Improvement of biosurfactant production by Rhodococcus ruber IEGM 231 grown on waste cooking oil, their emulsifying activity and antioxidant potential

Abstract

In recent years, the production of microbial biosurfactants, increasingly used in various industries and agriculture, from low-cost renewable substrates is intensively studied. Large volumes of food waste, in particular waste cooking oil, are promising substrates for the production of biosurfactants on an industrial scale. However, studies on the ability of Rhodococcus bacteria, well-known biosurfactant producers, to grow on this substrate and the functional characteristics of synthesized biosurfactants are still scarce. In this study, the possibility of improving biosurfactant production by Rhodococcus ruber IEGM 231 grown on waste cooking oil was investigated using multifactor analysis and response surface methodology (RSM). Using standardized parameters for the concentrations of carbon and nitrogen sources in the fermentation medium, a Pareto diagram was constructed, which shows a direct dependence of the crude biosurfactant yield on the oil concentration, while the concentrations of sugar and inorganic nitrogen salts were found insignificant. Using RSM, the optimal ratio of the medium parameters was determined for the maximum yield of crude biosurfactants: oil (+ 1), sugar (0), NH4- and NO3-containing salts (− 1) in the following concentrations: 5.0 vol%, 2.5 g/L, 0.1 and 0.2 g/L. Crude biosurfactants (40 g/L) obtained from excess waste oil under nitrogen limitation contained three glycolipid fractions, including predominant monoacyl trehalose, previously identified for this strain grown on n-hexadecane, and a considerable amount of residual non-converted acylglycerides. The synthesized biosurfactants produced stable emulsions from petroleum products, food and cosmetic oils and demonstrated high antioxidant capacity in the in vitro ABTS (2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid) radical scavenging test. Overexpression of the fbpB gene encoding mycolyltransferase, a key enzyme of the final stage of trehalolipid assembly was revealed in the cells grown on waste cooking oil, which indicated the induction of the synthesis of glycolipid biosurfactants. These findings open up the possibility of valorization of large-scale food oil waste into value-added biosurfactants using Rhodococcus.