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Objective scaling of taste of sake using taste sensor and glucose sensor
MtERlALE
SUEBKE& ElGlHEERIN8 MaterialsScience and Engineering C 4 ( 1996) 4549
C
Objective scali:ng of taste of sake using taste sensor and glucose sensor Satoru Iiyama a, Yuji Suzuki a, Shu Ezaki b, Yukihiko Arikawa ‘, Kiyoshi Toko d** aDepartment of Industrial Chemistry, Faculty of Engineering, Kinki University in Kyushu, Iizuka 820. Japan b Department of Electrical Engineering, Faculty of Engineering, Kinki University in Kyushu. Iizuka 820, Japan ’ Food Technology Research Institute of Nagano Prefecture, Nagano 380, Japan d Department of Electronics, Faculty of Engineering, Kyushu University 36, Fukuoka 812. Japan Received 30 June 1995; in revised form 29 September 1995
Abstract The taste of Japanese sake wals investigated using a taste sensor with eight kinds of lipid membranes and an enzymatic glucose sensor. The electric-potential pattern constructed of eight outputs from the taste sensor has information on the taste quality and intensity. A standard solution for sake measurement was synthesized. Therefore, chemical meanings were assigned to two perpendicular axes transformed from the nine variables of outputs from the two sensors; commercial brands of sake were scattered on this plane. The taste of sake was discussed objectively. Keywords: Taste sensor; Electric potential; Lipid membrane; Glucose sensor; Taste; Sake
1. Introduction The taste of sake is quite delicate and complicated. However, Sato et al. [ l] elucidated that the taste could be adequately represented by three major factors; sweetness (amakara), fullness (kosa) and cleanness (kireisa) . Moreover, they [ 21 proposed equations which relate the taste with two ingredients, i.e. the sugar content and the titratable acidity. The equations imply in general that the sweetness of sake could be approximated by the difference between the sugar content and the titratable acidity, and the fullness by the sum of both the ingredients. Hence, the two tastes of sake may be expressed by these two terms. Taste can be measured with a multichannel taste sensor which uses several kinds of lipid membranes [ 3-101. The sensor responds to different taste qualities by a unique pattern of output signals. Consequently, it can easily distinguish between different brands of beverages such as beer, coffee and aqueous drinks. In a previous paper [ 111, the taste of sake was studied with the sensor. As a result, it was revealed that the response of the sensor has a good correlation to the titratable acidity, which is one of the most important values for sake brewing. The titratable acidity is expressed as a function of pH, amino acid c:oncentration, organic acid concentration, etc.; i.e. it does not reflect the pH directly, and is an essential quantity related to “sourness” felt by humans. * Corresponding author. 0928-4931/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDJO928-4931(95)00128-X
The finding of high correlation between the sensor output and the titratable acidity [ 111 is very important from the viewpoint of the automatic fermentation process of sake-making. The sensor transforms taste information generated by chemical substances into the electric potential change of the receptor membranes. Hence, it does not respond well to nonelectrolytes which do not greatly affect the membrane potential. Evaluation of the taste of sake for sweetness is not easy using the taste sensor; hence, we used here an enzymatic glucose sensor simultaneously in a comprehensive evaluation of the sake taste, because the sweetness of sake is mainly produced by glucose.
