Abstract Agar is widely used in yogurt manufacture to improve the stability of milk gels. However, the formation and structural properties of acid-induced casein–agar gel have not been elucidated. In this study, different acidification temperatures were used to evaluate the evolution of acid-induced casein–agar double networks and their microstructure and penetration properties. Agar (0.25%, w/w) was mixed with a micellar casein dispersion (3%, w/w), acidified with glucono-δ-lactone at 30, 37, 42, or 45 °C, respectively, and then cooled to 10 °C. Compared with casein gel alone, the casein–agar gel had a higher firmness and water holding capacity (WHC). Moreover, these acid-induced casein–agar double networks formed different structures depending on the acidification temperature. When acidified below the agar transition temperature (at 30 or 37 °C), the agar formed a weak gel prior to casein network formation that hindered the movement of casein micelles. Therefore, the casein–agar mixtures formed double networks during the acidification stage with relatively low elasticity indexes (EIs). In contrast, casein–agar mixtures acidified above the agar transition temperature (at 42 or 45 °C) formed double networks during the cooling stage that exhibited higher EIs. Cryo-scanning electron microscopy showed denser networks in casein–agar gels acidified above the agar transition temperature. Furthermore, casein–agar gels acidified above the agar transition temperature showed a significantly higher firmness and WHC than those acidified below. Overall, acidification temperature significantly affected the gelation sequence and, thereby, the formation and structural properties of the acid-induced casein–agar double network.

中文翻译：

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酸诱导酪蛋白-琼脂双网络的形成和结构特性：凝胶序列的作用

摘要 琼脂广泛用于酸奶生产中，以提高乳凝胶的稳定性。然而，酸诱导酪蛋白琼脂凝胶的形成和结构特性尚未阐明。在这项研究中，使用不同的酸化温度来评估酸诱导酪蛋白-琼脂双网络的演变及其微观结构和渗透性能。琼脂 (0.25%, w/w) 与胶束酪蛋白分散体 (3%, w/w) 混合，分别在 30、37、42 或 45 °C 下用葡萄糖酸-δ-内酯酸化，然后冷却至10℃。与单独的酪蛋白凝胶相比，酪蛋白琼脂凝胶具有更高的硬度和保水能力（WHC）。此外，这些酸诱导的酪蛋白-琼脂双网络根据酸化温度形成不同的结构。当在琼脂转变温度（30 或 37 °C）以下酸化时，琼脂在酪蛋白网络形成之前形成了弱凝胶，阻碍了酪蛋白胶束的运动。因此，酪蛋白-琼脂混合物在酸化阶段形成双重网络，弹性指数 (EI) 相对较低。相比之下，在琼脂转变温度（42 或 45 °C）以上酸化的酪蛋白-琼脂混合物在冷却阶段形成双网络，表现出更高的 EI。冷冻扫描电子显微镜显示在琼脂转变温度以上酸化的酪蛋白 - 琼脂凝胶中更密集的网络。此外，在琼脂转变温度以上酸化的酪蛋白 - 琼脂凝胶显示出比以下酸化的显着更高的硬度和 WHC。总体而言，酸化温度显着影响胶凝顺序，因此，