and yeast. For therapeutic applications, these classes of biosurfactants have the utmost importance. The major cause for the respiration failure in prematurely born children is chiefly due to the deficiency of phospholipid protein complex [57].
4.6.8 Polymeric Biosurfactant
The best‐studied polymeric biosurfactants are emulsan, liposan, mannoprotein, and polysaccharide–protein complexes [26]. For hydrocarbons in water, even at a concentration as low as 0.001–0.01%, emulsan works as a very effective emulsifying agent. With a composition of 83% carbohydrate and 17% protein, liposan behaves as an extracellular water‐soluble emulsifier. Mannoproteins, which contain 44% amnnose and 17% protein, are produced in large quantity by Saccharomyces cerevisiae. For oil spills, organic solvents, and alkanes, these mannoproteins show excellent emulsifying activity.
4.6.9 Particulate Biosurfactants
An essential fragment for the remediation of alkanes by microbes is microemulsion, consisting of extracellular film vesicle segments of hydrocarbon. Protein, phospholipids, and lipopolysaccharides are the key components of vesicles formed by Acinetobacter sp. that have a thickness of 1.158 cg/cm3 and 20–50 nm diameter.
4.7 Factors Influencing Biosurfactants Production
Biosurfactant production and their chemical compositions are influenced by a number of factors. Environmental conditions (temperature, pH, air circulation, divalent cation, and saltiness) as well as the nature of the available energy source in the form of carbon and nitrogen, reaction media composition, and limited nutrient supply have a powerful impact on synthesized biosurfactants.
4.7.1 Environmental Factors
The biosurfactants production is either enhanced or inhibited by the reaction conditions. Therefore, for the large‐scale production of the desired biosurfactants, it is important to constantly upgrade the bioprocess as the item might be influenced by changes in reaction conditions, i.e. temperature, pH, air circulation, or unsettling speed. Most of the synthesized biosurfactants provide their best performance in a temperature range of 25–300 °C. A change in the biosurfactant composition can occur during temperature variations. Zinjarde and Pant [58] in their study on Yarrowia lipolytica illustrated the best biosurfactant production at pH 8.0, i.e. at regular ocean water pH, which is also the natural surrounding pH of Y. lipolytica. Rhamnolipid production by Pseudomonas sp. occurred at its most extreme pH (6–6.5) and decreases above pH 7.
4.7.2 Carbon and Nitrogen Sources for Biosurfactant Production
Scientists have used various carbon compounds as fundamental substrates and energy sources for the production of biosurfactants. The substrate composition, i.e. carbon and nitrogen source, influences the biosurfactant type, quality, and quantity [59]. Ilori et al. [60] in their study identify glucose, sucrose, glycerol, diesel, and raw petroleum as good sources of carbon for biosurfactant production. A nitrogen source is also very crucial for biosurfactant production. Medium containing nitrogen sources fulfills the basic requirement of multiplication and development of biosurfactant‐producing microorganisms for protein and chemical blends. Some of the distinctive sources of nitrogen used for biosurfactant production are ammonium sulfate, ammonium nitrate, sodium nitrate, meat concentrates, and malt extricate, etc.
4.8 Strategies for Commercial Biosurfactant Production
The high production cost, low yield, and sophisticated product recovery restricts widespread application of biosurfactants as compared to chemical surfactants. The inefficient bioprocess engineering, usage of a cost–credit substrate, and poor strain improvement are some of the major drawbacks behind high production costs of biosurfactants. For commercial applications of biosurfactants in various fields, these stated drawbacks should be solved. Therefore, the net economic gain between the production cost and application benefit will govern the future of biosurfactants. For large‐scale industrial production and applications, the links between the production parameters of these molecules, their structure and their functions, need to be optimized. The following proposed strategy can smooth cost‐effective biosurfactant production and application.
4.8.1 Raw Material: Low Cost from Renewable Resources
In their natural environment, the microbial population produces surface‐active agent in extremely minute quantities. Keeping in mind these microbial behaviors, researchers try to maximize the biomolecule yield and extract more and more concentrations of highly efficient biosurfactants.
In lowering the overall production cost of biosurfactants, a selection of suitable and efficient low‐cost raw materials is important. Raw materials with higher concentrations of carbohydrate, nitrogen, and lipids highlight the necessity for biosurfactants in commercial production. Utilization of agricultural wastes and byproduct materials that are available in abundant quantity along with the benefit of reduced environmental pollution chances serve as the best raw material for biosurfactants. In a study conducted by Ashby et al. [61] on the effect of raw materials on biosurfactant cost, the authors found that approximately 75% of the total operating cost accounted for 90.7 million kg of sophorolipid production biosurfactant was due to glucose and oleic acid as the raw materials. The sophorolipid production costs vary depending on the raw material used; for example, when glucose and high oleic sunflower oil were used, the cost was estimated to be $2.95/kg and when glucose and oleic acid were used, it was reported to be $2.54/kg. This estimated high cost of sophorolipids production can be reduced after replacing the costly substrate with a low‐cost industrial and agro‐based byproduct. In another study by Rodrigues et al. [62], authors utilized low‐cost materials for production of biosurfactants and the yields were increased by 1.5 times to that of the original cost and a 60–80% reduction in the medium cost was observed.
4.8.2 Production Process: Engineered for Low Capital and Operating Costs
Currently, only very few biosurfactants have been used in metal ion remediation processes on a commercial scale due to lack of cost‐effective production processes. Due to the high costs of producing biosurfactants, their industrial application has been hindered.
Biosurfactant production on a large scale seemed to be very effective, but there is an urgent need to overcome competitiveness with their synthetic counterparts. Scientists made few attempts to develop large‐scale biosurfactants. The Bacillus subtilis FE‐2 strain has been used by Veenanadig et al. [50] to produce biosurfactants in a packed column bioreactor with a volumetric capacity of 30 l. In another study conducted by Daniels et al. [63] for large‐scale production of rhamnose and 3‐hydroxydecanoic acid from Pseudomonas sp., they claimed, in their patent, to produce in a defined culture medium that contained corn oil as a carbon source a high level of rhamnolipids at a concentration from about 30 g/l to about 50 g/l.
4.8.3 Improved Bioprocess Engineering
Process optimization plays a crucial role in cost reduction of large‐scale biosurfactant production. Synthesis of biosurfactant can be categorized into four foremost types:
1 Biosurfactant production associated with the growth medium and substrate utilization.
2 Under a growth‐limiting condition, biosurfactant production, e.g. P. aeruginosa shows an overproduction of biosurfactants when nitrogen and iron are limited.
3 Resting or immobilized cell utilization in biosurfactant production. This type of biosurfactant production shows high efficiency
