Antioxidant capacity of olive pruning phosphoric acid prehydrolyzates

Biomass obtained from olive tree pruning is a lignocellulosic residue lacking currently of industrial applications whose use as raw material for ethanol production has been proposed. Its relatively low content of sugars (50% db, with 25% glucan) and high content of extractives (20%), make it interesting the development of other processes for taking full advantage of the residue and to obtain alternative end products.

In this context, and considering ethanol production as the main process, this work examines the antioxidant capacity of the liquid fractions issued from olive tree pruning pretreatment (prehydrolyzates). Antioxidant compounds obtained from natural sources have a potential market in the preservative food industry as a replacement of fossil-derived antioxidants.

The raw material was submitted to dilute phosphoric acid pretreatment in a Parr reactor under selected conditions according to a factorial experimental design, with temperature (170-210 °C) and acid concentration (0.5-1.5% w/v) as variables. After filtration the liquid fractions were used for determining their antioxidant capacity by two free radical scavenging methods (DPPH and ABTS) and by ferric thiocyanate and TBARS assays. The antioxidant capacity of the prehydrolyzates was compared to those of natural and synthetic antioxidants (hydroxytyrosol, α-tocopherol, BHT and Trolox).

Taking into account their phenolic content results show that the prehydrolyzates have a high antioxidant capacity. For example, against ABTS radical, prehydrolyzates showed to be approximately 8-fold more potent than Trolox. The average effective concentration (EC₅₀) was 0.04 versus 0.36 g GAE/L for Trolox. Therefore the prehydrolyzates could be recovered to produce antioxidant extracts.