Protein; Tofu (Soybean Curd) Production by an Electro-Reaction

Food Sci. Technol. Res., 10 (4), 424-427, 2004

Electrochemical Coagulation of Soybean Protein; Tofu (Soybean Curd) Production by an Electro-Reaction

Yoshiro KAMATA, Michiko YAMAKI and Mio ONODERA

Food Science Laboratory Miyagi University ofEducation, 149 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-0845, Japan

Received March 29, 2004; Accepted July 7, 2004

Effect of direct current on soymilk coagulation was studied to establish a new processing method for food. Soymilk was treated with direct electric current (60 mA) in an anode chamber. The treated soymilk gave tofu-like gel only by simple heating. The obtained tofu-like gel showed appearance and texture similar to a commercial kinu-tofu. Sodium dodecyl sulfate gel electrophoresis showed that the electric treatment did not change the state of covalent cross linkage including disulfide bridges among the soybean protein molecules. Therefore, non-covalent intermolecular interactions may play an important role in the gel network formation. Though the oxidation of the soymilk in the anode chamber cannot be ruled out, the treated soymilk caused almost no change on peroxide value of the oil component, the active chloride content or the oxidation- reduction potential. Nor was any mutagenicity observed for the treated soymilk. Therefore, the electric treatment with direct current is considered a safe technique for food processing.

Keywords: soybean, soymilk, electric treatment, tofu gel, coagulation, mutagenicity

Introduction method may become a new and effective technique for food Soybean protein is the most popular vegetable protein industries because it is simple, easy to control, and requires because its usage is very remarkable not only in traditional no chemical agent. This paper describes a new electro- foods but also in modern foods such as fabricated foods. chemical method for coagulation of soymilk. The protein has abilities to form gel, emulsion, foam etc. (Kinsella, 1979). Among these functional properties, the Materials and Methods gel-forming ability has long been used to make many tra- Materials Soybean (var. Tachinagaha) harvested in ditional foods such as tofu, yuba etc. (Okubo, 1992). Also, Miyagi, Japan in 2001 was used throughout this work. All the protein has been used as a gelling agent for meat analog reagents were of the highest grade. (Hoshi, 1992). Preparation ofsoymilk The soybeans (300 g) were The gelling properties have been studied widely allowed to swell overnight. The soaking water was dis- (Kinsella, 1979), especially in heat setting gel. Among the carded and 960 ml of fresh water was added. The mixture heat gels, tofu (soybean curd) possesses unique gelling was crushed followed by heating at weak boiling (92 :!: mechanisms. Nigari, that is a traditional coagulant for tofu, 1'C) for 5 min. Soymilk was obtained by filtering it through is made from seawater and primarily contains magnesium a sheet of cotton cloth. salts. Pure calcium salt or magnesium salt or both are also Systemfor electric treatment Anode (platinum) was used for this purpose. The ion bridges with these divalent placed in a sample chamber (600 ml capacity) containing cations are thought to be the important mechanism of 500 ml of soymilk. The soymilk was constantly stirred network structure formation in tofu coagulation (Hashizume during this process. Cathode (platinum) was placed in and Ka, 1978). Glucon0-6-lactone (GDL) is also used for another chamber (100 ml) containing sodium chloride tofu production but acid coagulation is the main mecha- solution (0.2 M). The two chambers were connected with nism in this case (Hashizume and Ka, 1978). a few sheets of filter paper as a salt bridge. Sixty mA of Electrochemical methods have seldom been applied to direct current was then applied. food production systems. The electrolytic reduction of Preparation oftofu gel The soymilk was treated elec- soybean protein has been used to improve the functional trically for 1, 2, 3, 4 and 5 hrs. One hundred ml of treated properties of the protein (Komeyasu and Miura, 1981; soymilk was heated in a weakly boiling water-bath (93 i Miura and Komeyasu, 1 982). Another usage was to acidify 1'C) for 30 min. The obtained gels were cooled and stored soybean protein solution to obtain acid precipitated protein at 5~C overnight. (Bazinet et al., 1998). Electro-coagulation of protein in Rheologicalproperties oftofu gel The obtained gels wastewater from fish paste manufacturing has also been and commercial tofu were cut as a cylinder by 23 mm in attempted (Ohtake et al., 1977). The electrochemical diameter and 20 mm in thickness. Breaking stress was measured by compression at a rate of 6 cm/min with a rheometer (Rheometer NRM-2002J, Rheotech, Tokyo)

