INRAE, UMR STLO, Agrocampus Ouest, F-35000 Rennes, France.
CNIEL, F-75009 Paris, France.
Food Res Int. 2020 Mar;129:108847. doi: 10.1016/j.foodres.2019.108847. Epub 2019 Dec 3.
The rheological properties and microstructure of dairy gels involve the connectivity between milk fat globules (MFG) and casein micelles that is affected by technological processes such as milk homogenization and heat treatment. The underlying mechanisms require further quantification of the interactions at the nanoscale level to be fully understood and controlled. In this study, we examined the adhesion of homogenized MFG to milk proteins and evaluated the role of ultra-high temperature (UHT) heat treatment and pH. The combination of physico-chemical analysis, rheology and microscopy observations at different scale levels associated to atomic force microscopy (AFM) force spectroscopy were used. AFM experiments performed at the particle scale level showed that adhesion of individual homogenized MFG to milk proteins (1) is increased upon acidification at pH 4.5: 1.4 fold for unheated samples and 3.5 fold for UHT samples, and (2) is enhanced by about 1.7 fold at pH 4.5 after UHT heat treatment of milk, from 176 pN to 296 pN, thanks to highly-reactive heat-denatured whey proteins located at the surface of MFG and caseins. The increased inter-particle adhesion forces accounted for more connected structures and stiffer UHT milk acid gels, compared to unheated-milk gels. Using a multiscale approach, this study showed that heat treatment of milk markedly affected the interactions occurring at the particle's surface level with consequences on the bulk structural and rheological properties of acid gels. Such findings will be useful for manufacturers to modulate the texture of fermented dairy products through the tailoring of heat-induced complexation of proteins and the connectivity of homogenized MFG with the protein network. This work will also contribute in a better understanding of the impact of process-induced changes on the digestibility and metabolic fate of proteins and lipids.
乳制品凝胶的流变特性和微观结构涉及乳脂肪球（MFG）和酪蛋白胶束之间的连接性，而这种连接性受均质化和热处理等工艺过程的影响。为了充分理解和控制这些过程，需要进一步量化纳米尺度水平上的相互作用机制。在本研究中，我们研究了均质化 MFG 与乳蛋白的粘附作用，并评估了超高温（UHT）热处理和 pH 的作用。我们采用物理化学分析、流变学和显微镜观察相结合的方法，并在不同的尺度水平上进行研究，包括原子力显微镜（AFM）力谱学。在颗粒尺度水平上进行的 AFM 实验表明，（1）在 pH4.5 下酸化时，单个均质化 MFG 与乳蛋白的粘附作用增强：未加热样品增加了 1.4 倍，UHT 样品增加了 3.5 倍；（2）在 UHT 热处理后的 pH4.5 下，由于 MFG 和酪蛋白表面存在高度反应性的热变性乳清蛋白，粘附力增强了约 1.7 倍，从 176 pN 增加到 296 pN。这种增强的颗粒间粘附力导致了更多的连接结构和更硬的 UHT 乳酸性凝胶，与未加热的乳酸性凝胶相比。通过多尺度方法，本研究表明，牛奶的热处理显著影响了发生在颗粒表面的相互作用，从而影响了酸性凝胶的整体结构和流变特性。这些发现将有助于制造商通过调整蛋白质的热诱导复合作用以及均质化 MFG 与蛋白质网络的连接性来调节发酵乳制品的质地。本工作还将有助于更好地理解工艺诱导变化对蛋白质和脂肪消化率和代谢命运的影响。
