High-Cell-Density Yeast Oil Production with Diluted Substrates Imitating Microalgae Hydrolysate Using a Membrane Bioreactor

Abstract

Microbial oil production from industrial waste streams and biogenic resources, such as biomass hydrolysates, is emerging as a sustainable alternative to use of fossil and vegetable oils. However, the carbon sources of these substrates are typically diluted, leading to low product concentrations and, therefore, high fermentation and downstream processing costs. In this study, high-cell-density yeast oil production with a defined medium, which imitated the sugar composition of a diluted substrate, a typical microalgal biomass hydrolysate, is carried out on a 50 L scale using a membrane bioreactor (MBR) consisting of a microfiltration unit suited for industrial application. The process was run on a semi-continuous mode to reduce operational costs. Oleaginous yeast Cutaneotrichosporon oleaginosus was used as a biocatalyst and lipid production was induced by phosphate deficiency in the medium with a C/P ratio of 3515 g g−1. In this way, high cellular lipid contents of up to 76.5% (w/w) of dry cell mass, an average lipid yield of 32% (w/w), and a lipid space–time yield (STY) of up to 8.88 g L−1 d−1 were achieved with final high cell densities of up to 116 g L−1 dry biomass. Furthermore, use of a defined medium and elemental analysis of the yeast cells and yeast oil enabled drawing an accurate carbon mass balance of the production system. Carbon conversion efficiencies—fraction of total carbon supplied in the form of sugars converted into lipids at the end of the process—of up to 61.5% were achieved from diluted substrates using the MBR with total cell retention. Considering these results, it is concluded that utilization of an MBR on a semi-continuous mode would be very reasonable for yeast oil production, enabling high productivities with diluted sugar substrates.