3.5. FTIR
Untreated and treated samples (6h) were analyzed with FTIR (Fig. 6).
Fig. 6a shows the hydrogen stretching region, including the range of
2800-3050 cm-1. Liquid, gel, and solid fractions
showed an increase in the number of single bonds (2930
cm-1, 2860 cm-1 symmetric and
asymmetric stretching bands), and a reduction of double bonds (3010
cm-1, =C-H stretching band). Suggesting that HVACP
treated samples as previously explained, might have undergone possible
hydrogenation or dimerization/polymerization reaction. Further
information about bond modifications is included in the fingerprint
region (Fig. 6b), in the range of 800-1600 cm-1. The
peak observed at 914 cm-1 corresponds to the
cis-olefinic group, which shows a reduction from untreated to treated
samples (Guillén & Cabo, 1997). However, an increase of trans- double
bonds bending vibration was also observed in the 968
cm-1 bands, with a higher peak for PHO. These results
are in accordance with the fatty acid composition.
Fig. 6 – about here
Hexane was used to separate the liquid oil from the insoluble fraction
by using Soxhlet extraction. The extracted insoluble fraction (Solid-F
in Fig. 6) was characterized by a reduced content of double bonds (peak
at 3010 cm-1) and a higher content of single bonds.
This is probably due to the Diels-Alder (Smith, 2017; Yao et al., 2008)
reaction between the conjugated double bonds formed by HVACP treatment
in one triglyceride with the one double bond of another triglyceride
molecule. Therefore, two double bonds are disappearing by the reaction
and a reduction in absorbance of the mentioned peak. Besides of an
increase of absorbance in the peak observed at 1465
cm-1 that corresponds to bending vibration of the
methylene group, and an increase in absorbance in the peak observed at
1377 cm-1 that correspond to symmetrical bending
vibration of methyl groups (Guillén & Cabo, 1997).
As seen in Figure 6, there are no additional peaks that provide
information about structure modifications rather than changes in single
and double bonds. It is proposed that the formation of this
oligomer/polymer may occur through the Diels Alder (Smith, 2017)
reaction mechanism, as seen in Fig. 4. As a result, the triglyceride
fatty acid chains are linked by cyclic structures. A rearrangement of
the double bonds from cis- to trans-, as a consequence of the reaction,
is also reported (Muik, Lendl, Molina-Díaz, & Ayora-Cañada, 2005). This
effect can be observed in results from FTIR and fatty acid composition,
by an increase of the trans isomer of 18:1 and 18:2.
HVACP treatment has not only been used to modify chemical structures but
has been studied mainly as a processing technique to reduce the
microbial load of food (Misra et al., 2019). The mechanisms of microbial
inactivation may include the production of UV light, oxidation of
membrane lipids, or protein oxidation (Liao et al., 2017). Therefore,
this technology is known as an oxidizer tool. In this study, the goal is
under the HVACP treatment to use hydrogen gas as a source of hydrogen
species and their effect on unsaturated fatty acids. Consequently, the
presence of oxygen was avoided because any trace could form reactive
oxygen species that may initiate oxidation reactions. Hydrogen gas was
flushed for 5 minutes, to reduce oxygen content below 0.01%. However,
treated samples of soybean oil (1.9-5.0 meq/kg) showed a higher peroxide
value (PV) than untreated (0.19 meq/kg). These values were below 10
meq/kg, which is the limit of peroxide content requirement for fats and
oils (Pegg, 2005). PV is a critical parameter that should be monitored.
Further precautions can be adopted to remove oxygen from the plasma
chamber, or it would be required the use of antioxidants as additives.
Interestingly, results showed that longer treatment times reduce the PV.
Solid fractions of the samples treated for 6h have a PV value of
2.71±0.6 meq/kg, being lower than 5.0±0.9 meq/kg for the 2h-solid
treatment. Samples treated for 6h have less amount of double bonds due
to the possible dimerization via Diels-Alder (Ionescu & Petrovic, 2009;
Mihail & S., 2012; Smith, 2017) reaction as presented in Fig. 3
available to react with oxygen, which means that that samples treated
for longer treatment times are less susceptible to lipid oxidation.
CONCLUSIONS
In this study, we investigated the modifications of soybean oil using
HVACP hydrogen plasma. Soybean oil was treated with H2species formed by HVACP treatment for 2, 4, and 6. Samples were
characterized by techniques such as gas chromatography (GC), FTIR,1H-NMR, 13C-NMR, 2D-NMR, oxidation,
and thermal properties.
HVACP treatment of soybean oil using hydrogen gas can transform the
liquid oil into a solid product. This study found that HVACP can
catalyze hydrogenation of double bonds of the oil and polymerize
triglycerides. Hydrogen species formed by HVACP treatment can convert
isolated double bonds which contain bis-allylic hydrogens into
conjugated double bonds. Besides, those triglycerides can be
hydrogenated or polymerized (via Diels Alder) by hydrogen species form
under cold plasma conditions. HVACP is an environmentally friendly
technique to hydrogenate plant oils without the generation of trans
fatty acids. The use of cold plasma to modify chemical structures can
contribute to the production of sustainable products obtained from green
processes.
ACKNOWLEDGEMENTS
This work was funded by the Department
of Food Science of Purdue University. Ximena Yepez gratefully
acknowledges the financial support of the office of the Secretary of
Higher Education, Science, Technology, and Innovation of the government
of Ecuador for her graduate studies. This research did not receive any
specific grant from funding agencies in the public, commercial, or
not-for-profit sectors.
REFERENCES
AOAC. (2005). Official Methods of Analysis of AOAC International .