Relationship between Calcium-binding Ability and Structure of Soybean Protein Based on the Change of Preparation

Soybean protein was a high-quality plant-based alternative protein, yet its calcium content was lower than animal protein. This study measured the calcium-binding ability of soybean protein under different extraction, heat treatment, and enzymatic hydrolysis conditions. The differences in calcium-binding ability, subunit composition, secondary structure, tertiary structure and hydrophobicity of soybean protein under four different preparation methods were compared. The relationship between the calcium-binding ability and the structure of soybean protein were discussed. The results showed that the binding of soybean protein and calcium ions was related to both the protein's secondary structure composition and its hydrophobicity. Heat treatment and enzymatic hydrolysis unfolded the secondary structure, leading to the enhancement of hydrophobicity and the exposure of the calcium-binding sites. Enzymolysis led to the aggregation of small molecular weight proteins, causing a spatial restructuring of the protein. These changes resulted in a 53% increase in bound calcium compared to heat treatment. The heat-enzyme combined treatment was the most effective among various treatments, which could increase the Ca2+ of the protein by about 2 folds and decrease the precipitation rate by 43%. Optimal soybean protein with high calcium-binding ability preparation involved dissolution at pH8 and 25 ℃, with acid-precipitated proteins dissolved in four times water at pH8. Subsequent steps included heating at 100 ℃ for 30 minutes, and enzymatic treatment using 1000 U/g flavor protease for three hours.