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 .