Enhanced production of yeast biosurfactant sophorolipids using yeast extract or the alternative nitrogen source soybean meal

Microbial-derived biosurfactants hold great promise in biorefinery applications, yet their widespread use is hindered by high production costs and limited yields. This study aimed to address this challenge by investigating the use of low-cost nitrogen sources, specifically soybean meal or yeast extract, to enhance the production of sophorolipids (SLs). In a significant breakthrough, this research uncovered a gene cluster responsible for SL biosynthesis in the non-pathogenic yeast cell factory, Starmerella riodocensis. Culture optimisation using the Box–Behnken Design (BBD) further boosted SL production. Cultivating these yeasts with 1% yeast extract or soybean meal at pH 4.5 in flasks resulted in impressive SL yields of 26.41 or 22.34 g/L for S. riodocensis GT-2564R and 21.40 or 19.91 g/L for Starmerella bombicola BCC5426, respectively. Upon upscaling to 5 L bioreactors using soybean meal as an alternative nitrogen source, both strains demonstrated remarkable improvements, with SL yields reaching 36.6 to 40.82 g/L and high productivity of 0.22–0.24 g/L/h. This enhancement correlated with the upregulated expression of genes encoding key enzymes involved in SL biosynthesis. Chromatographic analysis of SLs revealed the presence of acidic and lactonic forms, along with various derivatives. Minimal surface tensions and desirable critical micelle concentrations indicated excellent biosurfactant properties. In conclusion, the utilisation of cost-effective substrates such as soybean meal shows great potential for advancing industrial-scale production and commercialisation of biosurfactants.