Linoleic induction period

The singlet oxygen reacts directly with the double bond by addition, and shifts the double bond one carbon away. The singlet oxygen attack on linoleate produces four hydroperoxides as shown in Figure 2-20. Photooxidation has no induction period, but the reaction can be quenched by carotenoids... 

Although several studies have been performed to determine the characteristics of regular sunflower oil for use in frying, few were focused on the in situ stability of an oil contained in fried foods. In one smdy, french fries fried in high-linoleic sunflower oil and crashed to different sizes, were subjected to fast-aging in OSI equipment at 110°C and the induction period was determined (84). The values obtained differed from those for pure sunflower oil. This method is a more realistic... 

The comparison of the antioxidant potency of hydroxytyrosol, tyrosol, oleuropein with respect to vitamin E and BHT and the synergistic effect with vitamin E were studied in thermal initiated oxidation of methyl linoleate in the presence of azo-compounds, in heptanol or propanohwater. The activity of the phenols was expressed as increase of induction period and extent of reaction inhibition [47]. The azo compounds initiated oxidation is a free radical chain process. Confirming the results from a previous study [43], this study showed that hydroxytyrosol and oleuropein were much more effective than BHT and vitamin E in extending the induction period. Tyrosol was devoid of activity and no synergistic effect on the preservation of methyl linoleate was foxmd when vitamin E was used together with tyrosol. 

Although phospholipid bilayers are better mimics of biomembranes than are micelles, there are few reliable quantitative data on flavonoid antioxidant activities in lipid bilayers. Terao and coworkers compared the antioxidant efficiency of quercetin and catechins (epicatechin and epicatechin gallate) with that of a-Toc in egg yolk PC liposomes using initiation by the water-soluble initiator, ABAP, and analysis of hydroperoxide formation and antioxidant consumption by HPLC. Based on the length of the induction periods and the profile of suppressed hydroperoxide formation, they concluded that quercetin and the catechins were more efficient antioxidants than a-Toc in these bilayers. Apparently the unique behavior of a-Toc in bUayers is responsible for these results (vide supra). In hexane and alcohols solution during suppressed peroxidation of methyl linoleate, the relative antioxidant activities reversed so that the flavonoids were 5-20 times less active... 

The relative rates of autoxidation of different unsaturated fatty acids and esters were compared on the basis of oxygen absorption measurements (Table 1.1). In neat systems without added initiator, linoleate was 40 times more reactive than oleate, linolenate was 2.4 times more reactive than linoleate, and arachidonate was 2 times more reactive than linolenate. The oxidizability of polyunsaturated fatty acid (PUFA) esters was also compared on the basis of oxygen uptake measured kinetically by the induction period method described above, in solution in the presence of azo initiators. The oxidizability of 18 2,18 3,20 4, and 22 6 was linearly related to the number of bis-aUyUc positions present in the fatty esters. From this relationship, the oxidizability of each PUFA was increased approximately two fold for each active bis-allylic methylene group. Thus, the oxidizability of 22 6 was 5 times greater than that of 18 2. 

The autoxidation of trilinolein was more complicated than that of the simple fatty esters and did not follow the same rate equation. The order of the reaction was about 0.84 compared to 1.0 for the simple fatty esters. The efficiency of the initiators used, DMVN, was increased to 100% in the trilinolein compared to about 75% in the simple fatty esters. The difference in kinetic behavior between trilinolein and methyl linoleate was attributed to the tendency of the triacylglycerol to form aggregates. The kinetic induction period approach used to measure oxidizability may be subject to errors because of the changes in efficiency of some of the artificial initiators used according to the system and the lipid substrate. Phenolic antioxidants such as a-tocopherol are also affected by the colloidal properties of the lipids used in the oxidation test system employed (Chapter 9). 

The adsorption of linoleic acid onto silica gel from petroleum ether solution conformed to a Langmuir isotherm, consistent with the formation of a monolayer (Porter etal. 1972). Confirming the finding of Honn etal. (1951) with soybean oil, it was found that the most rapid uptake of oxygen occmred at a linoleic acid-silica ratio close to that for the monolayer. Without included antioxidant, oxidation commenced at a nearly linear rate without observable induction period. Time for consumption of one-half mole of oxygen per mole of linoleic acid was ca. 60 min on acid-washed silica. If very small amounts of a-tocopherol were included in the layer, virtually no oxygen uptake measurable in this system occurred during the induction pe-... 

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