Development of continuous cultivation process for oil production through bioconversion of minimally treated waste streams from second-generation bioethanol production

BACKGROUND: Efficient conversion of inhibitor containing pentose rich stream into bioethanol or biogas is still challenging due to several technological challenges; however, this stream can be efficiently assimilated into lipids by oleaginous yeasts such as Rhodotorula. In this work, a continuous and nonsterile process was developed for bioconversion of waste streams into lipids for biodiesel using growth-inhibitor-tolerant oleaginous yeast. RESULTS: An inhibitor-tolerating property of the strain was used to develop a cyclic and continuous near axenic cultivation process in a 2 L nonsterile bioreactor to enhance valorization of waste streams and to reduce energy requirements for media and vessel sterilization. After media optimization, continuous cultivation resulted in significant increase in biomass (14.90 g L ⁻¹ day ⁻¹ ) and lipid productivity (6.15 g L ⁻¹ day ⁻¹ ) as compared to flask (biomass ∼3 g L ⁻¹ day ⁻¹ and lipid productivity ∼0.6 g L ⁻¹ day ⁻¹ ) results. Palmitic acid (C16:0) and oleic acid (C18:1) were the major fatty acids, accounting for almost 80% of total lipids, thus making lipid produced in the process suitable for biodiesel production. CONCLUSION: Inhibitors present in waste streams successfully maintained near axenic conditions during the cultivation. Maximum dilution rate of 1 was achieved after media optimization, which resulted in complete valorization of nutrients into biomass and lipids with high productivity.