membrane potential. An
umami substance was selected as one of the mixed substances. The reason is that umami,
which was recently added as a fifth basic taste,< 10J shows a synergistic effect that is aremarkable enhancement of the umami taste by the combination of two types of umami
substances.<' 'l The synergistic effect between umami substances could be detected by animpedance change similar to that reported in a lipid impregnated into the pores of filter
paper.OZ)

2.

Materials and Methods

2.1 Lipid/polymer membranes

The lipid/polymer hybrid membranes, which have been used as a transducer for thetaste sensor, consist of synthetic lipid, polyvinyl chloride (PVC) as the forming polymer,
and di-n-octylphenylphosphonate (DOPP) as the plasticizer.
The lipids are abbreviated as follows: dioctyl phosphate, DOP; trioctylmethylammonium
chloride, TOMA; oleyl amine, OAm; decyl alcohol, DA; oleic acid, OA. The mixtures of
DOP and TOMA are named 9:1, 5:5 and 3:7, indicating the ratio of molar concentrationsof the components. Each lipid, 400 mg of PVC and 0.5 ml of DOPP were mixed together
in a test tube with 10 ml of tetrahydrofuran as the solvent.
A membrane to measure the electric potential was obtained by evaporation of solvent
from a glass plate. The membranes formed, which were transparent and soft films ca. 200µm thick, were placed on the surface of a multichannel electrode together with an inner
solution (1 M KC!).

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

The 200-µm-thick membrane was not suitable to measure the impedance, because theimpedance change with taste substances was very small. Therefore, a thin membrane tomeasure the impedance was formed by dip-coating directly on a platinum electrode 2 mm
in diameter. The estimated thickness of the membrane was less than 1 µm.

2.2 Measurement of membrane impedance
The membrane impedance was measured using an alternating electric current source
applied to the lipid/polymer membrane in the test solution. Figure 1 shows the experi­mental apparatus for measuring the impedance of the membrane.
A frequency response analyzer (FRA; NF electronic instruments, 5020) produced asinusoidal voltage wave. The sinusoidal electric current converted from the voltage wasapplied from a counter electrode to the sensing electrode across the lipid/polymer
membrane. The voltage response between the sensing electrode and the referenceelectrode was fed into the FRA. The FRA estimated the complex values of the membrane
impedance from the input-output characteristics of gain and the phase shift.
2.3 Measurement of membrane potential
The membrane potential was led using the multichannel electrode with the lipid/
polymer membranes and a reference electrode with an inner solution of saturated KCl.
The structure of the electrodes in the test solution is as follows: Ag/AgCl wire I 1 M KC!
I membrane I test solution I saturated KCl solution I Ag/AgCl wire.

Frequency response
analyzer
V/I converter

Counter electrode
Reference- electrode
Lipid/polymer
membraneStirrerD
PC=

Stirring bar

Fig. 1. Experimental apparatus to measure the impedance of the lipid/polymer membrane.

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

The electric potential signals from the membranes were sent to a scanner throughhigh-input impedance amplifiers. The signals selected sequentially by the scanner wereconverted to a digital code by a digital voltmeter and collected in a computer.

3.

Results and Discussion

3.1 Impedance paths and equivalent circuits
Figure 2 shows the impedance paths of the lipid/polymer membrane in a complex
plane. The input frequency was swept from 1 Hz to 10 kHz.
Approximately circular arcs were obtained for DOP, DA, OAm and OA membranes asshown in Fig. 2(a); therefore these membranes may be approximated, except at a very low
frequency range, by the equivalent circuit of a parallel connection of an electric resistanceand, an electric capacitance .
. 'The values of equivalent resistance and equivalent capacitance were estimated fromthe impedance at a specified frequency where the resistance component and the reactancecomponent ofthe membrane impedance were almost equal, i.e., the middle point of the2

Resistance
64

8(Xl0Q)
10"'Cl0......

(a)

X

u

Q)

0

(b)

.,,Cl
0
......

-2

\OA

-4

0
0

Resistance1(X 10 Q)
2

X

u
Q)

u
-1
Fig. 2. Paths of the membrane impedance in a complex plane with input frequency changes from 1
to 10 kHz. (a) DOP, DA, OAm and OA membranes; (b) 9:1, 5:5, 3:7 and TOMA membranes.

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

451

circular arc. For example, the frequency was adjusted to 100 Hz for a certain measurementof the OA membrane. The resistance and the capacitance of this membrane were
estimated to be 6.5 MQ and 0.25 nF, respectively.
On the other hand, the impedances of the membranes containing TOMA, i.e., 9: 1, 5:5,
3:7 and TOMA, were quite small, and the circular shapes in the paths of these membranes
were limited to a narrow range as shown in Fig. 2(b). The equivalent resistance and thecapacitance were therefore hard to estimate in these membranes.

