Oxidative treatments of solid olive residues: Effects on phenolic and fatty acid fractions

In the present study, we considered the effects of several chemical treatments of three different olive residues (pomace) of diverse origins, which all involve H2O2. Diluted (40vol) and concentrated (130vol; 30% wt) H2O2 was used alone or in combination with Fe sulfate (the Fenton system). The main goal was to oxidize the phenolic substances, as only a dephenolyzed sample can be safely used for applications like soil improvement or as biomass for biogas generation. The o-diphenols were always oxidized in higher amount than single phenols as demonstrated by data on tyrosol and hydroxytyrosol. The Fenton system appears to be a method that can oxidize the phenolic fraction, while leaving only a trace of residual H2O2 inside the pomace. We also considered the effects of these oxidative treatments on the triglycerides. Linoleic acid was largely oxidized, although only in pomace with a low water content and in the presence of the Fenton treatment. Finally, the whole lignocellulose matrix of all of the pomaces was recovered at the end of treatment without any large changes, as shown by the IR spectra of oxidized samples. Practical applications: Modern two-phase olive-oil extraction systems have become widely used, and it can be expected that in the near future they will replace traditional extraction plants. Together with many advantages, they also have some disadvantages, such as a greater need for disposal of pomace. Such by-products can be of value for re-use in soil improvement, as biomass for generation of biogas, as new ingredients for livestock feed, and as raw material for energy purposes (stone). Often these uses are hindered by the presence of low amounts of derivatives that can cause problems such as inhibition of the biochemical pathways involved. The high amounts of phenols and fatty acids in pomaces represent a serious problem for these supplementary uses. A phenol-free pomace would allow alternative uses and provide additional economic return from these by-products. We have considered the effects of several chemical treatments, always involving H2O2, of pomaces of different origins, for the oxidising of both their phenols and their residual triglycerides. The Fenton system is the method that can oxidise the phenolic fraction, while leaving only traces of residual H2O2. Among the fatty acids contained in the triglyceride fraction, linoleic acid was largely oxidized, although only in pomace with a low water content and in the presence of the Fenton treatment. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.