Biological conversion of lignocellulosic biomass to ethanol

The ability to convertd-xylose to xylitol was screened in 44 yeasts from five genera. All but two of the strains produced some xylitol with varying rates and yields. The best xylitol producers were localized largely in the speciesCandida guilliermondii andC. tropicalis. Factors affecting xylitol production by a selectedC. guilliermondii strain, FTI-20037, were investigated. The results showed that xylitol yield by this strain was affected by the nitrogen source. Yield was highest at 30–35°C, and could be increased with decreasing aeration rate. Using high cell density and a defined medium under aerobic conditions, xylitol yield byC. guilliermondii FTI-20037 from 104 g/ld-xylose was found to be 77.2 g/l. This represented a yield of 81% of the theoretical value, which was computed to be 0.9 mol xylitol per mold-xylose.

Screening of yeasts for production of xylitol fromd-xylose and some factors which affect xylitol yield inCandida guilliermondii

There are a number of process advantages which could be exploited through the use of thermophilic microorganisms for ethanol production. Energy savings through reduced cooling costs, higher saccharification and fermentation rates, continuous ethanol removal and reduced contamination have stimulated a search for routes to thermophilic or thermotolerant yeasts. These routes have included screening existing culture collections, temperature adaptation, mutagenesis and molecular techniques and finally isolating new strains. Varying success has been achieved, however, the most thermotolerant yeasts have come from fresh isolations from environments which experience high temperatures. Thermotolerant yeasts have been investigated for the following potential applications: simultaneous saccharification and fermentation of cellulose, where the high fermentation temperature allows more rapid and efficient enzymatic cellulose hydrolysis; whey fermentation, where high salt and low fermentable substrate concentrations make conditions difficult; and fermentation of D-xylose and cellobiose, which is essential for efficient conversion of woody biomass to ethanol. Ethanol and temperature tolerance are important characteristics for commercial yeast strains. Both characteristics are interactive and generally decrease with increasing temperature and ethanol concentration. Considerable research has been directed towards investigation of fatty acid composition changes in response to these stresses and the role of heat shock proteins in tolerance mechanisms. If thermotolerant yeasts are to be used in commercial processes, bioreactor configuration will play an important part in the design of production processes. Batch and fed-batch systems have been shown to be useful in some circumstances as have continuous flow systems, however, some of the newly isolated thermotolerant yeasts such as Kluyveromyces marxianus do not show the high growth rate under anaerobic conditions that is characteristic of Saccharomyces cerevisiae. Various immobilization techniques appear to offer a means of presenting and maintaining high biomass in anaerobic continuous flow reactors.

Ethanol production at elevated temperatures and alcohol concentrations: Part I - Yeasts in general

https://www.fpl.fs.fed.us/documnts/pdf1990/alexa90a.pdf

Respiratory efficiency and metabolite partitioning as regulatory phenomena in yeasts

Xylose fermentation by Candida shehatae and Pichia stipitis: effects of pH, temperature and substrate concentration

The fermentation of D-xylose by theCandida sp. mutant XF217 was studied at different oxygen uptake rates by controlling dissolved oxygen tension or agitation rate. The ethanol yield increases as the oxygen uptake rate decreases. The ethanol production rate is maximized at an oxygen uptake rate of 9–12 millimole/L/hour. Xylitol and cell mass production and cell morphology are also affected by oxygen level.

Mary Welch Baillargeon

Papers

Effect of oxygen uptake rate on ethanol production by a xylose-fermenting yeast mutant,Candida sp. XF217