Background and overview[1]
Lecithin is a fat mixture containing chlorine and phosphorus that was first isolated from egg yolk by Frenchman Gobley in 1844. In the 1930s, soy lecithin was discovered in the by-products of soybean oil processing. The main components of lecithin include lecithin, inositol phospholipid, cephalin, phosphatidic acid, serine phospholipid and sphingomyelin; lecithin in the narrow sense refers to choline phosphoglyceride or phosphatidylcholine, and its triglyceride molecule The phosphate on it binds to choline. Soy lecithin, also known as soy lecithin, is a viscous, highly oily lecithin obtained when refining edible soybean oil. It is a type of lipid compound with important physiological functions and a type of natural surfactant. It is widely used in food, medicine, cosmetics and many other aspects. Since crude phospholipids contain 30% to 40% of oil, they are highly lipophilic and weakly hydrophilic, which limits their application in the above-mentioned industries. In recent years, soybean lecithin has developed rapidly abroad, and its refining, separation and deep processing and comprehensive utilization have developed rapidly, and people have paid more and more attention to it. my country is rich in soybean resources and has rich raw material advantages in developing soybean lecithin. The deep processing and transformation technology of agricultural products is also one of the technology development fields that the country currently supports nationwide. Therefore, vigorously developing soybean lecithin and its deep processing products can not only increase the added value and technological content of soybeans, but also increase the market prospects of its products. Very vast.
Chemical structure and composition[1]
Soy lecithin is a by-product obtained during the refining process of soybean oil. Its chemical structure is mainly composed of phosphate and choline groups (X) on the triglyceride molecule. The structural formula is shown in the figure. In the formula, R1 and R2 are hydrocarbon groups representing fatty acids, and the saturated fatty acids and unsaturated fatty acids in R1 and R2 are mixed and coordinated. Common ones include stearic acid, palmitic acid, oleic acid, linoleic acid, linolenic acid and arachidonic acid. The main ingredients of soy lecithin: 19% to 20% lecithin (phosphatidylcholine), 8% to 20% cephalin (phosphatidyl ethanolamine), 20% to 21% phosphatidyl inositol and phosphatidylserine, etc. .
Physical properties[1]
Soy lecithin is a light yellow to brown, odorless or slightly smelly viscous or powdery material. From its structural point of view, the two fatty acid chains are hydrophobic groups, and the phosphate and choline groups are hydrophilic groups. Therefore, it is a surfactant with a series of interface properties and colloidal properties. Soy lecithin is insoluble in water, easily soluble in a variety of organic solvents, and easily forms reverse micelles, that is, the hydrophobic group is on the outside and the hydrophilic group is on the inside. Emulsification is an important property of phospholipids. There are a large number of unsaturated fatty acids in soy lecithin molecules, which are easily oxidized by air. The increase in temperature not only accelerates the occurrence of this oxidation, but also gradually deepens the color. Soy lecithin can also form liquid crystals, including thermotropic liquid crystals and lyotropic liquid crystals.
Chemical Properties[1]
The chemical properties of soy lecithin are mainly reflected in its ester bonds, fatty acid chains and X substituents of phospholipids. Soy lecithin can undergo a complete hydrolysis reaction when heated or boiled under acidic or alkaline conditions to generate small molecule products such as free fatty acids, glycerol, inositol and phosphoric acid. Under the action of special phospholipases, soy lecithin can be partially hydrolyzed. For example, snake lecithin enzyme can specifically act on the unsaturated fatty acid ester bonds of phospholipids to decompose them. Since soy lecithin molecules contain unsaturated fatty acids, various addition reactions can occur in the unsaturated bonds. In the presence of organic acids such as lactic acid, soy lecithin reacts with hydrogen peroxide to partially hydroxylate its unsaturated bonds. In the presence of catalysts such as nickel, soy lecithin can undergo an addition reaction with hydrogen to generate saturated phospholipids. Under certain conditions, soy lecithin can undergo addition reactions with halogens, halogen acids, etc. to generate halogenated products. The amine group in the phosphatidylethanolamine molecule can react with acylating reagents such as acid anhydride to generate acylated products.
Apply[2]
Soy lecithin can be used as an emulsifier, lubricant, dispersant, anti-aging agent, foaming agent, growth promoter, etc. in food; it can be used as a drug carrier in the pharmaceutical industry to prevent and treat arteriosclerosis and improve fat metabolism. , prevent liver dysfunction, strengthen arterial blood vessel walls, reduce necrosis and other effects.
