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Determination of Selenium in Tofu by Hydride Generation―Atomic Absorption Spectrometry and the Relationship between Selenium Content

and Soybean OriginSatoko Tanaka*§　Sumiko Sakamoto**　Satoshi Iwatsuki**　Kenji Chayama**

　Selenium, an essential trace element, affects disease prevention and health promotion activity, particularly con-trolling the occurrence and metastasis of cancer. We measured the selenium content in processed soybean food by hydride generation―atomic absorption spectrometry. Measurement conditions were validated by analysis of the standard reference material, rice flour NIST―SRM �56� a. When the actual measurement value, 3� . � � . �　 g/�00 g, was compared to the certified value for standard reference, 3� . 0 � . 0　 g/�00 g, we concluded that measure-ment conditions were reasonable. When we measured the selenium content in �� different varieties of tofu, we found that the selenium content in tofu made from domestic soybeans was lower than those in tofu made from Ca-nadian and American soybeans. We found similar results from the measurement of the selenium content in kinako. The selenium content of baked kinako made from American soybeans was higher than that of powder made from Japanese soybeans. We concluded that selenium content measurement in processed soybean foods can be used to distinguish domestic products from foreign products.

Keyword： Selenium, hydride generation―atomic absorption spectrometry, processed soybean food, domestic products

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1．　Introduction

　The essentiality of selenium in living organisms was demonstrated by Schwartz et al. in �95�. As a constituent of glutathione peroxidase, selenium is involved in the deg-radation of lipid peroxides generated in vivo （Bowman et

al. �00�）. Selenium deficiency causes Keshan disease and Kashin―Beck disease and induces cancer （Bowman et al. �00�, Kenkou・eiyoujyohokenkyukai compilation �999, Su-zuki et al. �99�）. On the other hand, excess selenium in-take is known to cause various symptoms such as cardiac atrophy and loss of appetite （Bowman et al. �00�, Kenk-ou・eiyoujyohokenkyukai compilation �999, Suzuki et al. �99�）. At low concentrations, selenium is an essential nu-trient for humans and animals ;　however, at high concen-trations, it is toxic. Its narrow range of tolerance between toxicity and essentiality makes selenium an interesting trace element. The daily selenium requirement for hu-mans is �5―35 g for adult males and �0―30 g for adult females, and the intake limit for adults is �00―�50 g ac-cording to one food composition table （Kagawa �00�）. Se-

lenium is found in many fishes, meats, and grains ;　Japa-nese individuals whose staple foods are grain and fish seldom experience selenium deficiency （Suzuki et al. �99�）. However, due to the increasing consumption of processed and imported foods, selenium intake is as-sumed to be changing.　Analysis of food components on the scale of parts per billion （ppb）（ng/g levels） is necessary to determine trace amounts of components such as selenium. Fluores-cence spectrometry with � , 3―diaminonaphthalene （DAN ;　detection limit, 0 . 0�―0 . � ppb （Wang et al. �99�, Rodri-guez et al. �99�））, inductively coupled plasma mass spec-trometry （ICP―MS ;　detection limit, 0 . 06 ppb as �0Se de-termination （Japan Society for Analytical Chemistry �00�）, and hydride generation―atomic absorption spec-trometry （HG―ASS ;　detection limit, 0 . 0� ppb （Vande-casteele et al. �995）） are currently used determination methods of selenium, which can be measured with high accuracy at ng levels. Many examples of these measure-ment methods have been reported （Tanaka �9��, Naka-guchi et al. �9��, Tamari et al. �003, Tamari �999, Kana-mitsu et al. �00�, Noda et al. �9��, Choi et al. �009, Ferri et al. �00�, Navarro―Alarcon et al. �00�）, however, the former two methods are associated with some problems. In the HG―AAS method, selenium is determined by atomic absorption spectrometry as the amount of seleni-um hydride gases （H�Se） produced by the reduction of selenite acid, which is an inorganic form of selenium （IV）,

* Department of Food and Nutritional Science, Kobe Womenʼs Junior College

** Department of Chemistry Faculty of Science and Engineering, Konan University

　§ inquiry　 Department of Food and Nutritional Science, Kobe Womenʼs Junior College Kobe Womenʼs Junior College, �―�―� Minatojima―Nakamachi, Chuo, Kobe 650―00�6, Japan TEL 0��（�5�）6���　FAX 0��（�5�）��03

