Development status of biosurfactants.

As far as I know, surfactants are an important class of chemical raw materials, often referred to as the \"industrial flavor enhancer.\" They play a special and crucial role in many fields such as the petroleum industry, environmental engineering, the food industry, and fine chemicals. At present, almost all surfactants are chemically synthesized from petroleum, and chemically synthesized surfactants often cause serious environmental pollution problems during their production and use. Biosurfactants are a newer member of the surfactant family; they are biologically derived macromolecules with surface-active properties, produced by microorganisms. Compared with chemically synthesized surfactants, biological surfactants, in addition to having the same functions such as reducing surface tension, stabilizing emulsions, and increasing foam formation, also possess advantages such as being non-toxic and biodegradable, which are not characteristic of conventional chemically synthesized surfactants. These properties of biosurfactants make them particularly suitable for the petroleum industry and environmental engineering, such as the biological reduction of oil viscosity, improving crude oil recovery rates, and the biological remediation of soil contaminated by heavy oils. Furthermore, as natural additives, biosurfactants are gaining increasing popularity in various industries such as the food industry, fine chemicals, pharmaceuticals, and agriculture. As there is a growing appreciation for nature and an increasing awareness of environmental protection, bio-surfactants will have broader application prospects, and may become alternatives or upgrades to chemically synthesized surfactants. 1 History of research on biosurfactants: As early as the 1940s, Zobell, while studying the mechanism by which sulfate-reducing bacteria release crude oil from sand particles, pointed out that the production of surfactants by microorganisms is one of the main mechanisms for bacteria to drive oil out of reservoirs. In 1957, Dostálek and Spumy from the Czech Republic injected Desulfovibrio and Pseudomonas along with molasses into oil reservoirs, resulting in an increase in crude oil production. They believe that it may be the surfactants produced by bacteria that alter the interfacial tension of the rock-oil-water three-phase system. After the 1960s, the oil industry began to develop, and attention was drawn to the emulsification mechanism of hydrocarbons by microorganisms. Research on the extracted microbial surfactants focuses on their structure, properties, biosynthesis, and regulation. In 1976, Zajie and Panchal reviewed the sources and properties of microbial emulsifiers and predicted their potential for application. In 1980, Cooper and Zajie reviewed the chemical properties of microbial surfactants. In 1984, Zajie and Seffens described the physicochemical properties of microbial surfactants. In 1991, the axially symmetric droplet analysis (ADSA) conducted by Vander et al. proved to be very effective; other simpler methods include rapid droplet rupture experiments, thin-layer chromatography for detecting surfactin-producing colonies, and colorimetric assays for identifying hydrocarbon-degrading bacteria and rhamnolipid-producing bacteria. A research focus in recent years has been the development of methods for rapidly identifying high-yielding strains of surfactant-producing bacteria and evaluating their potential. The production of surfactants using microbial fermentation was developed in the late 1970s. Countries such as Canada, the United Kingdom, Germany, and the former Soviet Union are primarily engaged in the research and development of various new types of biological surfactants, the search for optimal production conditions, the analysis and modification of surfactant structures, the testing of their physicochemical properties, and physical simulations of oil displacement in laboratory settings. Several products from the Zajie laboratory have been commercialized. A bacterium developed by the Singer laboratory that uses n-alkanes as its sole carbon source to produce extracellular and intracellular glycolipid-based surfactants, which reduce the viscosity of heavy oil by over 95% and form stable oil-in-water emulsions. 2 Types of biosurfactants and their microbial sources 2.1 Types of biosurfactants Biosurfactants produced by microorganisms include many different types. Based on their chemical composition and microbial origin, they can be divided into five major categories: glycolipids, lipopeptides and lipoproteins, fatty acids and phospholipids, polymers, and the cell surface itself. 2,2 Sources of microorganisms that produce biosurfactants Biosurfactants are mostly produced by bacteria, yeasts, fungi (molds), etc. The microbial strains used for the production of biosurfactants through microbial fermentation can be roughly divided into three categories: one category consists of microorganisms that use alkanes as their sole carbon source, such as Corynebacterium sp.; another category includes microorganisms that use water-soluble substrates as their carbon source, such as Bacillus sp.; and the third category comprises microorganisms that can utilize both alkanes and water-soluble substrates as carbon sources, such as Pseudomonas sp. Table 1 lists the types of biosurfactants and their microbial sources. 