3.2 Impedance and potential change by taste substance

The value of the membrane impedance at every measurement in the initial solution
containing 10 mM KCl without added taste substances varied by ca. 30% (data notshown), whereas the membrane potential had high reproducibility. It may be inferred thatthe variation in the membrane impedance resulted from the difference in thickness. The
membrane thickness affects the impedance, whereas the potential of the lipid/polymer
membrane is theoretically independent of its thickness.( 13l The ordinates of resistance andcapacitance are, therefore, displayed by a relative value taken as unity as the initial value
in this paper.
Figure 3 shows changes in the equivalent resistance, the equivalent capacitance andthe electric potential of OA membranes with increasing concentration of the five basic
taste substances. The impedance changed due to the taste substances and produced astriking feature in the equivalent.circuit as follows: the resistance decreased with anincrease in the concentration of taste substances as shown in Fig. 3(a), whereas no change
appeared in the capacitance as shown in Fig. 3(b).
Figure 3(c) shows the dependence of the potential on the taste substances. The
membrane potential increased, because the OA membranes acted as positively charged
surfaces.
The membrane resistance and the membrane potential had something in common in
their response to the five basic substances. Taste substances could be put in increasingorder of the threshold in both the resistance shown in Fig. 3(a) and the potential shown in
Fig. 3(c). as follows: quinine hydrochloride (bitterness), HCl (sourness), monosodium L­
glutamate (MSG; umami), and NaCl (saltiness). Both the resistance and the potential
showed little sensitivity to sucrose (sweetness).
Figure 4 shows the changes in the resistance and the potential of OA membranes as theconcentration of two umami substances was increased. MSG as a kind of amino acid anddisodium inosine 5 '-monophosphate (IMP) as a kind of nucleotide were selected asumami substances in this work.
The resistance and the potential had less in common in response to MSG and IMP. The
potential responses to MSG and IMP were approximately equal in both the threshold (ca.
1 mM) and slope. On the other hand, the threshold of IMP was approximately ten times
lower than that of MSG in the resistance.
Thus,. the responses of the resistance and the potential to taste substances had somecharacteristics in common as well as some differences.

452

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

(b)

(a)
1.2

.:::

Q)

�

Q)

0.8

§ 0.6 .._

0.6

-� 0.4·g 0.4 §<U 0.2 .._

0.2
0

1.2 -----------�

.::::
't;l
�
� 0.8 -

0.001

0.1

10

1000

Concentration (mM)

0 '--'----'---'-----'---'----'---'
10
0.001
1000
0.1

Concentration (mM)(c),........_

250---<r-NaCl
Quinine-v-HCI---*- Sucrose-<>-MSG

200

-I',-

� 150

"i

100

-�... 50

0
-50

0.001

0.1

10

1000

Concentration (mM)

Fig. 3. Changes in electric characteristics of OA membranes due to basic taste substances: (a) theequivalent resistance, (b) the equivalent capacitance and (c) the electric potential.

3.3 Influence of the combination of two taste substances
The electric response to a solution of MSG and IMP in combination was measured.

Figure 5 shows the changes in the equivalent resistance and the potential of OA mem­branes with increasing concentrations of one umami substance in combination withanother umami substance in solution.
A decreasing resistance was augmented by the increasing concentration of the otherumami substances as shown in Figs. 5(a) and S(b). On the other hand, no remarkableeffect of the combination of umami substances appeared in the potential as shown in Figs.
5(c) and 5(d).

453Sensors and Materials, Vol. 11, No. 8 (I 999)

� 1.2

-�

,._,,

0.8

Q)

0.6

(a)
en

0.4

· rn 0.2
Q)

0
0.001

>a

--<>-MSG---o-JMP
0.1

10

0.1

10

Concentration

(mM)

250

,._,, 200(b)........

150

0..

50

·g 100
-�

Q)

�

0
-50
0.001

Concentration(mM)

Fig. 4. Changes in electric characteristics of OA membranes due to umami substances: (a) the
resistance and (b) the potential.