日本調理科学会誌 Vol. ��，No. 6，3�5～3�0（�0��）〔Article〕

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J. Cookery Sci. Jpn.　Vol. ��　No. 6（�0��）

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with sodium borohydride under acidic conditions. Accu-rate determination of selenium can be achieved without the effects of interfering substances and matrix contami-nation in the sample solution.　In this study, we examined the conditions for determin-ing the selenium content in food by HG―ASS and deter-mined the selenium content in tofu as a food sample. We have studied the mineral content in processed foods from soybeans by atomic absorption spectrometry and induc-tively coupled plasma―atomic emission spectrometry

（Tanaka et al. �00�, �006, �00�, �009）. In addition, by measuring trace minerals in agricultural products such as soybeans, differences between domestic and foreign prod-ucts may be distinguished （Yasui et al. �000, Ariyama et

al. �003, Kosaka et al. �006, Hattori et al. �009）. We exam-ined whether there were differences in the amount of se-lenium in soybean products by country of origin.

2．　Experiments

1）　Reagents　All chemical reagents were purchased from Wako Pure Chemicals. Selenium standard solution and sodium boro-hydride were graded for atomic absorption spectrometric analysis. Sodium selenate was a first grade product of 9�％ purity. Hydrochloric acid, nitric acid, and perchloric acid were of metal analytical grade. Sodium hydroxide was special grade. Rice flour SRM―�56� a from the Na-tional Institute of Standards and Technology （NIST） was used as a standard sample for optimization of measure-ment conditions of HG―AAS. A series of selenium （IV） solution was prepared by the appropriate dilutions of se-lenium standard solution. A series of sodium selenate so-lution was prepared by the appropriate dilutions of � , 000 parts per million （ppm） stock solution prepared by dis-solving 0 . �393 g of sodium selenate in 0 . 5％ nitric acid solution （�00 mL）.　Sodium borohydride solution （0 .�5 w/v％ in 0 . �％ NaOH） for hydride generation in the measurement of HG―AAS was prepared just before use. Hydrochloric acid

（analytical grade） was suitably diluted and prepared. All solutions were prepared using pure water obtained with a NANOpure II water purification system （Barnstead）.2）　Samples　Forty―one samples of commercially available tofu ［�� soft tofu （kinugoshi tofu）, �9 hard tofu （momen tofu）, and �� packed soft tofu （jyutenkinugoshi tofu）］ were purchased from a supermarket in Kobe City and ana-lyzed after freeze―drying. Four kinako were purchased

after confirmation of their country of origin. The samples that homogenization tofu of one packing were prepared for measurement by wet ashing and reduction by freeze―drying. After 0 . � to 0 . � g of freeze―dried material was accurately weighed, an acid mixture （3 mL HNO3 � mL HClO�） was added to the vial followed by heating on a hotplate. Hydrochloric acid （� .5 mL of �� M） was added to the ashed sample followed by heating to �0℃ for �0 min to reduce Se （VI） to Se （IV）. This solution was diluted to 3 . 6 M hydrochloric acid as a HG―AAS mea-surement sample.3）　Apparatus　A Hitachi Z―6�00 atomic absorption photometer equipped with a HFS―3 hydride generator was used for determining the selenium content of each sample. A sche-matic diagram of hydride generation is shown in Figure � and instrumental conditions of selenium determination are listed in Table �. An EYELA FD―�000 benchtop freeze dryer （Tokyo Rika, Japan） was used for lyophili-zation of the samples.

3．　Results�and�Discussion

　We first examined wet―ashing conditions. We added 5 mL of nitric acid to an accurately weighed standard sample and heated it to ��0℃. When the solution turned clear, we added � . 5 mL of perchloric acid and heated it to �60℃. During the wet―ashing procedure, we added nitric acid ensure that the sample volume was never less than � mL. We reduced the wet―ashed sample by adding � . 5 mL of �� M hydrochloric acid and heating it to �0℃ for �0 min, followed by adjusting the final hydrochloric acid concentration to 3 . 6 M to prepare the HG―AAS samples by previously reported methods. As a result, only 90％ of the standard value was observed. This result

Figure 1．　 Schematic diagram of seleniumhydride generation （HFS―3）.

