Effects of Different Salt Ions on the Gel Properties and Molecular Interactions of Quinoa Protein

This research studied the effects of different salt ions on the gel properties of quinoa protein, and explored its molecular mechanisms, so as to provide a theoretical basis for the processing of quinoa protein gels.

Quinoa protein was extracted by alkali extraction and acid precipitation. Quinoa protein solution (20%, w/v) was prepared at pH 7.0. NaCl, CaCl₂, CaSO₄ and MgCl₂ was added in quinoa protein solution till the concentration was 50 mmol∙L^-1, and then the solution was heated in a water bath to prepare quinoa protein gels. The effects of salt ions on the texture, water retention, color properties and water distribution of quinoa protein gels were analyzed. Meanwhile, the effects of salt ions on the microstructure and rheological properties of quinoa protein gels were studied by scanning electron microscopy and rheometer. The effects of salt ions on the molecular interactions and secondary structure of protein gels were also analyzed.

The addition of salt ions significantly decreased the hardness and water holding capacity, while increased the springiness of quinoa protein gels under pH 7.0. Quinoa protein gels with MgCl₂ showed the lowest hardness and water holding capacity. NaCl addition had no significant influence on the color properties of protein gels. However, the addition of bivalent salt ions significantly improved the lightness and whiteness of quinoa protein gels, and their whiteness increased from 59.62 to 67.80 with the addition of CaCl₂. Furthermore, the addition of salt ions promoted granular aggregation of quinoa protein, which made the gel network structure become coarse. Coarse and larger gaps were observed in the microstructure of quinoa protein gels when divalent salt ions were added. Meanwhile, compared with blank gels and gels added with NaCl, the addition of divalent salt ions significantly decreased the content of disulfide bond, and weakened the electrostatic interactions within quinoa protein gels. Furthermore, the addition of salt ions decreased the contents of β-sheets and β-turns, increased the contents of α-helix and random coil, which affected the orderliness of protein secondary structure.

Under neutral conditions, the gel properties of quinoa protein and microstructure of gels were affected by the presence of different salt ions to various degrees. Compared with the gel prepared with NaCl, quinoa protein gels with the same concentration of CaCl₂, CaSO₄, and MgCl₂ showed rougher microstructure, lower gel hardness and water holding capacity, as divalent salt ions significantly decreased the disulfide bond content and weakened the electrostatic interactions within quinoa protein gels.