Research progress on the physicochemical properties, sources, modification and applications of lecithin in multiple fields
1 Introduction
Lecithin is a phospholipid compound that is widely present in organisms. As a natural zwitterionic surfactant , it has important research value and application prospects in the fields of food, medicine, and cosmetics [12]. Phospholipids are an important component of biological membranes, a basic substance of life, and an active component of all biological cells, playing an important role in cell permeation and metabolism [11]. Phospholipid molecules are arranged in a bilayer to form a selective and protective barrier, controlling cell life and playing a very important role in activating cells, maintaining metabolism, resisting oxidative damage, and delaying aging [11].
In recent years, with the increasing demand from consumers for natural, green, and safe products, lecithin has received significant attention from academia and industry due to its advantages such as natural origin, excellent biocompatibility, and outstanding multifunctionality. This article aims to systematically review the latest research progress on lecithin, providing a reference for researchers and practitioners in related fields.
2. Chemical Structure and Physicochemical Properties
2.1 Chemical Structure
Lecithin broadly refers to an aggregate of various phospholipids, including phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylserine (PS), etc. [4][9]. In a narrow sense, lecithin mainly refers to its main component, phosphatidylcholine (PC) [4].
Lecithin molecules are composed of a hydrophilic glycerol phosphate head and a lipophilic fatty acid tail, making them an amphoteric molecule with natural surfactant properties [3]. This unique amphiphilic structure allows them to bind to both water-soluble and oil-soluble components, making them easy to bind to both water-soluble and oil-soluble components [11].
2.2 Physicochemical Properties
Lecithin is a pale yellow to brown translucent viscous substance with a slightly beany smell. It is insoluble in water but can swell into a colloidal solution in water . It is soluble in chloroform, ether, and petroleum ether[10]. As a natural surfactant, lecithin has emulsifying, dispersing, and wetting properties and is widely used in the chemical, food, and pharmaceutical industries[3].
Differences in different sources and preparation processes result in significant differences in the content and types of the main components of lecithin. In addition, different lecithins also have significant differences in fatty acid composition [13]. These differences further affect their application performance in different fields.
2.3 Pharmacopoeia Standards of Various Countries
The pharmacopoeias of various countries have different requirements for the content of “acetone insoluble matter” in lecithin: the United States Pharmacopeia (USP) requires no less than 50%, the European Union (EU) requires no less than 60%, and the Japanese Pharmacopoeia (JP) requires no less than 40%[4].
3. Sources and Extraction Processes
3.1 Main Sources
The main sources of lecithin include soybeans, sunflowers, egg yolks, milk, etc. [2]. Among them, the most important source of high-quality phospholipids is soybeans, and other sources include sunflowers, eggs, etc. [2]. Crude lecithin contains more than 60% acetone-insoluble matter (a mixture of various phospholipids) and has an oil content as high as 40% [4].
3.2 Extraction and Purification Process
The extraction process of lecithin varies depending on the source. Taking soybean lecithin as an example, its preparation process is as follows: steam is passed through crude soybean oil to precipitate phospholipids, and then the separated gum is extracted, refined and vacuum dried to obtain a paste product [10].
phospholipid components rich in PC ( about 35% PC ) from crude lecithin , and further enrichment by chromatography can obtain natural high-purity PC ( PC>70% ) [14] .
3.3 Differences in composition from different sources
Soybean lecithin, egg yolk lecithin, and egg yolk phosphatidylcholine have significant differences in composition and fatty acid composition due to their different sources and preparation processes [13]:
• Soybean lecithin : PC content is about 80%, mainly linoleic acid (about 60%), with a high proportion of unsaturated fatty acids, which makes it very easy to oxidize [13].
• Egg yolk lecithin : PC content is about 70%, PE content is about 20%, fatty acid composition is mainly palmitic acid and oleic acid, and the proportion of unsaturated fatty acids is relatively small, and its stability is better than that of soybean lecithin[13].
