Soybean Phytate Content and Its Influence on Tofu Texture - ACS

Chapter 16

Downloaded by OHIO STATE UNIV LIBRARIES on June 24, 2012 | http://pubs.acs.org Publication Date (Web): December 14, 2010 | doi: 10.1021/bk-2010-1059.ch016

Soybean Phytate Content and Its Influence on Tofu Texture Takahiro Ishiguro*,1 and Tomotada Ono2 1Food

Research Center, Asahimatsu Foods Co. Ltd., 1008 Dashina, Iida, Nagano, Japan 2Faculty of Agriculture, Iwate University, Ueda 3, Morioka, Iwate, Japan *[email protected]

The variation factor in soybean phytate content and its influence on tofu texture was investigated. Twenty-seven soybean varieties were grown in a drained paddy field and upland field, and then their protein and phytate contents were determined using the FT-IR method. 12 soybean varieties grown in the drained paddy field contained significantly more phytate than those in the upland field. Variation in the phytate contents due to the different varieties was also observed. Therefore phytate content was affected by both environmental and hereditary factors. The increase of phytate concentration in soymilk shifted optimal coagulant concentration in both Mg and GDL tofu. The data showed the variation of phytate content influenced on tofu texture.

Introduction Tofu has been a popular food in some Asian countries since ancient times. Its consumption is now increasing as it is recognized as a wholesome food in all over the world. The quality of tofu is mainly evaluated on the basis of its texture properties. Phytate is considered to affect the tofu texture by reacting with protein and coagulants such as the magnesium or calcium salts (1) . Phytate occurs in many grains and legumes. Binding of phytate to soy protein has been reported to influence the physicochemical properties of the protein. Katoh et.al. (2) reported that the removal of phytate from soy protein increased the surface hydrophobicity and emulsifiability of the protein. Those phenomena suggest that the linkage of © 2010 American Chemical Society In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


phytate to protein could change the existing form of protein in soymilk and then affect tofu texture. A few studies have discussed the influence of phytate on tofu texture. Hou and Chang (3) reported that phytate contents in soymilk have little effect on pressed tofu. Saio et.al. (4) reported that increasing the phytate contents in soymilk caused a decrease in hardness and an increase in yield of tofu. Liu and Chang (5) have reported that phytate contents in soymilk have a correlation with optimal coagulant concentration. Soybean cultivation is increasing in fields that have been converted from paddy fields for rice (called “drained paddy field” in this study) in Japan. However, drained paddy fields may be different from the primary soybean field (called “upland field” in this study) based on some conditions such as the soil properties. For example, it is known that soil in the drained paddy field have higher phosphate adsorption coefficient than in the upland field. The difference in the cultivation environments may affect several characteristics of the soybean. Therefore, it will then affect the tofu quality. In this study, in order to investigate the effect of soybean variety and growing condition on phytate contents, we cultivated 27 varieties of soybean in both a drained paddy field and upland field, and then their phytate contents were determined. Then we further investigated the influence of the soybean phytate content on the tofu texture.

Materials and Methods Materials Twenty-seven Japanese varieties of soybeans (Figure 1) were grown in a drained paddy field and upland field. These soybeans were used to investigate the changes of the phytate contents. A variety of Suzuyutaka was used for examining the influence of the phytate content on the tofu texture.

Determination of Phytate in Soybean The phytate content was measured by the method of Ishiguro et al. (6). Each soybean (about 10 g) was soaked in deionized water for 18 h at 4°C. The swollen bean was ground (11,400 rpm, 4 min) into a homogenate with 8 times the water vs. the soybean dry weight using an Oster blender (Oster Co, Milwaukee, USA), and the homogenate was then filtered through a defatted cotton sheet. The filtrate was designated as the raw soymilk. Phytate in the raw soymilk was completely precipitated by the addition of calcium under alkaline conditions (pH 11.5). The precipitate was dissolved in citrate buffer (pH 6.0) and then the IR spectra were measured with the ATR accessory. The phytate content was determined by the absorbance at 1070cm-1. 250 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Preparation of Tofu The soybeans were soaked in deionized water for 18 h at 4°C. Each swollen bean was ground (11,400 rpm, 4 min) into a homogenate with 6 times the water vs. the soybean weight using the Oster blender (Oster Co, Milwaukee, WI), then an antifoam-emulsion (Antifoam-AF-Emulsion, Nakarai Chemicals Ltd.) was added (about 80 mg for 200ml of soymilk) during the grinding. The homogenate was filtered through a defatted cotton sheet. The filtrate is the raw soymilk. In order to prepare the soymilks having different phytate contents, the potassium phytate solution (pH 7.0) was added to the raw soymilk. Those soymilks have same composition of protein, calcium, magnesium, and other contents expect phytate. Soymilk was prepared by heating the raw soymilks in a boiling bath for 5 min above 95 °C, and then quickly cooled to room temperature with iced water. The Soymilk (25 mL) was degassed by vacuum-aspiration and cooled to 4 °C. Each 1.0 mL of various concentrations of MgCl2 or Glucono-1-5-lactone (GDL) solutions was mixed into the soymilk and the mixture was placed in a water bath (Isotemp Fisher general purpose water bath, Fisher Scientific, Boston, MA, USA) at 90 °C for 1 h. Measurement of Tofu Texture The tofu was placed in a refrigerator (4°C) for over 18 hours. When measuring the curd texture, the temperature of the tofu curd was at room temperature (23°C). The tofu, 13 mm height and 20 mm diameter, placed on a measuring plate. A compression test was carried out at the compression rate of 2 cm / min using a RHEOTECH FUDOH RHEOMETER (Rheotech Co., Tokyo, Japan) with a 25 mm diameter cylindrical plunger. Statistical Analyses Significant differences between group means were analyzed by a t-test (p > 0:05) using the WinSTAT program.

