Production, Characterisation and Applications of Biosurfactants-Review
Pattanathu K.S.M. Rahman
Biosurfactants are surface active compounds
released by microorganisms. They are biodegradable non-toxic and ecofreindly
materials. In this review we have updated the information about different
microbial surfactants. The biosurfactant production depends on the fermentation
conditions, environmental factors and nutrient availability. The extraction
of the biosurfactants from the cell-free supernatant using the solvent
extraction procedure and the qualitative and quantitative analysis has
been discussed with appropriate equipment details. The application of
the biosurfactant includes biomedical, cosmetic and bioremediation. Rhamnolipid
biosurfactants produced by Pseudomonas aeruginosa DS10-129 showed
significant applications in the bioremediation of hydrocarbons in gasoline-spilled
soil and petroleum oily sludge. Rhamnolipid biosurfactant enhanced the
bioremediation process by releasing the weathered oil from the soil matrices
and enhanced the bioavailability of hydrocarbons for microbial degradation.
It is having potential applications in the remediation of hydrocarbon
Biosurfactants are amphiphilic biological compounds produced
extracellularly or as part of the cell membrane by a variety of yeast,
bacteria and filamentous fungi (Chen et al., 2007; Mata-Sandoval
et al., 1999, 2000) from various substances including sugars, oils
and wastes. However, carbohydrates and vegetable oils are among the most
widely used substrates for research on biosurfactant production by Pseudomonas
aeruginosa strains (Rahman et al., 2002a, b, 2003; Raza et
al., 2007). The amphiphiles that form micelles can be potentially
used for surface chemical works, are termed as SURFace ACTive AgeNTS or
SURFACTANTS. Soaps and detergents can be described as having similar characteristics
as surfactants. All surfactants have two ends namely, a hydrocarbon part
which is less soluble in water (hydrophobic end). The hydrophobic part
of the molecule is a long-chain of fatty acids, hydroxy fatty aids, hydroxyl
fatty acids or α-alkyl-β-hydroxy fatty acids. The water soluble
end (hydrophilic) can be a carbohydrate, amino acid, cyclic peptide, phosphate,
carboxylic acid or alcohol. Additionally, the hydrophobic moiety is usually
a C8 to C22 alkyl chain or alkylaryl that may be linear or branched (Van
The unique properties of biosurfactants allow their use
and possible replacement of chemically synthesised surfactants in a number
of industrial operations (Kosaric, 1992). Biosurfactants reduce surface
tension, Critical Micelle Concentration (CMC) and interfacial tension
in both aqueous solutions and hydrocarbon mixtures (Rahman et al.,
2002c, d; Banat, 1995).
GENERAL CLASSIFICATION OF BIOSURFACTANTS
Surfactants can be classified according to the nature
of the charge on individual polar moiety. Anionic surfactants are negatively
charged usually due to a sulphonate or sulphur group. Non-ionic surfactants
lack ionic constituent and the majority of all non-ionics are polymerisation
products of 1, 2-epoxyethane. Cationic surfactants are characterised by
a quaternary ammonium group which is positively charged. Lastly, amphoteric
surfactants have both positively and negatively charged moieties in the
same molecule (Van Ginkel, 1989). Biosurfactants can also be grouped into
two categories namely; low-molecular-mass molecules with lower surface
and interfacial tensions and high-molecular-mass polymers, which bind
tightly to surfaces (Rosenberg and Ron, 1999). Examples of low-molecular-mass
molecules are rhamnolipids (Lang and Wullbrandt, 1999; Cohen and Exerowa,
2007), sophorolipids (Davila et al., 1997) whilst food emulsifiers
(Sheperd et al., 1995) and biodispersan (Rosenberg, 1993) are some
of the examples of high-molecular-mass polymers.