2. Materials and methods 2.1. Lipid membranes The taste sensor with a multichannel electrode was similar to that previously reported [3-l 11, as mentioned below. The lipids are abbreviated as follows: dioctyl phosphate, DOP; trioctylmethylammonium chloride, TOMA; oleyl amine, OAm; decyl alcohol, DA; oleic acid, OA. Hybrid lipid membranes such as DOP:TOMA= 9:1, DOP:TOMA =5:5 and DOPTOMA = 3:7, which indicate the mixture of two lipids with the ratio of molar concentration, were also used. Each lipid was mixed with 400 mg polyvinyl chloride and 0.5 ml
S. Iiyama et al. /Materials Science and Engineering C 4 (1996) 45-49
46
Table 1 Lipids used for membranes
of channels Scanner
Channel
Lipids
1 2 3 4 5
DOP:TOMA = 37 TOMA DOP:TOMA= 5:5 OAm
6
DOP:TOMA
I 8
DA OA
Personal computer
Enzyme electrode
DOP = 9: 1
dioctyl phenylphosphonate (plasticizer), which was dissolved in 10 ml tetrahydrofuran, and dried on a glass plate. The membrane was a transparent, colorless and soft film of 150-200 pm thickness. Eight lipid membranes were stuck on one electrode with channels l-8, which are listed in Table 1. The same lipid membranes were used for half a year throughout the present work. 2.2. Measurement
Llpld membrane electrode
-
using a taste sensor and a glucose sensor
The measuring apparatus used in this work with a taste sensor and a glucose sensor is shown in Fig. 1. The structure of the taste sensor is as follows: Ag/AgCl electrode in 100 mM KC1 solution )membrane 1reference electrode in taste solution. The electric signal from each membrane was converted to a digital code by a digital voltmeter (Advantest, R65.51) through a high-input-impedance amplifier and an eight-channel scanner, and taken into a computer (NEC, PC9801). Hydrophilic and hydrophobic interactions of lipid membranes with taste substances are reflected by membrane potential changes. Since each membrane responds to taste substances by different ways from other membranes, the response pattern constructed of eight outputs can specify the taste quality and intensity. Glucose concentration was measured by an enzyme electrode (Denki Kagaku Keiki, 7820L). The enzyme electrode consists of an oxygen electrode and an enzyme membrane containing glucose oxidase. The enzyme membrane was fixed on an oxygen-permeable membrane of the oxygen electrode. The enzyme electrode was dipped in the solution, which was bubbled by air to saturate it with dissolved oxygen. A constant electric voltage of - 0.7 V was applied to the enzyme electrode. The electric voltage proportional to the electric current flowing in the enzyme electrode was scanned continuously, together with the outputs of the multichannel lipid-membrane electrode. 2.3. Standard solution and Japanese sake The standard solution for sake, which should provide an objective origin of various kinds of sake, was prepared by taking account of a synthetic sake [ 11. Its ingredients are as follows (amounts per liter) : glucose 42.0 g, starch syrup 5.55 g, succinic acid 1.11 g, sodium succinate 0.22 g, lactic acid
Reference electrode
Fig. 1. Measuring apparatus. Lipid-membrane electrode and enzyme electrode belong to the taSte sensor and the glucose sensor, respectively.
0.33 ml, monosodium glutamate (MSG) 0.22 g, glycine 0.11 g, alanine 0.11 g, NaCl 0.15 g, KI-I*PO, 0.06 g, CaHPO, 0.06 g, tyrosol 0.20 g, ethanol 200 ml. This solution was called “standard solution A”, and was used as an initial reference in the measurement. Some ingredients of solution A were changed with the progress of the experiment, as mentioned below. 17 brands of Japanese sake were prepared: Hana no Tsuyu Ryosen ( 1)) Hana no Tsuyu Ginjo (2)) Kiku Tama no I (3), Ume Nishiki Akiokoshi (4), Suishin Namaginjo (5), Shiratsuyu Honjozo (6), Gekkeikan (7), Ginban Namaginjo (8)) Chiyo no Haru Jumnai Ginjo (9), Tsuruhime Ginjo ( lo), Hakutsuru Kizake ( 11) ,Chiyo no Haru Namachozoshu ( 12), Hakutsuru Josen ( 13), Wakatake ( 14), Reiho ( 15)) Yoshinoyama ( 16) and Hakutsuru Namachozoshu (17).
3. Results 3.1. Standard solution A In previous studies, when the taste of beverages such as coffee and beer was investigated with the taste sensor, a particular commercial brand of the same type was used as a standard solution [ 5-71. In the measurement of the taste of sake, too, a certain brand of sake was used as the standard solution [ 111. Selection of the standard solution is very important, because electric output potentials for the measured samples are obtained by taking the output for the standard solution as the zero level of response potential. Furthermore, the sensitivity and the reproducibility of the sensor output are affected by the standard solution. In this meaning, the same quality of standard solution must be provided always. However, the same commercial brand is not always available. In addition, its ingredients are not known. Hence, we tried to prepare a standard solution for sake. Fig. 2 shows the response of the taste sensor to a commercial sake, no. 12, when the standard solution A was used as reference. The standard deviations (mV) were 0.38, 0.28, 0.13,0.40,0.25,0.14,0.09,0.21 for channels 1 to 8, respec-
S. Iiyama et al. /Materials Science and Engineering C 4 (I 996) 45-49
47
This solution, called “standard solution B”, was also used as reference in the measurement of commercial sake (Fig. 4). Although the responses of channels 5-8 approached the zero line as expected, those of channels l-4 deviated from it. 3.3. Standard solution C
I
2
3
4
5
6
8
7
Channel Fig. 2. Response potential of the taste sensor to commercial sake no. 12. Standard solution A was used as reference.
