Green Chemistry and Biosurfactant Research - Biosurfactants: Research Trends and Applications - page 8

BIOSURFACTANTS
Microbial surface-active compounds are a group of structurally diverse molecules
produced by different microorganisms and are mainly classified by their chemical
structure and their microbial origin. They are made up of a hydrophilic moiety,
comprising an acid, peptide cations, or anions, mono-, di-, or polysaccharides, and
a hydrophobic moiety of unsaturated or saturated hydrocarbon chains or fatty acids.
These structures confer a wide range of properties, including the ability to lower
surface and interfacial tension of liquids and to form micelles and microemulsions
between two different phases. These compounds can be roughly divided into two
main classes (Neu, 1996): low-molecular-weight compounds called biosurfactants,
such as lipopeptides, glycolipids, and proteins, and high-molecular-weight poly-
mers of polysaccharides, lipopolysaccharide proteins, or lipoproteins that are col-
lectively called bioemulsans (Rosenberg and Ron, 1997) or bioemulsifiers (Smyth
et al., 2010). The best studied microbial surfactants are glycolipids. Among these,
the best-known compounds are rhamnolipids, trehalolipids, sophorolipids, and
mannosylerythritol lipids (MELs). Rhamnolipid production by Pseudomonas spe-
cies has been extensively studied, and potential applications have been proposed
(Maier and Soberón-Chávez, 2000). Rhamnolipids from Pseudomonas aerugi-
nosa are currently commercialized by Jeneil Biosurfactant, USA, mainly as a fun-
gicide for agricultural purposes or an additive to enhance bioremediation activities
(Banat et al., 2010).
PRODUCTION AND OPTIMIZATION
BSs are amphiphilic compounds produced on living surfaces, mostly microbial cell
surfaces, or excreted extracellularly. Microorganisms utilize a variety of organic
compounds as the source of carbon and energy for their growth. When the carbon
source is an insoluble substrate like a hydrocarbon (C x H y ), microorganisms facili-
tate their diffusion into the cell by producing a variety of substances, the BSs. Some
bacteria and yeasts excrete ionic surfactants that emulsify the C x H y substrate in the
growth medium. Some examples of this group of BS are rhamnolipids, which are
produced by different Pseudomonas sp., or the sophorolipids that are produced by
several Torulopsis sp. Some other microorganisms are capable of changing the struc-
ture of their cell wall, which they achieve by synthesizing lipopolysaccharides or
nonionic surfactants in their cell wall. These are lipopolysaccharides, such as emul-
san, synthesized by Acinetobacter sp. and lipoproteins or lipopeptides, such as sur-
factin and subtilisin, produced by Bacillus subtilis . Microorganisms also produce
surfactants that are in some cases a combination of many chemical types: referred to
as the polymeric microbial surfactants.
Several developments in the optimization of culture conditions and downstream
processing have been published recently. The use of agroindustrial by-products has
been reported for both yeasts and bacteria (Makkar and Cameotra, 2002). Sobrinho
et al. (2008) used groundnut oil refinery residues and corn steep liquor as substrates
for anionic glycolipid production by Candida sphaerica , while the biosynthesis of
glycolipids by P. aeruginosa was obtained using cashew apple juice as substrate