Results and Discussion Effect of Variety and Field Difference on Soybean Phytate Content Phytate is considered to one important component in soybean that affects the tofu texture. 27 soybean varieties were cultivated in both the drained paddy field and upland field. 12 soybean varieties grown in the drained paddy field contained significantly more phytate than those in the upland field (Figure 1). The phytate contents of the other 14 verities showed no significant difference between the drained paddy and upland field. Only 1 variety of soybean grown in the upland field contained more phytate than that from the drained paddy field. The variation in the phytate contents was also observed among the different varieties. These results show that 251 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


the phytate content is affected by both environmental and hereditary factors. This variation in phytate may have caused the scatter in the tofu texture. The Enrei and Oosuzu varieties are known to be well suited for tofu making. On the contrary, the Hatayutaka and Tachinagaha varieties are known to be difficult for tofu making. Based on the phytate contents, Enrei and Oosuzu have no difference in the phytate contents between the drained paddy field and upland field. For Hatayutaka and Tachinagaha, the beans grown in the drained paddy field contained more phytate than those grown in the upland field. Enrei and Oosuzu have lower variation of the phytate content may cause a stable tofu making. Effect of Soymilk Phytate Concentration on Mg Tofu Texture Then we investigated that the influence of the variation of phytate content in soybean on tofu texture. MgCl2 is a coagulant used most frequently in Japan. We measured the breaking stress of tofu made from soymilks containing various concentrations (3.3, 4.8, and 6.2 mM) of phytate with various MgCl2 concentrations (5-30 mM). Soymilk containing 3.3 mM phytate reached maximum breaking stress at 15 mM of magnesium concentration, while soymilk containing 6.2 mM phytate reached maximum breaking stress at 26 mM of magnesium concentration (Figure 2). Magnesium concentration for maximum breaking stress shifted from 15 mM to 26 mM depending upon increase of phytate content from 3.3mM to 6.2 mM (Figure 2). This coagulant concentration is recognized as optimal. It showed that fluctuation of phytate content in soybean affects Mg tofu texture.

Figure 1. Phytate contents of soymilks made from soybeans grown in upland (□) and drained paddy fields (▪). Raw soymilk was prepared from each soybean to measure contents of soybean. Phytate content was measured using the method of Ishiguro et al. (5) 252 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Breaking stress of Mg-tofu prepared from soymilks having various phytate contents

Figure 3. Breaking stress of GDL-tofu prepared from soymilks having various phytate contents Liu and Chang (7) have developed the facile titration method for the examination of soymilk coagulant requirement, and they (5) observed that phytate content in soymilk was correlated to coagulant requirement. We confirmed this phenomenon with actual tofu and its texture. 253 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Effect of Soymilk Phytate Concentration on GDL Tofu Texture GDL (Glucono-1-5-lactone) is a coagulant that induces acid coagulation. It is used mainly for packed tofu. In order to examine the influence of phytate on GDL tofu texture, we measured the breaking stress of tofu made from soymilks containing various concentrations of phytate (3.3, 4.8, and 6.2 mM) with various GDL concentrations (0.3-0.9%) Tofu containing 3.3mM phytate reached maximum breaking stress at 0.4% of GDL concentration, while tofu containing 6.2mM phytate reached maximum breaking stress at 0.7% GDL concentration (Figure 3). Since phytate have pH buffering effect, GDL requirement may be increased in high-phytate soymilk. This result showed that phytate content in soybean also affects GDL tofu texture.

Conclusion We concluded that phytate content in soybean was influenced by both environmental and hereditary factors. The variation of phytate content should be regarded as one of the reasons causing the variation in the tofu texture.

References 1.

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Toda, K.; Ono, T.; Kitamura, K.; Hajika, M.; Takahashi, K.; Nakamura, Y. Seed protein content and consistency of tofu prepared with different magnesium chloride concentrations in six Japanese soybean varieties. Breed. Sci. 2003, 53, 217–223. Katoh, M.; Jin, DH.; Naganuma, T; Ogawa, T; Muramoto, K. Removal of phytic acid from soy protein isolates and its effect on protein functionality. Daizu Tannpakushitsu Kenkyu 2002, 5, 41–46, in Japanese. Hou, H. J.; Chang, KC. Yield and textual properties of tofu as affected by the changes of phytate content during soybean storage. J. Food Sci. 2003, 68, 1185–1191. Saio, K.; Koyama, E.; Yagasaki, S.; Watanabe, T. Protein-calcium-phytic acid relationship in soybean. Part III. Effect of phytic acid on coagulative reaction in tofu-making. Agric. Biol. Chem. 1969, 33, 36–42. Liu, Z.; Chang, KC. Effect of soy milk characteristics and cooking conditions on coagulant requirements for making filled tofu. J. Agric. Food Chem. 2004, 52, 3405–3411. Ishiguro, T.; Ono, T.; Nakasato, K.; Tsukamoto, C.; Shimada, S. Rapid measurement of phytate in raw soymilk by mid-infrared spectroscopy. Biosci., Biotechnol., Biochem. 2003, 67, 752–757. Liu, Z; Chang, KC. Development of a titration method for predicting optimal coagulant concentration for filled tofu. J .Agric. Food Chem. 2003, 51, 5214–5221.

254 In Chemistry, Texture, and Flavor of Soy; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Soybean Phytate Content and Its Influence on Tofu Texture - ACS

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