2.2Ogll
I
-0
I
/’
,0----
I .67g/l
In order to bring the standard solution closer to commercial sake, the concentrations of various ingredients were changed, and the effects on the response pattern were investigated. Fig. 5 shows that tyrosol is most effective in lowering the responses of all channels equally. The concentrations of the standard solution were changed as follows: glucose 20.0 g, succinic acid 0.50 g, lactic acid 0 ml, tyrosol2.00 g, ethanol 150 ml. The other ingredients were the same as those of the standard solution A. This solution was called “standard solution C”, and was used as reference in the measurement of commercial sake. Fig. 6 shows that solution C is quite adequate as reference, because the responses of channels to sake No. 12 are well distributed on both sides of the baseline.
__________________
5:
-lOI. I
2
-z 1 3
5 4 Channel
6
7
8
2 5
a
Fig. 3. Effect of succinic acid ‘onthe response of the taste sensor.
tively. This sample showed an intermediate, normal pattern among 17 measured samples, and hence it was assumed to be a typical one. Thus, it is desirable that the response of each channel is located near zero mV. However, the responses in Fig. 2 are far apart from it, especially in channels 3, 4, 6, 7 and 8.
I. I
2
3
4 5 Channel
6
7
8
Fig. 4. Response potential of the taste sensor to commercial sake No.12. Standard solution B was used as reference.
3.2. Standard solution B Channels 1, 2 and 4 are made from positively charged membranes, channel 3 noncharged, and those of channels 58 negatively charged ones. Among the ingredients, acids usually affect the negatively charged membranes. Therefore, a series of solutions with different concentrations of succinic acid were prepared. Fig. 3 :shows their effects on the taste sensor. Succinic acid markedly affected the negatively charged membranes of channels 5-8. This result suggests that the concentration of succinic acid should be decreased in order to bring the standard solution closer to commercial sake. In the experiment with the standard solution A, on the other hand, it was prepared so as to equal the average concentrations of glucose and ethanol in commercial sake. Accordingly, the concentrations of glucose, succinic acid and ethanol were decreased to 20.0 g, 0.20 g and 150 ml, respectively.
2
3
4 5 Channel
6
7
8
Fig. 5. Effects of several ingredients on the response of the taste sensor. The abbreviations are: Su.Na, 0.07 g sodium succinate; Ca.P, 0.015 g CaHPO.,; S.S, 0.55 g starch syrup; Ala, 1.10 g alanine; K.P, 0.6 g KH#G,; Tyrs, 2.0 g tyrosol; MSG, 0.66 g MSG. The origin of the response potential was taken to be a modified solution B, where succinic acid and lactic acid were replaced by 0.5 g and 0 ml, respectively, from standard solution B.
S. Iiyama et al. /Materials Science and Engineering C 4 (1996) 4549
48
-1011’
3
2
4
5
6
7
8
Channel
Fig. 6. Response potential of the taste sensor to commercial sake no. 12. Standard solution C was used as reference.
3.4. Quantification of taste of sake For quantitative scales for acidity and sugar content, we prepared comparative solutions which contained different concentrations of succinic acid and glucose, respectively, in standard solution C. This method is similar to that used for the study of the taste of tomatoes [ 71. The concentrations (amounts per liter) of succinic acid were chosen as 0.125 g, 0.25 g, 0.5 g, 1.O g, 2.0 g (series 1) , and those of glucose 5 g, 10 g, 15 g, 20 g, 25 g, 30 g, 35 g (series 2). The comparative solutions and commercial sake were measured with the taste sensor using standard solution C as reference. Principal component analysis [ 121 was applied to data obtained for the solutions of series 1 using the taste sensor. Factor loadings (ali for i= 1...8) to PC1 were obtained by this process. Let us assign the transformation coefficient of the output of the glucose sensor for series 2 to u2g, then the transformation matrix M from the data by the taste sensor and glucose sensor defined by
leads to
M=
(2)
Here, T, and T, are the abscissa and ordinate of the transformed two-dimensional plane, respectively, u, . ..u8 designate the outputs from channels 1-g of the taste sensor, and ug the output of the glucose sensor, where the average was subtracted from the output so that the response potential for the standard solution might be located at the origin. In the present case, M has the explicit form M=
0.00
(0
-0.05 0
0.49
0.22
0.08 0.36
0.52 0.54 0
0
0
0
0
0
0
11
(3)
Using Eq. ( 1) with Eq. (3)) response potentials for several brands of commercial sake were transformed to the two-
--d
Acidamtent
(mv)
Fig. 7. Taste maps of Japanese sake: (a). (b) and (c) were obtained from the taste and glucose sensors using standard solutions A, B and C, respectiveiy. 0, commercial sake; 0, artificial solutions with different glucose concentrations; 0, solutions with different succinic acid concentrations. Attached numbers indicate different brands of sake (see text).
