Xanthan Gum Effects on Solubility and Emulsification Properties of Soy Protein Isolate

Abstract

The effect of xanthan gum (XG) on solubility and emulsifying properties of soy protein isolate (SPI) was evaluated. The solubility of SPI was increased by addition of XG (P<0.05). The emulsifying activity of SPI-XG was 4 times higher than that of SPI or XG alone (P<0.05) and similar to that of bovine serum albumin (BSA) (P>0.05). The emulsifying stability of SPI-XG dispersions was respectively 3 and 2 times higher than that of SPI and BSA (P<0.05). The solubility and emulsifying properties of SPI-XG dispersions were stable over a wide range of pH (3.0 to 9.0), ionic strength (0.1 to 1.0M NaCl), and heat (85°C, 1 hr). Key Words: soy protein, xanthan gum, emulsifying activity, emulsion stability tained from the Dept. of Agronomy, Univ. of Arkansas. Reagents were of analytical grade and purchased from Fisher Scientific (Pittsburgh, PA) and Sigma Chemical Co. (St. Louis, MO). Preparation of SPIThe SPI was prepared from defatted soybean flour by the method of Wolf and Cowan (1975). Soybeans were cracked in a blender (Osterizer Galaxie Dual Range 14, Oster Corp., Milwaukee, WI), dehulled in a vertical aspiration unit (Seedburo Equipment Co., Chicago, IL), and milled (UDY Cyclone Sample Mill, UDY Corp., Fort Collins, CO) to provide flour. The soybean flour was defatted twice in hexane for 10 min (1:3, flour:hexane) to remove oil and dried overnight under a hood to remove residual hexane. Defatted soybean flour was suspended in water (1 part flour to 10 parts water), followed by alkaline extraction at pH 9.0 for 30 min. After centrifugation at 10,000 × g for 10 min, the supernatant pH was adjusted to 4.5 (isoelectric point of soy protein) with 1N HCl to precipitate proteins, followed by centrifugation at 10,000 × g for 10 min. The protein precipitate was washed with water, and pH was adjusted to 7. It was then frozen at -15°C, freeze-dried and stored at 5°C until used. Preparation of SPI-XG dispersionsSix 1-g portions of SPI and 0.3, 0.5, 1, 2, 3, or 4g of XG were dry-mixed manually to prepare the respective SPI-XG blends in the weight ratios of SPI/XG = 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4. Six SPI-XG dispersions were prepared by transferring the blends into 0.1 M sodium phosphate buffer (pH 7.4) while stirring with a magnetic stirrer (Mistral Pyro Multi-Stirrer, Lab-Line Instruments, Inc., Melrose Park, IL) for 60 min at ambient temperature to provide each dispersion with 0.1% soy protein. These dispersions were used for determination of viscosity and emulsifying properties. Based on preliminary data, SPI-XG in the weight ratio of 1:2, which gave the optimum emulsifying properties, was chosen to evaluate effects of pH, heat, and salt treatments. Determination of viscosityThe viscosity of prepared SPI-XG dispersions (SPI/XG = 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4) in 0.1M sodium phosphate buffer (pH 7.4) was measured by a Brookfield viscometer (Model RVF, Brookfield Engineering Laboratories, Inc., Stoughton, MA). All measurements were carried out at ambient temperature using spindle #3 at a speed of 20 rpm. Nitrogen solubility d...

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