Formation of 5-(hydroxymethyl)furfural during subcritical water extraction of natural matrices. Is it bioactivity-relevant?

Subcritical water extraction (SWE) has demonstrated a great potential for the extraction of bioactive compounds from natural matrices. The application of high extraction temperatures and pressures, while keeping the water in the liquid state, allows the attainment of high extraction yields in fast extraction processes. Although the applications of SWE to obtain bioactives from natural matrices are increasing, the studies dealing with the processes that might take place during the extraction are still scarce. Recently, we demonstrated how under SWE conditions not only the solubility of the analytes might be improved but also neoformed antioxidants can be obtained. Indeed, it was observed how the occurrence of reactions like Maillard and caramelization reactions during SWE resulted on the formation of neoantioxidants. This fact can be advantageous although the possible toxicity of the compounds formed during these non-enzymatic reactions should be carefully considered. Formerly, we observed that during the extraction of olive tree leaves at high extraction temperatures (200ºC), more active extracts in terms of antioxidant and in-vitro antiproliferative activities were obtained. Chemical characterization of those extracts revealed not only the presence of phenolic bioactives but also 5-(hydroxymethyl)furfural (HMF), a compound related to Maillard reaction. For this reason, in this work, a more exhaustive study has been devised to observe the influence of the temperature in the formation of HMF in SWE extracts of olive leaves. The final aim was to determine the relevance of the HMF present on different SWE extracts in terms of bioactivity. Thus, extracts obtained at different extraction temperatures were produced (50, 75, 100, 125, 150, 175 and 200 ºC) and chemically characterized by LC-MS. HMF was quantified in the extracts by HPLC-DAD. Moreover, the antioxidant activity, the amount of total phenols (Folin method) as well as the anticancer activity of both, extracts an