Yeast oil production by high cell-density cultivation from rice straw hydrolysate for bio-polyurethane foam making

Abstract

The objectives of this research were to: 1) select the highest-performance yeast for yeast oil (YO) production using xylose as a carbon source, 2) study the influence of carbon source utilization and organic acid supplementation on YO production, 3) optimize the YO production from rice straw hydrolysate (RSH), 4) improve the YO production performance from RSH by fed-batch process, 5) estimate the kinetic of the YO production under various cultural conditions using mathematical models, and 6) examine the feasibility of biopolyurethane (BPU) foam production using crude YO as a feedstock. Firstly, the five yeast strains were cultured in a xylose-based oil production medium (X-OPM) to determine the YO production efficiency and fatty acid (FA) profile using GCMS analysis. Then, the selected yeast strain was cultured in X-OPM at various initial xylose vconcentrations between 10 and 130 g/L. The effect of co-carbon source and acid supplementation on YO production was explored. Subsequently, the Box-Behnken Design (BBD) was conducted to investigate the optimum condition of YO production from RSH by varying concentrations of RSH, (NH4)2SO4, and KH2PO4. Fed-batch processes were performed in feeding modes of RSH-based medium and RSH. Afterward, mathematical modeling was employed to simulate the YO production profile for estimating the kinetic parameters of yeast growth, substrate consumption, YO production, and acid consumption. Finally, the YO was converted to rigid and semi-rigid BPU foams. The BPU chemical structures and properties were investigated by SEM, FTIR, density (ASTM D729), and water absorption (ASTM D570). The study found that Pseudozyma parantarctica CHC28 was the most effective yeast among other strains by producing 3.36 ± 0.01 g/L of YO (oil content of 28.4 ± 0.5%) at 144 h using 40 g/L of xylose as a carbon source. Additionally, the YO-producing parameters (YO concentration, YP/S, and oil content) obtained under various glucose-xylose mixing ratios were not significantly different (p-value > .05). Adding 1 g/L of acetic acid enhanced YO and oil content to 4.39 ± 0.30 g/L and 42.1 ± 2.82%, respectively. The FA composition of P. parantarctica YO exhibited slight variations due to different cultural conditions, with C:16 and C:18 FAs as the main components. The optimum conditions of YO production from RSH were 10 g/L of RSH, 0.5 g/L of (NH4)2SO4, and 9 g/L of KH2PO4, resulting in biomass and YO production of 4.67 ± 0.09 g/L and 1.02 ± 0.05 g/L, respectively. Moreover, the fed-batch process operated by RSH-based medium and RSH feeding could increase the YO to 3.76 ± 0.24 and 3.98 ± 0.16 g/L, respectively. In addition, fed-batch cultivation by both feeding modes could promote the YP/S of 0.13 and oil contents exceeding 51%. The main FAs of RSH-based YO were C:16 and C:18, greater than 88.2%. Furthermore, the unconstructed mathematical models provided goodness-of-fit adequately to describe the oleaginous cultivation profiles. The YOs could be converted into BPU foam in rigid and semi-rigid forms. The SEM image of BPU foam revealed that the surface of rigid BPU was smooth with a few pores. In contrast, the semi-rigid BPU presented a rough surface with many non-uniform pores. The FTIR analysis demonstrated that the YO was successfully transformed into BPU. The density and water absorption were higher than 0.86 g/cm3 and 23.3%. These results summarized that converting YO to BPU provided an alternative way for biopolymer production using xylose and RSH as primary carbon sources.