南豆腐加工过程中品质及蛋白质结构的变化

以大豆为原材料，采用传统豆腐制作工艺，探讨加工工艺对南豆腐品质的影响，并利用傅里叶变换红外光谱和十二烷基硫酸钠-聚丙烯酰胺凝胶电泳研究加工过程中大豆蛋白结构的变化。以豆腐得率、保水性、质构特性和色差为评价指标，当凝固剂质量分数1.2%、蹲脑时间20 min和压型力度3.0 g/cm2时，制得的豆腐得率和保水性均较高，硬度和咀嚼性较好，色泽偏白，符合南豆腐“嫩”的特征。未经处理的大豆蛋白二级结构中以β-折叠（46.36%）为主，其次为β-转角（18.48%）和无规卷曲（15.45%），浸泡、打浆会导致β-折叠含量下降、β-转角含量增加，蛋白质构象由收缩的紧密结构逐渐转变为展开状态；而点脑、蹲脑以及加压操作后，大豆蛋白的β-折叠含量不断回升，无序结构减少，蛋白结构较为稳定。豆腐加工操作对大豆蛋白的亚基影响较小，只有豆腐脑与南豆腐蛋白的电泳条带灰度变浅，而成型后的南豆腐含有一定量的7S亚基以及11S亚基，说明在加工过程中蛋白质的保持率较好。

Effect of Processing Conditions on Quality and Protein Secondary Structure in Southern Tofu

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.