|| Type and microbial origin of biosurfactants (Mulligan,
Various micro-organisms are known to produce specific
kind of biosurfactants. This depends on mainly the molecular composition
of the type of biosurfactant produced. For instance, Pseudomonas aeruginosa
DS10-129 was used to produce rhamnolipid (Rahman et al., 2002a,
b, 2003), sophorose lipid by Torulopsis bombicola and Bacillus
subtilis ATCC 2132 which was used by Davis et al. (2001) to
produce surfactin. Kosaric (1992) classified biosurfactants based on their
structure namely; hydroxylated and cross-linked fatty acids, polysaccharide-lipid
complexes, glycolipids, lipoproteins-lipopeptides, phospholipids and complete
cell surfaces. On the other hand, Biermann et al. (1987) group
biosurfactants as glycolipids, lipopeptides, phospholipids, fatty acids,
neutral lipids, polymeric and particulate compounds (Table
1). Lastly, Healy et al. (1996) group biosurfactants into four
main categories namely, glycolipids, phospholipids, lipoproteins/ lipopepetides
TYPES OF BIOSURFACTANTS
There are many types of biosurfactants each produced
by a specific micro-organism. The following are some of the various types
Glycolipids: Most known biosurfactants are glycolipids.
They consist of mono-, di-, tri- and tetrasaccharides which include glucose,
mannose, galactose, glucuronic acid, rhamnose and galactose sulphate.
The fatty acid component usually has a composition similar to that of
phospholipids of the same micro-organism (Veenanadig et al., 2000;
Chen et al., 2007). Also, they are made up of carbohydrates in
combination with long-chain aliphatic acids or hydroxyaliphatic acids
(Desai and Banat, 1997). Among the glycolipids, the best known are the
rhamnolipids, trehalolipids and sophorolipids (Desai and Banat, 1997;
Karanth et al., 1999) and the best-studied glycolipid bioemulsifiers,
rhamnolipds, trehalolipids and sophorolipids are disaccharides that are
acylated with long-chain fatty acids or hydroxyl fatty acids (Rosenberg
and Ron, 1999).
Rhamnolipids: Bacteria of the genus Pseudomonas
are known to produce glycolipid surfactant containing rhamnose and 3-hydroxy
fatty acids (Lang and Wullbrandt, 1999; Rahman et al., 2002b).
Rhamnolipids produced by Pseudomonas aeruginosa have been widely
studied and reported as a mixture of homologous species RL1 (RhC10C10),
RL2 (RhC10), RL3 (Rh2C10C10)
and RL4 (Rh2C10) (Syldatk and Wagner, 1987; Lang
and Wagner, 1987; Rahman et al., 2002b). Using virgin olive oil
(Healy et al., 1996), a rhamnolipid was produced by Pseudomonas
fluorescens NCIMB 11712 that is a methyl pentose monosaccharide. Disaccharide
rhamnolipids are formed by condensing two moles of rhamnose sugar and
an acetal group links the hydrophobic group. However, the lipid part of
the molecule contains ester and carboxyl groups. Rhamnolipids produced
by Pseudomonas aeruginosa strains are among the most effective
surfactants when applied for the removal of hydrophobic compounds from
contaminated soils (Rahman et al., 2006). They posses low average
minimum surface tension of (30-32 mN m-1); high average emulsifying
activity of (10.4-15.5 U mL-1 filtrate), low critical micelle
concentration (CMC) (5-65 mg L-1) and high affinity for hydrophobic
organic molecules (Van Dyke et al., 1993).
Sophorolipids: They are group of biosurfactants
produced by Torulopsis sp. Sophorolipids (SLs) consist of a dimeric
sugar (sophorose) and a hydroxyl fatty acid, linked by a β-glycosidic
bond (Asmer et al., 1988). According to Hu and Ju, (2001) there
are two types of SLs namely, the acidic (non-lactonic) SLs and the lactonic
SLs. The hydroxyl fatty acid moiety of the acidic SLs has a free carboxylic
acid functional group whilst that of the lactonic SLs forms a macrocyclic
lactone ring with the 4-hydroxyl group of the sophorose by intramolecular
esterificaion. Until recently, lactonic SLs have been reported to have
attracted more commercial and scientific attention than their acidic counterparts.