1) Application in food industry
With the advancement of science and technology, the application of lecithin has attracted more and more attention, especially in the food industry. Soybean lecithin has a variety of properties, and its application in food lies in its functions of emulsification, moistening, stabilization, demoulding, separation and mediation, antioxidant and prevention of starch aging. As a food additive, it is used in baked goods to increase dough volume, uniformity and shortening, and extend the shelf life of food. It is used in candies, instant foods, margarine and cold drinks, etc. It can emulsify, disperse and moisten; it can also be used in meat, poultry and egg foods, dairy products, and various convenience foods, all of which can produce some special effects. Soybean lecithin can improve the processing performance of baked goods: for example, it can enhance the absorption of water, make the dough soft and easy to process, and make the product puffy and have a special flavor. In order to increase the protein content of bread, add about 12% defatted soybean flour to the flour, and then add 1% phospholipid.It can also promote hair growth and reduce gray hair.
5) Applications in other aspects
In addition to the above applications, soybean lecithin is also widely used in textiles, rubber, plastics, leather, dyes, paints, petrochemicals, pesticides and other industries.
Preparation[1-2]
Lecithin is commonly found in resources such as egg yolk, animal brain, soybeans, corn, cottonseed, etc. In practical applications, it is mainly extracted from soybeans and egg yolks. Extracting high-purity lecithin has always been an important topic in phospholipid research. The oily residue of soybean oil processing contains 40% to 50% phospholipids, and the lecithin content in soybean lecithin is about 16% to 20%. The main extraction and separation methods of soy lecithin are:
1) Organic solvent method
Traditional lecithin production mostly uses organic solvents for extraction. The principle is to achieve separation based on the difference in solubility of each component in the mixed phospholipids in the solvent. The solvents used are generally C1 to C4 low-carbon alcohols and n-hexane. , petroleum ether, diethyl ether, ethyl acetate, etc. The key to the solvent extraction method is to find a good solvent or solvent system. The temperature, solvent dosage, solvent concentration, etc. must be controlled during extraction. Organic solvent extraction methods are divided into single solvent extraction methods and mixed solvent extraction methods.
2) Organic solvent and inorganic salt composite precipitation method
This method utilizes the property of lecithin to form precipitates with certain inorganic salts to extract lecithin from phospholipids, thereby achieving the purpose of separating it from other phospholipids and removing proteins and fats. Several inorganic salts, such as CdCl2, MgCl2, CaCl2, ZnCl2 are used to extract soybeans After conducting comparative experiments with lecithin, it was concluded that ZnCl2 is an ideal precipitating agent.
3) Chromatographic Separation Technique
Chromatography is an efficient separation technology that was mostly used in laboratories in the past. In the past 20 years, it has been gradually scaled up and applied to industry. Commonly used chromatographic separation techniques in soy lecithin extraction include thin layer chromatography, column chromatography and high performance liquid chromatography.
4) Thin layer chromatography separation technology
Soy lecithin was purified by centrifugal thin-layer chromatography (CTLC) and identified by high-performance liquid chromatography and thin-layer chromatography. The result was that 716 mg of pure SPC could be obtained with an injection volume of 1000 mg. The CTLC method is simple and has a large preparation volume. The product is free of neutral impurities and other phospholipids, and the SPC purity reaches 99.99%.
5) Column chromatography separation technology
Column chromatography uses an adsorbent as a stationary phase. When the solutes in the mobile phase pass through the stationary phase, they achieve separation due to their different adsorption and desorption abilities. Li Wei et al. [8] used silica gel as the adsorbent and used a mixture of methanol and chloroform with a gradient difference of (1:2) to (2:1) to perform concave gradient elution to elute lecithin. The amount of eluent used is only 5 to 6 times the column volume, and the elution time is only 6 hours.
6) Supercritical extraction technology
Supercritical extraction technology is a new technology that has developed rapidly in recent years. The commonly used CO2 supercritical fluid can fully retain the nutritional and functional properties of the product. Through orthogonal experiments, the optimal process conditions for the preparation of lecithin components in Polygonum Multiflori by supercritical CO2 extraction were studied. The optimal process conditions were selected as follows: extraction pressure 32MPa, extraction temperature 50°C, analysis kettle pressure 6MPa, analysis kettle temperature 55°C.
7) Membrane separation method
This method separates lecithin from the mixture based on the molecular weight of different components in lecithin and the difficulty in passing through the semipermeable membrane. If a phospholipid solution dissolved in a mixture of ethane and isopropyl alcohol is passed through a polypropylene semipermeable membrane, the concentration of lecithin can be increased from 25% to 51%.