Sample

HCl

Ar

NaBH4

Peristalticpump

Reactioncoil

Drain

Separator

H2Se

AAS

�9

Determination of Selenium in Tofu by Hydride Generation―Atomic Absorption Spectrometry and the Relationship between Selenium Content and Soybean Origin

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showed that selenium reduction was inadequate.　Thus, we reduced the initial amount of nitric acid to 3 mL and increased perchloric acid to � mL. During wet―ashing, to minimize nitric acid addition, a mixture of ni-tric acid and perchloric acid was added to maintain the sample volume of � mL. After reduction and dilution, the selenium amount was determined to be 3� . � � . � g/�00 g （measured five times）, which was in good agree-ment with the standard value （3� .0 � . 0 g/�00 g）. These results were consistent with previously reported results （Tanaka et al. �009）, in which excess nitric acid inhibited complete selenium reduction.　We tested whether the form of the tofu sample before wet―ashing influenced the measurement. Food samples would typically be stored in a freezer. However, because the water content of tofu is very high, a large amount of sample was needed to determine trace amounts of seleni-um.　We compared the determination results between freeze―drying and raw or frozen samples. The weights of freeze―dried samples were corrected to wet weight based on the water content of the samples. As a result, the selenium contents of tofu made from Canadian and American soybeans were � . �� 0 . �9 g/�00 g of raw tofu, � . �3 0 . �� g/�00 g of frozen tofu, and � . 500 . 0� g/�00 g of freeze―dried tofu, respectively. We con-cluded that the freeze―dried tofu samples were adequate for determining the selenium content without any prob-lems.　We measured the selenium contents in tofu and baked soybean flour samples using HG―ASS under the wet―ash-ing conditions previously described. The measurement results of �� tofu samples with the water contents and the wet―weights calculated from water contents are list-ed in Table �.From these results, selenium amounts differed with each

different soybean origin. The average amount of selenium in tofu samples was 0 . 5� g/�00 g of Japanese soybeans, 3 . 05 g/�00 g of mixed Canadian and American soy-beans, 0 . �3 g/�00 g of Chinese soybeans, � . �� g/�00 g of mixed Chinese and American soybeans, and � . 09 g/�00 g of soybeans of unknown origin. Selenium contents of tofu currently available in Japan, from a mixed of Ca-nadian and American soybeans （sample ��,�9,��,�3,��） were close to thevalue of 6 . � g/�00 g which was report-ed by Choi et al （Choi et al. �009）.　Thus, the difference in the average values was tested using the t―test. We found that the selenium amount in the tofu samples made from Japanese soybeans was sig-nificantly lower than that from mixed Canadian and American soybeans or mixed Chinese and American soy-beans （P＜0.00�）. On the other hand, no significant dif-ference was observed between tofu made from Japanese soybeans and tofu made from Chinese soybeans.　Tofu samples made from Chinese soybeans also showed significantly lower selenium contents compared with those from mixed Canadian and American soybeans or mixed Chinese and American soybeans. On the other hand, the selenium contents in tofu samples made from mixed Canadian and American soybeans were significant-ly higher compared with the others （P＜0 .05）. From these results, we found that Canadian and American soy-beans contain a high concentration of selenium.　A significant difference in the selenium content was found between Chinese and American soybeans and soy-beans of unknown country of origin. It may be possible that the tofu made from soybeans of unknown country of origin was made from a mixture of Chinese and Ameri-can soybeans.　The selenium content of soft tofu was believed to be the lowest among the soft, hard, and packed tofu prod-ucts. We examined this hypothesis by the t―test method

Table 1．　Instrumental conditions of selenium analysis by HG―AAS

AAS （Hitachi, model Z 6�00）Wavelength of hollow cathode lampElectric current of HCLAcetylene pressureAir pressure

�96 .0 nm�� .5 mA�5 kPa�60 kPa

Hydride generation apparatus （Hitachi, model HFS―3）Sampling timeReaction timeAcid carrierReducing reagentSample solution

5 .0 s�0 .0 s� . � M HCla）

0 .�5 w/v％ NaBH� in 0 . �％ NaOH3 .6 M HCla）

a） Optimized in the present study.

