Efficient Production of Acid-Form Sophorolipids from Waste Glycerol and Fatty Acid Methyl Esters by Candida floricola

We discovered that Candida floricola ZM1502 is capable of selectively producing the promising hydrophilic biosurfactants, acid-form sophorolipids (SLs), from glycerol. However, productivity was very low (approximately 3.5 g L^-1) under the initial culture conditions. Here, we describe the design of culture medium for abundant production of acid-form SLs by C. floricola ZM1502 using waste glycerol and hydrophobic substrates in order to develop a method for SL production and disposal of waste glycerol produced by oleo-chemical industries. Urea provided the best nitrogen source for acid-form SL production from glycerol among four nitrogen sources tested [urea, NaNO₃, NH₄NO₃, and (NH₄)₂SO₄]. Among carbon sources we compared, hydrophobic substrates (soybean oil and oleic acid) led to productivities of approximately 20 g L^-1, indicating that hydrophobic substrates provided fatty acid moieties for SL production. Addition of olive oil and oleic acid to waste glycerol enhanced acid-form SL production to 42.1 ± 0.9 and 37.5 ± 3.4 g L^-1, respectively. To develop a potential industrial process, we explored other suitable hydrophobic substrates for SL production, which were obtained on site from oleo-chemical industries. Alkyl C₁₈ esters (Pastell M-182), along with waste glycerol, increased acid-form SL production to 48.0 ± 3.4 g L^-1 over a 7-d period. Furthermore, we demonstrated abundant production of acidic SLs at the mini-jar fermenter scale, obtaining 169 g L^-1 over 180 h using a fed-batch cultivation technique. Efficient acid-form SL production by C. floricola could have a great impact on the development of bio-industrial processes using waste glycerol as a substrate.