Casein is a group of milk proteins with high nutritional value, and the exploitation of its techno-functional potentials has been investigated for decades. In this study, acid casein powder was dissolved in 0.1 mol/L phosphate buffers with different pH, resulting in casein solutions with pH 5.9, 6.6 and 7.3. During preparation and storage (40 °C) of the samples, casein hydrolysis was observed in size exclusion chromatography and gel electrophoresis. The degree of hydrolysis increased with increasing pH, and treatment of casein with commercial plasmin resulted in similar polypeptides, suggesting that the hydrolysis was caused by residual indigenous plasmin present in the acid casein powder. Most polypeptides could be cross-linked by microbial transglutaminase, except for one particular fraction which appeared at constant intensity in the chromatograms. The stiffness of acid-induced gels as determined in small amplitude oscillatory shear rheology decreased with increasing degree of hydrolysis, and was also lower for cross-linked samples when the preceding casein hydrolysis was more pronounced. Enzymatic cross-linking increased the resistance of casein against plasmin-related hydrolysis, presumably because of the resulting lysine modification. However, one particular fraction of polypeptides was released by hydrolysis in spite of cross-linking, suggesting that they did not contain lysine residues that are susceptible for mTGase. The results indicate that plasmin-related hydrolysis should be taken into account for the application of acid casein or sodium caseinate as additive in food design.

中文翻译：

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水解原浆蛋白：非胶束酪蛋白的酶促交联和酸诱导的凝胶形成的后果。

酪蛋白是一组具有高营养价值的牛奶蛋白，数十年来一直研究其技术功能潜力的开发。在这项研究中，将酸性酪蛋白粉末溶解在具有不同pH值的0.1 mol / L磷酸盐缓冲液中，得到的酪蛋白溶液的pH值为5.9、6.6和7.3。在样品的制备和储存（40°C）过程中，在尺寸排阻色谱和凝胶电泳中观察到酪蛋白水解。水解程度随pH值的增加而增加，酪蛋白用商业纤溶酶处理会产生相似的多肽，这表明水解是由酸性酪蛋白粉末中残留的天然纤溶酶引起的。大多数多肽可以通过微生物转谷氨酰胺酶交联，除了在色谱图中以恒定强度出现的一个特定馏分。在小振幅振荡剪切流变学中测定的酸诱导的凝胶的刚度随水解度的增加而降低，并且当前述酪蛋白水解更明显时，对于交联样品也较低。酶促交联增加了酪蛋白对纤溶酶相关水解的抵抗力，这可能是由于产生的赖氨酸修饰。然而，尽管交联，但是多肽的一小部分还是通过水解释放的，这表明它们不包含对mTGase敏感的赖氨酸残基。结果表明，在食品设计中使用酪蛋白酸酪蛋白或酪蛋白酸钠作为添加剂时，应考虑纤溶酶相关的水解。在小振幅振荡剪切流变学中测定的酸诱导凝胶的刚度随水解度的增加而降低，而当前述酪蛋白水解更明显时，交联样品的刚度也较低。酶促交联增加了酪蛋白对纤溶酶相关水解的抵抗力，这可能是由于产生的赖氨酸修饰。然而，尽管交联，但是多肽的一小部分还是通过水解释放的，这表明它们不包含对mTGase敏感的赖氨酸残基。结果表明，在食品设计中使用酪蛋白酸酪蛋白或酪蛋白酸钠作为添加剂时，应考虑纤溶酶相关的水解。在小振幅振荡剪切流变学中测定的酸诱导凝胶的刚度随水解度的增加而降低，而当前述酪蛋白水解更明显时，交联样品的刚度也较低。酶促交联增加了酪蛋白对纤溶酶相关水解的抵抗力，这可能是由于产生的赖氨酸修饰所致。然而，尽管交联，但是多肽的一小部分还是通过水解释放的，这表明它们不包含对mTGase敏感的赖氨酸残基。结果表明，在食品设计中使用酪蛋白酸酪蛋白或酪蛋白酸钠作为添加剂时，应考虑纤溶酶相关的水解。