ABSTRACT
High voltage atmospheric cold plasma (HVACP) treatment generates
reactive gas species that induce inter and intramolecular reactions in
soybean oil. The goal of this study is to analyze the effect of HVACP
treatment on the chemical structure of soybean oil in a hydrogen gas
environment at atmospheric pressure. HVACP was used to treat soybean oil
(15g), for up to 6h by triplicate. Plasma generated reactive gas species
interact with the sample producing three distinct fractions identified
as a liquid, gel, and solid. Fatty acid profile, FTIR,1H-NMR/13C-NMR, GPC, thermal
properties and peroxide value, were used to characterize the chemical
structure. Results indicated a lower content of polyunsaturated fatty
acids, increased content of saturated fatty acids, and the presence of
isomers. An insoluble portion was observed in the solid fraction and
increase with treatment time up to 42% in the 6h treated samples.
Plasma species may cause two main reactions: polymerization and
hydrogenation.
Keywords: cold plasma, soybean oil, hydrogenation, polymerization, oil
modification
INTRODUCTION
As a food ingredient, soybean oil is used mainly as cooking oil or salad
dressing. The high content of unsaturated fatty acids makes liquid
soybean oil susceptible to oxidation and limits its range of
applications. The partial hydrogenation of soybean oil reduces the
polyunsaturated fatty acid content leading to a semi-solid fat that is
less susceptible to oxidation. However, this process isomerizes the
monounsaturated fatty acid, generating 25-40% of trans fatty acids
(Gunstone, Harwood, & Harwood, 2007). The consumption of trans fatty
acids is linked to an increased risk of cardiovascular diseases, and
nowadays, partially hydrogenated oils (PHO) are not allowed to be used
as food ingredient (FDA, 2015).
Vegetable oils are also an essential source for the production of
lubricants and polymers because of their availability and low cost.
Diverse technologies have been studied to modify vegetable oils to
obtain epoxides, polyurethanes, polyamides, polyols, and many other
structures with valuable physical and chemical properties (Miao, Wang,
Su, & Zhang, 2014). The polymerization reactions occur mainly in the
carboxylic end of fatty acids and the double bonds of triglycerides.
Additionally, cold plasma has been studied as a processing aid to
functionalize organic compounds (Pankaj et al., 2017). Collisions of
electrons with molecular or atomic gases generate a series of reactions
that lead to the formation of ions, radicals, and other excited plasma
species that are highly reactive. This technology has been used to add
chemical groups (deposition) to organic structures, and it has been
studied as a method to modify edible films, polymers, or graphene
(Ostrikov, Neyts, & Meyyappan, 2013). Nitrogen-doped graphene has been
synthesized using ammonia or nitrogen gas (Wang, Maiyalagan, & Wang,
2012). Hydrogen plasma species can saturate a monolayer of carbon atoms
tightly packed, by exposure to a low-pressure cold plasma (Burgess et
al., 2011). Plasma hydrogenation of graphene converts a highly
conductive material into an insulator, by the deposition of hydrogen
atoms on its structure. These are examples of the versatility of this
technology to form new products by adding specific atoms or molecules.
The reaction products can be controlled by gas composition, input
energy, frequency, or treatment time.
High voltage atmospheric cold plasma (HVACP) is a technology that has
been studied to decontaminate food products and to modify food chemical
structures. This technology is designed to treat a sample contained
within a fixed gas volume under a dielectric barrier discharge high
electric field (Misra et al., 2019). The soybean oil treatment
temperature is maintained lower than 50°C, which is considered ‘cold’ in
comparison with thermal plasma that can reach temperatures of
2000-3000°C. The treatment of soybean oil using cold plasma is a
potential processing technology to reduce the formation of trans fatty
acids (Yepez & Keener, 2016).
The goal of this study is to analyze the effect of HVACP treatment on
the chemical structure of soybean oil in a hydrogen gas environment at
atmospheric pressure. Fatty acid composition is measured to determine
changes in the unsaturation degree. FTIR, 1H-NMR, and13C-NMR and GPC techniques were used to acquire
information related to structural modifications. Thermal properties of
all the samples were also investigated.
MATERIALS AND METHODS