59D-6 |
The effect of surfactants on the charge, rheology, and solubility of soy protein isolate |
A. MALHOTRA and J. N. Coupland. Dept. of Food Science, The Pennsylvania State Univ., 103 Borland Lab., University Park, PA 16802 Soy proteins are important functional and nutritional ingredients in many foods. Frequently their functionality is limited by their relatively poor solubility, particularly close to their isoelectric point. We are concerned with understanding the mechanism of soy protein insolubility and determining ways to improve it. Our objective was to study the effects of added surfactant on functional and molecular properties of soy protein isolate. Soy protein isolate (0.05 wt%) was dispersed in water, and the pH adjusted with small volumes of acid or base. The zeta-potential of the soy protein particles was measured by phase-analysis light scattering. The protein was titrated with either surfactant (sodium dodecyl sulfate, SDS, or polyoxyethylene sorbitan monolaurate, Tween 20) at a constant pH (4 or 8) or with acid at a constant surfactant concentration (0 or 5 wt% SDS or Tween 20). The solubility and viscosity of more concentrated (5 wt%) protein samples was measured by established techniques. The solubility of the surfactant-free and Tween 20 containing samples showed a typical U-shaped dependence on pH with a minimum (~10% soluble) at their isoelectric point (3.5-4.5) while the SDS containing samples were highly soluble (>90%) at all pH values. The zeta-potential of the Tween and surfactant free sample increased from a negative to a positive value with decreasing pH while the SDS containing samples were equally highly charged (~ -52 mV) at all pH values. Together, these results suggest a high charge is essential for good soy protein solubility. Titrations with surfactant were used to study the stochiometry of complex formation. While we do not expect to use SDS as a food ingredient the controlling influence of charge on soy protein solubility is clearly illustrated. Other charged complexing agents may be better candidates to prepare soluble soy protein ingredients.
Session 59D, Food Chemistry: Proteins and Physicochemical Properties
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