Soy Protein Isolate and Process for Its Manufacture

isolated soy protein A process for producing a soy protein isolate, the process comprising: dispersing white flakes produced from soybeans in a liquid to produce a soy protein extract; separating insoluble material from the soy protein extract to form a soluble soy protein extract; adjusting the pH of the soluble soy protein extract to about the isoelectric point of soy protein with an acid to form a precipitated soy protein mixture; centrifuging the precipitated soy protein mixture and decanting the supernatant to form a soy protein curd; diluting the soy protein curd with water to form a soy protein slurry; adjusting the pH of the soy protein slurry to a pH of from about 9.5 to about 10.5 with a base to form a pH-adjusted soy protein slurry; heating and reacting the pH-adjusted soy protein slurry with an enzyme without maintaining the pH level to form an enzyme hydrolyzed soy protein mixture; and adjusting the pH of the enzyme hydrolyzed soy protein mixture with an acid to a pH of from about 7.0 to about 7.6; wherein the enzyme hydrolyzed soy protein isolate has an average molecular weight of from about 12,000 Daltons to about 18,000 Daltons and a degree of hydrolysis of from about 2.9% to about 5.2%, wherein the soy protein isolate has a soluble solids index of from about 80% to about 100% at a pH of from about 7.0 to about 7.8, an average particle size of from about 15 .mu.M to about 60 .mu.M, and greater than 99% homogeneity in water at a pH of from about 7.0 to about 7.8 after 30 minutes, and wherein the soy protein isolate has greater than 90% homogeneity in water at a pH of from about 7.0 to about 7.8 after 2 hours.Once the STNBS value is determined for the soy protein material in a given soy protein isolate sample, the percent degree of hydrolysis can be determined by the following formula, if so desired: Degree of Hydrolysis (%)=((STNBS Value.sub.Sample-24)/885)(100) wherein 24 is the TNBS correction for lysyl amino group of non-hydrolyzed isolated soy protein and 885 is the theoretical total TNBS value for the complete isolated soy protein hydrolysate (derived from the total amino acid profile of isolated soy proteins).[0049] Preferably, the soy protein isolate of the present invention is comprised of soy protein material having a degree of hydrolysis of about 50 STNBS to about 70 STNBS. More preferably, the soy protein material has a degree of hydrolysis of about 54 to about 65 STNBS. Still more preferably, the soy protein material has a degree of hydrolysis of about 55 STNBS to about 62 STNBS.Measurement of the Soluble Solids Index of the Soy Protein Isolate[0050] A ready-to-drink acidic beverage comprised, in part, of a soy protein isolate will have less sediment where the soy protein isolate has a higher solubility and/or higher suspendability. At present, there is more than one procedure available to determine solubility. The High Speed Solubility ("HSS") method separates the insoluble protein by centrifugation for 20 minutes at 45,500.times.g, whereas the Nitrogen Solubility Index ("NSI") method operates at 500.times.g for 10 minutes. Obtaining results from both the HSS and the NSI methods takes a relatively long period of time because total protein and soluble protein is measured by time consuming methods such as the Biuret, Kjeldahl, or LECO combustion procedures.[0051] As such, an alternative method, the Soluble Solids Index ("SSI") method, was developed as a rapid method for determining the solubility of soy protein isolates. Using the SSI method, the solubility is measured based on solids, as opposed to total protein and soluble protein. For soy protein isolates, the SSI value correlates to NSI values, and is, therefore, just as valuable as the NSI values for monitoring soy protein solubility as affected by process treatments or during storage.[0052] To calculate SSI, 12.5 g of the soy protein isolate of the present invention is weighed in a weigh boat. Deionized water (487.5 g) is then added to a blender jar. A defoamer (2-3 drops, Dow Corning Antifoam B Emulsion 1:1 with H.sub.2O) is added to the water. The blender rheostat is adjusted such that the stirring speed produces a moderate vortex (approximately 14,000 rpm). The soy protein isolate sample is added within 30 seconds of the creation of the vortex, and is blended for 60 seconds. The soy protein isolate sample slurry is then transferred to a 500 ml beaker, and the beaker is covered and stirred for 30 minutes at a moderate speed. Then, two 200 g portions of the soy protein isolate sample slurry are transferred into two centrifuge bottles. The remainder portion is kept for total solids calculation. The centrifuge bottles containing the soy protein isolate sample slurry are centrifuged at 500.times.g for 10 minutes. The supernatant (50 ml) is withdrawn from the centrifuge bottles and placed into plastic cups. Equal portions of the soluble solids supernatant (5.0 g) and the total solids remainder portion (5.0 g) are dried at 130.degree. C. for 2 hours and weighed to determine soluble solids and total solids, respectively. The SSI is then calculated using the following formula: SSI(%)=(Soluble Solids/Total Solids).times.100 The average from the two supernatant samples is then calculated to determine reported SSI values.