Utilization of palm sludge oil for rhamnolipid biosynthesis by Pseudomonas aeruginosa USM-AR2 in a stirred tank reactor

The study focused on rhamnolipid production by batch fermentation of Pseudomonas aeruginosa USM-AR2 in a 3-L stirred-tank reactor (STR) using palm sludge oil (PSO) as the sole carbon source. The impact of various agitation rates towards the dispersion of PSO in the medium was evaluated to improve biomass growth and rhamnolipid production. A mechanical foam collection and recycling system was designed and retrofitted to the STR to overcome severe foam formation during fermentation. The maximum biomass produced was 11.29 ± 0.20 g/L obtained at 400 rpm, while the maximum rhamnolipid production was 5.06 ± 1.17 g/L at 600 rpm, giving a rhamnolipid productivity of 0.023 g/L/h. High agitation enhances substrate availability by breaking the hydrophobic semi-solid PSO into smaller substrate particles, increasing surface contact area, thus facilitating the PSO utilisation by P. aeruginosa USM-AR2, thereby inducing rhamnolipid production. This study further demonstrates the ability of rhamnolipid to solubilize and disperse sludge oil, which typically remains a solid at room temperature, in the liquid medium. GCMS analysis showed that five fatty acids, namely palmitic acid, myristic acid, stearic acid, methyl ester and linoleic acid, have been utilised. The rhamnolipid showed an oil spreading test result of 160 mm of waste engine oil displacement compared to control using distilled water that remained non-displaced, and a critical micelle concentration (CMC) of 17 mg/L. In emulsification index (E24) assay, the rhamnolipid was shown to emulsify toluene (66.7% ± 7.2), waste engine oil (58.3% ± 7.2), kerosene (41.8% ± 4.8) and n-hexane (33.1% ± 5.7). UPLC analysis on rhamnolipid revealed a congener mixture of rhamnolipid, namely di-rhamnolipid and mono-rhamnolipid mixture. This is the first report on the employment of an integrated foam control reactor system with PSO as the carbon source for rhamnolipid production by P. aeruginosa USM-AR2 culture.