The soymilk shows astringent taste. However, undesirable astringent taste of soymilk disappears when tofu is curdled. Tofu curd usually shows less or not astringent taste but some tofus in a market showing astringent taste. There is no report indicating the reason of less objectionable sensation in tofu curd. Therefore, the aims of this research were,
i)To identify the real components responsible for the astringent taste in soy foods.
ii)To clear the relation between the taste characteristics and chemical components in soymilk and tofu curd.
iii)Establishment of a method of identifying astringency by sensory evaluations.
iv)Proposal of new methods of preparing the soymilk having low astringent characteristics.

In order to achieve the objectives the total works were performed in three steps as follows:

Experiment 1) Identify the real components responsible for the astringent taste in soy foods.

In order to get reliable result by sensory evaluation, ten panelists were selected who are able to recognize the astringent sensation at low concentration. Isoflavone enriched extract (about 39% isoflavones) showed no astringency. Some soy foods having high amount of isoflavone showed less astringent taste but some soy foods having less isoflavone showed strong astringency. Therefore, it was assumed that the components with astringency are solubilized in soymilk but are insoluble (attached with large component) in tofu curd, which create tofu with less/no astringency. To determine the location of isoflavone in soymilk and tofu curd, dialysis was performed. About 80% of isoflavones exist soluble form (dialyzable) in both soymilk and tofu. The location of phytates in both soymilk and tofu curd was also investigated. Phytates are known to be bound with soy proteins during the tofu curd formation. About 55% of total phytates existed freely (soluble/dialyzable) in the soymilk but only 6-13% of total phytates existed freely (soluble/dialyzable) in the tofu curds coagulated with Ca-/Mg-salts). In addition, 1% K-phytate solution at pH 7 showed the very similar astringency to soymilk, however Ca-phytate at the same concentration and pH showed no undesirable sensation. Thus, above results showed that neither isoflavones but k-phytate may responsible for astringency in soymilk and tofu curd.

Experiment 2) Decrease mechanism of undesirable astringent taste of soymilk during tofu curd formation.

The astringency of the whey fractions of different tofu coagulum was investigated. About 50% phytate (about 120mg/100g tofu) was remained in GDL tofu whey (tofu curd was lightly centrifuged and separated the supernatant fraction). Whereas, Ca-tofu whey contained negligible phytate content. To obtain desire amount of phytate in the tofu whey for sensory evaluation, tofu curds were prepared with mix-coagulant (GDL + calcium sulfate) where the final coagulant concentrations were 0.3%. Phytates in the whey were decreased to 0.05 to 0.01% by increasing calcium sulfate concentration. Four of ten panels recognized the concentration difference of 0.04% by triangle tests. GDL-coagulant itself may influence to the whey testing. Therefore, tofu curds were prepared by only calcium sulfate coagulant with different concentrations. The intensity of astringency of the tofu whey dramatically decreased with 0.1, 0.2, and 0.3% calcium sulfate concentration. Phytate of the wheys were 0.13, 0.06, and 0.03%, respectively. All panel members were able to identify the concentration of 0.1% difference of phytate by triangle test. In the same way, seven of ten were distinguished the 0.07% difference but none of them were distinguished the 0.03% difference. Above results showed that astringency of soymilk caused by K-phytate decreased due to addition of coagulant to make tofu curd. As the high potassium content (118mg/100g) was detected in tofu whey, if the phytate remains free even at low concentration (about 0.04%) in the curd under certain circumstances, that may create tofu with astringency.

Experiment 3) Improvement of astringency in soymilk and tofu curd.

The triangle test method in this study proved that all panel members could differ 0.02% tannic acid, and 0.1% potassium phytate against water. Those solutions were the lowest concentrations that ten panelists were correctly discriminated against water. Tannic acid was taken as standard astringent sample and potassium phytate as proposed astringent solution in this dissertation. Then it was thought whether panelists able to distinguish tannic acid from other solutions except water. Sensory comparison of 0.02% tannic acid and 0.1% potassium phyate was performed by triangle test method. Seven of ten panels failed to discriminate the difference among these samples. If those sample showed different taste then more panels should differ among them. So, it can be assumed that 0.1% potassium phytate at pH 7 perhaps shows the similar astringency to 0.02% tannic acid. Taste evaluation of potassium-, calcium-, and sodium phytate (0.1% solution) were performed by descriptive taste method. All panels expressed that 0.1% potassium phytate had an astringent taste but those of calcium / sodium phytate had no astringent taste. De-phytate soymilk showed no astringency by descriptive sensory evaluation.

　It can be suggested that astringency caused by potassium phytate can eliminate by using null or less phytic acid contained seeds, and that certain concentration of calcium coagulant may work as a masking agent of traditional soymilk astringency. A "drinking tofu" can be thought for non-traditional people (unfamiliar to drink traditional soymilk) in which taste should be as like tofu but physical structure as like as traditional soymilk. Control of free form phytate by using Ca-/Mg- coagulant to form a tofu curd should serve improved taste.