Conclusions
The above study demonstrates the efficacy of T2(spin-spin) transversal relaxation time of the alkyl chain tail to follow the chemical and structural changes occurring during LSO aging via a thermal autoxidation process over a temperature of 25 to 120C. For example aliphatic chains tail’s T2 energy relaxation TD of LSO samples heated and exposed to air’s O2 up to 40oC clearly correlates with an increase rate of peroxides production that is not immediately followed with a significant rate of aldehydes accumulation, nor crosslinking polymerization. Heating of LSO at 60oC, however shows that this is the turning point in which the oxidation process is completed by forming aldehydes and terminates with polymerization products. As heating temperatures are increased the rate of LSO oxidation increase dramatically.
The above results demonstrate that 1H LF-NMR T2 transversal energy relaxation times of the PUFA’s terminal alkyl chain can be used to readily, with at-line facile instruments, to determine the different stages of LSO oxidation, beyond what is currently available. The T2 relaxation time values in the early stages of oxidation change from the non-oxidized samples as a function of the temperature of autoxidation. At the lowest reported temperatures of autoxidation (25oC and 40oC) an initiation and propagation phase of oxidation result, without the termination phase, giving molecular structures, as graphically shown, with the lowest viscosities and higher T2 values due to increased chain mobility because of chain decomposition. At higher temperatures of LSO oxidation, starting at 60oC there are the stages of initiation, propagation and termination of oxidation wherein the latter is the formation of polymerization phases with high viscosity. Thus with increasing temperature above 60oC (80, 100 and 120oC) a rapid polymerization process results in significantly lower T2 relaxation times due to increase of viscosity. Hydroperoxides are rapidly converted to other products such as aldehydes and polymers. Polymerization is reflected in changes in the T2 relaxation times of alkyl chains, higher viscosities and lower self-diffusion constants. Thus this rapid polymerization process at higher oxidation temperatures results in significantly lower T2 at a material stage that has lower peroxide values than found at lower oxidation temperatures.
These lower temperatures of oxidation have a T2 peaking over time that is postulated to represents increasing mobility because of chain decomposition and then a relatively slow decrease in T2 values possibly due to increased molecular crosslinking and appearance of viscous gel-like product.
These results show the versatility of selective T2relaxation time assessment of LSO’s rich in alkyl chain of omega-3 PUFA and most likely in other oils, to readily determine the state of oxidation. Therefore, it is postulated that selective determination of LSO tail T2 relaxation times can be used as a facile and accurate marker of the omega-3 PUFA-rich oil oxidative aging process.
Acknowledgments The authors would like to acknowledge Prof. John van Duynhoven and Mr. Donny Merkx from Wageningen University for their support with HR NMR study and valuable comments and suggestions. We also would like to thank Dr. Z. Abramovich for the technical work with GC, Mrs. Shoshana Kravchik and all the members of PLBL for general laboratory assistance. This study was partially supported by a grant from Ministry of Science and Technology, Israel.