Production of Biosurfactant by Streptomyces luridus So3.2 Using Commercial and Recycled Frying Oils in a Stirred-Tank Bioreactor

Abstract

Biosurfactants are sustainable alternatives to petroleum-derived surfactants, yet their industrial application is often constrained by production costs and process efficiency. This study aimed to evaluate biosurfactant production by the psychrotolerant Antarctic bacterium Streptomyces luridus So3.2 using low-cost recycled frying oils under mild cultivation conditions and to validate the process in stirred-tank bioreactors. Cultivation conditions were optimized using response surface methodology (RSM), and the same optimized pH and carbon source concentration were subsequently applied in stirred-tank reactors at laboratory scales of 2 L and 20 L, while aeration (0, 0.5 and 0.9 vvm) and agitation speed (100, 150, and 200 rpm) were evaluated as reactor-operational variables. Filtered and centrifuged recycled frying oil yielded the highest biosurfactant performance (emulsification indices exceeding 80%, enhanced oil displacement, and surface tension values below 40 mN m−1) compared to commercial oils. Biosurfactant production was growth-associated, with detectable surface activity within the first 24 h. RSM identified optimal cultivation parameters at pH 8.0, 3% (w/v) inoculum, and 2% (w/v) oil concentration. The biosurfactant exhibited a critical micelle concentration (CMC) of 29 mg L−1 and a critical micelle dilution (CMD) of 43.2, yielding an estimated broth concentration of 1.25 g L−1. At the 2 L reactor scale, moderate aeration (0.5 vvm) combined with intermediate agitation (150 rpm) preserved high surface activity, yielding emulsification indices above 83%, oil displacement halos of 12.5 cm, and surface tension values as low as 35.5 mN m−1. This performance was also maintained during validation at 20 L. FTIR and TLC analyses indicated lipid- and peptide-associated functional groups. These findings were further complemented by HPLC, MALDI-TOF MS and genome-mining analyses (antiSMASH), revealing a complex molecular profile and multiple NRPS/NRPS-like biosynthetic gene clusters, supporting the interpretation of the recovered product as a putative lipopeptide-associated surface-active extract. Overall, this work demonstrates the feasibility of producing a biosurfactant-associated surface-active extract from recycled frying oils using S. luridus So3.2 under mild conditions.