Emulsification Science: Oil, Water, and Stabilizers
Emulsions are mixtures of two immiscible liquids, typically oil and water, that are stabilized by an emulsifying agent. In food, emulsions are everywhere: mayonnaise, salad dressings, milk, cream, butter, ice cream, chocolate, and countless sauces depend on emulsification for their texture, stability, and mouthfeel. Understanding the science of emulsification allows cooks and food producers to create stable, consistent emulsions and to troubleshoot when emulsions break or separate.
Oil and water do not naturally mix because water molecules are polar — they have a slight positive charge on one side and a slight negative charge on the other — while oil molecules are nonpolar and have no charge separation. The strong hydrogen bonding between water molecules excludes oil molecules, causing them to separate into distinct phases. This thermodynamic barrier to mixing is overcome by adding energy through agitation and by introducing emulsifiers that reduce the interfacial tension between the two liquids. Research in the Journal of Food Engineering has shown that the stability of food emulsions depends on a delicate balance of droplet size, emulsifier concentration, continuous phase viscosity, and environmental conditions.
Types of Emulsions
Emulsions are classified by which liquid forms the dispersed droplets and which forms the continuous phase surrounding them. The two basic types are oil-in-water and water-in-oil emulsions.
Oil-in-Water Emulsions
In an oil-in-water emulsion, tiny oil droplets are dispersed throughout a continuous water phase. Mayonnaise, milk, cream, vinaigrettes, and most sauces are oil-in-water emulsions. The properties of an oil-in-water emulsion are dominated by the water phase — they are water-thinnable, conduct electricity, and feel cool and refreshing on the tongue. The oil droplets contribute richness, creaminess, and flavor without making the emulsion feel greasy.
The stability of oil-in-water emulsions depends on preventing the oil droplets from coalescing. Droplets in motion continually collide with each other. If the interfacial film surrounding each droplet is strong enough, the droplets bounce off each other and remain separate. If the film is weak, the droplets merge upon contact, growing larger until the oil phase separates completely. Emulsifiers strengthen the interfacial film by positioning themselves at the oil-water interface, with their water-loving heads in the water and their oil-loving tails in the oil.
Water-in-Oil Emulsions
In a water-in-oil emulsion, water droplets are dispersed throughout a continuous oil phase. Butter, margarine, and some spreads are water-in-oil emulsions. These emulsions are dominated by the properties of the oil phase — they are greasy, water-repellent, and do not conduct electricity. The water droplets in butter contribute to its mouthfeel and spreadability, while the continuous fat phase provides structure and stability.
Water-in-oil emulsions are generally more stable than oil-in-water emulsions because the continuous oil phase is more viscous, which slows droplet movement and coalescence. However, they are more susceptible to temperature changes because the continuous fat phase melts at a relatively low temperature. Butter, for example, is a stable water-in-oil emulsion at refrigeration temperatures but becomes soft and may separate at room temperature.
Emulsifiers
Emulsifiers are molecules that have both water-loving (hydrophilic) and oil-loving (lipophilic) regions. This dual nature allows them to position themselves at the oil-water interface, where they reduce interfacial tension and create a protective film around dispersed droplets. Food emulsifiers come from both natural and synthetic sources.
Lecithin
Lecithin is a phospholipid found naturally in egg yolks, soybeans, and sunflower seeds. It is the primary emulsifier in mayonnaise and hollandaise sauce. The phospholipid molecules in lecithin have a phosphate-containing head that is water-soluble and two fatty acid tails that are oil-soluble. When egg yolk is whisked into oil, the lecithin molecules surround each oil droplet with their tails oriented toward the oil and their heads oriented toward the water, creating a stable emulsion.
Egg yolk also contains lipoproteins and proteins that contribute additional emulsifying capacity. The combination of lecithin, proteins, and other yolk components makes egg yolk one of the most effective natural emulsifiers available. One egg yolk can emulsify approximately one cup of oil before the emulsion becomes saturated and risks breaking. The presence of mustard in mayonnaise recipes adds mucilage, a polysaccharide that also has emulsifying properties and contributes additional stability.
