The effect of citric acid on the phenolic contents of olive oil

Response surface methodology was used to optimise the combined effects of malaxation time (t) and aqueous citric acid solution (C-A) added at the beginning of the malaxation step on total polyphenols (TP) and o-diphenols (OD) content and the antiradical power (ARP) of the olive oil extracted from the Italian olive fruits of Coratina cultivar. Different tests were performed according to a 3(2) full factorial design, varying t from 30 to 90 min and the C-A from 5 to 15 ml/kg(paste). Overall optimal conditions identified by a numerical optimisation for the three responses were found to be at t = 30 min and C-A = 13.79 ml/kg(paste) under which the model predicted TP = 604.52 mu gCAE/g(oil), OD = 80.44 mu gCAE/g(oil) and ARP = 28.73 mu gDPPH/mu lextract. There were also linear correlations between TP (R-2 = 0.8176) and OD (R-2 = 0.8633) values of olive oil and waste water. The results of this study demonstrate that considerably short malaxation time and relatively small amounts of citric acid were required to enhance the quality of olive oil. The outcome of our study will therefore be of great value for the commercial production of olive oil with high level of polyphenols and o-diphenols. (C) 2009 Elsevier Ltd. All rights reserved.