Emulsions are fascinating mixtures that play a crucial role in our daily lives, often without us even realizing it. They are formed when two normally unmixable liquids, like oil and water, are combined in a way that one liquid is dispersed as tiny droplets throughout the other. While both oil-in-water (O/W) and water-in-oil (W/O) emulsions exist, O/W emulsions are particularly prevalent, especially in the food we eat and the products we use. In this article, we’ll explore five common examples of oil-in-water emulsions, uncovering the science behind their formation and the important role of emulsifiers in their stability.
Milk
Milk is a classic example of a naturally occurring O/W emulsion and a nutritional staple consumed globally. It’s a complex mixture consisting primarily of water (around 87%), within which tiny globules of milk fat (about 3-4%) are dispersed. These fat globules are primarily composed of triglycerides. Also suspended in this water phase are proteins (like casein and whey), lactose (milk sugar), and various vitamins (such as Vitamin A and D) and minerals (like calcium).
The natural emulsifier in milk is primarily casein, a family of phosphoproteins that make up about 80% of the total protein content in cow’s milk. Casein molecules have both hydrophobic and hydrophilic regions, allowing them to effectively surround the fat globules and keep them suspended in the water phase. They form a protective layer around the fat globules, preventing them from coalescing and separating out. Other milk proteins, like whey proteins, also contribute to emulsion stability, although to a lesser extent.
To further enhance the stability of milk and prevent the fat globules from rising to the top (a phenomenon known as creaming), commercial milk is usually homogenized. Homogenization is a mechanical process that forces milk through small openings under high pressure (around 10-25 MPa). This process drastically reduces the size of the fat globules from an average of 3-4 micrometers down to less than 2 micrometers, and often less than 1 micrometer. This reduction in size significantly increases the total surface area of the fat globules, allowing more casein and other milk proteins to adsorb to their surfaces, further stabilizing the emulsion and preventing creaming.
Mayonnaise
Mayonnaise is a staple condiment found in kitchens worldwide and a prime example of a stable O/W emulsion created with the help of a powerful natural emulsifier. It’s essentially a thick, creamy mixture made by dispersing a large volume of oil (typically 70-80% vegetable oil) as tiny droplets within a smaller volume of water (provided by vinegar or lemon juice, which is about 5-8%, and egg yolk).
The key to mayonnaise’s stability lies in egg yolk, which contains about 18-20% lecithin, a potent phospholipid emulsifier. Lecithin molecules possess a hydrophilic phosphate “head” and hydrophobic fatty acid “tails.” When oil is slowly added to the egg yolk and vinegar mixture while whisking vigorously, the lecithin molecules quickly migrate to the oil-water interface. Their hydrophobic tails embed themselves in the oil droplets, while their hydrophilic heads extend into the surrounding water. This creates a strong, flexible film around each oil droplet, preventing them from coming into contact and coalescing.
The process of making mayonnaise requires a slow and gradual addition of oil, particularly in the initial stages. This allows the lecithin to effectively coat the newly formed oil droplets before they have a chance to merge. Vigorous whisking is also essential, as it provides the mechanical energy needed to break down the oil into tiny droplets and distribute them evenly throughout the water phase. The smaller the oil droplets, the larger the surface area for the lecithin to cover, and the more stable the emulsion.
Ice Cream
Ice cream is a more complex example of an O/W emulsion, which is also classified as a foam. It’s a frozen, aerated mixture containing a complex interplay of tiny ice crystals, air bubbles, fat globules, and a liquid water phase. This liquid phase contains dissolved sugars (like sucrose and lactose, around 15-20%), proteins (like casein and whey, around 4-5%), flavors, and stabilizers.
The smooth texture of ice cream relies on controlling the size of the ice crystals and air bubbles, as well as stabilizing the fat globules. This is where a combination of emulsifiers comes into play. Commonly used emulsifiers include mono- and diglycerides (around 0.2-0.5%), lecithin, and sometimes polysorbates like Tween 80. These emulsifiers help to create a stable emulsion by coating the fat globules and preventing them from clumping together during the freezing process. They also help to incorporate and stabilize air bubbles, which contribute to the lightness and volume of the ice cream.
