Protease hydrolysis of soy protein substrates can lead to weaker gels.

The targeted enzymatic modification of proteins could result in ingredients that have functionality tailored to specific food applications. For soy proteins, stronger gelation and high viscosity are preferable properties for comminuted meat products, soups and gravies.

At the same time, baby foods, yogurts and soups need a less viscous product mix and weaker gelation. Scientists at Iowa State University think that a minimal enzyme treatment of soy proteins retains thermo-rheological properties that could have use in certain applications. Their objective was to investigate the effect of a minimal protease treatment of soy protein products on such rheological properties as gelation and viscosity.

Investigators undertook a limited enzymatic hydrolysis: 2% and 4% degree of hydrolysis (DH) of soy protein isolate, concentrate, extruder-expelled (EE) flour and hexane-extracted soy flour. The work was carried out at pH 7 and at 50 C with endoprotease bromelain in a 250-mL glass reactor. The rheological properties of freeze-dried hydrolysates (10% w/w protein dispersions) were evaluated with a commercial rheometer. Dynamic shear (1% oscillatory strain, 0.1 Hz) was monitored in a temperature sweep of 25 C to 90 C, then cooled to 25 C. Apparent viscosity was followed through the shear rate of 10 to 500 s-1.

The non-treated protein isolate produced the strongest gel, followed by concentrate, EE meal and flour. The results showed that soy protein hydrolysates retained their gelling ability even after undergoing protease hydrolysis at a 2% and a 4% DH. Up to a 4% DH caused a dramatic loss of gelling ability--from 8- to 40-fold--for all substrates.

The storage modulus (G') increased when the isolates were cooled after they had been heated to 90 C. Hydrolysis increased the flow index and decreased the consistency coefficient, resulting in less viscous dispersions. The results suggest that the limited protease hydrolysis of different soy protein substrates at a low DH results in weaker gels and thinner dispersions. They could be useful in applications that utilize such properties as baby foods, yogurts and soups.

Further information. Lawrence Johnson, Center for Crops Utilization Research, Iowa State University, Department of Food Science and Human Nutrition, 1041 Food Sciences Building, Ames, IA 50011; phone: [phone]; fax: [phone]; email: [e-mail].