Optically pure lactic acid production from lignocellulosic biomass
Abstract: Lactic acid is a platform chemical that can be used for many applications. More recently, optically pure lactic acid—L- or D-lactic acid enantiomers—have been used for the polymerisation to poly-lactic acid (PLA). The possibility of tailoring the properties of this bioplastic by adjusting the ratio of each enantiomer might help to accelerate the substitution of petroleum-based plastics with bio-based and compostable alternatives. Today, PLA is produced primarily from raw materials that can be used as food—sugarcane and corn. To decrease the utilisation of food crops for producing bioplastics, alternative processes usinglignocellulosic biomass and organic waste are under development. In this thesis, optically pure lactic acid production from lignocellulosic biomass was examined using softwood (spruce) and post-consumer waste viscose.Softwood was subjected to a SO2-catalysed steam explosion pretreatment to saccharify hemicellulose and open the solid structure of cellulose and lignin. The pretreated softwood was separated into two fractions (liquid and solid). The hemicellulose hydrolysate (liquid fraction) was used without further treatment as a carbon source in the bacterial cultivations, whereas the solid fraction of pretreated softwood and the waste viscose (a man-made cellulosic fibre) had to undergo enzymatic hydrolysis before use in cultivation studies.Two strains of Pediococcus acidilactici were used in this work. Each strain was genetically modified to produce only one enantiomer: P. acidilactici TY112 generates L-lactic acid, and P. acidilactici ZP26 produces D-lactic acid. The two strains were used to produce the respective enantiomers from the liquid fraction of softwood, with similar results. Subsequently, their resistance to the inhibitory compounds generated during the pretreatment was evaluated. A fed-batch strategy for co-consumption of glucose and mannose and conversion of hydroxymethylfurfural (HMF) and furfural was studied and reported. Simultaneous saccharification and fermentation (SSF) was used in cultivations with the softwood solid fraction as the carbon source. Despite initial results of a long lag phase of 48 h under several conditions, this adaptation phase was decreased by starting the cultivation with monomeric sugars before the addition of the solid fraction and enzymatic blend for SSF. P. acidilactici ZP26 also showed promising results in cultivations using saccharified waste viscose, despite the presence of dyes and likely other additives and contaminants in the media.These results add knowledge to a broader research project that aims to use P. acidilactici to produce optically pure lactic acid and high-quality PLA from several sources of lignocellulosic biomass. These modified bacteria synthesise their respective enantiomers with high yields, even in the presence of possibly toxic compounds in softwood and waste viscose.
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