Biosurfactants are surfactant molecules of microbial origin which can be produced on the basis of vegetable oil and sugar substrates. Like surfactants of non-microbial origin, biosurfactants are amphiphilic, i.e. they possess both a hydrophobic and a hydrophilic molecular part. Thanks to this amphiphilic character, they are used to reduce surface tensions or interfacial tensions.
Biosurfactants are currently produced and used only in very small quantities because they are not economically competitive with petroleum-based synthetic surfactants. However, their Critical Micelle Concentration (CMC) is technically comparable to conventional nonionic surfactants.
Surface-active substances of microbial origin can be divided into chemically different groups. Glycolipids, lipopeptides and lipoamino acids, lipoproteins and lipopolysaccharides as well as phospholipids, mono- and diglycerides and fatty acids are of particular importance.
Glycolipids are the most widely distributed group of low molecular weight biosurfactants and can be divided into:
- Trehalose and other mycolic acid containing glycolipids
- Cellobiose and mannosylerythritol lipids
The hydrophilic “molecular head” can be both non-ionic and ionic in nature. This includes mono-, di- and polysaccharides as well as carboxylic acid, amino acid and peptide groups. The hydrophobic “molecular tail” usually consists of unsaturated, saturated or hydroxylated fatty acids.
Biotechnological production of biosurfactants
Biosurfactants produced with the help of bacteria or fungi are of great interest for industrial biotechnology. One advantage over the petrochemical production of surfactants is that biosurfactants can be produced on the basis of renewable raw materials (e.g. vegetable oils, sugar in the form of thick juice or molasses). A high degree of surfactant efficiency and good biodegradability are further reasons in favour of the use of biosurfactants.
The biotechnological production of biosurfactants has been established for some time, but due to the high production costs they have so far only been used in niche areas. For example, rhamnose lipids (rhamnolipid) are mainly produced by Pseudomonas aeruginosa, surfactin by Bacillus subtilis, emulsan by Acinetobacter calcoaceticus and liposan by Candida lipolytica as well as sophorose lipid (sophorolipid) by Torulopsis bombicola. Cellobiose lipids can be formed in the presence of alkanes or triglycerides by the corn boll blight(Ustilago maydis) while corynomycolates and trehalose lipids are produced by bacteria of the genera Corynebacterium and Arthrobacter. Particularly high yields are achieved in the production of sophorolipids with more than 400 g/l suspension while for most other biosurfactants up to 110 mg/l suspension can be achieved.
Process optimization is a focus of current biotechnological research on biosurfactants. Due to the very good surfactant properties, the problem of foaming also arises. Although good chemical antifoaming agents (especially silicone oils) exist, they can influence the product quality. In current research, therefore, alternatives are being tested, such as mechanical foam destruction.
Some biosurfactant-producing organisms, such as Pseudomonas aeruginosa, the only producer of significant amounts of rhamnolipids, are opportunistic pathogens and are therefore classified as potentially dangerous microorganisms. Their handling is linked to appropriate technical measures and is very costly; alternative producers are currently being researched. The surfactin of Bacillus subtilis has very good surfactant properties, but due to its haemolytic effect it is not used.
Application potential of biosurfactants
The application of biosurfactants basically spans the application areas of chemically synthesized surfactants. Due to the potentially better biodegradability of biosurfactants, additional special, environmentally relevant areas of application can be developed which are not possible for chemically synthesized surfactants. Corresponding areas of application for biosurfactants are, for example, uses in the field of bioremediation or environmental remediation (e.g. treatment with trehalosedicorynomycolate after major oil disasters) and tertiary oil production (“enhanced oil recovery”). The main applications, however, correspond to those of synthetic surfactants in household and cleaning products, cosmetics, medicine, in food processing technology and in agriculture and crop protection, in which surfactants from oleochemical production (direct chemical conversion of vegetable oils) are also used.
Rhamnolipids in particular find applications in household cleaners (Henkel, Ecover), sophorolipids are used in skin creams, especially in Japan.
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