The hydrothermal pretreatment of olive mill solid waste amended with 0.6 M organic acids was studied
at temperatures between 100 and 170 C. Acetic and formic acids which are endogenous intermediates of
hemiacetyl splitting at subcritical conditions were tested. Formic acid, with smaller molecular size and
lower pKa, was found to be more effective than acetic in the entire range of temperatures tested. Yield
of enzymatic hydrolysis was significantly enhanced (>2 folds) at temperatures above 140 C.
Concentration of aldehyde byproducts in the medium increased with temperature and pressure and addition
of organic acids, however, the highest concentration detected (ca 1 g/L) did not surpass values
reported as inhibitory of sugars fermentation to ethanol by either yeast or bacteria. Aldehyde production
was more affected by temperature than by acid addition. Concluding, addition of formic acid to
hydrothermal pretreatment at relatively mild temperatures (140–170 C) and pressure (10–13 atm)
improved saccharification yield while saving energy.