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