E-mail: y-kama @ staff . miyakyo-u . ac . j p using a circular-plate plunger 25 mm in diameter. Electrochemical Production of Tofu 425 Electrophoresis Sodium dodecyl sulfate (SDS) gel electrophoresis (Laemmeli et al., 1970) was applied with a commercial pre-made gel for 1.5 hr at 100 V. Oxidation-reduction potential (ORP), active chloride and peroxide value (POV) The ORP of soymilk was measured by an automatic titrater (AT-400, Kyoto Denshi Kogyo, Kyoto) with an ORP electrode (M271, Kyoto Denshi Kogyo). 1 hr 2 hr The 2,2'-azinobis(3-ethylbenzothiazolin-6-sulfonic acid) method was used with an active-chloride test kit (Active C1-Test Wako, Wako Chemical, Osaka) for active chloride. To remove the disturbance by protein particles and oil droplets, 10 ml of the soymilk was dialyzed against 90 ml of water for 24 hrs. The outside water was analyzed. Its absorbance at 630 nm was measured, from which the amount of active chloride was obtained by comparison with 3 hr standards of calcium hypochlorite solution. The POV was determined by the ferric thiocyanate method (Iwami et al., 1987). The soymilk sample was lyophilized and a 20 mg portion was applied to the analysis. Mutagenicity test Mutagenicity was evaluated by the Umu-test (Umu-lac kit, Japan Antibody Laboratory, Takasaki) which is an improved Ames test (Oda et al., 1984; Maron and Ames, 1983). The Umu-test positive was defined as follows: a sample that shows P-galactosidase activity two fold higher than control-sample activity. The S9 mix Fig. 1. Tofu-like gels obtained from the electro-treated soymilk. Soymilk was treated electrically from I to 5 hours followed by heating was also used to evaluate the activation of mutagenicity for 30 min in a boiling water bath (93 ~: 1'C). Time (hrs) for electric by metabolism. treatment is shown in photographs.

Results 2000 Soymilk coagulation (tofu) by electro-reaction Five 1 800 hundred ml of soymilk was placed in an anode chamber. Direct current was applied to the system from one to five 1 600 hours under a constant current condition of 60 mA. The a) 1 400 treated soymilk was heated for 30 min in a weakly boiling I'> 1 200

water-bath (93 :!: 1"C). A11 samples gave gels that were very (')*o 1 ooO similar to tofu in their appearance (Fig. 1), while the ~co untreated soymilk did not show any coagulation under the o) 800 1~ same condition. clS 600 ~ Figure 2 shows the breaking stress of the soymilk gels OO 400 and two kinds of commercial tofu. The soymilk gels made 200 from electro-treated soymilk were compared to a com- o mercial kinu-tofu and a commercial momen-tofu. Though 1 hr 2 hr 3 hr 5 hr Momen Kinu the data were scattered widely depending on the sampling Fig. 2. Comparison of breaking stress of tofu-like gels and two kinds position, a considerably similar range of breaking stress of commercial tofu. Cylindrical samples (23 mm in diameter and 20 mm can be observed for the electro-treated soymilk gels in height) were cut from the gels followed by a compression test. Averages compared to the commercial kinu-tofu. and standard deviations are shown. The pH of the soymilk was lowered by the electric treatment (Table I ). The range of the pH was 5 .6-6.4. Therefore, Table 1. Changeof soymilk pH by electro-treatment followed by there is a possibility that the lowered pH causes the gelation. heating.

