SummaryIn the context of climate change and the depletion of fossil fuels, there is a great need for alternatives to petroleum in the transport sector. This review provides an overview of the production of second-generation bioethanol, which is distinguished from first-generation and subsequent generations of biofuels by its use of lignocellulosic biomass as raw material. The structural components of the lignocellulosic biomass such as cellulose, hemicellulose and lignin, are presented along with technological unit steps including pre-treatment, enzymatic hydrolysis, fermentation distillation and dehydration. The purpose of the pre-treatment step is to increase the surface area of carbohydrate available for enzymatic saccharification, while minimizing inhibitors. Performing the enzymatic hydrolysis releases fermentable sugars, which are converted by microbial catalysts into ethanol. The hydrolysates obtained after pre-treatment and enzymatic hydrolysis contain a wide spectrum of sugars, predominantly glucose and xylose.Genetically engineered microorganisms are therefore needed to carry out co-fermentation. The excess of harmful inhibitors in the hydrolysate, such as weak organic acids, furan derivatives and phenol components, can be removed by detoxification before fermentation. Effective saccharification further requires using co-acting exogenous hemicellulases and cellulolytic www.ftb.com.hrPlease note that this is an unedited version of the manuscript that has been accepted for publication. This version will undergo copyediting and typesetting before its final form for publication. We are providing this version as a service to our readers. The published version will differ from this one as a result of linguistic and technical corrections and layout editing.enzymes. Conventional species of distillers' yeast are unable to ferment pentoses into ethanol, and only a very few natural microorganisms, including yeast species like Candida shehatae, Pichia stipites, Scheffersomyces and Pachysolen tannophilus, metabolize xylose to ethanol.Enzymatic hydrolysis and fermentation can be performed in a number of ways, by separate saccharification and fermentation, simultaneous saccharification and fermentation or consolidated bioprocessing. Pentose-fermenting microorganisms can be obtained through genetic engineering, by introducing xylose-encoding genes into metabolism of a selected microorganism to optimize its use of xylose accumulated in the hydrolysate.
Pretreatment is a necessary step when lignocellulosic biomass is to be converted to simple sugars; however single-stage pretreatment is often insufficient to guarantee full availability of polymeric sugars from raw material to hydrolyzing enzymes. In this work, the two-stage pretreatment with use of acid (H2SO4, HNO3) and alkali (NaOH) was applied in order to increase the susceptibility of Jerusalem artichoke stalks (JAS) and oat straw (OS) biomass on the enzymatic attack. The effect of the concentration of reagents (2% and 5% w/v) and the order of acid and alkali sequence on the composition of remaining solids and the efficiency of enzymatic hydrolysis was evaluated. It was found that after combined pretreatment process, due to the removal of hemicellulose and lignin, the content of cellulose in pretreated biomass increased to a large extent, reaching almost 90% d.m. and 95% d.m., in the case of JAS and OS, respectively. The enzymatic hydrolysis of solids remaining after pretreatment resulted in the formation of up to 45 g/L of glucose, for both JAS and OS. The highest glucose yield was achieved after pretreatment with 5% nitric acid followed by NaOH, and 90.6% and 97.6% of efficiency were obtained, respectively for JAS and OS.
This research shows the effect of dilute acid pretreatment with various sulfuric acid concentrations (0.5-2.0% [wt/vol]) on enzymatic saccharification and fermentation yield of rye straw. After pretreatment, solids of rye straw were suspended in Na citrate buffer or post-pretreatment liquids (prehydrolysates) containing sugars liberated after hemicellulose hydrolysis. Saccharification was conducted using enzymes dosage of 15 or 25 FPU/g cellulose. Cellulose saccharification rate after rye straw pretreatment was enhanced by performing enzymatic hydrolysis in sodium citrate buffer in comparison with hemicellulose prehydrolysate. The maximum cellulose saccharification rate (69%) was reached in sodium citrate buffer (biomass pretreated with 2.0%[wt/vol] H 2 SO 4 ). Lignocellulosic complex of rye straw after pretreatment was subjected to separate hydrolysis and fermentation (SHF) or separate hydrolysis and cofermentation (SHCF). The SHF processes conducted in the sodium citrate buffer using monoculture of Saccharomyces cerevisiae (Ethanol Red) were more efficient compared to hemicellulose prehydrolysate in respect with ethanol yields. Maximum fermentation efficiency of SHF processes obtained after rye straw pretreatment at 1.5% [wt/vol] H 2 SO 4 and saccharification using enzymes dosage of 25 FPU/g in sodium citrate buffer, achieving 40.6% of theoretical yield. However, SHCF process using cocultures of pentose-fermenting yeast, after pretreatment of raw material at 1.5% [wt/vol] H 2 SO 4 and hydrolysis using enzymes dosage of 25 FPU/g, resulted in the highest ethanol yield among studied methods, achieving 9.4 g/L of ethanol, corresponding to 55% of theoretical yield. K E Y W O R D S dilute sulfuric acid pretreatment, enzymatic saccharification, ethanol production, fermentation, rye straw Additional supporting information may be found online in the Supporting Information section at the end of this article. How to cite this article: Robak K, Balcerek M, Dzieko nska-Kubczak U, Dziugan P. Effect of dilute acid pretreatment on the saccharification and fermentation of rye straw. Biotechnol
In addition to saccharose, sugar beet root contains a lignocellulosic fraction, which is not used in the process of sugar production and remains in sugar beet pulp. There is a great interest in using the polysaccharides (cellulose, hemicellulose) present in this raw material for the production of bioethanol. The objective of this study was to assess the effect of the enzymatic treatment of sugar beet biomass on the hydrolysis of the cellulose and hemicellulose present in its cell walls, as well as its effect on the efficiency of alcoholic fermentation of saccharose and sugars liberated from structural polysaccharides. Its effect on the efficiency of the process of inoculating the fermentation medium with a monoculture or a co-culture of yeast strains fermenting hexose and pentose sugars was also investigated. Our results reveal that in order to enable the utilization of all fermentable sugars in the sugar beet root biomass (saccharose as well as monosaccharides bound in structural polysaccharides), initial enzymatic treatment should be applied, followed by alcoholic fermentation using sequential inoculation with a co-culture of Saccharomyces cerevisiae and Pichia stipitis. These conditions ensure the utilization of hexoses and pentoses (xylose) in alcoholic fermentation, thus enabling the production of 9.9±0.4 kg of ethanol from 100 kg of sugar beet biomass.
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