## 1 Introduction  
Petroleum hydrocarbons are common environmental pollutants of concern. They contaminate soil and water primarily during oil spills and eventually enter the food web. Remediation of hydrocarbon-contaminated soils conventionally is achieved through biological degradation for moderate contamination, or through physico-chemical soil-washing techniques [1]. Surfactants aid in the removal of pollutants adsorbed to the soil by the stimulation of mass transport of these pollutants from the soil to the aqueous phase. This is achieved by the reduction of surface tension and micellar solubilisation, emulsification of liquid pollutant and facilitated transport [2]. Due to the adsorption of the molecules of surfactants on the soil contaminant surface, a repulsion occurs between the head groups of the surfactants and the soil particles. This facilitates the removal of contaminant molecules from the soil surface in extractive soil remediation [3, 4]. Regulations increasingly restrict the use of chemical agents for soil/water recovery [5], favouring more sustainable green remediation approaches. The natural biodegradation of hydrocarbons in soil has been reported to be remarkably enhanced by the in situ production of biosurfactants derived from soil microorganisms or their addition [2, 6]. Biosurfactants may be used as surface-active soil-cleaning agents that are more environmentally friendly for physico-chemical soil washing. A variety of bacteria, yeast and filamentous fungi produce amphiphilic biological compounds, known as biosurfactants, extracellularly or as part of their cell membrane [7; 8; 9; 10]. They can be of low molecular weight such as glycolipids or lipopeptides, or of high molecular weight such as exoellular polymeric surfactants. The latter include polysaccharides, lipopolysaccharide, proteins, lipoproteins or complexes of these biopolymers [11]. Sophorolipids form a class of glycolipid biosurfactants isolated from non-pathogenic yeasts such as _Candida bombicola_. The non-pathogenic nature of sophorolipids and the very high yield of over 400 g/L that can be achieved nowadays is increasingly making it a more attractive alternative over petroleum-based surfactants [12; 13]. The two yeasts most commonly studied for their ability to produce sophorolipids are _Candida bombicola_ and _Candida lipolytica_[14]. The remarkable differences between structural classes of sophorolipids are the basis of the wide variation in their physicochemical properties. For example, the sophorolipids synthesized by _Candida bombicola_ actually are a mixture of related molecules with different chain length, saturation and hydroxylation in the fatty acid part and different patterns of lactonisation and acetylation [12]. These compounds are readily biodegradable and have low cytotoxicity levels [15], properties that are very favourable for application in soil contaminant remediation. At present, the expenses associated with biosurfactant-based soil remediation are high. Production cost of sophorolipids are in the 2-5 \(\upepsilon\)/kg range, which is higher than that of ethoxylate or alkyl polyglycoside (USS 1-3/kg) [16]. The cost of biosurfactant production has to be brought down to \(\upepsilon\)1.70/L to outperform chemical surfactants in the market [17]. More economical ways of biosurfactant production must be explored, as this is the primary hurdle in the way of their commercialisation. Until now, rhamnids and surfactin, a cyclic lipopeptide, are the only commercially available biosurfactants [18]. Cheaper substrates such as dairy and distillery wastes, and sugarcane molasses, could be used for the biosurfactant production process. Also recycling of surfactants is considered to reduce the overall costs of remediation [19]. In the year 2012, the total production of surfactants was approximately 12 million tons, of which biosurfactant production had a share of 3.5 million tons [20]. Patents for some biosurfactant producing microorganisms such as _Candida bombicola_ ATCC2214, have also been issued in recent years [21].