Sensors and Materials, Vol. 13, No. 3 (2001) 145-154
MYUTokyo
S &M0438

Fundamental Study of Various Taste Solutions
by Ultrasonics
Yohichiro Kojima* and Hatsuyoshi Kato
Tomakomai National College of Technology
443 Nishikioka, Tomakomai 059-1275, Japan(Received October 7, 2000: accepted February 10, 2001)
Key words:

taste solution, taste sensing, ultrasonics, layered medium, resonant transmission

In this paper, the possibility of taste sensing is considered for five basic taste solutions
and marketed beverages by making use of their ultrasonic characteristics. First, soundvelocities are discussed in basic taste solutions and beverages at frequencies of approxi­
mately 2 MHz. Second, a novel layered medium is proposed for acoustic impedance
matching devices with metals, and experimental results are shown for ultrasonic pulse
transmission. Finally, the feasibility of taste sensing using the ultrasonic characteristics ofa layered medium is discussed.

1.

Introduction

As elucidations about the chemical senses of human beings are advanced, biosensors
and chemical sensors are being developed which imitate the structure of chemical senses.
Further, "taste sensors" and "odor sensors" are put to practical use with various lipid
membranes.< 1, J Taste sensors have been studied using surface plasmon resonance andsurface photo voltages.<3,4l In particular, the application of surface acoustic devices as
ultrasonic devices to the sensors is reported not only for discrete tastes but also forsolutions of taste substances.<5) Ultrasonic sound propagates with different characteristics
in various materials. Its propagation characteristics reflect structures and properties of thematerials. Therefore, research on the propagation of ultrasonic sound is very important forevaluating physical properties of liquids or solutions. There is the possibility that ultra­
sonic sound may become an effective method of taste sensing.
2*E-mail: kojisan@ gt.tomakomai-ct.ac.jp

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

Two kinds of experiments are reported in this paper. As experiment I, ultrasonic pulses
are directed incident to some taste solutions, and we examine the possibility of taste
sensing from changes in velocities of the ultrasonic pulses. The taste and deliciousness offood which humans sense are composed of many factors: sweetness, acidity, saltiness,
bitterness, and umami, which are the five basic tastes. In previous studies, the five basic
tastes were detected by the electric response and optical response of lipid membranes. We
also used solutions of these five tastes as standards. Moreover, interactions of tastes whichstrengthen each other and weaken each other are known in several taste mixtures. A
counterbalance effect and a contrast effect were examined as interactions of tastes in our
experiments. Sound velocities were also measured by sound propagation in marketed
beverages.
An acoustic impedance matching device is necessary to improve the transmission of theultrasonic sound to water or taste solutions. For this purpose, a novel layered medium was
fabricated. Ultrasonic pulses were directed incident to water by placing the layered
medium between the pulser and water in experiment IL

2.

Experiment I

2.1 Materials and methods

We used five taste solutions which contained chemicals typical of each basic taste, i.e.,
HCl for sourness; NaCl for saltiness; quinine for bitterness; sucrose for sweetness; Na­glutamate for umami. Concentrations of those solutions ranged from 0.1 to 1000 mM.
Beverages were made by Kirin Beverage Company: "straight tea in the afternoon," "teawith lemon," "earl grey," "milk tea," "orange juice" and "tomato juice." The temperatureof the solutions was set to 22±1 °C during experiments. The outline of the measurement
system is as follows: An electric impulse is produced by a pulser (made by PANAMETRICS
Company, Model 500PR) and is transformed into a ultrasonic pulse by a transducer (madeby PANAMETRICS Company) at frequencies around 2.25 MHz. The ultrasonic pulse istransmitted into a solution from the transducer. The pulse propagates in the solution and isreflected at the end of a cell filled with the solution. The reflected ultrasonic pulse istransformed to an electric signal by the same transducer. The signal is observed with adigital oscilloscope (made by SONY Tektronix Company, model TDS3012) and analyzed
by a computer. Sound velocities are calculated from propagated distances divided by thetime intervals between the ultrasonic pulses received.