dimensional plane (T,, T,). The same procedure as above was also applied to standard solutions A and B for comparison. Results are summarized in Fig. 7. When standard solution C was used as reference, all the brands of commercial sake are found to be distributed around the origin of the T, and T, axes in Fig. 7(c) . The abscissa and ordinate axes imply the acid content and the sugar content, respectively. Therefore, both the contents of acid and sugar in sake can be easily found with this map. The correlation coefficient was 0.999 between the abscissa T, and the logarithm of succinic acid concentration in Fig. 7 (c) . It implies that the abscissa (mV) can be easily transformed into the succinic acid concentration (g I- ’ ) . Since other acid components are included in commercial sake, the abscissa of the taste map in Fig. 7 can be interpreted as the equivalent concentration in terms of succinic acid.
4. Discussion The taste sensor is very useful in food discrimination, and it has sensitivity, durability and reproducibility superior to those of humans [3-l 11. When we tested a certain beverage such as beer, a particular commercial brand was used as a standard solution hitherto [5]. This is not a convenient method, because the same brand is not always available.
S. liyama et al. /Materials Science and Engineering C 4 (1996) 45-49
Hence, we here synthesize:d a standard solution for sake. Experimental difficulties will decrease with this standard solution, because we may obtain or dispose of it at any time. Various kinds of solutions with different compositions were made based on the standard solution, and chemical meanings were assigned to the two axes. The transformation matrix obtained was utilized to place the measured sake on this two-dimensional map. As shown in Fig. 7(c), each sake brand is discriminated on the map, whose axes correspond to acid and sugar contents. Hence, the fullness and sweetness of each sake may be estimated to some extent. According to Sato et al. [Z!], the sweetness is approximated by the difference between sugar and titratable acidity, and the fullness by the sum of both ingredients, as can be understood from the following equations: [sweetness] = 0.86 X [sugar X [ titratable
[fullness]
= 0.42 X [sugar
49
here using a transformation matrix can be applied to other foodstuffs. A previous paper [ 111 showed that the titratable acidity can be quantified with a negatively charged lipid membrane such as dioctyl phosphate and oleic acid. It should be noted that the t&ratable acidity is a complex quantity affected by pH, amino acids, organic acids, etc. The titratable acidity can be directly expressed if the abscissa is rewritten using the output from these kinds of membranes, although more detailed experiments may be necessary. Another important term, “cleanness” , for the taste of sake can also be expressed using output patterns from the taste sensor [ 111. We are confident that quantification of the taste of sake will become possible using a convenient multichannel sensor constructed from a taste sensor with lipid membranes and a glucose sensor.
content] -1.16 acidity] + const.
References
content] + 1.88 [l] S. Sato, M. Tadenuma, K. Takahaahi, Y. Azuma, K. Tanzawa and H.
X [ titratable
acidity] + const.
(4)
Therefore, sake in the upper right section of the map should have a sweet-thick taste; sake in the lower right section shows a dry-thick taste. Sensory evaluation by humans was not made in the present study; however, commercial brands of sake seem to have tastes somewhat different from the standard solution C, as can be guessed from their ingredients, although they are well distributed around it in Fig. 7(c). This apparent conflict may imply insufficiency of the present standard solution to quantify the absolute value of taste of sake. In other words, the origin of taste is not adequately reconstructed yet, but we should consider that only a relative scale of taste was obtained in Fig. 7(c). A more adequate choice of standard solution may be left as a future task by taking into account human sensory evaluation. Nevertheless, this result is worth noting as the first trial to provide an objective scale of the taste of sake. In addition, the method proposed
Edamum, J. Sot. Brew. Jpn., 69 (1974) 771. [2] S. Sato, H. Kawaahima and Y. Murayama, J. Sot. Brew. Jpn., 69 (1974) 774. [ 31 K. Hayaahi, M. Yamanaka, K. Toko and K. Yamafuji, Sensors Act.B, 2 (1990) 205. [4] T. Mumta, K. Hayaahi, K. Toko. H. Ikezaki, K. Sato, R. Toukubo and K. Yamafuji, Sensors Mater., 4 (1992) 81. [5] K. Toko, T. Murata, T. Matsuno, Y. Kikkawaand K. Yamafuji, Sensors Mater., 4 (1992) 145. [6] K. Toko, T. Matsuno, K. Yamafuji, K. Hayaahi, H. lkezaki, K. Sato, R. Toukubo and S. Kawarai, Biosens. Bioelectron., 9 (1994) 359. [7] Y. Kikkawa, K. Toko and K. Yamafuji, Sensors Mater., 5 (1993) 83. [8] S. Iiyama, K. Toko, T. Matauno and K. Yamafuji, Chem. Senses, 19 (1994) 87. [9] Y. Kikkawa, K. Toko, T. Matsuno and K. Yamafuji, Jpn. J. Appl. Phys., 32 (1993) 5731. [ 101 S. Iiyama, M. Yahiro and K. Toko, Sensors Mater., 7 (1995) 191. [ 1 l] Y. Arikawa, K. Toko, H. Ikezaki, Y. Shinha, T. Ito, I. Oguri and S. Baba, Sensors Mater., 7 (1995) 261. [ 121 W.R. Dillon and M. Goldstein, Multivariate Analysis, Wiley, New York, 1984.
