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New Crops News, Spring 1991, Vol. 1 No. 1

Transforming Soybeans to Improve Tofu

Tofu, or soybean curd, is the principle source of protein in Japan and other nations of the Orient. Cream colored with a texture not unlike firm custard, tofu is notably low in saturated fat and has no cholesterol. These characteristics are leading to greater acceptance in the increasingly diet conscious American market.

Niels Nielsen of the Purdue University Agronomy Department is developing soybean lines by conventional breeding practices that are suitable for producing high quality tofu. Fundamental to this objective is the need to develop criteria to identify those individuals in a breeding population that have superior tofu making characteristics. With this in mind, a rapid, small scale procedure has been developed by Evan Evans, a postdoctorate fellow from Australia, which permits tofu to be produced with 80g of seed meal. Tofu made by this procedure from accepted tofu varieties, such as 'Beeson' and 'Sennari' (Japan), gave a product whose physical characteristics closely resemble those of commercially prepared Japanese tofu (Mori-Nu Inc, intermediate between the soft and firm products). Sufficient material is obtained from the procedure so that the following characteristics can be monitored: recovery of solids and protein, color, pH, and the mouth feel parameters, fracturability and hardness. Comparison of a number of tofu preparations made from samples from the same variety revealed that these characteristics were highly reproducible.

The usefulness of the method was further tested by preparing tofu from a number of soybean varieties and advanced breeding lines. The results of this study revealed that substantial differences exist among the soybean lines tested in their ability to make high quality tofu. At present, we are using this method to survey 180 commercial varieties and advanced breeding lines from the Vartest trial at West Lafayette. It is envisaged that lines that combine high yield and useful tofu making attributes will be identified.

Despite a long history of tofu consumption in Japan, the characteristic beany taste of soy products is regarded as a characteristic that is discriminated against by the consumer. In this regard, elimination of the L2 and L3 lipoxygenase isozymes from soybean seeds has been demonstrated to effectively reduce the beany aroma and flavor. Taste tests conducted in Japan showed that soymilk prepared from L2L3-less 'Century' soybeans was preferred by consumers over soymilk prepared from standard varieties. The age of the consumer had no effect on the soymilk scores, but did have a marked influence on score for tofu quality. The older population preferred a traditional tofu with the stronger beany taste, whereas the younger population preferred the milder tasting tofu prepared from the L2L3-less 'Century' beans. Taste tests conducted with American consumers showed preference for the L2L3-less tofu. The tests further showed that the L2L3-less 'Century' soybeans produced tofu that was too yellow in color. The yellow color of this tofu will cause it to be discriminated against because tofu becomes more yellow as it ages.

Steps have been initiated to identify the compound(s) responsible for the yellow color of tofu made from the L2L3-less 'Century' beans. The compounds responsible for the yellow color of the L2L3-less are extractable into organic solvents. A comparison of the differences between UV spectra of extracts of tofu from L2L3-less and 'Burlison' (a white tofu) indicate that the responsible compound(s) are probably isoflavones. Experiments are now under way to establish the identity of these compounds. It is anticipated that their identification will enable the development of a rapid assay that selects beans which produce white tofu.

While a number of soybean varieties have been obtained that are considered to produce superior tofu by a large Japanese manufacturer, these varieties do not perform well in Indiana. In general, the yields of the Japanese lines are too low to be of commercial value, and the plants are especially prone to insect damage and pod shattering at maturity. We have begun to intercross Japanese, L2L3-less and the acceptable tofu producing American soybean varieties to develop germplasm with adequate protein content and yield, a large seed size, and which produce tofu with acceptable texture, appearance and taste. The value added soybeans are targeted for export to the Japanese tofu industry and the expanding domestic market.

Evan Evans
Department of Agronomy
Purdue University
West Lafayette, IN 47907-1165

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