They have measurable biocide activity (Lang et al., 1989), whilst
the acetylated lactonic SLs have been applied in cosmetics as antidandruff,
bacteriostatic agents and deodorants (Mager et al., 1987).
Trehalolipids: Another group of glycolipids are
the trehalolipids, the serpentine group seen in many members of the genus
Mycobacterium is due to the presence of trehalose esters on the
cell surface (Asselineau and Asselineau, 1978). Disaccharide trehalose
linked at C-6 and C-6` to mycolic acid is associated with most species
of Mycobacterium, Norcardia and Corynebacterium.
Mycolic acids are long-chain, α-branched-β-hydroxy fatty acids.
Trehalolipids from different organisms differ in the size and structure
of mycolic acid, the number of carbon atoms and the degree of unsaturation
(Desai and Banat, 1997). Trehalose lipids from Rhodococcus erythropolis
and Arthrobacter sp. were found to lower the surface and interfacial
tensions in culture broth from 25-40 and 1-5 mN m-1, respectively
(Li et al., 1984).
Lipoproteins and Lipopeptides: Lipopepetides called
surfactin are produced by Bacillus sp. containing seven amino acids
bonded to a carboxyl and hydroxyl groups of a 14-carbon acid. Surfactin
just as any other biosurfractant reduces surface tension from 72-27 mN
m-1 with concentrations as low as 0.005%, making surfactin
one of the most powerful biosurfactants (Kakinuma et al., 1969).
The cyclic lipopeptide surfactin produced by Bacillus subtilis
ATCC 21332 is an example of one of the most powerful biosurfactants. Another
important characteristic of surfactin is its ability to lyse mammalian
erythrocytes and to form spheroplasts (Bernheimer and Avigad, 1970). This
property is been used to detect surfactin production through haemolysis
on blood agar.
Fatty acids: Fatty acids produced from alkanes
as a result of microbial oxidations have been considered as surfactants
(Rehn and Reiff, 1981). In addition to the straight-chain acids, micro-organisms
produce complex fatty acids containing OH groups and alkyl branches. Examples
of such complex acids include Corynomucolic acids that are also surfactants
(Kretschner et al., 1982). The hydrophilic or lipophilic balance
of fatty acids is clearly related to the length of the hydrocarbon chain.
For lowering surface and interfacial tensions, the most active saturated
fatty acids are in the range of C12-C14 (Rosenberg and Ron, 1999).
Phospholipids: Phospholipids are known to form
major components of microbial membranes. When certain hydrocarbon-degrading
bacteria or yeast are grown on alkane substrates, the level of phospholid
increases greatly. For instance, using hexadecane-grown Acinetobacter
sp. HO1-N, phospholipids (mainly phosphatidylethanolamine) rich vesicles
were produced (Kaeppeli and Finnerty, 1979). Phospholipids have been quantitatively
produced from Thiobacillus thiooxidans that are responsible for
wetting elemental sulphur necessary for growth (Beeba and Umbriet, 1971).
Phosphatidylethanolamine produced by Rhodococcus erythropolis
grown on n-alkane resulted in the lowering of interfacial tension between
water and hexadecane to less than 1 mN m-1 and CMC of 30 mg
L-1 (Kretschner et al., 1982).
Polymeric biosurfactants: Emulsan, liposan, mannoprotein
and polysaccharide-protein complexes are known to be the best-studied
polymeric biosurfactants (Desai and Banat, 1997). Using Acinetobacter
calcoaceticus RAG-1, Rosenberg et al. (1979) extracted a potent
polyanionic amphipathic heteropolysaccharide bioemulsifier called emulsan.
