Seed Protein Content and Consistency of Tofu Prepared with Different Magnesium Chloride Concentrations in Six Japanese Soybean Varieties

Abstract

The relationship between the protein content of soybean seeds and the consistency of tofu was examined for six Japanese soybean varieties, Enrei, Fukuyutaka, Sachiyutaka, Ayakogane, Hatayutaka and Tachinagaha. The seed protein content was estimated by determining the nitrogen content using the Dumas method. Tofu was prepared from a raw homogenate of water-soaked soybeans by heating and by the addition of MgCl₂ as a coagulant. The tofu consistency was evaluated by measuring the breaking stress of tofu curd using a Creep meter. The breaking stress of tofu increased when the concentrations of MgCl₂ in soymilk increased above 0.20 %. The breaking stress reached a maximum value at concentrations of around 0.40 %, with differences among soybean varieties and cultivation conditions of the soybeans. There was a significant positive correlation (r = 0.87) between the maximum breaking stress of tofu and the seed protein content for the six varieties. In contrast, the breaking stress of tofu prepared with 0.25 % MgCl₂ did not show a significant correlation (r = 0.27) with the seed protein content for the six varieties but was significantly correlated (r = 0.52), when the data of Sachiyutaka were excluded. Fukuyutaka and Ayakogane required a lower MgCl₂ concentration for the maximum breaking stress of tofu than Sachiyutaka, Enrei, Tachinagaha and Hatayutaka, which required a MgCl₂ concentration above 0.40 % for the maximum breaking stress of tofu. Especially, Sachiyutaka required the highest MgCl₂ concentration, 0.45 % on the average, for the maximum breaking stress of tofu among the six varieties. Sachiyutaka-tofu showed the lowest breaking stress on the average at a concentration of 0.25 % MgCl₂, which is the concentration generally used by manufacturers, in spite of its high content in seed protein. In contrast, Fukuyutaka required the lowest MgCl₂ concentration, 0.34 % on the average, for the maximum breaking stress and the highest breaking stress of tofu prepared with 0.25 % MgCl₂. That is one of reasons why manufacturers prefer to use Fukuyutaka for producing tofu. Concentration of a coagulant for the maximum breaking stress as well as seed protein content could become criteria for quality evaluation of soybeans for tofu processing.