3.1 Fatty acid composition
An increase in saturated and a decrease in unsaturated fatty acids were observed in all treated samples. Solid fractions had higher exposure to reactive plasma species (especially H radicals and H+) (El-Zeer et al., 2013) and showed more considerable changes in their composition. A significant increase in stearic acid (C18:0) was observed in solid fractions from 4.5 to 10.4%. The total saturated fatty acids increased from 16.7% to 22.5%. Linolenic acid decreased from 8.4 ±0.4% to 1.5±0.5%, and linoleic acid from 53.4±1.8% to 17.7±2.6%. Results from the fatty acid composition are shown in Fig. A1 in the appendix data section. According to the results, it can be said that the decrease in unsaturated fatty acids cannot be directly related to the increase of saturated fatty acids.
Oleic acid (C18:1-9c) decreased significantly over treatment time from 20.6% to 13.8%. Two new peaks were detected in the chromatogram of HVACP treated samples, corresponding to elaidic acid (C18:1-9t) and linoelaidic acid (C18:2-9t,12t), that reached 0.8% and 2.6% as the highest values for 6h treated samples, respectively. Traditional partial hydrogenation of soybean oil may form 25-40% of trans fatty acid (Jovanovic, 1998), mostly as elaidic acid (C18:1-9t). Trans isomers are formed through incomplete hydrogenation, as a product of the absorption-desorption of unsaturated fatty acids from the catalyst surface, and it is isomerized to its lower energy positional structure. The fatty acid composition of a traditional PHO with an iodine value of 84 is shown in Fig. A1 (appendix data section), with a total trans fatty acid content of 31.8%. Even though HVACP treatment using hydrogen gas can reduce IV to the level of a traditional PHO, it has a different fatty acid composition and a remarkable lower content of trans fatty acids in the range of 2.0-2.7%. Total trans fatty acid content is much lower for HVACP treated samples, mostly formed by linoelaidic acid (C18:2-9t,12t). In contrast with PHO that mainly has elaidic acid (C18:1-9t). The mechanism of the formation of trans isomers in HVACP treatment is not well understood because there is not a catalyst surface where the double bonds are opened, neither absorption-desorption.
Yepez and Keener reported the absence of trans fatty acids in cold plasma treatment of soybean oil that reached an iodine value of 92, using a modified atmosphere of nitrogen-hydrogen gas for a 12h treatment time (Yepez & Keener, 2016). In this study, a modified atmosphere of pure hydrogen gas was used as an approach to accelerate the reactions. The gel fraction treated for 4h achieved a similar iodine value as the 12h sample treated with nitrogen-hydrogen gas; however, it showed a 2% content of trans fatty acids. Therefore, soybean oil treated with hydrogen gas may increase the isomerization of cis- into trans- double bonds due to the higher reactivity of hydrogen species and energy applied by cold plasma treatment (Dijkstra, 2006). It is known that the bond dissociation energy of a nitrogen molecule requires more energy than a hydrogen molecule does.
In summary, the fatty acid composition for the 6h solid fraction formed after HVACP treated sample showed a saturated fatty acid content of 38.8%, the monounsaturated fatty acids (MUFA) 24.7%, and the polyunsaturated fatty acid (PUFA) with a 36.5%. Including a total trans fatty acid content of 4.8%. The iodine value of a 6h solid sample is 86.6, calculated from the fatty acid composition. In contrast, the fatty acid composition of a PHO with a similar iodine value has a saturated fatty acid content of 22.5%, 57.3% of MUFA, 20.4% of PUFA, including 31.8% of trans fatty acids. HVACP treatment produced a fatty acid composition of soybean oil with a different profile compared to a PHO.
A portion of the samples treated with HVACP under hydrogen gas atmosphere was insoluble in hexane. This insoluble portion was filtered before the gas chromatography analysis; therefore, the reported fatty acid composition corresponds to the soluble portion of the samples. The amount of insoluble portion of the solid fraction increased consistently with a prolonged treatment time, reaching up to 41.6% with respect to the solid fraction treated for 6h. It is suggested that the formation of insoluble material may be correlated to the reduction of unsaturated fatty acids and subsequently polymer formation.
Fig. 2 – about here
It has been reported that HVACP can degrade the H2molecules forming H+, H3+, and H species by 0.0001%, 0.1%, and 1 %, respectively (El-Zeer et al., 2013). Since the H radicals are more abundant in the process, it can be suggested that H radicals are the most active species and can be responsible for any possible reaction. Indeed, the H+ and H radicals are the species that lack of electrons and their stabilities are similar. So, both species can be responsible for the mechanism suggested. Schneider (Schneider, 2008) reported an updated mechanism for lipid peroxidation, mentioning the formation of conjugated systems from isolated double bonds. The suggested mechanism depends on bis-allylic radical stability. Behr et al. also presented the formation of a conjugated system from linoleic acid by a catalytic reaction leading to five different structural isomers (Behr, Witte, & Bayrak, 2013). As aforementioned, HVACP treated H2 molecules form H radicals and cations, and these species can react with unsaturated lipids converting isolated double bonds into conjugated ones. Fig. 2 illustrates a possible mechanism for the formation of a conjugated system via interaction between one of the bis-allylic hydrogens (see hydrogens in red color) in the structure of linolenic acid with H radicals. Linolenic acid has given as an example for the fatty acids which contain bis-allylic hydrogens. The triglyceride that we have studied had 8.4 % of linolenic acid, and, as aforementioned, its quantity decreased to 1,5 % after HVACP treatment. This can happen in linoleic acid as well.
Figure 3 – about here