It is a very effective emulsifying agent for hydrocarbons in water even
at a concentration as low as 0.001-0.01%. Additionally, it is noted as
one of the most powerful emulsion stabilizers known with the ability to
resist inversion even at a water-to-oil ratio of 1:4 (Zosim et al.,
1982). Ciriglian and Carman (1984) synthesised liposan, an extracellular
water-soluble emulsifier using Candida lipolytica. It is composed
of 83% carbohydrate and 17% protein with the carbohydrate portion being
a heteropolysaccharide consisting of glucose, galactose, galactosamine
and galactoronic acid. Cameron et al. (1988) demonstrated the production
of large amounts of mannoprotein by Saccharomyces cerevisiae. When
purified, the emulsifier contains 44% mannose and 17% protein. The mannoprotein
exhibited excellent emulsifying activity toward several oils, alkanes
and organic solvents. Other polymeric biosurfactants such as biodispersan,
alasan, food emulsifiers, protein complexes and insectides emulsifiers
have also been reported.
Biosurfactants are usually produced extracellularly or
as part of cell membrane by yeast, bacteria or filamentous fungi (Mata-Sandoval
et al., 1999). Different kinds of bacteria have been employed
by many researchers in producing biosurfactant using culture media. Most
of such bacteria used are isolated from contaminated sites usually containing
petroleum hydrocarbon by products and/or industrial wastes (Rahman et
al., 2006; Benincasa 2007).
Factors affecting biosurfactant production: A
number of factors affect the production of biosurfactants. These factors
include environmental factors as well as source of carbon substrate among
Environmental factors: Biosurfactant production
like any other chemical reaction is affected by a number of factors that
either increase its productivity or inhibit it. Accordingly, environmental
factors such as pH, salinity and temperature affect biosurfactant production
(Rahman et al., 2002b; Ilori et al., 2005; Raza et al.,
2007). During In situ applications, bacteria for Microbially Enhanced
Oil Recovery (MEOR) must be able to grow under extreme conditions encountered
in oil reservoirs such as high temperature, pressure, salinity and low
oxygen level. Additionally, it was found out that biosurfactant produced
from Pseudomonas strains MEOR 171 and MEOR 172 were not affected
by temperature, pH and Ca, Mg concentration in the ranges found in many
oil reservoirs (Karanth et al., 1999). Desai and Banat (1997) also
affirm the fact that environmental factors and growth conditions such
as pH, temperature, agitation and oxygen availability also affect biosurfactant
production through their effects on cellular growth or activity. Salt
concentrations also affect biosurfactant production depending on its effect
on cellular activity. Some biosurfactants however, were not affected by
salt concentrations up to 10% (w/v), although slight reductions in the
CMCs were detected (Abu-Ruwaida et al., 1991).
Carbon substrates for biosurfactant production: A
number of carbon substrates have been used in many researches during biosurfactant
production. Indeed the type, Quality and quantity of biosurfactant production
are affected and influenced by the nature of the carbon substrate (Singer,
1985; Raza et al., 2007). Diesel and crude oil were identified
to be good sources of carbon for biosurfactant production by organisms
(Ilori et al., 2005). Other water soluble compounds such as glucose,
sucrose and glycerol have also been reported to be a source of carbon
substrate for biosurfactant production (Desai and Banat, 1997; Rahman
et al., 2002a). In the treatment of wastewater (Pagilla et al.,
2002) used soluble acetate and sparingly soluble hexadecane as carbon
substrate for Gordonia amarae growth and biosurfactant production
in large scale batch reactors. It has become evident that the importance
of carbon substrates does have a major role to play on the biosurfactant
production. It was noted that carbon sources such as nutrient concentrations,
pH and age of the culture affects the yield of rhamnolipid production.