SUEBKE& ElGlHEERIN8 MaterialsScience and Engineering C 4 ( 1996) 4549
C
Objective scali:ng of taste of sake using taste sensor and glucose sensor Satoru Iiyama a, Yuji Suzuki a, Shu Ezaki b, Yukihiko Arikawa ‘, Kiyoshi Toko d** aDepartment of Industrial Chemistry, Faculty of Engineering, Kinki University in Kyushu, Iizuka 820. Japan b Department of Electrical Engineering, Faculty of Engineering, Kinki University in Kyushu. Iizuka 820, Japan ’ Food Technology Research Institute of Nagano Prefecture, Nagano 380, Japan d Department of Electronics, Faculty of Engineering, Kyushu University 36, Fukuoka 812. Japan Received 30 June 1995; in revised form 29 September 1995
Abstract The taste of Japanese sake wals investigated using a taste sensor with eight kinds of lipid membranes and an enzymatic glucose sensor. The electric-potential pattern constructed of eight outputs from the taste sensor has information on the taste quality and intensity. A standard solution for sake measurement was synthesized. Therefore, chemical meanings were assigned to two perpendicular axes transformed from the nine variables of outputs from the two sensors; commercial brands of sake were scattered on this plane. The taste of sake was discussed objectively. Keywords: Taste sensor; Electric potential; Lipid membrane; Glucose sensor; Taste; Sake
1. Introduction The taste of sake is quite delicate and complicated. However, Sato et al. [ l] elucidated that the taste could be adequately represented by three major factors; sweetness (amakara), fullness (kosa) and cleanness (kireisa) . Moreover, they [ 21 proposed equations which relate the taste with two ingredients, i.e. the sugar content and the titratable acidity. The equations imply in general that the sweetness of sake could be approximated by the difference between the sugar content and the titratable acidity, and the fullness by the sum of both the ingredients. Hence, the two tastes of sake may be expressed by these two terms. Taste can be measured with a multichannel taste sensor which uses several kinds of lipid membranes [ 3-101. The sensor responds to different taste qualities by a unique pattern of output signals. Consequently, it can easily distinguish between different brands of beverages such as beer, coffee and aqueous drinks. In a previous paper [ 111, the taste of sake was studied with the sensor. As a result, it was revealed that the response of the sensor has a good correlation to the titratable acidity, which is one of the most important values for sake brewing. The titratable acidity is expressed as a function of pH, amino acid c:oncentration, organic acid concentration, etc.; i.e. it does not reflect the pH directly, and is an essential quantity related to “sourness” felt by humans. * Corresponding author. 0928-4931/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDJO928-4931(95)00128-X
The finding of high correlation between the sensor output and the titratable acidity [ 111 is very important from the viewpoint of the automatic fermentation process of sake-making. The sensor transforms taste information generated by chemical substances into the electric potential change of the receptor membranes. Hence, it does not respond well to nonelectrolytes which do not greatly affect the membrane potential. Evaluation of the taste of sake for sweetness is not easy using the taste sensor; hence, we used here an enzymatic glucose sensor simultaneously in a comprehensive evaluation of the sake taste, because the sweetness of sake is mainly produced by glucose.
