Effects of water, steam and shear conditioning on the protein quality of soy grits.

M. Thomas, E. M. R. A. H. van Kol, S. Tamminga, M. W. A., Verstegen and A. F. B. van der Poel

Department of Animal Nutrition, Wageningen Institute of Animal Sciences (WIAS), Marijkeweg 40, 6709 PG Wageningen, The Netherlands


The effect of adding steam (at 0•6 and 1•2 bar) and water (1•9% and 64% of the flux of soy grits) in a conventional barrel type conditioner and its effects on protein dispersibility index (PDI), nitrogen solubility in 0•2% KOH and trypsin inhibitor activity (TIA) of commercial soy grits was studied. In addition, conditioned soy grits were subjected to expander treatment at two different screw speeds (2 and 3 rps) and PDI, NSI and TIA were measured. Analysis of variance was used to determine the effects of steam pressure, water addition and screw speed on the mentioned protein quality parameters. In addition, resultant temperature from steam addition was used in an analysis of covariance to determine the effects of temperature as a covariate, water addition and screw speed on protein quality parameters. A significant interaction for steam pressure and water addition was found on all parameters. The highest denaturation or inactivation of the protein was found at the combination incorporating high steam and high water addition. Screw speed during expander treatment did not significantly affect protein quality parameters. It was concluded both from this study and from literature that PDI is a better parameter to evaluate technological treatments when low amounts of motor power are dissipated (<110 kJ kg-1) and that NSI in 0•2% KOH is a better parameter to discriminate between technological treatments when moderate to high amounts of motor power are dissipated in soy grits. TIA decreased with increasing water or steam content. The effect of trypsine inhibitor inactivation in soy grits was highest at higher temperatures and was not affected by screw speed of the expander.

Keywords: protein quality; protein dispersibility; nitrogen solubility; trypsin inhibitor activity; soy grits; conditioner; expander; water; steam; processing

Journal of the Science of Food and Agriculture, Volume 74, Issue 3, July 1997, pages 392–400

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