On a positive note, hydrophobic substrates like corn oil, lard (rich in
unsaturated and saturated fat) and long chain alcohols maximized biosurfactant
production (100-165 mg g-1 substrate). Contrarily, hydrophilic
substrates like glucose and succinate delivered poor yields (12-36 mg
g-1 substrate) (Mata-Sandoval et al., 2000). Lastly,
Robert et al. (1989) attests to the fact that Pseudomonas aeruginosa
can be produced from a variety of carbon sources such as C11 and C12 alkanes,
succinate, pyruvate, citrate, fructose, glycerol, olive oil, glucose and
Estimation of biosurfactant activity: This involves
measuring the changes in surface and interfacial tensions, stabilization/destabilization
of emulsions and hydrophilic-lipophilic balance (HLB). Using a tensiometer,
the surface tension at air/water and oil/water interfaces can be easily
determined. The surface tension of distilled water is noted to be 72 mN
m-1 and an addition of biosurfactant lowers it to as low as
28 mN m-1 (Rahman et al., 2006). Thus adding a biosurfactant
to water reduces its surface tension to a critical level above which amphiphilic
molecules readily form supramolecular structures like micelles, bilayers
and vesicles known as Critical Micelle Concentration (CMC). CMC is therefore
defined as the ability of a biosurfactant within an aqueous phase and
is commonly used to measure the efficiency of a biosurfactant (Desai and
Analytical methods: A number of analytical methods
have been employed by many researchers in their analyses and in some cases
characterisation of biosurfactants. In the Table 2,
the type of biosurfactant, bacteria, solvent, supporting references and
type of analytical method used are shown.
|| Analytical methods used for the qualitative and quantitative
analysis of biosurfactant
|TLC = Thin Layer chromatography; HPLC = High Performance
Liquid Chromatography; FTIR = Fourier transform infrared spectroscopy;
GC/MS = gas Chromatography with Mass Spectroscopy
APPLICATIONS OF BIOSURFACTANT
A number of applications of biosurfactants have been
researched into and published. Its usefulness to man in most aspects of
human life can not be over emphasised. The enormous market demand for
surfactants is currently met numerous synthetic, mainly petroleum-based
chemical surfactants. These compounds are usually toxic to the environment
and as well as been non-biodegradable. Furthermore, they may bio-accumulate
and their production, processes and by-products can be environmentally
hazardous. It has become necessary that tightening environmental regulations
and increasing awareness for the need to protect the ecosystem have effectively
resulted in an increasing interest in biosurfactants as possible alternates
to chemical surfactants (Banat et al., 2000; Benincasa, 2007).
Biosurfactants are beginning to acquire a status as potential performance-effective
molecules in various fields. Presently, biosurfactants are mainly used
in studies on enhanced oil recovery and hydrocarbon bioremediation (Rahman
et al., 2004, 2006). The worldwide production of surfactants amounted
to 17 million metric tonnes (t) in 2000 (including soaps) with expected
future growth rates of 3-4% year-1 globally and 1.5-2.0% in
the EU (Whalley, 1995). Industrial applications of surfactants are classified
according to how they are applied. These are surfactants used in detergents
and cleaners (54%); as auxiliaries for textiles, leather and paper (13%);
in chemical processes (10%); in cosmetics and pharmaceuticals (10%); in
the food industry (3%); in agriculture (2%) and in others (8%).
Biosurfactants and bioremediation: Oil spillage
during offshore production (drilling) and its transport from one location
to another is seriously affecting aquatic life. An explicit example is
the massive oil spillage as well as release during the Gulf War from 1991
to 1992. It was estimated that some 11 million barrels of oil was released
into the Arabian Gulf from January to May 1991, polluting more than 800
miles of Kuwait and Saudi Arabian coastline. The cost of clean-up has
been estimated at more than $700 million. The oil released in to the Gulf
produced devastating consequences on the marine wildlife of the area,
including endangering hawksbill and green turtles, thousands of cormorants
(a type of marine bird) as well as 400-500 tons of fishes died in the
Gulf as a result of exposure to oil or polluted water. Additionally, (Shaw,
1992; Burns et al., 1993; Burger, 1993) it was identified that
several oil pollution accidents at high seas and on beaches have resulted
in enormous ecological and social catastrophes. Rahman et al. (2003,
2004, 2006) examined the bioremediation of n-alkanes in petroleum sludge
containing an oil and grease content of 87.4%. Remarkably, 10% of the
sludge constituting C8-C11 alkanes were degraded 100%; whilst C12-C21,
83-98%; C22-C31 between 80-85% and finally C32-C40, 57-73% after 56 days
with addition of a bacterial consortium, nutrients and rhamnolipids. In
another experiment, (Hayes et al., 1986) demonstrated that when
Boscan Venezuelan heavy crude oil was treated with emulsan, oil viscosity
was reduced from 200,000 to 100 Cp. Hence, it became visible to pump heavy
oil 26,000 miles in a commercial pipeline after this treatment although
conventional chemical surfactant treatment failed. Biosurfactants are
also used in bioremediation of sites contaminated with toxic heavy metals
like uranium, cadmium and lead (Miller, 1995; Mulligan and Wang, 2006).
