Loss of tocopherols and formation of degradation compounds at frying temperatures in oils differing in degree of unsaturation and natural antioxidant content

Abstract

Samples of oils of different degrees of unsaturation, namely palm olein, olive oil, highlinoleic sunflower oil, high-oleic sunflower oil, rapeseed oil and soybean oil, were heated at 180°C for 2, 4, 6, 8 and 10 h in the presence or absence of their natural antioxidants. Also, tocopherol-stripped oils were supplemented with a-tocopherol (500 mg kg À1 ), d-tocopherol (500 mg kg À1 ) or a mixture of a-, b-, g-and d-tocopherols (250 mg kg À1 each) and heated under the same conditions. Losses of tocopherols and formation of polymeric triacylglycerols were followed. Total polar compounds were also evaluated after 10 h of heating. Results demonstrated that tocopherols were lost very rapidly, in the expected order, with a-tocopherol being the least stable. Polymeric and polar compound formation during heating was inhibited to a variable extent, being more dependent on the natural content and type of tocopherols than on the degree of unsaturation of the oil. For example, polymeric and polar compound contents in soybean oil were significantly lower than those found in high-linoleic sunflower oil. However, the expected influence of the degree of unsaturation was evident when oils were unprotected or possessed identical initial antioxidant contents. Finally, levels of degradation compounds after 10 h of heating were not dependent on the remaining content of antioxidants. INTRODUCTIONIn a previous study we stressed the interest in defining the action of natural antioxidants, particularly concerning the relationships between loss of antioxidants and formation of new compounds at the high temperature of processes such as frying. 1 With this aim we initiated a series of experiments to determine the influence of tocopherols on the alteration of the lipid substrate without the influence of minor uncontrolled compounds normally present in the oils, which might exert an antioxidant or pro-oxidant effect. We selected three model systems constituted by monoacid triacylglycerols, ie triolein, trilinolein and a 50:50 mixture of the two, of clearly differentiated degrees of unsaturation, and both loss of tocopherols and triacylglycerol degradation were analysed after heating at frying temperature for different periods of time. The results obtained indicated that, under the conditions used, addition of tocopherols had a protective action at high temperature, particularly when a mixture of the four

Loss in tocopherols and oxidative stability during the frying of frozen cassava chips

“…These differences were expected given that auto-oxidation is a process which occurs between molecular oxygen and unsaturated fatty acids and, consequently, oxidative stability is intrinsically related to the unsaturation degree of the oil. Jorge et al (1996a) and Barrera-Arellano et al (2002), when analyzing the initial oxidative stability of sunflower oil found similar values to the ones presented in this study -9.0 and 9.9 hours, respectively. With respect to palm and cottonseed oil, it is important to note that the oils used in this study contain TBHQ as an added antioxidant and the values may be higher than those found for refined oils without additives.…”

“…Thus, Barrera-Arellano et al (2002) verified that after heating at 180°C, tocopherol degradation occurred regardless of the oil type. Nevertheless, they observed that after 10 hours heating, significant total tocopherol quantities still remained in polyunsaturated oils such as sunflower, soybean and canola oils, whereas in monounsaturated oils, palm olein, high oleic sunflower oil and olive oil, the presence of tocopherols was not detected.…”

Use of polar and nonpolar fractions as additional information sources for studying thermoxidized virgin olive oils by FTIR

“…The observed inflection at 20 h also coincides with the moment in which the concentration of o-diphenols abruptly decreases to 8.71 mg·kg −1 , which means 6.5% of the initial concentration ( Table 1). After that moment, the oil is hardly protected against oxidation by means of antioxidants, and the oil undergoes oxidation at a higher rate, which depends on its composition of fatty acids (Barrera-Arellano et al, 2002). Similar inflections in the increasing trend of polar polymers and oxidized triacylglycerols are observed after 20 h (Table 1).…”

Frying performance of two virgin oils from <em>Cornicabra</em> olives with different ripeness indices

“…In this respect, the similar frying performance of the two oils, mainly differing in the contents of antioxidants, could be related to the rapid degradation of both polyphenols and tocopherols at frying conditions. Barrera-Arellano et al (2002) suggested that the degradation of tocopherols at frying temperatures seems to be predominant over their antioxidative role, as the degradation of tocopherols added to different purified vegetable oils was independent of the degree of unsaturation of the oil. These authors suggested that the protection effect of naturally occurring tocopherols was a secondary mode of action.…”