Biosurfactants for a Sustainable Future. Группа авторов

and quality of the carbon substrates and other media components (i.e. nitrogen, phosphorus, magnesium, iron, and manganese) mostly determine the type of microbial biosurfactant [16,26–28]. Similarly, microbial growth parameters, i.e. pH, temperature, agitation, and dilution rate, also influence the nature of biosurfactant produced in fermentation [29].

3.4.1 Bacteria

      Bacteria plays an essential role in the biosynthesis of biosurfactants on the industrial scale. Pseudomonas has been reported to be the leading genus, followed by others for biosurfactant production [30]. Pseudomonas nautica, isolated from a Mediterranean coastal area, was reported to produce extracellular biosurfactants with excellent emulsifying behavior [31]. Based on the various kinds of carbon as well as hydrocarbons, microorganisms can yield different types of emulsifiers [32]. A group of researchers [33] proved this practically by providing various hydrocarbons as a carbon source to hydrocarbon‐degrading Pseudomonas fluorescens for the biosynthesis of trehalose lipid‐o‐dialkyl monoglycerides‐protein emulsifier [34].

Bacillus sp. are mostly recognized for the biosynthesis of lipopeptides (lipid connected to a peptide) [35], lichenysin (anionic cyclic lipoheptapeptide biosurfactant), surfactin (bacterial cyclic lipopeptide) [36], lipid–protein complex [37] and subtilisin (a protein‐digesting enzyme) [38]. In 1983, Jenneman et al. [39] documented the application of thermotolerant and halotolerant Bacillus licheniformis JF2 in microbial‐enhanced oil recovery. Similarly, Bacillus brevis and Bacillus polymyxa were documented to produce a large number of cyclic lipopeptides by using agro wastes [40]. Horowitz and Griffin [41] reported that biosurfactant BL‐86, which has been produced by B. licheniformis, is useful for various applications and is capable of remediating heavy metal contaminated soil. BL‐86 is capable of reducing surface tension and emulsification of hydrocarbons.

      There have already been 160 biosurfactants producing bacterial strains identified in soils contaminated with petroleum [12, 42, 43]. Abouseoud et al. [44] reported the potential of P. fluorescens Migula 1895‐DSMZ for biosurfactant production from olive oil. In their study, it was concluded that P. fluorescens Migula 1895‐DSMZ is the primary organism responsible for the production of biosurfactants from various industrial wastes.

      Acinetobacter sp. are readily available in nature and are among the most frequently available marine microbes in ocean habitats [45]. Due to their separate existence, Acinetobacter sp. have received significant attention from many scientists in the last few years. Hydrocarbon degrading Acinetobacter sp. perform a significant role in the bioremediation processes of various hydrocarbons in the natural environment [46]. Choi et al. [47] isolated Acinetobacter calcoaceticus RAG‐1, which produces a commercially important biosurfactant called “emulsan,” from the Mediterranean Sea, by using oil industry waste.

      Rhodococcus sp. are prominent for producing glycolipids like surface‐active molecules [48]. Peng et al. [49] reported the abundance of Rhodococcus erythropolis strain 3C‐9 in Xiamen Island coastal area soil, and they can remediate the oil‐contaminated soil in the area. A few scientists [13, 50] documented the production of various types of biosurfactants (glycolipid, polysaccharides, free fatty acids, and trehalose dicorynomycolate) by R. erythropolis [51] and Rhodococcus sp. [52].

3.4.2 Fungi and Yeast

      Many scholars [53–57] reported the ability of different fungi to produce surfactants by using diverse nutrient sources. Earlier, it was documented that Candida sp. were the most common fungal species used for biosurfactant production compared to other fungi. Dubey et al. [58] outlined the application of Candida bombicola in sophorolipid production by using various carbon sources from industrial byproducts. Yarrowia lipolytica is a well‐known fungus used for the synthesis of bioemulsifiers based on lipid, carbohydrate, and proteins [59]. This polysaccharides bioemulsifier improved the cellular hydrophobicity throughout the developmental stages.

      Yeast‐based biosurfactant research (Candida sp. Pseudozyma sp. and Yarrowia sp.) has gained the growing interest of researchers [60]. The significant advantage of using yeast for biosurfactant production is its GRAS (generally recognized as safe) status, which includes Y. lipolytica, Saccharomyces cerevisiae, and Kluyveromyces lactis [61]. GRAS designated organisms are not toxic or pathogenic so their products can be used in the FMCG and pharma companies. Zinjarde et al. [62] demonstrated that extracellular bioemulsifier production occurs once cells reach a stagnant growth phase. It has been shown that Y. lipolytica NCIM 3589 screened from the ocean sites produces a cell wall‐associated emulsifier (a mixture of carbohydrate, lipid, and protein) using alkanes or crude oil. Fontes et al. [63] described another bioemulsifier type Yansan, produced from the Y. lipolytica wild strain, IMUFRJ 50682, in glucose‐enriched fermentation media of molasses.

Pseudozyma and Torulopis are the second most explored yeasts preceded by Candida for biosurfactant production. Stüwer et al. [64] reported the production of sophorolipids derived from Torulopis apicola on agricultural waste under lab conditions. Similarly, Vacheron et al. [65] also reported sophorolipid production from Torulopis petrophilum under different growth parameters by using waste materials. Morita et al. (2007) reported Candida antarctica and Pseudozyma rugulosa as Glycolipid (Mannosylerythritol lipids) producing yeasts, which have superior vesicle‐producing and surface‐active properties. Sarubbo et al. [66] reported biosurfactant production by using Y. lipolytica strain in the fermentation medium enriched with protein (47%), carbohydrate (45%), and lipids (5%), where glucose was the primary carbon source utilized for biosurfactant production.

3.5 Bacterial Growth Conditions

Biosurfactant production in a bioreactor is a complicated process involving the close monitoring of the conditions involved in it. First of all, it is crucial to choose a suitable culture method (i.e. Continuous, Batch, or Fed‐batch), which depends on the type of organism, product, and type of bioreactor [67–69]. Atlić et al. [70] reported that every culture method has specific biomass kinetics, substrates, or final product or byproduct concentrations (Figure 3.2).

Figure 3.2 Key stages used for bioproduct formation in bioreactor.

      For example, researchers [71] described an advanced method for surfactin production in the stirred‐tank bioreactor with the help of Aureobasidium pullulans LB 83. Their group used a centrally ordered facial model intending to evaluate the impact of aeration levels (0.1–1.1 per min) and sucrose concentrations (20–80 g/l) on total biosurfactant productivity. Their results reported an increase in tensoactivity of 8.05 cm in an oil spreading test and productiveness of 0.0838 cm/h when all parameters were used at high levels.

3.5.1 Continuous Cultures

The continuous cultivation of microbes is one of the methodology of growing significance [72]. This methodology is primarily characterized by constant microbial growth in continuous culture, i.e. a constant rate of growth in a constant environment. In continuous culture, parameters such as pH, substrate concentrations, metabolic products, and oxygen that eventually shift during the “batch cultivation” growth cycle are all constant; however, the experimenter may individually monitor and control them [73–75]. These attributes of the continuous culture method make it a desirable option for research while offering the industrial microbiologist several benefits in terms of more affordable production techniques (Figure 3.3).