Screening of new yeast Pichia manchurica for arabitol production

High-level production of d-arabitol by Zygosaccharomyces rouxii from glucose: Metabolic engineering and process optimization

d-Arabitol is a top value-added compound with wide applications in the food, pharmaceutical and biochemical industries. Nevertheless, sustainable biosynthesis of d-arabitol is limited by lack of efficient strains and suitable fermentation process. Herein, metabolic engineering and process optimization were performed in Zygosaccharomyces rouxii to overcoming these limitations. Adopting systems metabolic engineering include enhancement of innate biosynthetic pathway, supply of precursor substrate d-ribulose-5P and cofactors regeneration, a novel recombinant strain ZR-5A with good performance was obtained, which boosted d-arabitol production up to 29.01 g/L, 59.31 % higher than the parent strain. Further with the optimum medium composition and fed-batch fermentation, the strain ZR-5A finally produced 149.10 g/L d-arabitol with the productivity of 1.04 g/L/h, which was the highest titer ever reported by Z.rouxii system. This is the first report on the use of metabolic engineering to construct Z. rouxii chassis for the sustainable production of d-arabitol.

Biocatalytic conversion of a lactose-rich dairy waste into D-tagatose, D-arabitol and galactitol using sequential whole cell and fermentation technologies

Dairy industry waste has been explored as a cheap and attractive raw material to produce various commercially important rare sugars. In this study, a lactose-rich dairy byproduct, namely cheese whey powder (CWP), was microbially converted into three low caloric sweeteners using whole-cell and fermentation technologies. Firstly, the simultaneous lactose hydrolysis and isomerization of lactose-derived D-galactose into D-tagatose was performed by an engineered Escherichia coli strain co-expressing β-galactosidase and L-arabinose isomerase, which eventually produced 68.35 g/L D-tagatose during sequential feeding of CWP. Subsequently, the mixed syrup containing lactose-derived D-glucose and residual D-galactose was subjected to fermentation by Metschnikowia pulcherrima E1, which produced 60.12 g/L D-arabitol and 28.26 g/L galactitol. The net titer of the three rare sugars was 156.73 g/L from 300 g/L lactose (equivalent to 428.57 g/L CWP), which was equivalent to 1.12 mol product/mol lactose and 52.24% conversion efficiency in terms of lactose.

High D-arabitol production with osmotic pressure control fed-batch fermentation by Yarrowia lipolytica and proteomic analysis under nitrogen source perturbation

D-arabitol, a five-carbon sugar alcohol, is widely used in food and pharmacy industry as a lower calorie sweetener or intermediate. Appropriate osmotic pressure was confirmed to facilitate polyol production by an osmophilic yeast strain of Yarrowia lipolytica with glycerol. In this study, an osmotic pressure control fed-batch fermentation strategy was used for high D-arabitol producing by Y. lipolytica ARA9 with crude glycerol. Glycerol was added to the broth quantitatively not only as a substrate but also as an osmotic agent. Meanwhile, NH₃·H₂O was fed as a nitrogen source and pH regulator. The maximum D-arabitol production reached 118.5 g/L at 108 h with the yield of 0.49 g/g and productivity of 1.10 g/L/h, respectively. Furthermore, a comparative proteomic analysis was used to study the cellular responses under excess and deficient nitrogen sources. Thirty-one differentially expressed protein spots belonging to seven different biological processes were identified. Excess nitrogen source enhanced gluconeogenesis and pentose phosphate pathways, both of which were involved in arabitol synthesis. In addition, cell growth was facilitated by increased expression of nucleotide and structural proteins. Enhanced energy and NADPH biosynthesis were employed to create a reductive environment and quell reactive oxygen species, improving D-arabitol production. Nitrogen deficiency resulted in cell rescue and stress response mechanisms such as reactive oxygen species elimination and heat shock protein response. The identified differentially expressed proteins provide information to reveal the mechanisms of the cellular responses under nitrogen source perturbation, and also provide guidance to improve D-arabitol production in metabolic engineering or process optimization methodologies.

The Improvement of Bioethanol Production by Pentose-Fermenting Yeasts Isolated from Herbal Preparations, the Gut of Dung Beetles, and Marula Wine

Efficient conversion of pentose sugars to ethanol is important for an economically viable lignocellulosic bioethanol process. Ten yeasts fermenting both D-xylose and L-arabinose were subjected to an adaptation process with L-arabinose as carbon source in a medium containing acetic acid. Four Meyerozyma caribbica-adapted strains were able to ferment L-arabinose to ethanol in the presence of 3 g/L acetic acid at 35°C. Meyerozyma caribbica Mu 2.2f fermented L-arabinose to produce 3.0 g/L ethanol compared to the parental strain with 1.0 g/L ethanol in the absence of acetic acid. The adapted M. caribbica Mu 2.2f strain produced 3.6 and 0.8 g/L ethanol on L-arabinose and D-xylose, respectively, in the presence of acetic acid while the parental strain failed to grow. In a bioreactor, the adapted M. caribbica Mu 2.2f strain produced 5.7 g/L ethanol in the presence of 3 g/L acetic acid with an ethanol yield and productivity of 0.338 g/g and 0.158 g/L/h, respectively, at a K _L a value of 3.3 h^-1. The adapted strain produced 26.7 g/L L-arabitol with a yield of 0.900 g/g at a K _L a value of 4.9 h^-1.