In addition to emulsifiers, ice cream often contains stabilizers like guar gum, carrageenan (E407), or xanthan gum. These hydrocolloids increase the viscosity of the water phase, further slowing down ice crystal growth and enhancing the overall stability of the emulsion. They also contribute to the smooth and creamy mouthfeel by preventing the formation of large, icy crystals. The specific combination and concentration of emulsifiers and stabilizers are carefully chosen to achieve the desired texture, overrun (amount of air incorporated), and melting properties of the ice cream.
Salad Dressing (Vinaigrette – When Emulsified)
A basic vinaigrette is a mixture of oil and vinegar, two ingredients that famously don’t mix. While a simple shaken vinaigrette will quickly separate back into its components, many commercially produced vinaigrettes are formulated to be stable O/W emulsions, remaining homogenous for extended periods.
To create a stable emulsified vinaigrette, emulsifiers are essential. Some natural ingredients, like mustard and honey, can act as mild emulsifiers due to the presence of proteins and other compounds with emulsifying properties. However, their emulsifying power is relatively weak.
For long-lasting stability, commercial vinaigrettes often rely on stronger, more effective emulsifiers like lecithin, xanthan gum, or mono- and diglycerides. These emulsifiers create a more robust barrier around the oil droplets, preventing separation and maintaining a homogenous appearance. The choice of emulsifier depends on factors like the desired texture, shelf life, and whether a “natural” label claim is desired.
Lotions and Creams (Cosmetics and Skincare)
Most lotions and creams used in cosmetics and skincare are O/W emulsions. They are designed to deliver both water-based and oil-based ingredients to the skin in a stable and aesthetically pleasing form. These products typically contain a mixture of water, oils (like mineral oil or plant-derived oils), emollients (which soften and smooth the skin), and active ingredients (like vitamins or antioxidants).
The cosmetic industry utilizes a vast array of emulsifiers, both natural and synthetic, to create stable O/W emulsions. Some common examples include:
- Stearic Acid: A naturally occurring fatty acid that can act as an emulsifier and thickener in cosmetic formulations.
- Cetyl Alcohol: A fatty alcohol that provides emulsifying and emollient properties, contributing to the smooth texture of creams and lotions.
- Glyceryl Stearate:A widely used emulsifier derived from glycerol and stearic acid, known for its effectiveness and safety in cosmetic applications.
- Polysorbates: A family of synthetic emulsifiers, including Tween 20 and Tween 80, renowned for their versatility and ability to create stable emulsions.
- Sodium Stearoyl Lactylate: An anionic emulsifier often used in combination with other emulsifiers to enhance stability and texture.
In lotions and creams, emulsifiers are essential for creating a homogenous and stable product that feels good on the skin. They allow for the even distribution of moisturizing ingredients, emollients, and other beneficial substances, ensuring that they are effectively delivered to the skin. The choice of emulsifiers also influences the product’s texture, viscosity, and overall sensory experience.
Conclusion
Oil-in-water (O/W) emulsions are incredibly common and play a vital role in creating a wide range of products with desirable textures, functionalities, and appearances. From the food we eat to the cosmetics we use, O/W emulsions are integral to our daily lives. Understanding the principles behind their formation and the crucial role of emulsifiers in their stability is essential for both product developers and informed consumers. As research into emulsifiers and their properties continues, we can expect further innovation in the development of new and improved O/W emulsions, leading to even more sophisticated and effective products in the future.
Sources
- McClements, D. J. (2015). Food emulsions: principles, practices, and techniques. CRC press.
- Kuhn, M. E. (2000). Emulsions. In Food Chemistry (pp. 399-444). Springer, Boston, MA.
- Damodaran, S., Parkin, K. L., & Fennema, O. R. (2007). Fennema’s food chemistry. CRC press.
- B. M., & Khan, T. (2018). Emulsifiers in food technology. In Handbook of food and beverage stability and shelf life (pp. 585-613). Woodhead Publishing.