Department of Food Science, University of Wisconsin-Madison, 1605 Linden Drive, Madison, Wisconsin 53706-1565, USA.
J Dairy Sci. 2010 May;93(5):1910-7. doi: 10.3168/jds.2009-2792.
We investigated the effect of altering temperature immediately after gels were formed at 37 degrees C. We defined instrumentally measurable gelation (IMG) as the point at which gels had a storage modulus (G') > or = 5Pa. Gels were made at constant incubation temperature (IT) of 37 degrees C up to IMG, and then cooled to 30 or 33.5, or heated to 40.5 or 44 degrees C, at a rate of 1 degrees C/min and maintained at those temperatures until pH 4.6. Control gel was made at 37 degrees C (i.e., no temperature change during gelation/gel development). Gel formation was monitored using small strain dynamic oscillatory rheology, and the resulting structure and physical properties at pH 4.6 were studied by fluorescence microscopy, large deformation rheology, whey separation (WS), and permeability (B). A single strain of Streptococcus thermophilus was used to avoid variations in the ratios of strains that could have resulted from changes in temperature during fermentation. Total time required to reach pH 4.6 was similar for samples made at constant IT of 37 degrees C or by cooling after IMG from 37 to either 30 or 33.5 degrees C, but gels heated to 40 or 44 degrees C needed less time to reach pH 4.6. Cooling gels after IMG resulted in an increase in G' values at pH 4.6, a decrease in LT(max), WS, and B, and an increase in the area of protein aggregates of micrographs compared with the control gel made at constant IT of 37 degrees C. Heating gels after IMG resulted in a decrease in G' values at pH 4.6 and an increase in LT(max) values and WS. The physical properties of acid milk gels were dominated by the temperature profile during the gel-strengthening phase that occurs after IMG. This study indicates that the final properties of yogurt greatly depend on the environmental conditions (e.g., temperature, time/rate of pH change) experienced by the casein particles/clusters during the critical early gel development phase when bonding between and within particles is still labile. Cooling of gels may encourage inter-cluster strand formation, whereas heating of gels may promote intra-cluster fusion and the breakage of strands between clusters.
我们研究了在 37°C 下形成凝胶后立即改变温度对其的影响。我们将仪器可测量的凝胶化（IMG）定义为凝胶的储能模量（G'）≥5Pa 时的点。凝胶在恒定的孵育温度（IT）37°C 下形成，直到 IMG，然后以 1°C/min 的速率冷却至 30 或 33.5°C，或加热至 40.5 或 44°C，并在这些温度下保持，直到 pH 值为 4.6。对照凝胶在 37°C 下形成（即在凝胶形成/凝胶发展过程中没有温度变化）。使用小应变动态振荡流变学监测凝胶形成，通过荧光显微镜、大变形流变学、乳清分离（WS）和渗透性（B）研究 pH 值为 4.6 时的结构和物理性质。使用单一的嗜热链球菌菌株以避免由于发酵过程中温度变化而导致的菌株比例变化。在恒定 IT 为 37°C 下形成的样品或在 IMG 后冷却至 30 或 33.5°C 所需的达到 pH 值为 4.6 的总时间相似，但加热至 40 或 44°C 的凝胶达到 pH 值为 4.6 的时间较短。在 IMG 后冷却凝胶会导致 pH 值为 4.6 时的 G'值增加，LT(max)、WS 和 B 减少，并且与在恒定 IT 为 37°C 下形成的对照凝胶相比，显微照片中蛋白质聚集体的面积增加。在 IMG 后加热凝胶会导致 pH 值为 4.6 时的 G'值降低，LT(max)值和 WS 增加。酸乳凝胶的物理性质主要由 IMG 后凝胶强化阶段的温度曲线决定。本研究表明，酸奶的最终性质在很大程度上取决于环境条件（例如温度、pH 值变化的时间/速率），这些条件在关键的早期凝胶发展阶段对乳蛋白颗粒/簇的影响，此时颗粒/簇之间的结合仍然不稳定。凝胶的冷却可能会促进簇间链的形成，而凝胶的加热可能会促进簇内融合和簇间链的断裂。