The acidified soymilk with I M hydrochloride also gave Electro-treatment pH of electro-treated pH of electro-treated gels but a self-standing gel was obtained only below pH5.5 time (hr) soymilk soymilk after heating (Fig. 3). Moreover, the heat gels made from acidified O 64 6.3 soymilk were too weak to detect its breaking stress. The 1 63 6.4 pH of soymilk treated for 3 hrs, which showed the hardest 2 61 6.2 gel, was merely 6.0. This pH value is not so different from 3 60 6.0 the commercial kinu-tofu (6.2) and the commercial momen- 4 58 5.8 5 5.7 tofu (6.3). Though the acidification may be one reason for 56 426 Y. KAMATA et al. the electrophoresis gels. The high molecular weight com-

= '~ ~~ ".~...;';i'~~~/;~ pounds were converted into subunit bands of the soybean globulins by the treatment with 2-mercaptoethanol (Fig. ~~~.'i~ 4b). The patterns were not changed by the period of electro- reaction. Therefore, the role of the disulfide bridge for- ~ mation by oxidation of cysteine residues during electro- reaction may be small. pH6.5 pH6.0 Changes in food integrity parameters during electro- reaction Table 2 shows some of the parameters affecting food integrity. The soymilk was electrically treated from one to five hours followed by analyses of POV, active chloride and ORP. Oxygen is produced at anodes in elec- trolysis of water. Therefore, the environment of the sample around the anode electrode must be oxidative. Soymilk contains a great deal of lipid that may be oxidized under this condition. The POV of the treated soymilk were very pH5.5 small and similar to the value of untreated soymilk, therefore, the lipid in soymilk is not oxidized by this treatment. Also, active chloride content was a little higher in the soymilk treated for five hours than in the others, but the value is almost similar to tap water (0.3 ppm) that was used for soymilk preparation. In addition, under the most suitable condition for tofu making (3 hrs) the value was the same as that of tap water. Hydrogen peroxide was also measured with a detecting strip (data not shown). No pH5.0 PH4.5 samples showed the existence of hydrogen peroxide. ORP Fig. 3. Coagulation of acidified soymilk. Soymilk was acidified with 1 M HCI followed by heating for 30 min in a boiling water bath (93 :!: value of soymilk is smaller than that of standard tap water 1~C). The adjusted pH of soymilk is shown in photographs. (around 450 mV), indicating that it is a rather reductive material. The electro-treatment does not affect the ORP of the coagulation, there may be additional reasons for for- the soymilk. This information confirms the fact that the mation of the strong gels. electro-treatment does not oxidize the component of Figure 4 shows the patterns of SDS gel electrophoresis soymilk. for electro-treated soymilk. The soymilk was already heat- denatured in the preparation process. Therefore, most of Table 2. Effect of electro-treatment on food integrity parameters. the subunits of soybean proteins are cross-linked, mainly Treating time ORP by disulfide bridges. Only weak subunit bands of soybean (hr) (meq/kg-oil)POV chloride Active (ppm) (mV) globulins were observed without using 2-mercaptoethanol O 69.0 O1 101 (Fig. 4a), because cross-1inked protein could not enter into 1 78.7 03 106 3 77.3 03 112 5 77.3 08 110 a b

The treated soymilk was also analyzed by the Umu-test.

