Biotechnological strategies to overcome inhibitors in lignocellulose hydrolysates for ethanol production: review

Abstract

One of the major challenges faced in commercial production of lignocellulosic bioethanol is the inhibitory compounds generated during the thermo-chemical pre-treatment step of biomass. These inhibitory compounds are toxic to fermenting micro-organisms. The ethanol yield and productivity obtained during fermentation of lignocellulosic hydrolysates is decreased due to the presence of inhibiting compounds, such as weak acids, furans and phenolic compounds formed or released during thermo-chemical pre-treatment step such as acid and steam explosion. This review describes the application and/or effect of biological detoxification (removal of inhibitors before fermentation) or use of bioreduction capability of fermenting yeasts on the fermentability of the hydrolysates. Inhibition of yeast fermentation by the inhibitor compounds in the lignocellulosic hydrolysates can be reduced by treatment with enzymes such as the lignolytic enzymes, for example, laccase and micro-organisms such as Trichoderma reesei, Coniochaeta ligniaria NRRL30616, Trametes versicolor, Pseudomonas putida Fu1, Candida guilliermondii, and Ureibacillus thermosphaericus. Microbial and enzymatic detoxifications of lignocellulosic hydrolysate are mild and more specific in their action. The efficiency of enzymatic process is quite comparable to other physical and chemical methods. Adaptation of the fermentation yeasts to the lignocellulosic hydrolysate prior to fermentation is suggested as an alternative approach to detoxification. Increases in fermentation rate and ethanol yield by adapted micro-organisms to acid pre-treated lignocellulosic hydrolysates have been reported in some studies. Another approach to alleviate the inhibition problem is to use genetic engineering to introduce increased tolerance by Saccharomyces cerevisiae, for example, by overexpressing genes encoding enzymes for resistance against specific inhibitors and altering co-factor balance. Cloning of the laccase gene followed by heterologous expression in yeasts was shown to provide higher enzyme yields and permit production of laccases with desired properties for detoxification of lignocellulose hydrolysates. A combination of more inhibitor-tolerant yeast strains with efficient feed strategies such as fed-batch will likely improve lignocellulose-to-ethanol process robustness.

Camptothecin-producing endophytic fungus Trichoderma atroviride LY357: isolation, identification, and fermentation conditions optimization for camptothecin production

“…Among them, MeJA is an optimum elicitor in view of that CPT yield increased 3.4-fold. Additionally, end-product feedback inhibition is another common problem during fermentation processes when the concentration of the end product reaches a certain level (Singh et al 2010;Parawira and Tekere 2011). Inclusion of an adsorbent resin in the production medium in combination with extractive fermentation is an effective approach to reduce end-product feedback inhibition and to increase the product titer.…”

Genetic engineering and enzyme research in lignocellulosic ethanol production

“…These inhibiting compounds, such as weak acids, furans and phenolic compounds, are formed or released during the thermo-chemical pre-treatment step such as acid and steam explosion (Parawira and Tekere, 2011). Inhibition of yeast fermentation by inhibitor compounds in lignocellulosic hydrolysates can be reduced by treating with lignolytic enzymes, for example, laccase, and microorganisms such as Trichoderma reesei, Coniochaeta ligniaria NRRL30616, Trametes versicolor, Pseudomonas putida Fu1, Candida guilliermondii, and Ureibacillus thermosphaericus (see Parawira and Tekere, 2011 for a full review). Genetic engineering is playing a major part in getting the best possible conversion rate in thermotolerant yeasts.…”

Enhanced Bioethanol Production from Industrial Xylose Residue Using Efficient Delignification

“…Organic acids, which are soluble in the lipids of the cell membrane and dissociate inside the yeast cells at physiological pH, are the main inhibitors generated in xylose production (Chandel et al 2013). These acids could potentially prolong the lag phase of fermentation, and they are expected to reduce the effect on enzymes associated with glycolysis (Kim et al 2008;Parawira and Tekere 2011). Therefore, pretreatment is a necessary step towards solving these issues.…”