Abstract: The effect of processing conditions on the quality of southern tofu prepared from soybeans by the traditional process was investigated. Changes in protein secondary structure were studied by Fourier infrared spectroscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). A coagulant dosage of 1.2%, a coagulation time of 20 min and a pressure of 3.0 g/cm2 were found to be the optimal conditions to obtain higher tofu yield and water-holding capacity (WHC) and better hardness and chewiness as well as a whiter color, which presented the tenderness characteristics of southern tofu. The predominant secondary structure of the untreated soybean protein was identified as β-sheet (46.36%), followed by β-turn (18.48%) and random coil (15.45%). After soaking and pulping, the β-sheet content decreased while the β-turn content increased, indicating that the protein conformation was transformed from a dense state to a loose state. However, after subsequent mixing with coagulant, coagulation and pressing, the β-sheet content continuously increased, while the content of random coil structure decreased, which indicates that the protein structure was stable. Little changes were found in soybean protein subunits during tofu processing. Only the protein bands of tofu jelly and southern tofu showed a lighter color. The protein structure of the finished tofu contained 7S and 11S subunits, which illustrates that soybean protein was well retained during tofu processing.

Key words: southern tofu, processing, quality, protein secondary structure, molecular mass