How are emulsifiers classified?

Emulsifiers are not a single substance, but a large family. Members of this family have diverse characteristics, origins, and areas of expertise. To truly understand emulsifiers, one must first understand how they are classified.

This article approaches the topic from three dimensions: source, ion type, and a core parameter that cannot be ignored when selecting emulsifiers—HLB value.

First dimension: By origin, natural vs. synthetic

Natural emulsifiers: Nature does it first.

Natural emulsifiers originate from plants, animals, or microorganisms, and their structures are the result of natural evolution and selection. They often possess both emulsifying functions and other physiological activities. There are three representative members:

Lecithin is one of the most well-known natural emulsifiers. It is abundant in egg yolks and soybeans, and its main component is phosphatidylcholine—the hydrophilic head consists of polar groups containing phosphate and choline, while the lipophilic tail consists of two fatty acid chains. This structure is highly similar to cell membrane phospholipids, resulting in excellent biocompatibility. Mayonnaise, chocolate, bread, infant formula… lecithin’s applications cover almost all foods requiring gentle emulsification. Soy lecithin is the most commonly used industrial source, extracted from a byproduct of soybean oil refining; it is inexpensive and has a stable supply.

Saponins are a class of natural surfactants widely found in plants. Their name comes from the Latin word for “soap”—their aqueous solutions produce persistent foam when shaken. Ancient people used to wash clothes directly from the roots and stems of saponin-containing plants. The hydrophilic portion of saponins is a sugar chain, while the lipophilic portion is a triterpenoid or steroidal structure . In the food industry, saponin extracts from yucca and quillaja are frequently used for foaming and emulsifying beverages, and are also one of the star ingredients in recent Pickering emulsion research.

Bile salts are emulsifiers produced by the human body. Synthesized by the liver and stored in the gallbladder, they are secreted into the small intestine after eating. The structure of bile salts differs from that of typical emulsifiers: their hydrophobic and hydrophilic sides are distributed on opposite sides of the molecule, like a coin with two different sides, rather than having hydrophilic and lipophilic ends. This rigid structure allows them to efficiently emulsify fats in the intestines, breaking down dietary fats into microemulsion particles for digestion by lipases. Bile salts are reabsorbed and recycled within the body, making them one of the most efficient chemical systems in the human body.

Synthetic emulsifiers: formulated in the laboratory

Synthetic emulsifiers are manufactured using chemical methods. Their structure is adjustable and their performance is controllable, making them a mainstay of the modern food, chemical, and pharmaceutical industries. Compared to natural emulsifiers, the biggest advantage of synthetic emulsifiers lies in their “designability”: by changing the length and saturation of fatty acid chains and adjusting the type and number of hydrophilic groups, the HLB value, melting point, solubility, and interfacial behavior of the molecule can be systematically altered. The following sections will introduce them one by one according to their main categories.

Monoglycerides and diglycerides ( E471) are the most widely used synthetic emulsifiers in the food industry. Monoglycerides are formed by esterifying glycerol with one fatty acid, while diglycerides esterify two. They are often used together as “glycerol fatty acid esters.” Their HLB values are approximately 3–4, making them lipophilic and suitable for water-in-oil or low-aqueous-phase systems. In bread and pastries, monoglycerides form helical complexes with amylose, inhibiting starch recrystallization and significantly delaying product staling and hardening. In ice cream, they help fat particles undergo controlled partial polymerization during churning and freezing, forming a stable foam network, giving the product a light texture and good melt resistance . The raw materials for monoglycerides come from the oil and fat industry and can be animal fats or vegetable oils. For vegan or halal certified products, the choice of source is crucial.

Sucrose fatty acid esters (E473) are produced by esterification of sucrose and fatty acids under alkaline conditions. Sucrose has eight esterifiable hydroxyl groups, allowing for esterification from monoesters to octaesters . Lower degrees of esterification result in stronger hydrophilicity and higher HLB values. Monoesters can have HLB values exceeding 15, making them extremely strong oil-in-water emulsifiers; polyesters, on the other hand, are highly lipophilic and suitable for water-in-oil systems. This wide range of adjustability is a unique advantage of sucrose esters. Sucrose esters have a refreshing taste and no off-flavor, making them widely used in the Japanese food industry and a common food additive in China (permitted under GB 2760). Furthermore, sucrose esters have some inhibitory effect on Gram-positive bacteria and, in some applications, also possess mild preservative properties.

