A vinaigrette separates into distinct layers if it is not stirred because it is made up of ingredients with different densities. Heavier elements such as oil will rise to the top while water and vinegar, lighter, will stay underneath.
Oil is lighter than vinegar: its density is lower. In simple terms, for the same volume, oil weighs less. This is mainly due to the fact that oil is made up of fat molecules, which are less dense and lighter. In contrast, vinegar, which is mainly composed of water and acetic acid, has more tightly packed molecules and therefore weighs more for the same occupied space. Since oil naturally floats above vinegar, the two liquids separate when you leave them undisturbed.
When you shake oil with vinegar well, you get a temporary emulsion. The thing is, they don't stay mixed for long because they lack special substances: emulsifying agents. These agents, like lecithin found in egg yolk or mustard, act a bit like mediators. They have one part that can attach to the oil and another that attaches to the vinegar, forcing them to stay together longer. Vinaigrette usually doesn't contain these "mediators," so the molecules separate quickly, each going their own way again. As a result, you quickly get two clearly distinct phases again.
Molecules of oil are said to be nonpolar, meaning they have no electric charges distributed across their ends. In contrast, those of vinegar (which is mainly made of water and acetic acid) are polar, with a different charge distribution from one side of the molecule to the other. In short, polar molecules love to mix with each other because they can form special bonds called hydrogen bonds. Nonpolar molecules prefer to stay together, without interfering with others. As a result, oil and vinegar remain separate, and the mixture does not hold naturally.
Gravity plays a central role here: it naturally pulls down the most dense liquids. Since vinegar is denser than oil, it quietly settles at the bottom of the container. The lighter oil then forms a layer on top. This slow phenomenon of accumulation in superimposed layers is called decantation. Thus, two distinct layers are quickly obtained as soon as you stop stirring.
The vinaigrette separates mainly due to a combination of simple physical effects: differences in density cause the lighter oil to rise to the surface while the denser vinegar sinks to the bottom. At the same time, surface tension limits the stable mixing of the two liquids. When you shake it, you temporarily create an emulsion: very fine droplets that disperse in the other component. But since there is nothing to keep everything well mixed for a long time, these small droplets gradually merge together, recreating two distinct layers. This phenomenon is called coalescence, and it is what ultimately leads to a clearly separated mixture to the naked eye.
Mustard or egg yolk contain lecithin, a natural emulsifier that prevents the dressing from quickly separating into distinct layers.
Temperature influences the stability of emulsions. A vinaigrette stored in the refrigerator will tend to separate more slowly than at room temperature due to the higher viscosity of cold oil.
Shaking a vinaigrette well not only temporarily mixes the ingredients but also enhances the development of flavors by increasing the surface area of contact between the vinegar and the oil.
The difference in density between oil and vinegar explains why, after separation, oil always floats above vinegar: it is less dense and therefore lighter.
Absolutely, the separation is simply the natural result of differences in density and does not mean that the dressing has deteriorated. Just give it a shake before use to restore its homogeneity.
Industrial dressings generally contain emulsifying additives (such as lecithin, xanthan gum, or other food stabilizers) that help keep the substances mixed for a longer time without visible separation.
Yes, temperature influences molecular agitation and the viscosity of liquids. A slight increase in temperature can temporarily facilitate mixing and delay separation. Conversely, a cooler temperature can make separation occur more quickly.
Sure! Here’s the translation: "Yes, because each oil has slightly different densities and viscosities. For example, a very fluid oil (like olive oil) will often tend to separate differently than a thicker oil (like walnut or rapeseed oil)."
Yes, you just need to add a natural emulsifying agent like mustard, honey, or egg yolk. These ingredients help create better cohesion between the oil and vinegar mixture, slowing down or temporarily preventing their separation.
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