Abstract The objective of this study was to assess the influence of reducing pH from its native value on the heat-induced gelation behaviour of egg yolk by monitoring its linear viscoelastic properties. Three acids differing in their location in the Hofmeister series were used (hydrochloric, citric, and phosphoric acids) at pH between 2 and 6. The viscoelastic measurements were carried out under small amplitude oscillatory shear, using parallel plate geometry by means of: (i) stress sweep tests to delimit the linear viscoelastic range at different temperatures; (ii) temperature ramp tests to monitor egg yolk protein gelation; and (iii) frequency sweep tests at 20 °C, after the thermal cycle. The microstructure of gels was also evaluated by Cryo-scanning Electronic Microscopy (CryoSEM). Egg yolk undergo dramatic changes in rheological and microstructural properties, when processed at high temperature, depending on pH and the type of acid used. Generally, four different regions take place over heat treatment: (i) a fluid-like region showing a moderate decrease in viscoelastic properties with temperature (ii) a sol-gel transition region involving denaturation, aggregation of protein molecules and association of aggregates to form a gel network; (iii) a plateau region for G′ and G″ and (iv) a reinforcement of the gel network through the regeneration of physical interactions during cooling. This pattern may show a strong dependence on pH and the acid involved. Both effects tend to decrease as the thermal treatment proceeds. Heat treatment also reveals large differences in gel microstructure, depending on pH and on the type of acid used.

中文翻译：

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蛋黄的热诱导凝胶化作为 pH 值的函数。酸的类型有什么区别吗？

摘要 本研究的目的是通过监测蛋黄的线性粘弹性特性，评估从其天然值降低 pH 值对蛋黄热诱导胶凝行为的影响。使用了 Hofmeister 系列中位置不同的三种酸（盐酸、柠檬酸和磷酸），pH 值介于 2 和 6 之间。粘弹性测量是在小振幅振荡剪切下进行的，使用平行板几何形状，通过以下方式：（i ) 应力扫描测试，以划定不同温度下的线性粘弹性范围；(ii) 用于监测蛋黄蛋白凝胶化的温度斜坡测试；(iii) 热循环后，在 20 °C 下进行频率扫描测试。凝胶的微观结构也通过冷冻扫描电子显微镜（CryoSEM）进行评估。在高温下加工时，蛋黄的流变学和微观结构特性会发生巨大变化，具体取决于 pH 值和所用酸的类型。通常，在热处理过程中会发生四个不同的区域：(i) 显示粘弹性随温度适度降低的流体状区域 (ii) 溶胶-凝胶转变区域，涉及变性、蛋白质分子的聚集和聚集体的结合形成凝胶网络；(iii) G' 和 G'' 的平台区域和 (iv) 通过冷却过程中物理相互作用的再生增强凝胶网络。这种模式可能显示出对 pH 值和所涉及的酸的强烈依赖性。随着热处理的进行，这两种效果都趋于降低。热处理还揭示了凝胶微观结构的巨大差异，