2.2 Results and discussion

Concentrations of the five basic taste solutions were set at the unique value of 100 mM
by measuring the velocity of sound passing through them. Figure 1 shows sound velocities
in the five basic taste solutions and in water which is not mixed with any taste substances.
The water was passed through an ion-exchange membrane after distillation. We observed
definite differences between sound velocities in the sweet solution and those in the salty
solution. The sweet solution is nonelectrolytic. In contrast, the salty solution is strongly
electrolytic. We conclude that the difference between the sound velocities is caused by theelectrolytic characteristics of the two solutions. The sweet solution and bitter solution

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SensorsandMaterials, Vol.13,No. 3 (2001)

1520

·u

1500
1480
1460
<?;-<,,

��

cs

&

�CJ

��

�

4'-

0

�l� <>���
..._<?.i-<,,Fig. 1. Velocity of sound in water and in solutions of the five basic tastes (100 mM).

showed a similar tendency in sound velocities. However, some foods are mixtures of bitter
solutions at a concentration of about 100 mM. We sense bitterness at concentrations lessthan 100 mM, and sense sweetness at concentrations higher than 100 mM. For practicaltaste sensing, the sound velocities may change from the values shown in Fig. 1. Therefore,
these two tastes are easily identified. In acidic solutions and salty solutions, solutions
which are both monovalent, the sound velocities are slightly different. In this case,differences between the ions of Na+ and H+ may be evident.
Figure 2 shows sound velocities in solutions with various concentrations of sweetness(sucrose). As the concentration increases, the sound velocity increases as well. On theother hand, sound velocities in water and in solutions with concentrations less than 1 mM
are comparable. Therefore, the identification of those solutions on the basis of soundvelocity is difficult. However, changes in sound velocities are detectable, because thesweetness threshold for humans is near 100 mM.
Practically, humans can sense complex tastes. We measured sound velocities in taste
solutions which were mixtures of two kinds of solutions. Figure 3 shows one example of
the results. We can see combinations of tastes that are known as the comparison effect andthe counterbalancing effect. The sweet solution at 1 M and the salty solution at 1 M wereused to evaluate the comparison effect. The mixture of 0.1 M sweet solution and 0.1 M
bitter solution was used to evaluate the counterbalance effect. The solutions were mixed
with the same concentrations. Each sound velocity in the single taste solution is shown in
the figure for the sake of comparison. The velocity of sound in the mixed taste solutions
shows a tendency different from the velocity of sound in single taste solutions. We could
not identify which taste substances influence the sound velocities in these mixed taste
solutions. We consider that the differences between the sound velocities are caused by theelectrolytic characteristics of the two solutions. Sound velocity C in a liquid can beformulated as follows:

C= {K
�p'

(1)

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

1580..... 1540

> 1500
1460

0.1

10

100

Concentration(rnM)

1000

Fig. 2. Velocity of sound to density of sweetness solution (Sucrose).

1580
1540

·u
> 15001460

Fig. 3. Velocity of sound in mixed taste solution.

· where K is bulk modulus and pis density. From this formula, it seems that differences inthese values affect the mixed taste solutions. Therefore, there is a possibility that theinteraction of the tastes can be expressed by an appropriate method using the velocity ofultrasonic sound. There is another effect called the synergy effect that describes theinteraction of the tastes. The synergy effect appears when several kinds of umami
substances are mixed. Further studies are necessary on this effect.
Many taste substances are included in marketed beverages. Sound velocities in fourkinds of tea and two kinds of juice are shown in Fig. 4 and Fig. 5, respectively. The soundvelocities in straighttea and in Earl Grey have values comparable to the velocity in water.
Various taste substances may be mixed in the tea. As a result, the sound velocities in milk
tea and in tea with lemon are higher than the sound velocities in straight tea. We cannot
identify which taste substances influenced the sound velocities in these teas. However,many acidic elements and sweet elements may be present in the lemon tea. Many kinds ofSensors and Materials, Vol. 13, No. 3 (2001)

,-...
ell

149

1520

]_ 1500·oc..9 1480
1460

Fig. 4. Velocity of sound in beverages (tea) on the market.