• Egg yolk phosphatidylcholine : PC content is as high as 98% or more, while PE and PI content is very low, resulting in weak emulsifying ability when used alone as an emulsifier[13].
4 Modification Technology
4.1 Hydrogenation Modification
Hydrogenated lecithin is a stable emulsifier and moisturizer formed by hydrogenation of lecithin under the action of a catalyst. The stability and emulsifying properties of hydrogenated lecithin are improved, and it is widely used in various industries such as food, pharmaceuticals, and cosmetics[11].
Hydrogenation treatment preserves the active components of lecithin well [11] . In terms of production process, researchers use supercritical CO₂ technology and Pd/C as catalyst to carry out the addition reaction of hydrogenated lecithin; some studies have also used impregnation method to prepare Pd/C catalyst, and used the change of iodine value before and after the hydrogenation reaction of soybean lecithin as an indicator to investigate the effect of preparation conditions on catalyst activity [11] .
Hydrogenated lecithin is a white, free-flowing powder with no odor and is a novel emulsifier [11] . Because high-purity lecithin has poor oxidative stability and is not easy to store, it is hydrogenated to make it stable [16] .
4.2 Differences between hydrogenated and non-hydrogenated lecithin
Hydrogenated and non-hydrogenated lecithin have different mechanisms of action on the skin[14]:
• Hydrogenated lecithin : It has a compact structure that penetrates only into the stratum corneum, stabilizing the skin barrier and transepidermal water loss (TEWL). It forms a waterproof lipid layer within the stratum corneum to prevent skin dehydration.
• Non-hydrogenated lecithin : It has a loose structure and can penetrate below the stratum corneum, which can increase penetration and slightly enhance transdermal water loss.
4.3 Other modification directions
In addition to hydrogenation, enzymatic reactions can be used to prepare lysophosphatidylcholine (LPC), which utilizes enzymatic hydrolysis of fatty acids to form micelle structures, thereby enhancing solubilization and emulsification capabilities [14]. Furthermore, lecithin/cholate mixed micelle systems have also been studied for solubilizing poorly soluble substances, representing a new generation of natural solubilizers and emulsifiers with high dermatological compatibility [5].
5. Emulsifying properties and liquid crystal phase behavior
5.1 Emulsification Mechanism
Lecithin is composed of a hydrophilic head and a hydrophobic tail. It is an amphoteric molecule and a natural surfactant with emulsifying, dispersing, and wetting properties [3]. Due to the compatibility of phospholipids with cell membranes, lecithin can provide better moisturizing performance and reduce the irritation of surfactants to the skin .
30 parts petrolatum under liquid crystal emulsification conditions , and even with ordinary emulsification methods, it can emulsify about 10 parts of oils, and it can also emulsify silicone oils that are relatively difficult to emulsify .
5.2 Comparison of emulsifying abilities of different phospholipids
Studies have shown that lecithin from different sources has significantly different emulsifying abilities[13]:
| emulsifier | Average particle size / nm | Particle size distribution/PDI | ζ potential /mV |
| Soy lecithin | 201.8~211.3 | 0.111~0.149 | -26.88~-28.94 |
| Egg yolk lecithin | 177.5~182.9 | 0.098~0.117 | -28.81~-30.18 |
| Egg yolk phosphatidylcholine | 325.6~340.8 | 0.891~0.924 | -8.99 to -11.52 |
Egg yolk lecithin has the best emulsifying performance (small particle size and high stability), while high-purity egg yolk phosphatidylcholine has the worst emulsifying ability due to the lack of auxiliary phospholipids such as PE [13] . Appropriately increasing the PE content in phospholipids is beneficial to the formation of a denser emulsion film and can improve the stability of the emulsion [13] .
5.3 Liquid Crystal Phase Behavior
Lecithin can self-assemble into liquid crystal systems in solvents, exhibiting different phase behaviors such as lamellar, cubic, and hexagonal [12]. Liquid crystals are a special state of matter, possessing both the anisotropy of crystals and the fluidity of liquids. Lamellar liquid crystals are introduced into emulsion systems to form liquid crystal emulsions, which have better moisturizing, stabilizing, and excellent properties due to their unique structure [12].
