β-carotene is bioactive and a precursor for vitamin A synthesis in nature. It can be extracted from plant materials or be produced by chemical synthesis using Wittig reaction. A new bioprocess has been developed to produce β-carotene from the xylose portion of bioenergy sorghum. Bioenergy sorghum was hydrothermally pretreated in a pilot-scale continuous reactor with 50% solids loading at 190ºC for 10 min and followed by disc refining. Xylose was extracted from the pretreated energy sorghum by dilute acid hydrolysis. Different temperatures (100ºC, 120ºC, 140ºC, and 180ºC), reaction times (5 min and 10 min), and sulfuric acid loadings (0%, 1%, and 2%) were examined to optimize the xylose recovery. The highest xylose yield of 64.87% (17.43 g/L) was obtained from the hydrolysis at 120
oC for 5 min with 2% sulfuric acid. The non-concentrated hydrolysate (NCH) was concentrated to 32 g xylose/L (medium-concentrated hydrolysate, MCH) and 66 g xylose/L (high-concentrated hydrolysate, HCH). The engineered
Saccharomyces cerevisiae strain SR8B was used to ferment the neutralized non-, medium-, and high-concentrated xylose syrup to β-carotene. In HCB, MCB, and NCB cultures, the yeast produced β-carotene titers of 114.50 mg/L, 93.56 mg/L, and 82.50 mg/L, which correspond to specific yeast biomass productions of 7.32 mg/g DCW, 8.10 mg/g DCW, and 8.29 mg/g DCW, respectively. This bioprocess of bioenergy sorghum combining the hydrothermal pretreatment, optimized dilute-acid hydrolysis for the extraction of xylose, and the xylose fermentation with engineered
Saccharomyces cerevisiae resulted in higher β-carotene yields compared to previous reports in which β-carotene was produced at 2.83 mg/g (Sun et al., 2016), 5.9 mg/g (Verwaal et al., 2007), and 6.3 mg/g (Yan et al., 2012) by engineered
S. cerevisiae from refined glucose.
References:
Sun, Y., Sun, L., Shang, F., Yang, G., 2016. Enhanced production of β-carotene in recombinant Saccharomyces cerevisiae by inverse metabolic engineering with supplementation of unsaturated fatty acids. Process Biochem., 51, 568-577.
Verwaal, R., Wang, J., Meijnen, J., Visser, H., Sandmann, G., van den Berg, J., van Ooyen, A., 2007. High-level production of beta-carotene in Saccharomyces cerevisiae by successive transformation with carotenogenic genes from Xanthophyllomyces dendrorhous. Appl. Environ. Microbiol., 73, 4342-4350.
Yan, G., Wen, K., Duan, C., 2012. Enhancement of β-carotene production by over-expression if HMG-CoA reductase couples with addition of ergosterol biosynthesis inhibitors in recombinant Saccharomyces cerevisiae. Curr. Microbiol., 64, 159-163.
