Induction period, fatty acid, unsaturated

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 oxygen uptake of an unsaturated fatty acid as a function of time is shown in Fig. 3.18. Studying this figure helps in the understanding of the elementary steps involved in autoxidation. The extreme case 1 demonstrates what has invariably been found in food the initial oxidation products are detectable only after a certain elapsed storage time. When this induction period, which is typical for a given autoxidation process, has expired, a steep rise occurs in the reaction rate. The proox-... 

Our data show that plants have evolved a low temperature-induced desaturation (LTD) process that results in the very rapid desaturation of fatty acids of newly formed lipids [3]. This is followed by a much slower basm level desaturation (BLD). The two processes result in a final level of desaturation that is determined by a balance between the level of the rapid initial phase and the subsequent slower BUD. We can show that the LTD is induced at low temperature but is thermolabile at high temperatures, thus providing a mechanism for controlling the rate of desaturation of newly formed lipid. The loss and induction of desaturase activity is much more rapid than the changes in level of unsaturation of the bulk membrane lipid. We beueve that it is the control of desaturation of newly formed lipid which allows the plant to respond slowly to the environmental temperature over extended periods of time rather than to react immediately to transient fluctuations in diurnal temperature. 

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