The authors are grateful to the Foundation for Science and Technology (FCT, Portugal) for financial support to CIMO (PEst-OE/AGR/UI0690/2014), REQUIMTE (UID/QUI/50006/2013) and J. Pinela (SFRH/BD/92994/2013); to FCT/MEC and FEDER under Programme PT2020 for financial support to LSRE (UID/EQU/50020/2013), and to QREN, ON2 and FEDER (Project NORTE-07-0162-FEDER-000050); to the Xunta de Galicia for financial support for the post-doctoral researcher of M.A. Prieto.
Tomato is the second most important vegetable crop worldwide and a rich source of industrially interesting antioxidants. Hence, the microwave-assisted extraction of hydrophilic (H) and lipophilic (L) antioxidants from a surplus tomato crop was optimized using response surface methodology. The relevant independent variables were temperature (T), extraction time (t), ethanol concentration (Et) and solid/liquid ratio (S/L). The concentration-time response methods of crocin and β-carotene bleaching were applied, since they are suitable in vitro assays to evaluate the antioxidant activity of H and L matrices, respectively. The optimum operating conditions that maximized the extraction were as follows: t, 2.25 min; T, 149.2 ºC; Et, 99.1 %; and S/L, 45.0 g/L for H antioxidants; and t, 15.4 min; T, 60.0 ºC; Et, 33.0 %; and S/L, 15.0 g/L for L antioxidants. This industrial approach indicated that surplus tomatoes possess a high content of antioxidants, offering an alternative source for obtaining natural value-added compounds. Additionally, by testing the relationship between the polarity of the extraction solvent and the antioxidant activity of the extracts in H and L media (polarity-activity relationship), useful information for the study of complex natural extracts containing components with variable degrees of polarity was obtained.