DATEM (Diacetyl Tartaric Acid Ester of Monoglycerides ) is a specialized emulsifier used in the baking industry . It is produced by reacting monoglycerides with diacetyl tartaric anhydride . It exhibits significantly stronger hydrophilicity than ordinary monoglycerides , with an HLB value of approximately 8–9. DATEM’s core role in dough is to strengthen the gluten network: it interacts with gliadin, improving the dough’s gas retention capacity, resulting in larger, more uniformly textured bread. It is found in almost every type of bread, hamburger bun, and English muffin recipe.

SSL and CSL ( sodium/calcium stearoyl lactylate , E481/E482) are anionic synthetic emulsifiers, produced by the condensation of lactic acid and stearic acid followed by salt formation with sodium or calcium . They have similar properties, with SSL (sodium stearoyl lactylate ) having slightly better water solubility, while CSL (calcium stearoyl lactylate) is more resistant to hard water. In dough, they work simultaneously with gluten proteins and starches, strengthening the network structure and delaying aging. They are often used in combination with DATEM to create a synergistic effect. Compared to DATEM, SSL/CSL is cheaper and more commonly found in cost-sensitive bulk baking products.

Tween (polysorbate, E432–E436) is a series of nonionic emulsifiers, formed by the esterification of sorbitol with fatty acids followed by polymerization with ethylene oxide . Its hydrophilicity is imparted by the polyethylene glycol chain. The numbers following the name represent the type of fatty acid: Tween 20 corresponds to lauric acid, Tween 40 to palmitic acid, Tween 60 to stearic acid, and Tween 80 to oleic acid. The shorter and more unsaturated the fatty acid chain, the stronger the overall hydrophilicity of the molecule. Tween generally has a high HLB value (14–17), making it a powerful oil-in-water emulsifier widely used in beverage turbidity agents, ice cream, and cosmetic emulsions. In the pharmaceutical field, Tween 80 is one of the most commonly used stabilizers in vaccine adjuvants and injectable formulations.

Span (sorbitan fatty acid ester, E491–E496) is a precursor to Tween: also made from sorbitol and fatty acids, but without polyethylene glycol conversion, thus exhibiting strong lipophilicity and a low HLB value (1.8–8.6). Span 80 (monoleate) is the most commonly used member, with an HLB of approximately 4.3, making it a classic choice for preparing water-in-oil emulsions. The combination of Tween and Span is a textbook formulation strategy: using Tween 80 (HLB 15) and Span 80 (HLB 4.3) in different mass ratios, the HLB value of the system can be continuously adjusted from 4.3 to 15, covering most emulsification needs, while their chemical skeletons are similar and exhibit excellent compatibility.

Modified lecithin products (modified soybean lecithin) represent the intersection of natural and synthetic processes. After enzymatic hydrolysis (lysophospholipids) or chemical modification (hydroxylated phospholipids), the hydrophilicity of natural lecithin is significantly enhanced, with its HLB value increasing from approximately 4 to over 8. Its emulsifying ability and thermal stability are also greatly improved. Modified lecithin retains the “clean label” image of natural raw materials while compensating for the insufficient hydrophilicity of natural lecithin, making it increasingly popular in organic foods and high-end cosmetics.

Polyglycerol esters (E475) are formed by the esterification of polyglycerol with fatty acids. The degree of polymerization of polyglycerol (2 to 10 glycerol units) and the types of fatty acids together determine the HLB value of the final product, with a very wide range (approximately 3–13), making it highly versatile in its applications. Polyglycerol esters hold a special place in the chocolate industry: polyglycerol ricinoleate (PGPR, E476) can significantly reduce the viscosity of melted chocolate, reduce the amount of cocoa butter used, lower production costs, and improve mold release properties. In a typical chocolate recipe, PGPR is usually used in only 0.1%–0.3%, yet it can bring significant processing benefits.

Alkyl polyglycosides ( APG ) are rapidly developing “green synthetic emulsifiers” in recent years. They are formed by the condensation of naturally derived glucose and fatty alcohols under acidic conditions. The raw materials are renewable and biodegradable. APG is nonionic, gentle on the skin, has good foaming properties, and excellent compatibility with other surfactants. It is showing a clear trend of replacing traditional synthetic surfactants in personal care products (facial cleansers, shower gels) and household cleaners. It is also one of the new generation raw materials that formulators are paying attention to in the “clean label” movement.

The types of synthetic emulsifiers are far more numerous than those mentioned above, covering the mainstream varieties used in food, cosmetics, and industrial applications. Together, they form a diverse and complementary toolbox, and the formulator ‘s job is to find the most suitable combination within this toolbox.

Second dimension: Classified by ion type

Besides their source, emulsifiers can also be classified into four categories based on the charge properties of their hydrophilic heads. This classification method is extremely useful in industrial formulations because the ionic type directly affects the compatibility of the emulsifier with other components, its pH sensitivity, and its functional performance in specific scenarios.

