Detoxification of hemicellulosic hydrolyzate from olive tree pruning residue

Abstract

One of the major problems in commercial production of lignocellulosic ethanol and xylitol, are the toxic compounds generated during the hydrolytic process. A concentrated hemicellulosic hydrolyzate from olive tree pruning residue has been detoxified using several methods in order to minimize the presence of inhibitory compounds, improving subsequent fermentation stage with yeasts. Thus, in a first stage, activated charcoal has been used to study the influence of pH, agitation, temperature and the proportion of this type of adsorbent, using a 24 experimental design. Further, liming and overliming processes were performed with three different alkalis, CaO, Ca(OH)2 and NaOH. Finally, the use of three organic solvents, chloroform, n-hexane and ethyl acetate, in four different proportions, 2:1, 1:1, 1:2 and 1:3 (hydrolyzate:solvent, v/v), was tested to achieve the toxic compounds elimination. In all assays we have checked the reduction in acetic acid, total phenolic compounds, furfural, hydroxymethylfurfural and total sugars concentrations. The volume loss of hydrolyzate has also been determined. In the adsorption processes with activated charcoal, the variables that have proved most influential on the responses tested were the percentage of this compound and pH. It could be an excellent alternative to decrease inhibitors from olive tree pruning residue hydrolyzates (removing 46% of acetic acid, 81% of phenolic compounds and 98% of total furans). The detoxification treatments with solvents revealed that the experimental conditions which combined the best inhibitor removal and lower alteration of sugar yields could be achieved using ethyl acetate because this extractor agent was able to remove near 50% of total phenolic compounds and 57% of total furans. Less favorable results were obtained when n-hexane was employed. The liming and overliming processes allowed a significant reduction of acetic acid (in some case up to 53%), lignin-derived compounds (close to 72%) and total furans (near 83%) mainly furfural, but also hydroxymethylfurfural.