The study investigated the influence mechanism of co-emulsifiers and oils on the liquid crystal phase behavior of soybean lecithin system using binary phase diagrams and pseudo-ternary phase diagrams [12] :
1. In the SL/water binary system, 35% SL by mass forms a lamellar liquid crystal at 20~55℃ (XRD verification q ratio 1:2), and 45% SL forms a hexagonal phase liquid crystal (q ratio 1:√3:√4) [12].
2. After being compounded with cholesterol, soybean lecithin with a mass fraction of 20%~35% can form a regular and bright “Maltaic cross” layered liquid crystal at room temperature . The addition of cholesterol promotes the assembly and arrangement between molecules [12] .
3. Among single oils, ester oils (GTCC, JOJOBA) are conducive to the formation of liquid crystal structure emulsions, while white oils are not conducive to the formation of liquid crystal structures [12].
4. When 16%~20% of composite oil (white oil:GTCC:JOJOBA=1:1:1) is added, the liquid crystal structure emulsion area is the largest and the liquid crystal structure is bright [12] .
5. Liquid crystal emulsions have a longer linear viscoelastic range than ordinary emulsions and better stability[12].
6. Applications in cosmetics
6.1 Moisturizing and Skin Barrier Repair
Hydrogenated lecithin has strong hydrophilicity and moisturizing properties, and has a strong affinity for skin and mucous membranes. When used in cosmetic formulations, it can play a role in moisturizing, emulsifying and dispersing, and conditioning the skin[11]. As an excellent lipid barrier enhancer, hydrogenated lecithin provides additional lipids to make the barrier sufficiently impermeable to water . It is mainly composed of hydrogenated phosphatidylcholine (PC), which is the same lipid as the skin and mimics the layered structure of the extracellular matrix[4].
Phospholipids have a strong water-locking ability . One molecule of phospholipid can bind 23 water molecules, enhancing the water-locking ability of the skin surface and deep layers [2] . PC has hygroscopic properties. One molecule of PC binds about 20 water molecules and can be used as a moisture storage function [4] .
the PC content in topical products , the better the moisturizing effect on the skin ; the skin roughness of volunteers who used lecithin with high PC content decreased significantly [4] .
6.2 Antioxidant and Post-Sun Repair
Hydrogenated lecithin can improve the antioxidant effect of the skin: After volunteers used hydrogenated lecithin emulsions with concentrations of 0.5%, 1%, and 2%, the relative content of squalene hydroperoxide decreased from 9 to 4, 2, and 1, respectively. The higher the concentration, the more significant the antioxidant effect [11].
In terms of post-sun repair, applying 2% hydrogenated lecithin emulsion to the erythema area induced by ultraviolet radiation resulted in skin color indexes of 150 , 190 , 170 , and 160 after 8h , 16h , 24h , and 48h , respectively , indicating that hydrogenated lecithin has a significant effect on post-sun repair [11] .
6.3 Emulsification and Skin Feel Adjustment
Lecithin emulsifiers have the following advantages[4][2]:
1. Biomimetic emulsifier : Highly compatible with human cell membranes, providing a unique skin feel.
2. Perfect skin feel : Soft and skin-friendly, it acts as a skin feel modifier, providing a cashmere-like touch.
3. Natural and gentle : Suitable for sensitive skin and has high skin tolerance.
In formulation applications, the recommended dosage of hydrogenated lecithin as a primary emulsifier is 1%-5% , and the recommended dosage as a skin feel enhancer and co-emulsifier is 0.2% -0.5 % , with a recommended pH range of 5-8[4] .
6.4 Liposome Delivery System
Liposomes are lipid bilayer structures and are widely used as delivery systems for active ingredients in cosmetics. Nanoliposomes use natural lecithin as the wall material of liposomes. Lecithin is a component of biological cell membranes. In addition to its superior emulsifying ability, it also has the effects of promoting transdermal absorption, enhancing moisturizing effect and soothing irritation [15].