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J. Cookery Sci. Jpn.　Vol. ��　No. 6（�0��）

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using the data sets of tofu made from only Japanese soy-beans. The average values of selenium content of tofu were 0 . �6 g/�00 g of soft tofu, 0 . 59 g/�00 g of hard tofu, and 0 . 36 g/�00 g of packed tofu ;　a significant dif-ference between hard and packed tofu was observed

（P＜0 .05）. However, no significant difference was ob-served among other combinations. We also examined the correlation between the difference of coagulants and sele-nium content using the data sets of tofu made from only Japanese soybeans. The average values of selenium con-

tent in tofu were 0 . �� g/�00 g using MgCl� as a coagu-lant, 0 . 5� g/�00 g using MgCl�・CaCl� , and 0 . �0 g/�00 g using MgCl�・GDL （glucono― ―lacton）. There was no sig-nificant difference among these. Although the difference in coagulants was shown to be correlated with the calci-um and magnesium contents in tofu （Tanaka et al. �00�）, there was no correlation between selenium content and the difference in coagulants.　The measurement results of baked kinako are shown in Figure �. In particular, the selenium contents of flour

Table 2．　Selenium and water content of tofu and the soybean country of origin（n 5）

SampleNumber

Se g/�00 gwet weight

Water（％）

Soybean countryof origin Tofu variety

Coagulatingagent

��3�56��9�0�����3���5�6�����9�0�����3���5�6�����9303�3�333�35363�3�39�0��

0 .�� 0 . 0�0 . 63 0 . 0�0 . 55 0 . 060 . 6� 0 . 030 . �0 0 . 0�0 . �� 0 . 030 . �6 0 . 0�0 . �9 0 . 060 . 3� 0 . 0�0 . 36 0 . 0�0 . 6� 0 . 090 . 05 0 . 0�� . 96 0 . �0� . 00 0 . 06� . 6� 0 . 0�3 . �� 0 . 0�5 . �3 0 . 0�0 . 05 0 . 0�0 . �� 0 . 0�� . 03 0 . �9� . �9 0 . 0�� . 53 0 . ��6 . 0� 0 . 06� . �5 0 . �5� . 65 0 . �00 . �3 0 . 0�0 . �� 0 . 0�0 . �� 0 . 090 . 3� 0 . 03� . �� 0 . �60 . 5� 0 . 0�0 . 5� 0 . 0�� . �0 0 . 030 . 66 0 . 05� . �� 0 . ��0 . �� 0 . ��0 . 6� 0 . 0�� . 60 0 . 0�� . 0� 0 . 0�� . �� 0 . �00 . �� 0 . 0�

�� .9�� . ��� . ��� . ��� . 5�� . 6�� . 6�3 . 0�5 . 6�� . ��� . 5�6 . 3�� . 6�� . ��� . 0�6 . 0�� . 0�6 . ��� . �―

�� .��� .5�� . ��6 . 0�9 . ��5 . 6�� . ��3 . 0�� . 6�� . ��� . ��5 . 9�9 . ��� . ��6 . 5�9 . ��� . 0�� . 9�6 . 0�6 . ��� . 0

JapanJapanJapanJapanJapanJapanJapanJapanJapanJapanJapan

Japan （Hokkaido）Canada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・AmericaCanada・America

ChinaChinaChina

China・AmericaChina・AmericaChina・America

――――――――――

Soft tofuSoft tofuSoft tofuHard tofuHard tofuHard tofuHard tofuHard tofuHard tofu

Packed soft tofuPacked soft tofuPacked soft tofu

Soft tofuSoft tofuSoft tofuHard tofuHard tofuHard tofuHard tofuHard tofu

Packed soft tofuPacked soft tofuPacked soft tofuPacked soft tofuPacked soft tofu

Soft tofuHard tofuHard tofuSoft tofu

Packed soft tofuPacked soft tofu

Soft tofuSoft tofuSoft tofuHard tofuHard tofuHard tofuHard tofuHard tofuHard tofu

