Conversion of cardoon crop residues into single cell oils by Lipomyces tetrasporus and Cutaneotrichosporon curvatus: process optimizations to overcome the microbial inhibition of lignocellulosic hydrolysates

In the present work, the production of Single Cell Oils (SCOs) from undetoxified cardoon hydrolysates (UCH) was investigated. The raw material was pretreated by acid catalyzed steam explosion at 195 °C for 7.5 min and hydrolysed at high dry matter content to achieve a final hydrolysate containing 90 g L-1 glucose and 9 g L-1 xylose. The different ability of Cutaneotrichosporon curvatus and Lipomyces tetrasporus to overcome the toxic effect of the biomass degradation by-products, mainly furan derivatives, in different growth phases was investigated in different set-ups conditions including four hydrolysates concentrations, (glucose and xylose respectively 30 + 3, 45 + 4, 60 + 6, and 90 + 9 g L-1) and two fermentation modes, batch and fed batch. The results indicated that the inoculum age corresponded to a different metabolism of the furanic aldehydes only in L. tetrasporus. C. curvatus was severely inhibited by UCH in any growth phase. L. tetrasporus cells in the stationary phase (SP) grew in UCH at any concentration, while cells in the exponential phase (EP) grew only at the highest dilution rate. The maximum lipids yields and lipid cell content of 20% and 47% respectively (7.1 g L-1 lipids) were achieved in batch mode using SP-inocula and the highest dilution ratio. The dominant fatty acid was oleic acid (59%). The scale-up to the 2 L bioreactor, in fed-batch mode, increased the lipids concentration up to 12.7 g L-1 and the productivity up to 0.135 g L-1 h-1.