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.