1. Why Separation Does Not Mean a Dressing Is Defective
There are two main types of dressing. One stays mixed, like mayonnaise. The other separates unless you shake it. Japanese standards (JAS) list both the "emulsified" type, where oil is scattered as tiny particles in vinegar, and the "separated liquid" type as valid dressings. [1] The separated type is a proper member of the family according to the rules.
Some product pages for oil and vinegar dressings note that even if the boundary between the layers looks cloudy, it does not affect the taste or quality. [2] According to Kewpie’s history page, the company launched "Separate Dressing" in 1973. This was 15 years after they first manufactured and sold French dressing (red) in Japan in 1958. [3]
Separation is not a sign that something is broken. So, what changes when you shake it, and why does it return? Let’s look inside the bottle step by step.
2. Why Oil Rises to the Top of Water
Oil is a liquid that is slightly lighter than water. The specific gravity of edible oils, excluding solid fats, is about 0.91 to 0.92 at 25°C, regardless of the raw material. This is less than water’s 1.00. [4] If you compare the same volume, water weighs 100, while oil weighs about 91 to 92.
Vinegar is mostly water. In 100g of grain vinegar, 93.3g is water. [5] So, the weight of vinegar is very close to that of water. In the bottle, the lighter oil settles on top, and the water-like vinegar settles on the bottom.
It is important to separate two ideas here. The reason they do not mix and the reason one comes to the top are different. They do not mix easily because oil and water have different properties. Oil comes to the top because it is lighter. These two factors combine to form two layers. [4]
The difference in weight is only about 10%. This small difference is one of the factors that determines the "floating speed" we will discuss later.
3. How Shaking Breaks Oil into Tiny Droplets
When you shake the bottle, the oil breaks apart into small droplets. When one liquid is scattered as fine droplets through another liquid that does not mix with it, like oil and water, it is called an emulsion. To make this, you apply force, such as shaking or stirring, to break the oil into fine pieces. [6] The English Wikipedia lists oil and vinegar dressing (vinaigrette) as an example of an unstable emulsion that separates quickly unless shaken vigorously. [7]
The white cloudiness is caused by the droplets scattering light. According to explanations, emulsions with particle sizes of 1 to 10 μm (1 μm is 1/1000 of a mm) look pure white. If the particles are larger than 100 μm, the whiteness decreases and looks grayish. [6]
Turning oil into droplets suddenly increases the surface area between the oil and vinegar. If you take 10 mL of oil and turn it into round droplets with a diameter of 1 mm, the total surface area is about 600 cm², which is about one A4 sheet of paper (approx. 624 cm²). If the droplets are 0.1 mm, the area is about 6,000 cm², or about 10 A4 sheets. This calculation assumes all droplets are the same round size; in reality, sizes vary.
An emulsion is described as a high-energy, unstable state. It is said to break down easily if left alone. [6] The oil droplets cannot hold the expanded boundary forever.
4. How Oil Droplets Rise, Join Together, and Form Two Layers
When you stop shaking, the oil droplets move back toward their original state. Generally, three movements overlap. First, because oil is lighter than water, the droplets float up and gather. This is called creaming. Second, the gathered droplets stick together. This is called coagulation. At this stage, the droplets themselves are not broken, so shaking or mixing can scatter them again. Third, the droplets merge to form larger droplets. This is called coalescence. As coalescence progresses, the mixture separates into an oil layer and a vinegar layer. [6] These processes can happen at the same time and do not always happen in this order.
Shaking separated dressing before use is normal. Shaking again breaks the oil into fine droplets, making it look white and cloudy again. This is similar to how water droplets on a window pane merge and grow larger. However, window droplets are water in air, which is a different phenomenon from oil in a bottle.
The speed of floating depends heavily on the size of the droplets. Researchers explain this with Stokes' Law: floating speed is proportional to the square of the droplet radius. It is faster when the density difference between oil and water is larger, and slower when the surrounding liquid is more viscous. [8] If the radius is halved, the floating speed drops to one-fourth. If the droplet is one-tenth the size, the speed drops to one-hundredth.
Let’s try a calculation. If the density difference is 0.08 g/cm³ (oil 0.92, water 1.00), the surrounding liquid is water with a viscosity of 1 mPa·s, and gravity is 9.8 m/s², the time to float 10 cm is about 4 minutes for a droplet with a diameter of 0.1 mm (100 μm). For a droplet with a diameter of 0.01 mm (10 μm), it takes about 6 hours. In a real bottle, droplet sizes are not uniform, so these are just rough estimates. According to the formula, larger droplets float first, and finer droplets remain longer. [8]
5. How to Slow Separation and How It Is Tested
Methods to delay separation can be read from the formula mentioned earlier: make the droplets smaller, reduce the density difference between oil and water, and make the surrounding liquid more viscous. Making the droplets smaller is the most effective method, and increasing the viscosity of the surroundings is also effective. [8]
A material used to make the surroundings viscous is a thickener. One such thickener is xanthan gum. It is often used in dressings and sauces. It is said that adding about 1% greatly increases the viscosity of the liquid. [9] The ingredient list for Kewpie’s separate-type products includes thickeners (xanthan gum), but no emulsifiers like egg yolk that wrap around and protect the droplets are found. [2] Thickeners are thought to slow floating by making the surroundings viscous, not by protecting the droplets. Separated types are not "empty"; some contain thickeners.
Conversely, some products contain both egg yolk and thickeners but still state, "Separation may be seen in the upper and lower parts." [10] Whether it separates does not seem to be decided by the names of the ingredients alone.
Traditionally, mustard is added to vinegar and oil dressings. The mucilage in mustard seeds is said to act as a natural emulsifier. [7][11]
The standards side has an interesting procedure. In the JAS test method, before measuring the moisture in separated liquid dressing, an emulsifier (polysorbate 80) is added at about 0.2 to 0.3% of the total amount. The container is shaken well to emulsify it, then mixed with a blender to make the sample. [1] The reason is not written in the standard, but perhaps because if it stays separated, the contents change depending on where you take the sample from. People measuring separated types also shake and mix first.
6. A Kitchen Experiment to See Dressing Separate
What happens in the bottle can be compared in your kitchen. Prepare two transparent containers with tight lids. Do this with an adult. Put water and oil in one, and vinegar and oil in the other, using the same amount (about 1 tablespoon each). Shake them the same number of times with the same strength. After stopping, use a stopwatch to measure the time until a clear oil layer appears on top. It is a trick to decide what counts as "separated" beforehand.
Will vinegar or water separate faster? What happens if you change the number of shakes? You can also try adding about 1/4 teaspoon of mustard to one side to compare.
Looking at the label of dressing in your refrigerator to see if it contains the word "thickener" is another way to explore. If you read materials, JAS 0955 section 5.2.1 [1] and the explanation by Mr. Takahashi in the Journal of the Japanese Society for Food Engineering [8] connect to the story we have looked at here.
For safety, close the lids tightly and do this with an adult. Do not drink or eat the liquids used in the experiment. Soak them in kitchen paper and dispose of them according to local waste disposal rules.