3 Properties of bio-surfactants: The molecular structure of bio-surfactants contains both polar and non-polar groups, making them a type of neutral amphiphilic molecule. Hydrophilic groups can be monosaccharides, disaccharides, polysaccharides, carboxyl groups, amino groups, or peptide chains in ionic or non-ionic form; hydrophobic groups, on the other hand, are composed of saturated fatty acids, unsaturated fatty acids, or fatty acids with hydroxyl groups. For some bio-surfactant molecules with larger molecular weights, such as protein-polysaccharide complexes, their hydrophilic and hydrophobic parts can be composed of different molecules. Biological surfactants can arrange themselves in an ordered manner at the interface between two phases to form a molecular layer, thereby reducing the energy of that interface, that is, the surface tension. Most biological surfactants can reduce the surface tension to 30 mN/m. They play a very important role in determining the rheological properties of the interface as well as the mass transfer between the two phases. Biological surfactants possess good thermal and chemical stability; for example, lipopeptides produced by Bacillus licheniformis can withstand temperatures of 75°C for at least 140 hours. Biological surfactants remain stable within a pH range of 5.5 to 12; when the pH is below 5.5, they gradually become inactive. Compared to chemically synthesized surfactants, bio-based surfactants have advantages: their reaction products are homogeneous, and it is possible to introduce new types of chemical groups, some of which are difficult to synthesize using chemical methods. Bio-based surfactants are safe, non-toxic, and require simple production processes; reactions can take place at normal temperature and pressure. 4 Applications of biosurfactants Biosurfactants have a very wide range of applications and can be used in industries such as oil extraction, environmental engineering, the food industry, agriculture, and fine chemicals. However, the most promising applications are in removing polluted oil tanks, transporting heavy oil, and improving crude oil recovery rates. 4.1 Application in the oil drilling industry (MEOR technology): Since the oil industry does not require high purity or specificity from biological surfactants, fermentation broths containing intact microbial cells can be used directly. Microbial enhanced oil recovery (MEOR) technology involves injecting certain microorganisms into the oil reservoir, along with nutrients necessary for their growth; these microorganisms produce biosurfactants as they grow. These biosurfactants can reduce the tension at the interface between crude oil and water, thereby increasing the oil field’s production volume. MEOR technology is an important application area for biosurfactants. In oil field exploitation, even after applying primary and secondary recovery techniques, approximately 70% of the crude oil remains in the reservoir. To further recover these substantial amounts of residual crude oil, chemically synthesized surfactants are typically injected into the oil wells to reduce the interfacial tension between crude oil and water, thereby enabling a large amount of crude oil trapped in the capillary pores of the formation to be released and increasing the crude oil recovery rate. However, chemically synthesized surfactants are usually difficult to biodegrade, causing severe environmental pollution. Biological surfactants can be degraded by microorganisms, so they do not cause pollution to the environment. The microbial stimulation method for oil recovery, which involves directly injecting into the ground microorganisms that produce surfactants along with appropriate nutrients, using underground oil as the sole carbon source and treating the oil reservoir as a bioreactor, is a challenge that researchers at home and abroad are striving to overcome. The key difference between microbial flooding and chemical flooding is that microorganisms can not only move in the direction of the water injection pressure gradient but also migrate deep within the oil reservoir, where they can interact with the crude oil that is difficult to reach using water flooding, thereby improving the efficiency of water flooding or chemical flooding. Furthermore, as microorganisms interact with crude oil, they produce metabolic products that help improve the oil recovery rate. In addition to bio-surfactants, these microorganisms also generate certain small molecular organic acids and organic solvents, which not only reduce the interfacial tension between oil and water but also increase the permeability of the oil reservoirs. On this basis, combining it with chemical flooding will maximize crude oil recovery rates while **reducing the costs of chemical flooding**. It is reported that the trehalolipids produced by Nocar디аrhodochro우с are used for the recovery of oil from underground sand and gravel, increasing the oil recovery rate by 30%. 4.2 Applications in environmental bioengineering In recent years, with the development of industrial and agricultural production, pollution of water bodies and soil caused by industrial wastewater and solid waste, pesticides used in farming, oil spills during extraction, and leaks during transportation has become increasingly severe, drawing people’s attention to this issue. Studies have shown that the addition of microorganisms or surfactants (biological or chemically synthesized) can enhance the hydrophilicity and bioavailability of hydrophobic compounds, thereby enabling the continuous degradation of pollutants in the environment. This technique is known as bioremediation, and biological surfactants are used in it. Soil microbial degradation of alkane compounds is the fundamental mechanism by which alkane pollutants are removed from the soil. To increase the