These effects of combination shown only in the resistance can be interpreted as asignificant increase in sensitivity to one umami substance caused by adding a minuteamount of another umami substance. This seems to be a synergistic taste effect between
umami substances recognized empirically.
The electric responses to four basic substances in combination with a small amount of
MSG were also measured. Figure 6 shows the change in the equivalent resistance and thepotential of OA membranes with increasing concentrations of quinine and HCl when 3
mM MSG was added to the solution.
The responses of the resistance and the potential to quinine are shown in Figs. 6(a) and
6(c), respectively. The threshold concentration of quinine in the graph of resistancedecreases well below 1 �LM in combination with MSG, whereas little effect of combina­
tion with MSG appeared for the potential as shown in Fig. 6(c).
The responses of the resistance and the potential to HCl are shown in Figs. 6(b) and
6(d), respectively. In the case of the responses to HCl, the threshold concentration roseSensors and Materials, Vol. 11, No. 8 (1999)

454

(a)

v
-�

(b)

1.2

-� 0.60.8
�-� 0.60.4

i:i::: 0.4

0.8

V

"'
<1)

0.2 �--�--�--�-�
0.1
0.001
10

100

0.01

Concentration (mM)Concentration (mM)

(d)

(c)

>

250

3 200
�

·,g 1500 MSG
�MSG+0.1 mMIMP-+-MSG+0.3 mMIMP

100

.;:Z

> 250 .....-----------,s-.....;;, 200
�
150
..g
8,100

·.g

50

.;:Z

0

W

-50

-G-IMP-®-IMP+ 1 mM MSG--11--IMP+ 3 mM MSG

� 50

0.01

100
Concentration (mM)

0-50 �--�--�--�-�
0.001
0.1
10
Concentration (mM)

Fig. 5. Changes in electric characteristics of OA membranes due to one umami substance in
combination with another. Resistance response to (a) MSG and (b) IMP; potential response to (c)MSG and (d) IMP.

both in the resistance and in the potential. It may be inferred that this result was
influenced by the pH change in the solution when MSG neutralized the acid.
The MSG at 3 mM concentration had little effect upon all responses of the resistanceand the potential to the remaining two basic taste substances, i.e., NaCl and sucrose (datanot shown).
The responses of the electric characteristics of OA membranes to taste substances areshown in Figs. 3 to 6. The DOP, DA and OAin membranes also responded both in theresistance and in the potential. The responses of these membranes were different fromthose of the OA membrane, and the synergistic effect also appeared only in the resistance455

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

(b)

(a)

-��

-��

d)

d)

--!::.-Quinine___..._ Quinine+3mMMSG

�
� 0.8
Q)

�
� 0.8

0

0

.;2

.;2

Q)

� 0.4

§ 0.6

§ 0.6
.....
r:r,

� 0.4
0.2

0.001

3
·.p

·50

-T-HCl+3mMMSG

0.01

100

Concentration (mM)

(d)

>

250

250

3 200

200

-� 150..... 150
0

0.2

10

Concentration (mM)

(c)

>

0.1

--v- HClo.. 100

-�
50
ra 0

100

0

ra 50

Q)

Q)

0
0.001

0.1

10

Concentration (mM)

-50

0.01

100

Concentration (mM)

Fig. 6. Changes in electric characteristics of OA membranes due to quinine hydrochloride and HCl
together with 3 mMMSG. Resistance response to (a) quinine hydrochloride and (b) HCl; potential
response to (c) quinine hydrochloride and (d) HCl.

as shown in Fig. 5 (data not shown). The stability of the impedance of these membranes
was, however, less than that for the OA membrane, and the reproducibility was not
sufficient. The temporal stability in the impedance measurement of membranes otherthan the OA one must be improved.
One type of umami substance raised the sensitivity of the lipid/polymer-membrane
impedance to the other umami substance as shown in Fig. 5. The MSG also raised thesensitivity to the bitter substance. It was pointed out that umami and bitter substancesabsorb into lipids.c2· 12l The measurement of the membrane impedance might be affected by
the strong interaction between the lipid/polymer membrane and the taste substances.

456

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

Summarizing the experimental results in this paper, a remarkable change in themembrane impedance in response to taste substances was obtained using a thin lipid/
polymer membrane formed by dip-coating, and the response characteristics of the mem­brane impedance added useful information for the development of a taste sensor. In
particular, the response difference between the impedance and the potential was notablein mixed solutions containing two taste substances. The synergistic taste effect could be
detected only in the impedance measurement.
The impedance and the potential of OA membranes did not respond to sucrose. An
improvement in the sensitivity to nonelectrolytes remains to be achieved. It is, however,difficult to make the characteristics of a lipid/polymer membrane completely coincidewith the sensation of human taste. A measurement including the potential and theimpedance of multiple membranes seems to be necessary to realize an artificial sense of
taste. It is also necessary to establish an analytical method for the extraction of theinformation about taste from multidimensional measurements.

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