Effectiveness of Sodium Borohydride Treatment on Acid Hydrolyzates from Olive-Tree Pruning Biomass for Bioethanol Production

The application of an effective removal treatment to minimize inhibitory compounds, present after hydrolysis treatment, might mean a great advance on renewable biofuel generation strategy since growth-inhibiting substances, such as phenolic compounds and furans, are one of the major problems in the development of fermentative step. Detoxification of olive-tree pruning hydrolyzates using NaBH4, as reducing agent, has had a positive effect on fermentative parameters using Pichia stipitis CBS 6054. Oxalic acid treatment conditions have been optimized (150 ∘C and 75 mM of oxalic acid in order to obtain the maximum fermentable sugar recovery without an excessive inhibitory compounds production) considering the combined severity factor, CSF, (CSF = 1.56 ± 0.05). Hydrolyzate characterization at optimal condition showed concentrations (in g/L) of 16.64 D-glucose, 15.05 D-xylose, 5.16 L-arabinose, 2.87 acetic acid, 3.18 phenolic compounds, and 0.96 furans. Sodium borohydride conditioning process has been studied using a response surface methodology. The use of NaBH4 (0.03 mol/L during 30 min at pH= 6.0) has been demonstrated to be a successful conditioning method for almost entirely furans elimination as well as 40% of phenolic compounds reduction. As a result, it has been possible to ferment sugary liquors which did not ferment without such treatment, obtaining a maximum ethanol yield of 27% (3.8 g/L) in treated hydrolyzates under optimum conditions.