Shafeeq et al. (1989) showed that hexadecane, octadecane and nanodecane
incubated for a 28 day period under laboratory conditions with the Pseudomonas
aeruginosa isolate S8 obtained from oil-polluted sea water degraded
the hydrocarbons by 47, 58, 73 and 60%, respectively. The application
of rhamnolipid produced by Pseudomonas aeruginosa DS10-129 along
with poultry litter and coir pith enhanced ex situ bioremediation
of a gasoline-contaminated soil (Rahman et al., 2002a). Benzene,
toluene, ethylbenzene, xylene and trimethylbenzene were degraded according
to Kosaric (2001) by adding microbial consortium to soil contaminated
with gasoline and enriched with nutrients and oxygen.
Other biosurfactant applications: Biosurfactants
have also been applied in food industries usually as food additives (emulsifiers).
For instance, lectin and its derivatives, fatty acid esters containing
glycerol, sorbitan or ethylene glycol and ethoxylated derivatives of monoglycerides
including recently synthesized oligopeptide (Bloomberg, 1991). These emulsifiers
have a long way to improving the flavour, taste and quality of products
with minimal health hazards. The agriculture industry has also benefited
from the production of biosurfactants. Stanghellini and Miller (1997)
demonstrated that rhamnolipids are highly effective against three representative
genera of zoosporic plant pathogens; Pythium aphanidermatum, Phytophthora
capsici and Plasmopara lactucea-radicis. Hence, purified mono-and
di-rhamnolipids with concentrations ranging from 5-30 mg L-1
caused cessation of motility and lysis of the entire zoospore population
in less than 1min. Bioemulsifiers are potentially used in various formulations
of herbicides and pesticides (Rosenberg and Ron, 1999). An example is
the use of bioemulsifiers (glycolipopeptides) produced by strains of Bacillus
for emulsifying immiscible organophosphorus pesticides (Patel and Gopinathan,
1986). Biosurfactant applications in cosmetic and pharmaceutical industries
have also been reported (Cameotra and Makkar, 2004).
BIOSURFACTANT AND CO2 EMISSIONS
Greenhouse effect is a naturally occurring process that
aids in heating the earth`s surface and atmospheric gases such as CO2,
water vapour and methane that are able to change the energy balance of
the planet by absorbing long wave radiation (infra red) emitted from the
earth`s surface. Studies have shown that biosurfactants a have a role
to play in the reduction, if not total elimination of CO2 emission
into the atmosphere. No
||Potential to substitute petrochemical by oleochemical
surfactants in the EU by 2010 (Patel, 2004)
|This table covers only the most important surfactants
while cationic, amphoteric and some of the nonionic surfactants are
excluded, RRM = Renewable raw materials; AE = Alcohol ethoxylate;
AES = Alcohol ether sulphate; AS = Alcohol sulphate; LAS = Linear
alkylbenzene sulphate; Pc = Petrochemical feedstock; PKO = Palm kernel
oil; CNO = Coconut oil; PO = Palm oil; SAS = Secondary alkane sulphonate,
* Containing 7 ethylene oxide (EO) units on average, Average
of 7 EO units based on PKO and CNO and 11 EO units based on PO
wonder the 1997 UNFCCC Kyoto Protocol was adopted to
curtail the emission of greenhouse gases (United Nations Framework Convention
on Climate Change, 1997). Assuming that the total surfactant production
remains constant until 2010 in EU, it was estimated that the amount of
oleochemical surfactants could be increased from about 880 kt in 1998
to approximately 1,100 kt in 2010, an increase of 24%. This substitution
reduces the life- cycle CO2 emissions from surfactants by 8%.