1520
1500
1480
1460

Fig. 5. Velocity of sound in beverages (juice) on the market.
milk fat are added to straight tea to make milk tea. Therefore, the differences among theseelements may influence the sound velocity. If we compare tomato juice with orange juice,the sound velocities in these juices have obvious differences. When we drank the juice, we
subjectively felt that the orange juice was sweeter than the tomato juice. In taste sensing by
lipid membranes which has been reported, the performance of the sensors in detecting the
sweetness of a solution is insufficient. By our method using ultrasonic sound, a result
indicating the feasibility of detecting sucrose was obtained as shown in Fig. 1. and Fig. 2.

3.

Experiment II

3.1 Basic theory of resonance

Recently, both theoretical and experimental studies of acoustic phonons have been
carried out as part of research on mesoscopic superlattices (layered media), which are made

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by stacking atomic monolayers.C6-15l In these studies, frequencies of acoustic phonons
range from several GHz to 1 THz. If we use frequencies of about several MHz, it ispossible to consider that the same results occur using ultrasonic sound and layered media a
few mm in size.
One part of the present study is to demonstrate the resonant transmission of acoustic
phonons in superlattices using ultrasonic sound and layered metal media.
We show a basic system for the theoretical consideration of this experiment in Fig. 6.
One free surface of the layered medium (LM) is immersed in liquid. The LM is made withsolid bilayers AB which are stacked alternately. For simplicity, we assume that the
acoustic impedance of the transducer is the same as those of the B layers. The ultrasonicsound is incident normal to the interfaces in the LM from one surface of the LM. After
travelling through the LM, the ultrasonic sounds is transmitted to the liquid from the otherLM surface, and a portion of the sound energy is reflected from the free surface back to the
LM. Transmitted sound pressure in the liquid is measured by a hydrophone.
Figure 7(a) shows transmission rates of ultrasonic sound from the LM into water as aliquid, where material A is Cu with thickness 0.5 mm and material B is Ag with thickness
0.5 mm. The total number of solid bilayers is eight. The acoustic impedances of the system
are 44.7 for Cu, 38.3 for Ag, and 1.48 for the water in units of 10 6 kg m-2s-1•
If we assume that the ultrasonic sound is a continuous wave, then there is a frequency(vR = 2.08 MHz) at which its transmission rate has a peak in spite of the large mismatch
between the acoustic impedances of Cu and Ag with water. The water has a value less than
one tenth of those of Cu and Ag. At this frequency vR, an infinite periodic layered medium
has a stop band which suppresses transmission of the sound. However, a finite LM has alocalized oscillation at the surface immersed in the water.C12i With this oscillation, theultrasonic sound is efficiently transmitted from the LM into the water.0 6l The time neededfromElectric pulser
toOscilloscope

Transducer

LiquidHydrophone

Fig. 6. Experimental instruments.

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151

1.0

P::.

(a)

0.8

.s 0.6p-� 0.4
p

0.2
2

3

6

4

Frequency (MHz)--;;,- 7

66
S

t::

-�

"§

(b)

5
4

3
2'" ._, ,., ""� o�������������������
0

2

3Frequncy (MHz)

4

5

6

Fig. 7. (a) Transmission rate and (b) transmission time through Cu/Ag layered medium.

for the ultrasonic sound to pass through the LM at this resonance is longer than that in thecase of nonresonance as in Fig. 7 (b).
The above resonance transmission is applicable to acoustic impedance matching
devices at the resonance frequency vR.

