Second-generation ethanol production from steam exploded olive tree pruning using phosphoric acid as catalyst

Over nine million ha of olive trees are cultivated worldwide, especially in Mediterranean countries. Olive tree pruning generates a huge amount of a lignocellulosic, renewable residue, which can be considered a suitable raw material for the production of ethanol due to its high content of potentially fermentable carbohydrates. In addition, it could be an interesting biomass for the production of other products of high added value, such as oligosaccharides and antioxidants, which can positively influence comprehensive waste utilization and reduction of ethanol production costs.

In this work, the effect of phosphoric acid (1% w/w) in steam explosion pretreatment of water extracted olive tree pruning at 175 ºC and 195 ºC was evaluated. The objective is to produce ethanol from all sugars (mainly glucose and xylose) contained in the pretreated material. The liquid fraction (prehydrolyzate), containing mainly xylose, was detoxified by alkali and ion-exchange resin and then fermented by the xylose fermenting yeast Scheffersomyces stipitis. Besides, simultaneous saccharification and fermentation (SSF) and presaccharification and simultaneous saccharification (PSSF) of slurry will be tested in laboratory experiments at high (w/w) consistency using commercial enzymes and a recombinant strain of Saccharomyces cerevisiae.

Preliminary results using industrial S. cerevisiae shows that ethanol yields in a SSF process are close to 80% when using solid fraction from slurry at 15% (w/w) substrate consistency. When prehydrolyzates detoxified by ion-exchange resins were used fermentation yield reaches about 70% of theoretical.

Financial support by Ministerio de Economía y Competitividad under Project ref. ENE2011-29112-C02-01 is gratefully acknowledged.