Solubility and Dispersion Viscosity of Soy Protein Isolates as Influenced by pH and Sucrose in Model and Food Systems

Three commercial soy isolates, Pro Fam S-970, Promine F and Supro 710, were studied. Part A involved the investigation of protein solubility and dispersion viscosity of the isolates in simple models at 3°C at pH levels of 3.8,3.3 and 2.8, attained by additions of citric acid solution. The effects of addition of sucrose at varying levels on solubility and dispersion viscosity of the isolates at the given pH levels were studied also. In Part B the findings of Part A were applied to functional performance of the isolate Supro 710 in a food system. A citrus flavored carbonated soy beverage was made from the isolate and subjected to objective and sensory evaluation. For the calculation of nitrogen solubility index (NSI), the total nitrogen contents of aqueous extracts and of the dry unextracted sample were determined by the standard microKjeldahl method. The objective determination of the apparent viscosity of dispersions of both the simple model and the food system was made with the Cannon-Feske Routine viscometer, #100. The Quantitative Descriptive Analysis (QDA) method was used for sensory evaluation of the product.

In Part A the main effects of pH and sucrose on NSI were significant for all soy protein isolates; the pH-sucrose interaction was significant for Pro Fam S-970 and Promine F. The solubility of each isolate increased as the dispersion pH was decreased from 3.8 to 2.8. Solubility of each isolate decreased with the addition of sucrose to the dispersions.

The apparent viscosity of isolate dispersions also was affected by both pH and sucrose. A small increase in apparent viscosity of the dispersions was found for each isolate as the pH of the dispersion was reduced from 3.8 to 2.8. The addition of sucrose to the dispersions also resulted in a small increase in the apparent viscosity.

In Part B, the results of the objective measurement of apparent viscosity were similar to those obtained for the model system in Part A. In the sensory evaluation, both pH and sucrose had significant effects on the individual parameters and on the overall acceptability of the beverage. As could be expected, decreased pH resulted in increased perception of tartness and decreased perception of sweetness; increased sucrose concentration resulted in decreased perception of tartness and increased perception of sweetness. The extent of the effects of sucrose on both tartness and overall acceptability depended on the pH level. The ratings of overall acceptability in relation to the values for specific parameters indicated that the panelists liked a low degree of tartness and a high degree of sweetness.

The combination of a sucrose concentration of either 12 percent with pH 3.8 or 15 percent with pH 3.8 or 3.3 appears to be optimum for this product. A beverage with 12 to 15 percent sucrose at pH 3.8 could contain about 2.2 g soluble protein per 100 ml and one with 15 percent sucrose at pH 3.3 could contain about 2.5 g soluble protein per 100 ml. The rating of borderline acceptability of a soy beverage with any of those pH-sucrose combinations suggests the feasibility of developing such a product.