Proteins
Many food proteins are effective emulsifiers because they have both hydrophilic and hydrophobic regions on their surface. Milk proteins, particularly caseins, are excellent emulsifiers used in dairy products, coffee creamers, and ice cream. The casein molecules adsorb to the surface of fat droplets, forming a stable layer that prevents coalescence. Whey proteins also have emulsifying properties and are used in a wide range of processed foods.
Soy protein, pea protein, and other plant proteins are increasingly used as emulsifiers in plant-based foods. These proteins can replace animal-derived emulsifiers in many applications, though they often require processing modifications to achieve equivalent performance. The effectiveness of protein emulsifiers depends on the protein concentration, pH, and ionic strength of the aqueous phase.
Synthetic Emulsifiers
Mono- and diglycerides are the most widely used synthetic food emulsifiers. They are produced by reacting glycerol with triglycerides, producing molecules that have excellent emulsifying properties. Mono- and diglycerides are used in baked goods to improve texture and extend shelf life, in margarine to ensure water dispersion, and in ice cream to control ice crystal size. Polysorbates, another class of synthetic emulsifiers, are used in ice cream and frozen desserts to improve texture and prevent butterfat from churning into butter during manufacturing.
The role of emulsifiers in processed foods is examined further in the food additives guide.
Emulsion Stability
An emulsion is thermodynamically unstable — given enough time, all emulsions will eventually separate. The goal of emulsion science is to slow the separation processes enough that the emulsion remains stable during its intended shelf life. Several physical processes contribute to emulsion breakdown.
Creaming and Sedimentation
Creaming occurs when dispersed droplets rise to the top of the emulsion due to density differences. Oil droplets are less dense than water and tend to rise, creating a concentrated cream layer at the top. Sedimentation is the same process in reverse — denser droplets sink to the bottom. Creaming does not necessarily break the emulsion because the droplets remain intact, but it produces an undesirable separation visible to consumers.
The rate of creaming is described by Stokes’ law, which shows that smaller droplets cream more slowly than larger droplets. Reducing the average droplet size through homogenization dramatically slows creaming. Increasing the viscosity of the continuous phase also slows creaming by resisting droplet movement. This is why salad dressings contain thickeners like xanthan gum — the increased viscosity keeps the oil droplets suspended.
Flocculation and Coalescence
Flocculation occurs when droplets aggregate without merging. They clump together but retain their individual identities. Flocculation can be reversible with gentle agitation. Coalescence is the merging of two or more droplets into a single larger droplet. Coalescence is irreversible and leads to complete phase separation if it proceeds unchecked.
Emulsifiers prevent coalescence by creating a strong film around each droplet that resists rupture when droplets collide. The thickness and mechanical strength of the emulsifier film determine how well it protects against coalescence. Proteins generally form thick, viscoelastic films that provide excellent protection, while small-molecule emulsifiers form thinner films that are more resistant to displacement but provide less mechanical strength.
Ostwald Ripening
Ostwald ripening is the process by which smaller droplets shrink and disappear while larger droplets grow. It occurs because the pressure inside a small droplet is higher than inside a large droplet, making the small droplet more soluble in the continuous phase. Molecules diffuse from small droplets through the continuous phase to large droplets. Ostwald ripening is most significant in emulsions where the dispersed phase has some solubility in the continuous phase. In food emulsions, it is a minor concern compared to creaming and coalescence.
Making Stable Emulsions
Creating a stable emulsion requires attention to the ingredient ratios, emulsifier selection, and processing conditions.
Ingredient Ratios
The ratio of oil to water determines the type and character of the emulsion. For oil-in-water emulsions, the oil phase can range from a few percent up to about seventy-four percent by volume — the maximum packing density of uniform spheres. Above this limit, the oil droplets are forced into contact and coalescence becomes inevitable. Mayonnaise, one of the most concentrated food emulsions, contains approximately seventy to eighty percent oil by weight. Achieving stability at such high oil concentrations requires very efficient emulsification and the use of multiple emulsifiers working together.
