Oxygen absorption of methyl esters of fat acids, and the effect of antioxidants

Abstract

K NOWLEDGE concerning the oxidation of the individual components of lard as well as their mixtures should be of value in disclosing the course of the development of oxidative rancidity. Furthermore the behavior of antioxidants with each component may help to reveal the nature of their protective action with lard.Although the exact individual components of lard are not available, they may be represented, approximately, by methyl oleate, methyl stearate, and methyl linoteate. A number of workers have investigated the oxygen absorption of fat acids and esters (1-12), but no single investigation has included all the methyl esters of the fat acids of lard and mixtures of these methyl esters.The usefulness of the Barcroft-Warburg apparatus in the study of the autoxidation of fats has been demonstrated (13,14) and it was our purpose to apply this convenient method to the methyl esters of fat acids representing the components of lard. Accordingly methyl oleate, methyl stearate, methyl linoleate, the distilled methyl esters of lard, and various mixtures of the individual methyl esters were prepared, and their oxygen absorption at 100 ° C. was determined. In addition, for general comparative purposes, measurements of the oxygen absorption of methyl linolenate were included.A group of antioxidants was evaluated with methyl linoleate, methyl oleate, methyl stearate, and the distilled methyl esters of lard. The antioxidants were a-tocopherol, nordihydroguaiaretic acid (NDGA), propyl gallate, benzylhydroquinone, and synergistic combinations with citric acid, d-isoascorbyl palmitate, and lecithin.As might be expected, methyl linolenate, methyl linoleate, the distilled methyl esters of lard, methyl oleate, and methyl stearate absorbed oxygen und developed peroxides, at relative rates in the order named. Mixtures of substrates absorbed oxygen at a rate which could be approximately predicted from the rate of oxygen absorption of the individual components. The order of effectiveness of the antioxidants varied somewhat with the different substrates but in each case the combinations of nordihydroguaiaretic acid and of propyl gallate with citric acid were the most powerful antioxidants. Preparation of the SubstratesN ETHYL oleate was prepared from the methyl esters of lard by a series of low-temperature fractional crystallizations and fractional vacuum distillations (15). The final product had an iodine nnmber of 85.5 (Wijs).Methyl stearate was prepared from hydrogenated soybean oil fat acids by esterification with methyl * One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United States Department of Agriculture.alcohol; sulfuric acid was used as a catalyst. After fractional vacuum distillation and recrystallization the methyl stearate had a saponification equivalent of 300.4; a melting point of 39.0-39.4 ° ; and an iodine number of zero (Wijs).Methyl linoleate was prepared from the unsaturated fat acids of cottonseed oil (16) by the method of Rollett (17). Methyl lin...

Surface studies of ultra-high molecular weight polyethylene irradiated with high-energy pulsed electron beams in air

“…The occurrence of carbon crosslinking throughout decomposition of hydroperoxides was reported 50 years ago in connection with the "drying" of linseed oil. [25][26][27] However, no further reports on this phenomenon and its mechanism have been published ever since. On the other hand, the observation that oxidation of UHMWPE does not occur during the initial stage of irradiation (i.e., up to 20 Mrad dose) reminds us observations of the induction period in thermal oxidation of polyethylene.…”

N-3 fatty acids for human nutrition: stability considerations

“…The oxidisability of these acids was found to increase approximately two-fold for each bis-allylic methylene group [40]. The relative rates of autoxidation of stearate, oleate, linoleate and linolenate were 1, 11, 114 and 179 [41].…”

Preparation of methyl linoleate hydroperoxide

“…The oil used in this work was over 80% linoleic acid; the remainder was made up of oleic acid and saturates. Since the rate of oxidation of oleic acid is ~6 to ~ 2 that of linoleic acid (3,5) and since it is present in an amount only ~ that of linoleie acid, the contamination of linoleate hydroperoxide with oleate hydroperoxide is indeed small.…”