36D-14

B. P. LAMSAL¹, S. Jung², P. A. Murphy², and L. A. Johnson³. (1) Food Sciences Dept., Iowa State Univ., 2312 FSHN, Ames, IA 50011, (2) Dept. of Food Science and Human Nutrition, Iowa State Univ., 2312 Food Sciences Bldg., Ames, IA 50011, (3) Center for Crops Utilization Research, Iowa State Univ., Dept. of Food Science and Human Nutrition, 1041 Food Sciences Bldg., Ames, IA 50011

Targeted enzymatic modification of proteins could result in ingredients with functionality tailored to specific food applications. For soy proteins, stronger gelation and high viscosity are preferable in applications for comminuted meat products, soups and gravies. At the same time, baby food applications, yogurts, and soups need less viscous product mix and weaker gelation. We hypothesize that minimal enzyme treatment of soy proteins retains thermo-rheological properties that could have use in target food applications. Our objective was to investigate the effect of minimal protease treatment of soy protein products on such rheological properties as gelation and viscosity. Limited enzymatic hydrolysis (2 and 4% degree of hydrolysis, DH) of soy protein isolate, concentrate, extruder-expelled (EE) flour, and hexane-extracted soy flour was carried out at pH 7, 50° C with endoprotease bromelain in a 250-mL glass reactor and monitored with a pH-STAT. Rheological properties of freeze-dried hydrolysates (10% w/w protein dispersion) was evaluated with RS 150 Haake rheometer. Dynamic shear (1% oscillatory strain, 0.1 Hz) was monitored in a temperature sweep (25 to 90° C, cooled to 25° C), while apparent viscosity was followed through the shear rate of 10 to 500 s^-1.The non-treated protein isolate produced strongest gel followed by concentrate, EE meal and flour. Our results showed that soy protein hydrolysates retained their gelling ability even after protease hydrolysis at 2 and 4 % DH. Hydrolysis up to 4% DH caused dramatic loss of gelling ability for all substrates ranging between 8 to 40 fold. Storage modulus (G') increased during cooling after heating to 90° C. G' values at 25° C were in the order: Control > DH2% > DH4%. Power law model provided excellent fit to hydrolysate dispersions showing shear-thinning behavior (n<1). Hydrolysis increased flow index and decreased consistency coefficient resulting in less viscous dispersions. The results suggest that limited protease hydrolysis of different soy protein substrates at low DH results in weaker gels and thinner dispersions. They could be useful in applications utilizing such properties like baby foods, yogurts, and soups.

Session 36D, Food Engineering: Rheology
8:30 AM - 12:00 PM, Monday AM Room Hall I-2

2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana