Θερμική οξείδωση και πολυμερισμός ελαίων: παρεμποδιστική δράση φυσικών προϊόντων πλούσιων σε αντιοξειδωτικά

Abstract

Thermal oxidation and oxidative polymerization are the main type of reactions occurring in oils heated at frying temperatures (180ºC). The products of the above reactions have not only a desirable, but also mainly an undesirable effect on the quality of heated oils. Therefore, the last years there is a lot of research regarding the potent inhibitory effect of natural products against the oil deterioration during heating. In the first part of the thesis, the effect of heating at 180ºC on the antioxidant activity of virgin olive oil (VOO), refined olive oil (ROO), sunflower oil (SO) and a commercial blend special for frying (BL) was examined by measuring the radical scavenging activity (RSA) toward 1,1-diphenyl-2-picrylhydrazyl radical (DPPH). The stability of heated oils was assessed by determining their total polar compound (TPC) content. VOO was found to be the most thermostable oil. RSA of VOO and ROO decreased rapidly during the first 5 h of heating. The other two oils retained their RSA to a relatively high extent after 10 h of heating, but in the meantime they reached rejection point (25-27 % TPC). The results demonstrated that VOO had a remarkable thermal stability but when a healthful effect is expected from the presence of phenolic compounds, heating has to be restricted as much as possible. In the second part of the thesis, the inhibitory effect of natural products or byproducts against thermal oxidation and oxidative polymerization of heated oils differing in unsaturation and natural antioxidant content was studied. The effect of ethanol and acetone extracts obtained from two plants belonging to Lamiaceae family, namely Greek sage (Salvia triloba L.) and summer savory (Satureja hortensis L.) on thermal stability of vegetable oils heated at frying temperature (180°C) was examined. VOO, ROO, SO and BL enriched with each extract obtained from the two plant materials at a concentration of 3 g/kg oil, were heated at 180°C for 10 h. Changes during heating were assessed by quantification of total polar materials and determination of p-anisidine value. The acetone extract obtained from Greek sage showed a better inhibitory effect against thermal oxidation of heated refined oils (BL, ROO, SO) than the respective ethanol extract, although the latter was found to have a relatively higher total phenol content. Both summer savory extracts retarded effectively the thermal oxidation reactions during oil heating, showing a more pronounced effect than the Greek sage acetone extract. The activity of acetone extract obtained from summer savory was stronger (SO, BL) or similar (ROO, VOO) to that of summer savory ethanol extract, although the latter was found to have a higher total phenol content. This was probably due to some less polar constituents (6-, 7-, 8- methoxy flavones) of the summer savory acetone extract, which may improve the thermal stability of the rich in polyunsaturated fatty acid and tocopherol oils. Sesame hulls, the main by-product of the conventional process for preparing sesame paste (tehina), sesame oil or other sesame-based products, are commonly disposed of or used as animal feed. Oil extracted from sesame hulls was inestigated for its inhibitory effect against oil deterioration. Mixtures of the above oil with refined olive oil were heated at 180°C and the changes in total polar compound content and p-anisidine value were determined. The unsaponifiable matter isolated from sesame hull oil was further examined for the respective effect against oil deterioration during prolonged heating. Two kinds of oils were used as substrates, a monounsaturated oil (refined olive oil) and a polyunsaturated, rich in a-tocopherol oil (sunflower oil). The inhibitory effect was pronounced for either lipid substrate used. In order to examine thoroughly the potent antioxidant and/or antipolymerization activity of sesame hull oil constituents, the two more important fractions of the unsaponifiable matter were isolated and studied for their inhibitory effect against oil deterioration during heating. The lipid substrate used was almost totally purified sunflower oil triacylglycerols. According to the levels of total polar compounds in the lipid substrate during heating, its p-anisidine values and its fatty acid methyl ester composition, sterol and lignin fractions were found to have a similar protective effect against either thermal oxidation or oxidative polymerization reactions. No synergistic effect was observed between both fractions. In conclusion, either Greek sage and summer savoury, or sesame hulls seem to be good sources of constituents showing antioxidant and antipolymerization properties, when added to vegetable oils heated at frying temperatures.

Σκοπός της διατριβής ήταν η διερεύνηση της ικανότητας πλούσιων σε αντιοξειδωτικά φυσικών προϊόντων να επιβραδύνουν τις αντιδράσεις θερμικής οξείδωσης και πολυμερισμού στα έλαια που θερμαίνονται στους 180ºC. Η παρακολούθηση των μεταβολών κατά τη θέρμανση των ελαίων έγινε με εξέταση της ικανότητάς τους να δεσμεύουν ελεύθερες ρίζες και κυρίως με μέτρηση του αριθμού ανισιδίνης, προσδιορισμό της περιεκτικότητας σε πολικά συστατικά και, κατά περίπτωση, εξακρίβωση της σύστασης σε λιπαρά οξέα. Μελετήθηκε η επίδραση της προσθήκης αιθανολικού ή ακετονικού εκχυλίσματος των φυτών Salvia triloba L. και Satureja hortensis L. στη θερμική σταθερότητα παρθένου ελαιολάδου, εξευγενισμένων ελαίων και κατάλληλου για τηγάνισμα μίγματος ελαίων. Τα εκχυλίσματα των φυτών επιβράδυναν ικανοποιητικά τη θερμική αλλοίωση των ελαίων, με το ακετονικό εκχύλισμα του S. hortensis L., αν και πτωχότερο σε φαινόλες, να προστατεύει αποτελεσματικότερα κυρίως τα πλούσια σε λινελαϊκό οξύ και τοκοφερόλες έλαια, εξαιτίας πιθανώς της ποιοτικής σύστασής του. Επίσης, μελετήθηκε η επίδραση των ασαπωνοποίητων συστατικών του ελαίου των φλοιών σησαμόσπορων (ΑΣ) στη θερμική σταθερότητα ελαίων πλούσιων σε ελαϊκό ή λινελαϊκό οξύ. Διερευνήθηκε αν η προσθήκη ελαίου φλοιών σε εξευγενισμένο ελαιόλαδο βελτιώνει τη θερμική σταθερότητα και ακολούθως η επίδραση της προσθήκης ΑΣ και των επιμέρους κλασμάτων τους στη θερμική σταθερότητα εξευγενισμένου ελαιολάδου ή ηλιελαίου και τριακυλογλυκερολών ηλιελαίου, αντίστοιχα. Τα ΑΣ παρεμπόδισαν σημαντικά τη θερμική υποβάθμιση των ελαίων. Τα κλάσματα των στερολών και των λιγνανών ήταν εξίσου δραστικά, ενώ δεν παρατηρήθηκε συνέργεια μεταξύ τους. Τα παραπάνω φυτά και οι φλοιοί σησαμόσπορων είναι πηγές συστατικών που συμβάλουν στη βελτίωση της θερμικής σταθερότητας ελαίων κατάλληλων για το τηγάνισμα τροφίμων.