Processing Conditions
The energy input during emulsification determines the droplet size. Higher energy input produces smaller droplets and more stable emulsions. Simple whisking produces droplets in the range of ten to one hundred micrometers. Immersion blenders and food processors can produce droplets of two to twenty micrometers. High-pressure homogenizers used in industrial food processing can produce droplets below one micrometer.
Temperature affects emulsification because it changes the viscosity of both phases and the solubility of the emulsifier. Warm oil is less viscous and breaks into smaller droplets more easily. However, excessive heat can denature protein emulsifiers, reducing their effectiveness. Most emulsions are best made at room temperature or slightly warm.
The texture of emulsions and how they interact with other structural elements of food is covered in the food texture science article.
Emulsions in Food Products
Mayonnaise
Mayonnaise is the archetypal food emulsion — an oil-in-water emulsion stabilized by egg yolk lecithin, with vinegar or lemon juice providing the aqueous phase. The egg yolk contains both emulsifiers and stabilizers, including lecithin, lipoproteins, and cholesterol. The vinegar or lemon juice lowers the pH, which helps the proteins in the yolk function more effectively as emulsifiers. The characteristically thick, spoonable texture of mayonnaise comes from the close packing of oil droplets at the high oil concentration.
Vinaigrette
A vinaigrette is a temporary oil-in-water emulsion of oil and vinegar, stabilized primarily by mustard and vigorous whisking. Unlike mayonnaise, vinaigrettes contain only a small amount of emulsifier and are inherently unstable — they separate within minutes to hours. The temporary nature of a vinaigrette is part of its character; the dressing is whisked or shaken just before serving to re-emulsify. Adding a small amount of mustard powder or paste provides enough emulsifier to extend the life of the emulsion without making it permanent.
Butter
Butter is a water-in-oil emulsion, the opposite of mayonnaise. Cream, which is an oil-in-water emulsion, is churned to break the emulsion and invert it, releasing the fat as butter. During churning, the mechanical agitation damages the fat globule membranes, allowing the fat droplets to coalesce into a continuous phase. The water and milk solids become dispersed as droplets within the continuous fat phase. The desired water content of butter is approximately sixteen to eighteen percent, regulated by law in many countries.
FAQ
Why does my mayonnaise break?
Mayonnaise breaks when the emulsion loses stability and the oil separates. Common causes include adding oil too quickly, using cold ingredients, not using enough emulsifier, or adding too much oil beyond the emulsifying capacity of the egg yolk. If mayonnaise breaks, it can often be rescued by whisking a new egg yolk with a tablespoon of water and slowly drizzling the broken mixture into it.
Can I make an emulsion without eggs?
Yes, several ingredients can emulsify without eggs. Mustard powder contains mucilage that stabilizes emulsions. Soy lecithin, available as granules or liquid, is an effective plant-based emulsifier. Xanthan gum and other hydrocolloids can stabilize emulsions by increasing the viscosity of the water phase. Aquafaba, the liquid from canned chickpeas, contains proteins that can form stable emulsions similar to egg whites.
What is the difference between homogenized and non-homogenized milk?
Homogenized milk has been forced through a small nozzle under high pressure to break the fat globules into smaller, uniform droplets. This prevents the cream from rising to the top. Non-homogenized milk allows the cream layer to separate naturally. The smaller fat droplets in homogenized milk also contribute to a creamier mouthfeel.
Why does hollandaise sauce separate when reheated?
Hollandaise is a heat-sensitive emulsion because the egg yolk emulsifiers are proteins that denature and lose their emulsifying capacity at high temperatures. Reheating hollandaise above 160 degrees Fahrenheit causes the proteins to coagulate and the emulsion to break. Reheat hollandaise gently over low heat while whisking constantly, or use a double boiler to avoid direct heat.
How do commercial salad dressings stay emulsified for months?
Commercial dressings use a combination of multiple emulsifiers, thickeners, and stabilizers. Xanthan gum and other hydrocolloids increase the viscosity of the water phase, slowing droplet movement and creaming. Polysorbate 60 and other synthetic emulsifiers form very stable interfacial films. High-pressure homogenization produces very small droplets that resist separation.