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2 Application of Microorganisms in Biosurfactant Production
Lorena Pedraza-Segura1, Luis V. Rodríguez-Durán2, Gerardo Saucedo-Castañeda3, and José de Jesús Cázares-Marinero4
1 Department of Chemical, Industrial and Food Engineering, Universidad Iberoamericana, Mexico City, Mexico
2 Multidisciplinary Academic Unit Mante, Universidad Autónoma de Tamaulipas, Tamaulipas, México
3 Biotechnology Department, Universidad Autónoma Metropolitana-Iztapalapa, Mexico City, Mexico
4 Department of Research & Development, Polioles, S.A. de C.V, Mexico
2.1 Biosurfactants Nature and Classification
Biosurfactants (BS) are amphiphilic molecules that, as any surfactant, consist in two contrasting moieties: (i) one hydrophilic part showing affinity to polar substances like water, and (ii) one lipophilic displaying attraction to non‐polar media such as oil and fats (Figure 2.1) [1]. The prefix “bio” refers to the fact that these surfactants are exclusively produced through biochemical transformations within microorganisms under certain conditions; this is why BS are also called microbial surfactants [2]. The adjustable binary hydrophilic–lipophilic feature confers surfactants appealing properties for applications in all types of industrial sectors.
Surfactants can be categorized according to their (i) origin, (ii) electrostatic status, and (iii) the ratio of their hydrophilic and lipophilic components (Table 2.1). In the first category, surfactants can be seen as synthetic, semi‐synthetic, and BS. Synthetic surfactants are those that have been manufactured on a large scale for many years from raw materials of petrochemical origin. Semi‐synthetic surfactants were introduced to the market to offer less hazardous alternatives to conventional surfactants. These are constituted by lipophilic fragments coming from biocompatible motifs like fatty esters, acids, or alcohols extracted from natural oils (oleum), ergo these surfactants are also called oleochemical surfactants. Nevertheless, further steps of conventional synthesis are always required to afford finished products (semi‐synthetic = half‐synthetic). Currently, it is estimated that >27% of consumed surfactants are oleochemicals. Europe and North America have dominated this particular market, and the increased consumption in recent years can be associated with the interest in products for personal care, home care, and agriculture.
In an attempt to solve relevant drawbacks related to low biodegradability profiles of synthetic surfactants, pollution of traditional chemical processes, and sacrifice of natural resources, new sustainable approaches have recently emerged, implying biotechnological means for surfactant obtention. In this context, BS are known to be secondary metabolites of living organisms just as bacteria and yeast and to be involved in some microbial physiological functions, e.g. cell development, biofilm formation, and osmotic pressure regulation. Furthermore, these play a key role in microbial survival in front of adverse situations such as lack of nutrients and the need to incorporate non‐soluble substances like hydrocarbons as the only carbon source [2]. These natural phenomena have been exploited by bioengineering in productive microbial fermentation, and since BS are achieved through complex enzymatic reactions, such biomolecules are endowed of robust chemical architectures, which are tunable from elegant processes of molecular biology, genetic engineering, or careful selection of culture conditions.
In the second category, surfactants are normally grouped according to the presence or absence of formal electrostatic charges in the hydrophilic moiety of the molecule. Thus, cationic surfactants contain formal positive charges (+) on the polar head of the surfactant molecule and are systematically escorted by negative counterions that neutralize the charges. Anionic surfactants have negative formal charges (−), an example of this latter is represented in Figure 2.1. The third class of ionic surfactants is those species comprising both positive and negative charges in the same body (±). These species are known as inner salts or zwitterions (from German zwitter = hybrid). The last case of this classification is the absence of formal charges in the surfactant molecule, so these are called nonionic surfactants.
Figure 2.1 3D chemical structure of lauryl sulfate as an example of a surfactant molecule. The hydrophilic head of sulfate (SO42−) is surrounded by water molecules, while the lipophilic tail avoids any contact with water.
Source: Based on [1].
