Emulsions: Why Mustard Holds a Vinaigrette Together

Put three spoons of olive oil and one of vinegar in a jar, shake hard, and the cloud does not last. By the time the leaves are dressed there is a clear gold layer floating on a dark one, and the first leaves take the oil while the last take the vinegar. Put a teaspoon of Dijon in the jar before the oil and shake again, and the same dressing stays cloudy far longer. The difference is what the mustard puts on the surface of the oil droplets, and once its job is clear the dressing stops depending on luck.

What an emulsion is

Oil and water do not dissolve in each other. What a dressing does instead is break one of them into droplets so small they hang suspended inside the other, and that suspension is an emulsion. Mayonnaise is the familiar example: the oil goes in slowly so that the yolk's lecithin can coat the small droplets, which stay suspended in the lemon juice or vinegar. Left alone, the droplets do what physics wants. They find each other, merge, and merge again, because a few large droplets have far less surface area than a fog of small ones, and less surface means less energy. J. Peter Clark, a consultant to the process industries writing for the Institute of Food Technologists, describes the failure as coalescence and, among the things that resist it, names an emulsifier at the droplet surface, small droplet size, and a viscous continuous phase. A shaken vinaigrette with no mustard relies on small droplets alone, and shaking cannot keep them small for long.

Why it breaks

Two oil droplets that touch in plain vinegar have nothing between them to stop them merging, and the layer at the top of the jar is the sum of those mergers. A lazy shake gives a dressing that separates before the salad is tossed, and adjusting the ratio does not address the cause. The problem is that nothing is standing on the surface of the droplets.

What mustard does

An emulsifier is a molecule with two ends: one that prefers water and one that prefers oil. It parks at the boundary between the two, oil end in, water end out, and once a droplet is coated, the coats on two droplets meet before the oil inside them can. Egg yolk carries lecithin, which is why a yolk makes mayonnaise. Clark names mustard as another emulsifier in the same sauce. Yellow mustard seed carries something less famous: a mucilage, a gummy polysaccharide found mostly in the seed coat, that thickens water. Lab work on the isolated mucilage points to three jobs.

The first is the coating. A 2015 study co-authored by an Agriculture and Agri-Food Canada scientist, on the water-soluble mucilage of yellow mustard seed, found that emulsions made with 1 and 2 percent mucilage showed no phase separation after 21 days at 25°C, and that a 1 percent mucilage solution had surface activity equal to a 15 percent solution of gum arabic, a commercial emulsifier. The second is body. The mucilage thickens the water phase and shows what researchers describe as weak-gel properties, and a more viscous water phase is one of Clark's stabilizing factors, the one a cook feels as body on the spoon. The third is droplet size: the same study found isolated yellow-mustard mucilage produced smaller droplets at pH 4 than at pH 6.5 or 9, and a vinegar dressing sits on the acid side of that range. Seed proteins and the fine ground particles of a mustard paste likely add to the effect, though the study measured only the mucilage.

Dijon, whole grain and yellow mustard work differently

The mucilage that the research measured comes from yellow mustard seed, and it lives in the seed coat. Dijon, by the French decree that governs the name, is made only from brown or black mustard seed, sieved so that no more than 2 percent of the finished product is seed coat, with at least 22 percent of its weight as dry extract from the seed. A smooth Dijon has had most of its bran sieved out, and the Saskatchewan Mustard Development Commission puts the mucilage in brown and Oriental seed at negligible levels. Mustard still works as an emulsifier, as Clark notes; for Dijon, the seed proteins and the finely milled paste are the plausible reason, though the mucilage study did not test them.

A whole-grain mustard keeps its hulls, so a spoon of it brings texture as well as body, and the seeds stay visible in the dressing. Plain yellow mustard is made from yellow seed, which the same commission credits with about 5.5 percent mucilage in its bran against negligible amounts in brown seed, and its milder flavor suits potato salad and slaw dressings. In practice: Dijon for a smooth, sharp dressing. Whole grain when the dressing wants texture and visible seed. A half-teaspoon of dry mustard powder, whisked into the vinegar first, when the mustard should not show in the dressing. The Dijon mustard complete guide goes deeper on Dijon itself.

Mustards, honey and yolk, side by side

The table assumes a standard 3:1 oil-to-vinegar dressing, made in a jar with a hard shake.

The pairing that earns its place is mustard with honey. The honey brings sweetness and body, the mustard brings the emulsifier, and that division of labor is the logic of a honey-mustard dressing.

The order and the whisk

The emulsifier goes in first, with the vinegar, the salt and any shallot or garlic, and the oil goes in last. Whisking the mustard into the vinegar spreads it through the liquid before any oil arrives, so it is already waiting when the droplets form. Then the oil, in a thin stream while whisking if the dressing should be thick and glossy, or all at once with a hard shake in a sealed jar if it only needs to hold through a meal.

The whisk matters because droplet size matters: in Clark's account, smaller droplets make a more stable emulsion, and agitation is what makes them small. A hard shake is the cheapest stabilizer in the kitchen. The ratio itself, and twelve dressings built on it, are in the vinaigrette ratio piece. The ratio describes the taste and the mustard describes the hold.

When it breaks anyway

A broken vinaigrette is only a vinaigrette that has been left alone too long. Shake it again and the droplets disperse once more. Olive oil will cloud and may solidify at fridge temperature, which is harmless; a while on the counter and a shake restores it.

The three jars this depends on

An emulsion only holds what it is given, so the oil, the vinegar and the mustard each decide the result. An extra virgin olive oil, such as Capezzana (shop Capezzana) or one chosen with the olive oil buying guide, brings the flavor of the oil into the dressing, where a neutral oil stays in the background. A sharp vinegar, chosen from the vinegar guide, is the water phase the whole structure hangs from. Sherry vinegar suits something roasted and white wine vinegar suits soft leaves, and the acid piece explains the reasoning. The mustard is the smallest quantity in the jar and the one that decides whether the other two stay together.

Sources & Further Reading

• J. Peter Clark, "Emulsions: When Oil and Water Do Mix," Food Technology (Institute of Food Technologists), August 2013
• Wu, Eskin, Cui and Pokharel, "Emulsifying properties of water soluble yellow mustard mucilage: A comparative study with gum Arabic and citrus pectin," Food Hydrocolloids 47 (2015), FAO AGRIS record
• Décret n° 2000-658 du 6 juillet 2000 relatif aux dénominations des moutardes, Légifrance
• Saskatchewan Mustard Development Commission, "Yellow Mustard Bran and Mucilage," Spread the Mustard
• The Pantry Dispatch: Dijon Mustard, The Complete Guide
• The Pantry Dispatch: The Vinaigrette Ratio
• The Pantry Dispatch: Does Olive Oil Go Bad?