Packed soft tofu

MgCl�MgCl�

MgCl�, CaSO�

MgCl�MgCl�MgCl�MgCl�

MgCl�, CaSO�

MgCl�, CaSO�

MgCl�MgCl�MgCl�MgCl�MgCl�

MgCl�, CaSO�

MgCl�MgCl�MgCl�MgCl�

MgCl�, CaSO�

MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�MgCl�

MgCl�, CaSO�

MgCl�MgCl�

MgCl�, CaSO�

MgCl�, CaSO�

CaSO�

CaSO�, GDLMgCl�, GDL

Canada・America :　mixed Canadian and American soybeansChina・America :　mixed Chinese and American soybeansGDL :　glucono― ―lacton

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Determination of Selenium in Tofu by Hydride Generation―Atomic Absorption Spectrometry and the Relationship between Selenium Content and Soybean Origin

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made from American soybeans showed the highest value （�5 .� � . 5� g/�00 g）. The selenium content of flour made from Japanese soybeans was relatively low （� .5�0 .�9 g/�00 g）.　In contrast, mineral content is low in Japanese soy-beans. This result is consistent with the report that, im-ported soybeans, mineral contents are highest in Ameri-can soybeans followed by Canadian soybeans （Yoshida et

al. �995）. It has been reported that the mineral content varies depending on minerals in the soil （Yoshida �00�）.

Conclusion

　We determined the amounts of selenium, which shows various physiological activities in tofu and baked soybean flour, by the HG―AAS method. Because the measurement value of NIST SRM―�56� a rice flour （3� .� � . � g/�00 g） was consistent with the certified value of 3� . 0� . 0 g/�00 g, we concluded that the manipulation and an-alytical conditions of this method were accurate. We de-termined the selenium contents in tofu and baked soy-bean flour using these analytical conditions. We obtained the results below.　In �� tofu samples, the average values of the selenium contents of tofu samples were 0 . 5� g/�00 g of Japanese soybeans, 3 . 05 g/�00 g of mixed Canadian and Ameri-can soybeans, 0 . �3 g/�00 g of Chinese soybeans, � . �� g/�00 g of mixed Chinese and American soybeans, and � . 09 g/�00 g of soybeans of unknown country of ori-gin. The selenium content of tofu made from Japanese soybeans was significantly low （P＜0.05）. On the other

hand, tofu made from mixed Canadian and American soy-beans showed a significantly high selenium value （P＜0 .05）. The selenium content of baked kinako or made from American soybeans was higher than that of kinako made from Japanese soybeans. According to this study, to eat �00 g of Japanese soybean tofu might supply about �/�6 of the daily selenium requirement for an adult, and �/� in the case of mixed Canada and America soybean tofu.　Therefore, soybean is thought to be a part of the re-source of selenium.

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Konan Univ., Sci. & Eng. Ser., 53（�）, �5―95

Figure�2．　 Relationship between selenium concentration of kinako and the country of origin

Country of origin

Se content (mg/100g)

Japan Canada America China

20

15

10

5

0

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（Received Apr, 6, �0��　Accepted Sep, ��, �0��）

水素化物発生原子吸光法による豆腐中セレン含有量と大豆原産国との関係

田 中 智 子*　坂本須美子**　岩 月 聡 史**　茶 山 健 二**

キーワード：セレン，水素化物原子吸光度法，大豆加工品，国産

* 神戸女子短期大学食物栄養学科 ** 甲南大学理工学部　§ 連絡先　 神戸女子短期大学　食物栄養学科

〒 650―00�6　神戸市中央区港島中町 �―�―�TEL 0��（�5�）6���　FAX 0��（�5�）��03

和文抄録　セレンは必須微量元素の � つで健康増進作用や疾病予防作用，とりわけガンの発生や転移を抑制することで，注目を集めている元素である。食品中のセレン含有量を水素化物発生原子吸光法を用い測定した。測定条件は，NIST SRM―�56� a の米粉を用い認証値 3�0 �0 ppb に対し，3�� �� ppb で妥当性を確認した。　�� 種類の豆腐中セレン含有量を測定した結果，国産と中国産大豆を用いた豆腐中セレン含有量は低く，カナダ・アメリカ産大豆を用いた豆腐は高かった。きな粉を測定した結果も，大豆の原産国がアメリカのセレン含有量は高く，国産大豆のきな粉は低くかった。