oe The test results did not show any mutagenicity for elec-

oe trically treated soymilk. p S Discussion A A variety of causes have been discussed for the heat gelation of protein. Most non-covalent linkages which B ,"#:=~ . form the native-structure of the protein collapse in heat denaturation. Though these linkages are reformed during cooling, a certain part becomes constructing intermolecular bonds forming the network structure. The formation of 1 2 3 4 5 1 2 3 4 5 disulfide bonds by the oxidation might take place and the Time for electro-treatment (hr) thiol-disulfide exchange reaction is also expected. Fig. 4. SDS gel electrophoresis of electro-treated soymilk. Subunits of Soymilk cannot be coagulated by heat treatment only. glycinin (S) and P-conglycinin (ce', oe and P) and polypeptide chains of glycinin subunits (A and B) are shown. a; without 2-mercaptoethanol. b; In the coagulation process of the tofu making (soybean with 2-mercaptoethanol. curd), electrostatic bridges with bivalent metallic ions are Electrochemical Production of Tofu 427 thought to be a main reason for the coagulation (Hashizume advantages in operability of the production line of food and Ka, 1978). The acid cohesion, which is caused by the materials. The technique is simple, easy to control and decrease in the pH due to GDL decomposition, is another needs no chemical agent. Based on the analysis results, it reason in the case of the GDL coagulation. However, the also gives a safe product. Therefore, it is expected to pH of the treated soymilk was not low compared to the become one of the important food-processing methods in tofu gel made by acidification. Though the acid cohesion the future. due to the pH decrease has some effect on the coagulation of the soymilk by electro-treatment, other mechanisms Acknowledgment This work was supported in part by a grant from must also play an important role. The effect of formation the Takano Life Science Research Foundation in 2002. of some covalent linkages may also be possible. However, the electrophoresis results of electro-treated soymilk References Bazinet, L., Lamarche, F, and lppersiel, D. (1998). Comparison of showed that the polymer formation through covalent bonds chemical and bipolar-membrane electrochemical acidification for including disulfide bridges by the electric treatment was precipitation of soybean proteins. J. Agric. Food Chem., 46, not remarkable. The introduction of non-covalent linkages 20 1 3-20 1 9 . may be the main reason for gelation. The reason for the Hashizume, K. and Ka, G. (1978). Difference between tofus coagulated with glucono-delta-lacton and calcium salts. Nippon gel formation is now under examination. Shokuhin Kogyo Gakkaishi, 25, 383-386 (in Japanese). The time of electro-treatment most suitable for gel for- Hoshi, Y. ( 1 992). Soybean processing, In "Soybean Science," Asakura- mation was a period of three hours under these experimental Shoten, Tokyo, pp.134-1 82 (in Japanese). conditions. Many small holes appeared in gels treated lwami, K., Hattori, M. and lbuki, F. (1987). Prominent antioxidant excessively. The excessive coagulation of soybean protein effect of wheat gliadin on linoleate peroxidation in powder model systems at high water activity. J. Agric. Food Chem. , 35, 628-63 1 . is expected to occur by prolonged electro-treatment. It Kinsella, J.E. (1979). Functional properties of soy proteins. J. Am. may disturb the formation of a uniform gel-network and, Oil Chemists ' Soc. , 56, 242-258. as a result, weaken gel strength. Komeyasu, M. and Miura, Y. (1981). Effects of electrolytic reduc- No strange taste was sensed for obtained tofu-like gels tion on suitability of soybean for making tofu. Nippon Shokuhin made by the suitable electric treatment, though excess Kogyo Gakkaishi, 28, 41-47 (in Japanese). Laemmeli, U.K. (1970). Cleavage of structural proteins during the electric current caused an astringent taste. The tastes of assembly of the bacteriophage T4. Nature, 227, 680-685. the obtained tofu gels were rather similar to that of the Maron, D.M. and Ames, B.N. (1983). Revised methods for the unmodified soymilk itself. A texture resembling commer- salmonella mutagenicity test. Mutat. Res., 113, 173-215. cial kinu-tofu was also felt in the gel. Miura, Y. and Komeyasu, M. (1982). Effect of electrolytic reduction As for the application of a new food-processing tech- on viscosity and gel formation of soybean protein solution. Nippon nology, safety is an indispensable requirement. If some Shokuhin Kogyo Gakkaishi, 29, 460-465 (in Japanese). Oda, Y., Nakamura, S., Oki, I., Nakada, A. and Shinagawa, H. ( 1984). chemical reactions are observed by electro-treatment, it is Short term screening method of environmental mutagen using necessary to examine the safety of the reaction product. umu-lac fusion gene. Kankyo Hen-i-gen Kenkyu, 6, 87-92 (in In the result of the Umu-test, the mutagenicity was not Japanese). shown. Moreover, there were no remarkable changes in Okubo, K. (1992). Soybean food science, In "Soybean Science," Asakura-Shoten, Tokyo, pp. 76-91 (in Japanese). ORP or active chlorine. The oxidation of oil and other com- Otake, S., Fukui, K., Teraoka, K. and Yoshida, H. (1977). ponents was not observed, though the sample may be Electrocoagulating treatment of waste water from fish paste-man- exposed to oxygen produced at the anode. ufacturing factories. Bull. Jpn. Soc. Sci. Fish., 43, 975-981 (in Food processing by electric treatment has excellent Japanese) .