Fermentation pentoses

Bacteria belonging to this genera are facultative anaerobes and require a rich medium containing growth factor and fermentable sugar for their development. Their optimum temperature is 25-30 °C with a pH value of 6. They are homofer-mentative, which means that all the glucose is metabolized into lactic acid and they do not ferment pentose. 

Bacteria will ferment pentoses to ethanol plus mixed acids [4], and various researchers have genetically engineered bacteria to improve ethanol production from pentoses. However, most industrial ethanol fermentations use S. cerevi-... 

Complete utilization of cellulose and hemicellulose requires selection or genetic modification of an organism that is able to ferment pentoses. In order to obtain monosaccharides from the raw material, several pretreatments and/or separations are required. First, the lignocellulosic material is mechanically treated and then delignified (pulped) by strong alkali or acid treatment. The (hemi)cellulose part becomes more accessible for enzymes at the same time. Subsequent enzymatic treatment mainly yields glucose and xylose and some arabinose. The enzymatic treatment and subsequent fermentation can be done in separate reactors or in one fermenter, in an SSF concept similar to starch SSF [57]. 

The strictly homofermentative lactobacilli do not ferment pentose, and form two molecules of lactic acid from one molecule of glucose by the Embden-Meyerhoff pathway. 

Recombinant Saccharomyees cerevisiae, able to ferment the pentoses D-xylose and L-arabinose, was modified for improved fermentation rates and yields. Pentose fermentation is relevant when low cost raw materials such as plant hydrolysates are fermented to ethanol. The two most widespread pentose sugars in our biosphere are D-xylose and L-arabinose. S. cerevisiae is unable to ferment pentoses but has been engineered to do so however rates and yields are low. The imbalance of redox cofactors (excess NADP and NADH are produced) is considered a major limiting factor. For the L-arabinose fermentation we identified an NADH-dependent L-xylulose reductase replacing the previously known NADPH-dependent enzyme. For D-xylose fermentation we introduced an NADP-dependent glyceraldehyde 3-phospate dehydrogenase to regenerate NADPH. 

Many LAB are able to ferment pentoses. They can only ferment heterofermen-tatively by entering the phosphogluconate pathway as either ribulose-5 phosphate or xylulose-5 phosphate (Kandler 1983) (Fig. 4). Pentoses (such as arabinose, ribose, xylose) are converted into lactate and acetate CO2 is not produced. The sum of the equation is... 

The fermentation of sulfite waste liquor was attacked in a novel way by Nord and co-workers and is reported by Nord and Mull (93). Fusarium Uni Bolley ferments pentoses and was able to grow on sulfite liquor provided the liquor was first freed of toxic substances by lime treatment or, alternatively, by a resin. The amount of alcohol produced was small. Attempts were made to ferment with Fusarium stillage from an alcohol plant operating on wheat, and an amount of alcohol equivalent to 6-12% of that obtained from yeast fermentation was obtained. It was shown that Fusarium mycelium was not toxic to mice, which would indicate there was no objection to the use of a Fusanam-fermented stillage as feed. 

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