Acid coagulation behavior of homogenized milk: effect of interacting and non-interacting droplets observed by rheology and diffusing wave spectroscopy

Abstract

This study analyzes the early stages of the acid coagulation behavior of milk containing homogenized fat globules. By addition of Tween 20 to homogenized milk, it was possible to create two similar colloidal systems with completely different interfacial properties. Control experiments using skim milk demonstrated that the presence of Tween did not overly affect the acid coagulation behavior of the casein micelles. The initial stages of aggregation were also similar for the two homogenized milk systems, indicating that the casein micelles were the main driving force behind early gel formation. For the case of homogenized milk without Tween, the fat globules were fully incorporated in the network. The stiffness of the gel was higher than the control and the overall spatial distribution of the fat droplets was not largely affected by the developing gel matrix. In contrast, the homogenized milk with added surfactant contained fat globules which did not interact directly with the casein micelles but rather became trapped inside the pores of the ensuing network. This gel showed a lower elastic modulus than the homogenized milk case and free-diffusing fat globules. Although the presence of interacting and non-interacting droplets did not overly affect the coagulation kinetics of the casein micelles, the acid gels did show different final properties which fully reflected the presence of “active” or “inert” fillers. The knowledge derived from this work will be the first step towards developing novel textures for dairy gels, modulating the extent of the interactions between the fat globules and the protein network.

应用流变学和散射波光谱法观察均质乳的酸凝乳特性

摘要 本文分析了均质牛乳酸凝胶形成初期的凝聚特性。将吐温-20加到均质牛乳中, 可产生具有完全不同界面性质的两种相似的胶体体系。以脱脂乳作对照组, 试验结果证明了吐温-20的存在对酪蛋白胶束酸凝聚的影响不太大。两种均质乳体系的初期凝聚过程非常相似, 表明酪蛋白胶束是凝胶形成的主要驱动力。未加吐温-20的脱脂乳, 脂肪球与酪蛋白胶束的网络完全溶为一体, 形成凝胶的硬度比对照组高, 而且形成的凝胶网络对脂肪液滴的空间分布没有太大的影响。相反, 加表面活性剂的均质乳中脂肪球并没有与酪蛋白胶束直接接触, 而是被包裹在形成的凝胶网络的内部空洞中。这种凝胶比均质乳的弹性模量低。尽管, 有相互作用和无相互作用脂肪球的存在并不影响酪蛋白胶束的凝聚动力学, 但是, 所形成的酸凝胶则具有不同的反射特性, 即存在“活性”和“惰性”的物质。基于本研究的结果, 可以初步探讨乳凝胶质构的形成过程, 有助于调整脂肪球和蛋白网络之间相互作用的程度。