Ethanolic fermentation of phosphoric acid hydrolysates from olive tree pruning

The objective of this work was to study the feasibility of using phosphoric acid to hydrolyze the hemicellulosic fraction of olive tree pruning, as a step in the bioconversion process to produce ethanol. Milled olive tree pruning was submitted to hydrolysis at 90 °C by phosphoric acid in a concentration range 0.3–8N for 240 min. The hydrolysates were then fermented by Pachysolen tannophilus. The hydrolysis stage was evaluated by the evolution of glucose and reducing sugars generated and by the conversion of hemicellulose fraction. The main parameters determined in the fermentation were: maximum specific growth rate, specific substrate consumption rate, specific ethanol production rate and ethanol yield. The maximum ethanol yield (0.38 kg/kg, equivalent to 74.5% of the theoretical yield) was obtained when hydrolysing with 0.5N phosphoric acid. Hemicellulose conversion is however incomplete at these operational conditions. Higher acid concentrations lead to higher hydrolysis of hemicellulose, but the ethanol yields resulting from the fermentation are lower.