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