Destribats Mathieu, Rouvet Martine, Gehin-Delval Cécile, Schmitt Christophe, Binks Bernard P
Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull. HU6 7RX, UK.
Soft Matter. 2014 Sep 28;10(36):6941-54. doi: 10.1039/c4sm00179f.
We have investigated a new class of food-grade particles, whey protein microgels, as stabilisers of triglyceride-water emulsions. The sub-micron particles stabilized oil-in-water emulsions at all pH with and without salt. All emulsions creamed but exhibited exceptional resistance to coalescence. Clear correlations exist between the properties of the microgels in aqueous dispersion and the resulting emulsion characteristics. For conditions in which the particles were uncharged, fluid emulsions with relatively large drops were stabilised, whereas emulsions stabilized by charged particles contained smaller flocculated drops. A combination of optical microscopy of the drops and spectrophotometry of the resolved aqueous phase allowed us to estimate the interfacial adsorption densities of the particles using the phenomenon of limited coalescence. We deduce two classes of particle arrangement. Complete adsorption of the particles was obtained when they were neutral or when their charges were screened by salt resulting in at least one particle monolayer at the interface. By contrast, only around 50% of the particles adsorbed when they were charged with emulsion drops being covered by less than half a monolayer. These findings were supported by direct visualization of drop interfaces using cryo-scanning electron microscopy. Uncharged particles were highly aggregated and formed a continuous 2-D network at the interface. Otherwise particles organized as individual aggregates separated by particle-free regions. In this case, we suggest that some particles spread at the interface leading to the formation of a continuous protein membrane. Charged particles displayed the ability to bridge opposing interfaces of neighbouring drops to form dense particle disks protecting drops against coalescence; this is the main reason for the flocculation and stability of emulsions containing sparsely covered drops.
我们研究了一类新型的食品级颗粒——乳清蛋白微凝胶，作为甘油三酯 - 水乳液的稳定剂。这些亚微米级颗粒在有盐和无盐的所有pH值条件下都能稳定水包油乳液。所有乳液都会出现分层现象，但具有出色的抗聚结能力。水性分散体中微凝胶的性质与所得乳液特性之间存在明显的相关性。在颗粒不带电的条件下，可稳定形成具有相对较大液滴的流动性乳液，而由带电颗粒稳定的乳液则含有较小的絮凝液滴。通过对液滴的光学显微镜观察和对分离出的水相进行分光光度测定，利用有限聚结现象，我们能够估算颗粒的界面吸附密度。我们推断出两类颗粒排列方式。当颗粒呈中性或其电荷被盐屏蔽时，颗粒会完全吸附，在界面处形成至少一个颗粒单层。相比之下，当颗粒带电时，只有约50%的颗粒会吸附，乳液液滴被覆盖的单层不到一半。这些发现通过低温扫描电子显微镜对液滴界面的直接观察得到了支持。不带电的颗粒高度聚集，在界面处形成连续的二维网络。否则，颗粒会组织成由无颗粒区域分隔的单个聚集体。在这种情况下，我们认为一些颗粒会在界面处铺展，导致形成连续的蛋白质膜。带电颗粒能够桥接相邻液滴的相对界面，形成致密的颗粒盘，保护液滴不发生聚结；这是含有稀疏覆盖液滴的乳液发生絮凝和具有稳定性的主要原因。