The theoretical maximum potential for total substitution is 37% (Table
3). Since the surfactant market is expected to grow, the avoided emissions
are expected to exceed 8% of the current life-cycle CO2 emissions
from surfactants. Furthermore, in 1998, an estimated 1.5 million tons
of CO2 emissions were avoided by the production of oleochemical
surfactants (Patel, 2004).
MERITS OF BIOSURFACTANTS
Researches have shown that biosurfactants exhibit many
advantages over chemically synthesized surfactants. The following are
some of the advantages of biosurfactants (Kosaric, 1992; Mulligan and
||Biodegradability: Biosurfactants are easily degraded
by bacteria and other microscopic organisms; hence they do not pose
much threat to the environment.
||Generally low toxicity: For instance glycolipids from Rhodococcus
sp. 413A were 50% less toxic than Tween 80 in naphthalene solubilization
tests (Kanga et al., 1997).
||Biocompatibility and digestibility: This ensures their application
in cosmetic, pharmaceuticals and as functional food additives.
||Availability of raw material: Biosurfactants can be produced
from cheap raw materials that are available in large quantities.
||Acceptable production economics: Depending on its application,
biosurfactants can also be produced from industrial wastes and by-products
and this is of particular interest for their bulk production.
||Use in environmental control: Biosurfactants can be efficiently
used in handling industrial emulsions, control of oil spills, biodegradation
and detoxification of industrial effluents and bioremediation of contaminated
||Specificity: Biosurfactants being complex organic molecules
with specific functional groups are often specific in their action.
This would be of particular interest in detoxification of specific
pollutants, de-emulsification of industrial emulsions, specific cosmetic,
pharmaceutical and food applications.
DEMERITS OF BIOSURFACTANTS
Despite the numerous advantages that biosurfactants have
been known to exhibit, it is also known to have the following associate
demerits (Kosaric, 1992).
||Large scale production of biosurfactants may be expensive.
However this problem could be overcome by coupling the process to
utilization of waste substrates, combating at the same time their
polluting effects that balance the overall costs.
||There is difficulty in obtaining pure substances (biosurfactants),
which is of particular importance in pharmaceutical, food and cosmetic
applications. This is because downstream processing of diluted broths
involved that may require multiple consecutive steps.
||Over producing strains of bacteria are rare and those found generally
display a low productivity. In addition, complex media need to be
applied to the sample.
||The regulation of biosurfactant synthesis is hardly understood,
seemingly it represent secondary metabolite regulation. Thus considering
a batch culture, secondary metabolite production begins when the culture
is stressed due to the depletion of a nutrient. This phenomenon is
closely correlated with the transition phase- slow growth rate of
culture and with the morphological changes that this phase implies.
Among others O2-limitation has been described as an essential
parameter to govern biosurfactant production.
||An improvement of the production yield is hampered by the strong
foam formation. Consequently, diluted media have to be applied and
only immobilised systems provide an increased productivity of about
3 gl-1 h-1 (Fiechter, 1992).
This review provides information about the biosurfactant
production by microorganisms. The scale-up of biosurfactants for industrial
production is still challenging. Since the composition of the final products
is affected by the nutrient, micronutrient and environmental factors,
it is obvious to find a right surfactant for industrial scale-up. In this
review we have provided an overview about the availability of various
analytical equipments to detect and quantify the biosurfactant. The requirement
of the purity of the biosurfactants depends on the application, for example
the surfactants used for environmental remediation should be free from
microbial loading but the quality of the product could be compromised.
But for pharmaceutical and cosmetic applications the biosurfactants should
meet the requirement of various regulatory standards. Few organisms in
the indigenous microbial flora are producing biosurfactants in the natural
environment to adapt to various adverse conditions. We are just trying
to exploit the process for the benefit of mankind. Still we need further
understanding of the microbial physiology and genetics of these microorganisms
to harness them for efficient industrial applications.
The authors wish to thank the University Research Fund
and University Enterprise Development Funds to support the biosurfactants
research at the University of Teesside.
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