2. Materials and methods 2.1. Lipid membranes The taste sensor with a multichannel electrode was similar to that previously reported [3-l 11, as mentioned below. The lipids are abbreviated as follows: dioctyl phosphate, DOP; trioctylmethylammonium chloride, TOMA; oleyl amine, OAm; decyl alcohol, DA; oleic acid, OA. Hybrid lipid membranes such as DOP:TOMA= 9:1, DOP:TOMA =5:5 and DOPTOMA = 3:7, which indicate the mixture of two lipids with the ratio of molar concentration, were also used. Each lipid was mixed with 400 mg polyvinyl chloride and 0.5 ml
S. Iiyama et al. /Materials Science and Engineering C 4 (1996) 45-49
46
Table 1 Lipids used for membranes
of channels Scanner
Channel
Lipids
1 2 3 4 5
DOP:TOMA = 37 TOMA DOP:TOMA= 5:5 OAm
6
DOP:TOMA
I 8
DA OA
Personal computer
Enzyme electrode
DOP = 9: 1
dioctyl phenylphosphonate (plasticizer), which was dissolved in 10 ml tetrahydrofuran, and dried on a glass plate. The membrane was a transparent, colorless and soft film of 150-200 pm thickness. Eight lipid membranes were stuck on one electrode with channels l-8, which are listed in Table 1. The same lipid membranes were used for half a year throughout the present work. 2.2. Measurement
Llpld membrane electrode
-
using a taste sensor and a glucose sensor
The measuring apparatus used in this work with a taste sensor and a glucose sensor is shown in Fig. 1. The structure of the taste sensor is as follows: Ag/AgCl electrode in 100 mM KC1 solution )membrane 1reference electrode in taste solution. The electric signal from each membrane was converted to a digital code by a digital voltmeter (Advantest, R65.51) through a high-input-impedance amplifier and an eight-channel scanner, and taken into a computer (NEC, PC9801). Hydrophilic and hydrophobic interactions of lipid membranes with taste substances are reflected by membrane potential changes. Since each membrane responds to taste substances by different ways from other membranes, the response pattern constructed of eight outputs can specify the taste quality and intensity. Glucose concentration was measured by an enzyme electrode (Denki Kagaku Keiki, 7820L). The enzyme electrode consists of an oxygen electrode and an enzyme membrane containing glucose oxidase. The enzyme membrane was fixed on an oxygen-permeable membrane of the oxygen electrode. The enzyme electrode was dipped in the solution, which was bubbled by air to saturate it with dissolved oxygen. A constant electric voltage of - 0.7 V was applied to the enzyme electrode. The electric voltage proportional to the electric current flowing in the enzyme electrode was scanned continuously, together with the outputs of the multichannel lipid-membrane electrode. 2.3. Standard solution and Japanese sake The standard solution for sake, which should provide an objective origin of various kinds of sake, was prepared by taking account of a synthetic sake [ 11. Its ingredients are as follows (amounts per liter) : glucose 42.0 g, starch syrup 5.55 g, succinic acid 1.11 g, sodium succinate 0.22 g, lactic acid
Reference electrode
Fig. 1. Measuring apparatus. Lipid-membrane electrode and enzyme electrode belong to the taSte sensor and the glucose sensor, respectively.
0.33 ml, monosodium glutamate (MSG) 0.22 g, glycine 0.11 g, alanine 0.11 g, NaCl 0.15 g, KI-I*PO, 0.06 g, CaHPO, 0.06 g, tyrosol 0.20 g, ethanol 200 ml. This solution was called “standard solution A”, and was used as an initial reference in the measurement. Some ingredients of solution A were changed with the progress of the experiment, as mentioned below. 17 brands of Japanese sake were prepared: Hana no Tsuyu Ryosen ( 1)) Hana no Tsuyu Ginjo (2)) Kiku Tama no I (3), Ume Nishiki Akiokoshi (4), Suishin Namaginjo (5), Shiratsuyu Honjozo (6), Gekkeikan (7), Ginban Namaginjo (8)) Chiyo no Haru Jumnai Ginjo (9), Tsuruhime Ginjo ( lo), Hakutsuru Kizake ( 11) ,Chiyo no Haru Namachozoshu ( 12), Hakutsuru Josen ( 13), Wakatake ( 14), Reiho ( 15)) Yoshinoyama ( 16) and Hakutsuru Namachozoshu (17).
