Biotechnical oxidation of D-xylose and hemicellulose hydrolyzates by Gluconobacter oxydans: Dissertation

Xylonic acid is attractive as a potential product from D xylose because the five carbon structure of xylose remains intact in the oxidation and hence the theoretical yield is high.Xylonic acid could possibly be used as sequestrant analogously to gluconic acid.In this study the biochemical oxidation mechanism and the biotechnical production of xylonic acid were studied as well as the suitability of steamed hemicellulose hydrolyzate as raw material for its production.Gluconobacter oxydans subsp. suboxydans ATCC 621 was found to produce xylonic acid efficiently from xylose if small amount of glucose was added to the medium to support growth of the organism.The highest xylonic acid yields obtained were 98 % of the theoretical (109 gl 1) on a 100 gl l xylose medium with an overall productivity of 2.5 gl 'h l.The oxidation of xylose was found to proceed in the periplasmic space by the action of a membrane bound aldose dehydrogenase, which appeared to be a quinoprotein, having pyrroloquinoline quinone (PQQ) as cofactor.The subsequent ~ lactonase, responsible for the hydrolysis of the primary reaction product xylono ~ lactone, was also detected in G. oxydans.Steamed hemicellulose hydrolyzate was a suitable carbon source for xylonic acid production only at concentrations of less than 45 gl l of the hydrolyzate.This was due to the existence of wood derived inhibitors in the hydrolyzate.As many as 29 monomeric aromatic compounds were detected in the hydrolyzate, the main compounds being 5 hydroxy methylfurfural, syringaldehyde and vanillin.The total amount of aromatic monomers and dimers was 0.8 % of the hydrolyzate dry weight.The steamed hemicellulose hydrolyzate was purified by various methods in order to increase its suitability as a biotechnical raw material.Of the treatments tested, ion exclusion was found to have the most pronounced effect.After ion exclusion G. oxydans was able to efficiently utilize the hemicellulose hydrolyzate with a xylose concentration of up to 100 gl l.G. oxydans was found to detoxify 5 hydroxymethylfurfural, vanillin and 4 hydroxybenzaldehyde by oxidizing them preferably to the corresponding acids, but also reducing them to the alcohols. Coniferyl alcohol was detoxified by oxidation to ferulic acid.The acids were found to be less toxic to G. oxydans than the aldehydes tested.Despite the detoxification mechanism present in G. oxydans, steamed hemicellulose hydrolyzate was not found to be a suitable carbon source at high concentrations without efficient purification.