Results of a study of the effect of calcium addition on polypeptide composition, hydrophobicity, sulfhydryl content, thermal stability, enthalpy of denaturation, and water solubility of soybean protein isolates suggest that the addition of small amounts of calcium (1.23-5.0 mg/g protein) induced the formation of α,α′ soluble aggregates, whereas large amounts (5.0-9.73 mg/g protein) induced the selective insolubilization of the glycinin fraction. The addition of this ion also produced thermal stabilization of the soybean proteins, mainly the glycinin fraction, and increased the enthalpy of denaturation. A decrease in the surface hydrophobicity of proteins with increasing calcium content was also observed. The results obtained suggested the existence of specific calcium-soy protein interactions, especially with the glycinin fraction.New food ingredients with good-quality protein and low cost currently are being sought. Many of those being investigated include soy protein because soybean seeds contain a high amount of protein with good nutritional quality. Furthermore, glycinin (11S) and β-conglycinin (7S) globulins, the major components of soy protein isolates, possess appropriate functional properties for food applications. The structure and conformation of these proteins as well as substances present during the isolation procedure influence these properties.Macromolecules are affected by salts. These effects will depend not only on salt concentration (ionic strength) but also on their nature (lyotropic effect). According to Hofmeister's series, which arranges ions according to the lyotropic effect they may have, the calcium ion is located at the end of this series, corresponding to those ions that promote protein saltingin, leading to destabilization of the native structure (1).Calcium is an essential mineral, either by itself or when replacing sodium. It participates in the etiology or control of two widespread diseases: osteoporosis and arterial hypertension (2). Moreover, calcium intakes are often far below the recommended levels. Accordingly, its incorporation into nondairy foods is an appropriate method for achieving adequate intake.Although the interactions between soy proteins and calcium have been studied (3-7), no consensus has been reached. Although some researchers support the existence of specific interactions (4,7), others disagree (5).The aims of this study were to obtain soy protein isolates enriched by calcium addition and to study the influence of this ion on some protein structural parameters and on the water solubility of the isolates.
MATERIALS AND METHODS
Preparation of isolates.Protein isolates were prepared from hexane-defatted soy flour (Bunge-Ceval, Brazil) containing 50% protein (NSI 78-80). The flour was extracted with water adjusted to pH 8.0 with 2 N NaOH (flour/water ratio of 1:10 wt/vol) at room temperature for 2 h. The pH was periodically readjusted to 8.0. This suspension was filtered through gauze, and the filtered material was centrifuged at 10,000 × g at 4°C for 30 min. The sup...