3. Results 3.1. Standard solution A In previous studies, when the taste of beverages such as coffee and beer was investigated with the taste sensor, a particular commercial brand of the same type was used as a standard solution [ 5-71. In the measurement of the taste of sake, too, a certain brand of sake was used as the standard solution [ 111. Selection of the standard solution is very important, because electric output potentials for the measured samples are obtained by taking the output for the standard solution as the zero level of response potential. Furthermore, the sensitivity and the reproducibility of the sensor output are affected by the standard solution. In this meaning, the same quality of standard solution must be provided always. However, the same commercial brand is not always available. In addition, its ingredients are not known. Hence, we tried to prepare a standard solution for sake. Fig. 2 shows the response of the taste sensor to a commercial sake, no. 12, when the standard solution A was used as reference. The standard deviations (mV) were 0.38, 0.28, 0.13,0.40,0.25,0.14,0.09,0.21 for channels 1 to 8, respec-
S. Iiyama et al. /Materials Science and Engineering C 4 (I 996) 45-49
47
This solution, called “standard solution B”, was also used as reference in the measurement of commercial sake (Fig. 4). Although the responses of channels 5-8 approached the zero line as expected, those of channels l-4 deviated from it. 3.3. Standard solution C
I
2
3
4
5
6
8
7
Channel Fig. 2. Response potential of the taste sensor to commercial sake no. 12. Standard solution A was used as reference.
2.2Ogll
I
-0
I
/’
,0----
I .67g/l
In order to bring the standard solution closer to commercial sake, the concentrations of various ingredients were changed, and the effects on the response pattern were investigated. Fig. 5 shows that tyrosol is most effective in lowering the responses of all channels equally. The concentrations of the standard solution were changed as follows: glucose 20.0 g, succinic acid 0.50 g, lactic acid 0 ml, tyrosol2.00 g, ethanol 150 ml. The other ingredients were the same as those of the standard solution A. This solution was called “standard solution C”, and was used as reference in the measurement of commercial sake. Fig. 6 shows that solution C is quite adequate as reference, because the responses of channels to sake No. 12 are well distributed on both sides of the baseline.
__________________
5:
-lOI. I
2
-z 1 3
5 4 Channel
6
7
8
2 5
a
Fig. 3. Effect of succinic acid ‘onthe response of the taste sensor.
tively. This sample showed an intermediate, normal pattern among 17 measured samples, and hence it was assumed to be a typical one. Thus, it is desirable that the response of each channel is located near zero mV. However, the responses in Fig. 2 are far apart from it, especially in channels 3, 4, 6, 7 and 8.
I. I
2
3
4 5 Channel
6
7
8
Fig. 4. Response potential of the taste sensor to commercial sake No.12. Standard solution B was used as reference.
3.2. Standard solution B Channels 1, 2 and 4 are made from positively charged membranes, channel 3 noncharged, and those of channels 58 negatively charged ones. Among the ingredients, acids usually affect the negatively charged membranes. Therefore, a series of solutions with different concentrations of succinic acid were prepared. Fig. 3 :shows their effects on the taste sensor. Succinic acid markedly affected the negatively charged membranes of channels 5-8. This result suggests that the concentration of succinic acid should be decreased in order to bring the standard solution closer to commercial sake. In the experiment with the standard solution A, on the other hand, it was prepared so as to equal the average concentrations of glucose and ethanol in commercial sake. Accordingly, the concentrations of glucose, succinic acid and ethanol were decreased to 20.0 g, 0.20 g and 150 ml, respectively.
2
3
4 5 Channel
6
7
8
Fig. 5. Effects of several ingredients on the response of the taste sensor. The abbreviations are: Su.Na, 0.07 g sodium succinate; Ca.P, 0.015 g CaHPO.,; S.S, 0.55 g starch syrup; Ala, 1.10 g alanine; K.P, 0.6 g KH#G,; Tyrs, 2.0 g tyrosol; MSG, 0.66 g MSG. The origin of the response potential was taken to be a modified solution B, where succinic acid and lactic acid were replaced by 0.5 g and 0 ml, respectively, from standard solution B.
S. Iiyama et al. /Materials Science and Engineering C 4 (1996) 4549
48
-1011’
3
2
4
5
6
7
8
Channel
Fig. 6. Response potential of the taste sensor to commercial sake no. 12. Standard solution C was used as reference.