Nanoliposomes prepared using hydrogenated lecithin as the wall material have better stability and can be prepared with emulsified particle sizes of less than 200 nm [15] . Liposomes prepared from lecithin and the like can be degraded in vivo and are non-immunogenic [9] .
6.5 Makeup Pigment Processing
Lecithin has good adaptability and permeability to the skin and can maintain the normal moisture balance of the skin. Hydrogenated lecithin is an amphoteric surfactant. When it interacts with powder, the polar end binds to the powder surface and the non-polar end is evenly distributed on the periphery, thereby improving the surface properties and dispersibility of the powder [16].
with hydrogenated lecithin has excellent skin affinity, enhances the softness, silkiness and moisturization of the pigment, is easily dispersed in the oil phase, has emulsifying properties, and is suitable for various moisturizing and hydrating liquid foundation products [ 7] . However, the cosmetic powder obtained by hydrogenated lecithin treatment does not have sebum resistance and has poor makeup retention . Natural non-GMO hydrogenated lecithin combined with perfluoroalkyl silane double treatment powder can achieve a long-lasting effect that is both moisturizing and hydrophobic and oleophobic [18] .
6.6 Transdermal Permeation Enhancement
Phospholipids are a class of low-toxicity, non-irritating permeation enhancers. Phospholipid complexes of drugs can alter the physicochemical properties of the original drug, prolong the duration of action, enhance pharmacological effects, and reduce toxic side effects [9]. Studies have conducted experiments on the in vitro permeation of puerarin and its phospholipid complexes using an improved Franz diffusion cell, and found that the puerarin-phospholipid complex has good permeability, easily binds to the dermis, and exerts a sustained-release effect [9].
7. Applications in the food and pharmaceutical fields
7.1 Applications in the food industry
Lecithin, as a natural emulsifier, is widely used in the food industry in dairy products, baked goods, chocolate, margarine and other products, and has multiple functions such as emulsification, dispersion, stabilization and antioxidation. The nutritional activity and functionality of soybean lecithin have received widespread attention[11].
7.2 Applications in the pharmaceutical field
Hydrogenated lecithin has long been used in the pharmaceutical field as an immune aid and a pharmaceutical carrier [11]. In intravenous emulsions, egg yolk lecithin has better stability than soybean lecithin due to its relatively low proportion of unsaturated fatty acids, and is mainly used as an emulsifier in oral and intravenous emulsions[13].
In transdermal drug delivery systems, drug – phospholipid complexes can be used to prepare transdermal absorption formulations with excellent sustained-release effects. Liposomes prepared from lecithin and other substances can be degraded in vivo and are non-immunogenic [9] .
8. Outlook and Challenges
Despite the widespread application of lecithin in various fields, several challenges remain to be addressed:
1. Stability issues : High-purity lecithin, especially soybean lecithin, contains a large amount of unsaturated fatty acids, which are easily oxidized and deteriorated. Although hydrogenation can improve stability, the hydrogenation process may change some of its functional properties [13][16].
2. Improved emulsification ability : The emulsification ability of a single lecithin emulsifier is limited, and it is usually necessary to combine it with other co-emulsifiers to prepare emulsions with good stability and fine texture [12]. Cholesterol, cetearyl alcohol and other co-emulsifiers have a positive impact on the formation of liquid crystal structure in lecithin system.
3. Multifunctional development direction : The development of new lecithin derivatives, such as lysophosphatidylcholine (LPC) and lecithin/cholesterol mixed micelle system, provides a new way to solubilize poorly soluble substances and control the release of active substances [14][5].
4. Sustainable sources : Non-GMO, sustainable lecithin products such as sunflower-derived lecithin are receiving increasing attention[4].
In the future, as the concepts of green chemistry and sustainable development gain wider acceptance, lecithin, as a natural multifunctional surfactant, will demonstrate greater application potential in fields such as biomimetic emulsification, active ingredient delivery systems, and green formulations.