Determination of optimal pre-treatment conditions for ethanol production from olive-pruning debris by simultaneous saccharification and fermentation
Introduction
The production of fuel-ethanol from lignocellulose biomass is of growing interest around the world because it can provide a number of environmental advantages over conventional fossil fuels, most notably a reduction in greenhouse-gas emissions. However, the higher production cost for bioethanol (1.3 dollars/dm3, from lignocellulose) as compared to gasoline, and its lower heating value (26,700 kJ/kg at ambient temperature) are some problems of this alternative fuel [1].
Traditionally, olive-tree is cultivated in Mediterranean countries (especially in Spain, Italy, Greece, Morocco and Tunisia) but in recent times it has been cultivated in regions of five continents (West Coast of USA, Mexico, Australia, Argentina, Chile, etc.) with a total area of more than 7 × 104 km2 [2]. In olive-tree cultivation, pruning is a necessary biennial operation to eliminate old branches and to regenerate the tree. Debris from olive-tree pruning represents a great volume of renewable biomass in Spain (between 4.3 × 109 and 7.5 × 109 kg/year) and a great potential resource for ethanol production (maximum of 1.3 × 106 m3/year).
The conversion of biomass to ethanol generally includes four steps: pre-treatment, hydrolysis of polysaccharides and oligosaccharides into monomer sugars, fermentation of sugars to ethanol and, finally, ethanol concentration to absolute alcohol (for use as motor fuel, ethanol must be concentrated to >99.8%).
Extensive literature is available on different pre-treatment methods to enhance the digestibility of lignocellulose materials [3]. Diluted-acid pre-treatment, at temperatures of 423.15–493.15 K, is one of the most important procedures and has been reviewed for different materials [4], [5], [6], working with sulfuric-acid or other acids (e.g. nitric acid, hydrochloric acid). During this thermal process, hemicellulose is depolymerized into a mixture of sugar oligomers and monomers, whereas less alteration is caused in lignin and cellulose [7]. Removal of hemicellulose increases porosity and therefore improves enzymatic digestibility of cellulose. Depending on the hydrolysis conditions, carbohydrates degradation products can form, and interfere with microbial activities.
SSF processes combine enzymatic hydrolysis of cellulose with simultaneous fermentation of the d-glucose obtained to ethanol [8]. The presence of yeast together with cellulases reduces the accumulation of d-glucose, thereby increasing the saccharification rate and ethanol yield. SSF also lowers capital costs and reduces the risk of microbial contamination [9].
This study evaluates the influence of pre-treatment conditions (temperature and acid concentration) on the simultaneous saccharification and fermentation of olive-pruning debris (OPD) using response-surface methodology and the best-known ethanol-fermenting yeast, Saccharomyces cerevisiae. The generation of sugars and ethanol from this raw material has been studied by using different pre-treatments: concentrated sulfuric-acid [10], concentrated phosphoric-acid [11] and steam explosion. This research can complete the available information on the utilization of olive-pruning debris for ethanol production.
Section snippets
Raw material
Olive-pruning debris (thin branches and leaves) were obtained from a local farm in the province of Jaén, Spain, air-dried at room temperature until reaching 8–10% moisture, milled, using a blade mill (Retsch, mod. SM1, Germany), to a particle size between 0.425 and 0.600 mm, homogenized and stored until used. The samples were characterized following ASTM methods [12], [13].
Cellulose and hemicellulose content was determined based on monomer content measured after an acid hydrolysis in two steps.
Raw material
Table 1 shows the composition of raw material (mean values and standard deviation of three determinations). Cellulose (as d-glucose) and hemicellulose (as sum of d-xylose, d-arabinose, d-galactose, and d-mannose) contents agree with those reported in previous studies [20] and make this biomass an adequate substrate for ethanol production.
Pre-treated materials characterization
The behaviour of OPD, during high-temperature diluted-acid hydrolysis, was similar to that of other lignocellulose materials. Regarding the percentage of
Conclusions
In this study of the conversion of olive-pruning debris to bio-fuel ethanol, via diluted-acid pre-treatment and the SSF process, was found that as the process temperature and sulfuric-acid concentration increased, hemicellulose was degraded to water-soluble compounds (in monomeric and oligomeric form). Pre-treatment step solubilized all the hemicellulose and increased the cellulose content to 46.3% (1.5 times more compared to the initial raw material). Simultaneous saccharification and
Acknowledgement
This work was financially supported by the Project 01272/2005 and by a mobility grant of the “Consejería de Innovación, Ciencia y Empresa, Junta de Andalucía (Spain)”.
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