3.4. Quantification of taste of sake For quantitative scales for acidity and sugar content, we prepared comparative solutions which contained different concentrations of succinic acid and glucose, respectively, in standard solution C. This method is similar to that used for the study of the taste of tomatoes [ 71. The concentrations (amounts per liter) of succinic acid were chosen as 0.125 g, 0.25 g, 0.5 g, 1.O g, 2.0 g (series 1) , and those of glucose 5 g, 10 g, 15 g, 20 g, 25 g, 30 g, 35 g (series 2). The comparative solutions and commercial sake were measured with the taste sensor using standard solution C as reference. Principal component analysis [ 121 was applied to data obtained for the solutions of series 1 using the taste sensor. Factor loadings (ali for i= 1...8) to PC1 were obtained by this process. Let us assign the transformation coefficient of the output of the glucose sensor for series 2 to u2g, then the transformation matrix M from the data by the taste sensor and glucose sensor defined by
leads to
M=
(2)
Here, T, and T, are the abscissa and ordinate of the transformed two-dimensional plane, respectively, u, . ..u8 designate the outputs from channels 1-g of the taste sensor, and ug the output of the glucose sensor, where the average was subtracted from the output so that the response potential for the standard solution might be located at the origin. In the present case, M has the explicit form M=
0.00
(0
-0.05 0
0.49
0.22
0.08 0.36
0.52 0.54 0
0
0
0
0
0
0
11
(3)
Using Eq. ( 1) with Eq. (3)) response potentials for several brands of commercial sake were transformed to the two-
--d
Acidamtent
(mv)
Fig. 7. Taste maps of Japanese sake: (a). (b) and (c) were obtained from the taste and glucose sensors using standard solutions A, B and C, respectiveiy. 0, commercial sake; 0, artificial solutions with different glucose concentrations; 0, solutions with different succinic acid concentrations. Attached numbers indicate different brands of sake (see text).
dimensional plane (T,, T,). The same procedure as above was also applied to standard solutions A and B for comparison. Results are summarized in Fig. 7. When standard solution C was used as reference, all the brands of commercial sake are found to be distributed around the origin of the T, and T, axes in Fig. 7(c) . The abscissa and ordinate axes imply the acid content and the sugar content, respectively. Therefore, both the contents of acid and sugar in sake can be easily found with this map. The correlation coefficient was 0.999 between the abscissa T, and the logarithm of succinic acid concentration in Fig. 7 (c) . It implies that the abscissa (mV) can be easily transformed into the succinic acid concentration (g I- ’ ) . Since other acid components are included in commercial sake, the abscissa of the taste map in Fig. 7 can be interpreted as the equivalent concentration in terms of succinic acid.
4. Discussion The taste sensor is very useful in food discrimination, and it has sensitivity, durability and reproducibility superior to those of humans [3-l 11. When we tested a certain beverage such as beer, a particular commercial brand was used as a standard solution hitherto [5]. This is not a convenient method, because the same brand is not always available.
S. liyama et al. /Materials Science and Engineering C 4 (1996) 45-49
Hence, we here synthesize:d a standard solution for sake. Experimental difficulties will decrease with this standard solution, because we may obtain or dispose of it at any time. Various kinds of solutions with different compositions were made based on the standard solution, and chemical meanings were assigned to the two axes. The transformation matrix obtained was utilized to place the measured sake on this two-dimensional map. As shown in Fig. 7(c), each sake brand is discriminated on the map, whose axes correspond to acid and sugar contents. Hence, the fullness and sweetness of each sake may be estimated to some extent. According to Sato et al. [Z!], the sweetness is approximated by the difference between sugar and titratable acidity, and the fullness by the sum of both ingredients, as can be understood from the following equations: [sweetness] = 0.86 X [sugar X [ titratable
[fullness]
= 0.42 X [sugar
49
here using a transformation matrix can be applied to other foodstuffs. A previous paper [ 111 showed that the titratable acidity can be quantified with a negatively charged lipid membrane such as dioctyl phosphate and oleic acid. It should be noted that the t&ratable acidity is a complex quantity affected by pH, amino acids, organic acids, etc. The titratable acidity can be directly expressed if the abscissa is rewritten using the output from these kinds of membranes, although more detailed experiments may be necessary. Another important term, “cleanness” , for the taste of sake can also be expressed using output patterns from the taste sensor [ 111. We are confident that quantification of the taste of sake will become possible using a convenient multichannel sensor constructed from a taste sensor with lipid membranes and a glucose sensor.
content] -1.16 acidity] + const.
References
content] + 1.88 [l] S. Sato, M. Tadenuma, K. Takahaahi, Y. Azuma, K. Tanzawa and H.
X [ titratable
acidity] + const.
(4)
Therefore, sake in the upper right section of the map should have a sweet-thick taste; sake in the lower right section shows a dry-thick taste. Sensory evaluation by humans was not made in the present study; however, commercial brands of sake seem to have tastes somewhat different from the standard solution C, as can be guessed from their ingredients, although they are well distributed around it in Fig. 7(c). This apparent conflict may imply insufficiency of the present standard solution to quantify the absolute value of taste of sake. In other words, the origin of taste is not adequately reconstructed yet, but we should consider that only a relative scale of taste was obtained in Fig. 7(c). A more adequate choice of standard solution may be left as a future task by taking into account human sensory evaluation. Nevertheless, this result is worth noting as the first trial to provide an objective scale of the taste of sake. In addition, the method proposed
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