degradation rate of alkanes, the effectiveness of inoculating non-soil microorganisms remains highly controversial. However, the degradation rate of alkanes can be increased by raising the number of microorganisms in the soil, and this increase in degradation rate is much greater than that achieved by adding a certain nutrient alone. Glycolipid biosurfactants can not only increase the removal rate of alkanes but also accelerate their mineralization, thereby reducing the adaptation time required for them to be utilized by microorganisms. Mannitol erythritol ester produced by Candida using Hua Zhaozhe et al. exhibits good degradation ability for n-alkanes. Studies by VanDyke et al. have shown that rhamnolipids synthesized by Pseudomonas aeruginosa, when added to sand or sandy loam, increase the removal rate of alkanes by 25%–70% and 40%–80%, respectively. Bio-surfactants can also be used to remediate soil contaminated by heavy metals, phenanthrene, and polychlorinated biphenyls. 4, 3 Applications in the food industry and fine chemicals: Meeting the requirements of functional foods and green food additives, biological surfactants have become widely used food additives in an era where people place a high value on nature and health. Sucrose esters, lecithin, sorbitan polymers, and others are all emulsifiers commonly used in the food industry today. In addition, biosurfactants can also be used in the food processing industry and fine chemical industry as humectants, preservatives, wetting agents, foaming agents, thickeners, lubricants, etc. It is reported that sucrose esters, when added to foods, can improve their processing properties, enhance their antioxidant and antifungal effects as well as their flavor quality. Sucrose esters have shown good results in preserving fruits such as oranges, apples, and pears. Bioemulsifiers synthesized by Candidautilis can be used as salad dressings. Acacia glycosides have good skin compatibility; they can be used as skin moisturizers in cosmetics, as well as in the production of detergents and to enhance the stability of photosensitive emulsions. Sucrose esters can also improve the washability of cosmetics, as well as enhance the smoothness and softness of the skin. 4,4 Applications in Other Areas In agriculture, bio-surfactants can be used for soil improvement, as fertilizers, for cleaning, in plant protection, and as insecticides. In the field of healthcare, biosurfactants can be used to treat certain diseases. It has been reported that the succinyl trehalolipid synthesized by Rhodococcus erythropolis thropopolis can inhibit herpes simplex virus and influenza virus. Phospholipoproteins produced by fermentation have an inhibitory effect on immunodeficiency virus in human cells. Bio-surfactants can also be used for efficient cell lysis and rapid determination of microbial counts. Since bio-surfactants can efficiently break down the cells of bacteria and fungi, the ATP inside these cells is released and reacts with the luciferase and fluorescein system to produce fluorescence. The amount of ATP is related to the number of cells; therefore, by measuring the amount of fluorescence generated, it is possible to determine the number of cells, thus enabling rapid detection. In addition, biosurfactants can also be used for monitoring the efficacy of bactericides and insecticides, as well as for tracking cell counts during fermentation processes in the fermentation industry. 5 Problems and Future Prospects At present, only a few products of bio-surfactants have reached the market; most varieties are still in the experimental research stage, and large-scale industrial production has not yet taken place, mainly due to their high production costs. It is estimated that biological surfactants cost 3 to 10 times more than chemical surfactants. To unlock the application potential of biosurfactants, reducing their production costs is a current focus and main goal of research and development. The main factors determining the production cost of biological surfactants include raw materials, fermentation processes, and downstream technologies. Therefore, there are three approaches to solving this problem: (1) Developing high-yield strains through selective breeding and genetic engineering, as well as creating methods for quickly identifying high-yield strains of surfactants and assessing their potential; (2) Finding inexpensive raw materials for fermentation, improving fermentation processes, and utilizing advanced downstream processing techniques to increase the yield of bio-surfactants during fermentation and extraction, thereby **reducing their production costs; (3) Taking advantage of the unique properties of bio-surfactants to develop secondary products from them and increase their added value. Such as in industries like cosmetics, food, and pharmaceuticals. In summary, bio-surfactants are a new type of surfactant that has been developed recently. They have the ability to reduce surface tension, are non-toxic to the environment, and possess good biodegradability; they belong to the category of natural additives. Apart from gaining attention in certain specialized fields such as the oil industry and environmental engineering, with the rapid development of biotechnology and related technologies, the price of bio-surfactants will gradually drop to levels acceptable to consumers. They will be used more and more widely in industries such as the food industry, fine chemicals, and healthcare, thus becoming closely linked to our daily lives. Research on biosurfactants and their applications holds broad prospects for development.