Anionic emulsifiers have a hydrophilic head that carries a negative charge in water. Common examples include sodium fatty acid (soap) and sodium dodecyl sulfate (SDS). They have strong emulsifying ability and produce rich foam, and are widely used in detergents and industrial cleaning. However, they are sensitive to water hardness (calcium and magnesium ions) and are prone to failure in hard water. They are also not suitable for use with cationic ingredients, as charge neutralization will occur, leading to precipitation and deactivation.

Cationic emulsifiers have a positively charged hydrophilic head, and are typically quaternary ammonium compounds. They have a strong affinity for negatively charged surfaces (such as hair, skin, and bacterial cell membranes), and are therefore widely used in hair conditioners, softeners, and antibacterial products. Because bacterial cell membranes are also negatively charged, cationic emulsifiers often possess some antibacterial activity, but they are relatively highly irritating and are generally not used in food.

Nonionic emulsifiers have uncharged hydrophilic heads, achieving their hydrophilicity through hydrogen bonds formed between the polyethylene glycol chain or polyhydroxyl structure and water. Examples include Tween, Span, and monoglycerides. The biggest advantage of nonionic emulsifiers is their “mildness”: they are unaffected by pH, have good compatibility with other ionic components, and exhibit low toxicity and irritation, making them the preferred choice in the food and cosmetics industries. They are also currently the most widely used and diverse class of emulsifiers.

Amphoteric emulsifiers have hydrophilic heads containing both positively and negatively charged groups, allowing them to behave as anions or cations under different pH conditions, while remaining nearly neutral near their isoelectric point. Lecithin is an example of this – its phosphate groups are negatively charged, and its choline groups are positively charged. Amphoteric emulsifiers combine the emulsifying power of anions with the adsorption properties of cations, exhibiting extremely low irritation and are commonly used in baby care products and high-end cosmetic formulations.

The third dimension: HLB value – a compass for selecting emulsifiers.

Now that we know the types of emulsifiers, the next question is: which one should we choose for a specific need?

Here is a key tool: the HLB value , which stands for Hydrophilic-Lipophilic Balance.

What is HLB value?

The HLB value, proposed by American chemist Griffin in 1949, is a numerical value describing the relative strength of the hydrophilicity and lipophilicity of emulsifier molecules, typically ranging from 0 to 20. A lower value indicates a more lipophilic molecule, while a higher value indicates a more hydrophilic molecule.

• HLB 1–3: Strongly lipophilic, suitable for use as an antifoaming agent.
• HLB 3–6: Primarily lipophilic, suitable for preparing water-in-oil (W/O) emulsions.
• HLB 7–9: Middle zone, suitable for wetting agents
• HLB 8–18: Primarily hydrophilic, suitable for preparing oil-in-water (O/W) emulsions.
• HLB 13–15: Suitable for use as a detergent
• HLB 15–18: Strongly hydrophilic, suitable for solubilizing.

How to select emulsifiers using HLB values

Each type of oil has a “required HLB value”—that is, the HLB range of the emulsifier best suited to emulsify it. For example, mineral oil requires an HLB of approximately 10, beeswax approximately 9, and silicone oil approximately 10–12. Formulators need to select emulsifiers with HLB values that match the oils to achieve stable emulsification.

In actual formulations, emulsifiers are often not used alone, but rather high-HLB and low-HLB emulsifiers are blended in proportion to achieve the target HLB value. The HLB value of the blended emulsion system can be calculated using a weighted average.

It is important to note that the HLB value is a necessary but not sufficient condition. Two emulsifiers with the same HLB value but different chemical structures may have vastly different emulsifying effects. The HLB value provides a direction for selection; the final formulation still needs to be experimentally verified, taking into account the chemical compatibility between the emulsifier and the oil, system temperature, pH, and the interactions of other components.

Limitations of HLB value

Griffin’s HLB system is most suitable for nonionic emulsifiers, but its accuracy is limited for ionic emulsifiers—because the behavior of charged emulsifiers is also strongly influenced by the ionic strength and pH of the system, and a single value cannot fully describe it. Subsequent researchers have proposed improved methods such as Davies HLB and PIT ( phase transition temperature), but Griffin HLB remains the most commonly used reference framework for industrial formulations due to its simplicity.

Three dimensions, a complete map

The source determines the raw material background and safety reputation of an emulsifier (natural ones are often more readily accepted by consumers, but their performance is not necessarily superior to synthetic ones); the ion type determines its compatibility with other components in the system and its applicable scenarios; the HLB value serves as a compass for quantitative selection, helping formulators quickly narrow down the candidate range from hundreds of emulsifiers. These three dimensions combined constitute a complete perspective on the emulsifier family.