Soy bean proteins have been utilized as an excellent edible protein source for a long time. In addition, since they have various functional properties such as emulsifying and gel-forming capabilities, they have been widely used as a raw material for foods or a material for improving quality of foods in edible meat products, fishery paste products, side dishes, bread, confectionery and a raw material for beverages. Further, recently, it has been elucidated that soybean proteins reduce blood cholesterol level and their nutritional and physiological functions have been noted.
On the other hand, in so-called acidic foods having pH below 4.6 (Ed. by Isao SHIBASAKI, “Sakkin and/or Jokin Ohyo Handbook (Sterilization and/or Disinfection Application Handbook)”, SCIENCE FORUM, p. 28), the use of soybean proteins is restricted because soybean proteins scarcely dissolve and do not exhibit their functional properties in a pH region where they are frequently used (pH 3.0 to 4.5). This is due to pH of acidic foods which is equal to or in the vicinity of the isoelectric point of a soybean protein (about pH 5).
Many techniques in the prior art relating to utilization of soybean proteins in acidic foods are mainly directed to prevention of aggregation and/or precipitation of soybean proteins in an acid region in the production of acidic beverages. For example, there have been known the addition of stabilizers such as pectin (JP 54-52754 A), and emulsifiers such as a sugar fatty acid ester having HLB of 13 or more (JP 59-41709 B).
The following will illustrate the conditions of a protein at the time of addition of a stabilizer. In a solution containing a soybean protein which is adjusted to pH 3.0 to 4.5, the protein in the system has a positive surface charge, but an absolute value of the charged amount is low because pH is in the vicinity of the isoelectric point. Then, the protein is liable to form aggregation and/or precipitation. A stabilizer, whose typical examples include polyanionic polysaccharides such as pectin, propylene glycol alginate, carboxymethylcellulose, etc., interacts with a protein having a positive charge and a stabilizer molecule adheres thereto to form protein granules each of which as a whole has a negative surface charge, thereby avoiding aggregation and/or precipitation owing to electric repulsion between the granules. However, these techniques using stabilizers or emulsifiers are not those for obtaining a dissolution state of a protein itself but those to be applied and utilized at the time of formulating a protein material and other materials. Then, a product having a transparent appearance can not be obtained and functional properties of a protein material itself such as emulsifying and gel-forming capabilities are hardly expected.
On the other hand, methods for suppressing aggregation due to passing the isoelectric point of a soybean protein have also been proposed (JP 7-16084 A and JP 12-77 A). However, since the addition of a stabilizer or emulsifier is required, the conditions of a protein are the same as those described above.
As a method for increasing solubility of a protein in an acidic region of below its isoelectric point, for a soybean protein, there is a method disclosed in JP 53-19669 B. In this method, a slurry of a soybean protein isolate having a solid content of 10 to 15% by weight is prepared at pH about 2.0 to about 4.2 and the slurry is subjected to a heat treatment at a temperature of about 120 to 160° C. by a continuous process.
However, a problem in solubility of a soybean protein in an acidic region still remains in this method. When a soybean protein slurry is adjusted to pH 3.0 to 3.5 and subjected to a heat treatment at a high temperature, the protein molecule forms a dispersed state, but it is a cloudy solution. Further, precipitation of the protein is caused during storage. This is not suitable for using the protein in protein foods in an acid region, particularly, in acidic protein beverages. Furthermore, a cloudy protein obtained by this method has poor functional properties such as emulsifying and gel-forming capabilities and its utility as a food improving material, which is expected for a normal soybean protein isolate, is remarkably restricted.
In addition, JP 55-29654 B discloses a method for isolating a soluble protein fraction, wherein a fraction soluble at pH below 4.6 can be isolated by combination of a phytase treatment and fractionation by pH adjustment. However, in this method, the product is obtained in low yield such as 14% by using a soybean protein isolate as a starting material. Therefore, this method is less practical.
JP 51-125300 A discloses a process for producing a protein having excellent solubility at pH 3 to 5 by washing defatted soybeans with an acid, treating the soybeans with an acidic phytase derived from a microorganism at pH 2 to 6 and separating a solubilized fraction. However, the protein obtained by this process is highly hydrolyzed by a protease. In addition, both solubilized and insolubilized fractions are formed and their separation is required. Then, the objective soybean protein having high solubility is obtained in low yield.
Thus, a soybean protein material, which can be utilized in an acidic food of pH lower than 4.6 and is soluble in a range of pH 3.0 to 4.5, and whose solution has preferred transparency in appearance and excellent storage stability together with functional properties such as emulsifying and gel-forming capabilities, has not been obtained heretofore in the prior art. Further, no process has been known for producing a soybean protein hydrolysate having high solubility and storage stability in the above pH range, efficiently.