3.2 Ultrasonic pulse transmission experiment

In the above subsection, we assume that ultrasonic sound is a continuous wave.
However, a pulsed wave is practical for applications of ultrasonic sound. In this section, an
experiment using ultrasonic pulses is discussed.
When an ultrasonic pulse was incident onto the LM as shown in Fig. 6, sound pressuresof the ultrasonic pulse in water were measured as in Fig. 8(a). The energy flux is calculated
from the pressures in water and is shown in Fig. 8(b); it is proportional to the power
spectrum of the pressures. For the sake of comparison, we replaced the LM with 8.0-mm­
thick isotropic bulk Cu plate. With this configuration, the pressure in water and the energy
flux became as shown in Figs. 9(a) and 9(b), respectively.
Incident ultrasonic pulses passing through the Cu plate penetrate gradually into thewater with repeating reflections between the two surfaces of the Cu plate. This property isclearly evident in Fig. 9(a). If we can assume that the phases of the ultrasonic pulses do notchange with the reflections, the power spectrum of the pressures (the energy flux) musthave peaks at the same frequencies as those of the following function:

152

Sensors and Materials, Vol. 13, No. 3-(2001)....."'·2 0,60.4,,-.., 0.8(a)..0 0.2.___,, 0.0
Q)

-0.2

"'"' -0.4
Q)
lo.

-0.6

p... -0.8

0

20

80

60

40

Time (µs)$ 1000

·2

800

(b)

� 600
i;::

400

�200
lo.
Q)

c::

0

2

3

Frequency (MHz)4

5

Fig. 8. (a) Sound pressures and (b) energy flux in water from Cu/Ag layered medium.

....."' 0.6.....

,,-.., 0.8

"8

(a)

0.4

0.2

ro 0.0.___,,....;::! -0.2"'"' -0.4.... -0.6
P-s
Q)

Q)

-0.8

0

20

60

40

Time (µs)

80

(b)

2

3

Frequency (MHz)4

5

Fig. 9. (a) Sound pressures and (b) energy flux in water from an isotropic Cu plate.

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IS(v)i =1 r-e4sinh 2 (r /2) + sin 2 nvr(2)

where vis frequency and yis the damping rate of the pressure pulses in the water as shownin Fig. 9(a). From this function, we see that the peaks are separated by the same frequency
interval 1/r. The value of r is calculated as 3.4 µs from Fig. 9(b). This value is almost thesame as the theoretical value of 3.8 µs which is derived from the round trip time in the 8.0mm-thick Cu plate.
Energy flux into the water from the LM is shown in Fig. 8(b). Peaks do not have thesame frequency intervals. This implies that the pulse reflection is not simple and its travel
time is different at the frequency shown in Fig. 7(b). However, near the resonancefrequency VR, both energy fluxes in Fig. 8(b) and Fig. 9(b) have peaks. Further, peaks in the
spectrum for the LM are at frequencies near the resonance frequency vR, when we compare
Fig. 8(b) with Fig. 9(b). This feature resembles the drawing effect in the field of nonlinear
phenomena.

4.

Summary

We collected basic data on taste sensing using the velocities of ultrasonic sound in fivebasic taste solutions and two marketed beverages from experiment I. For realistic
applications, we need more precise treatments. One method is to detect the absolute valuesof the energy flux of sound in the liquids. If this becomes possible, the damping
characteristics of ultrasonic sound in liquids will help to distinguish the taste solutions
precisely. To realize this, the hydrophone must be immersed in the taste solutions. In thiscase, we need ultrasonic sound to penetrate into the liquids more efficiently. Further,transducers must emit a stable sound spectrum which is not affected by loads, i.e., the kindof liquid. For this purpose, acoustic impedances should be matched sufficiently. However,transducers with a quarter-wave plate change their spectra if we replace the loads. To
render the spectrum stable, it is feasible to use the LM discussed in experiment II, which isa constituent of rigid metals.
This work was supported in part by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No.
11650062).

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