Effect of dielectric barrier discharge plasma on structural and physical characteristics of gum arabic

Introduction: Gum arabic is a branched polysaccharide and categorized as a protein-arabinogalactan hybrid. The presence of protein part along with the carbohydrate portion has led to its amphiphilic features (Dickinson, 2003). Structurally, it’s a disk structure that includes galactopyranose units in the main and lateral chains. This gum is widely used in the preparation of emulsions, encapsulation of flavors, and the production of hydrogels in the food and non-food industries (cosmetic and pharmaceutical products) (Williams and Phillips 2009). In recent decades, the modification of biopolymers has been one of the interesting topics, on the one hand, to solve their problems and, on the other hand, to develop new products. So far, numerous studies have been done to improve gum arabic properties by various chemical and physical approaches. Among physical methods, cold atmospheric pressure plasma is a new, green, and non-thermal method (De Geyter and Morent 2014). Plasma is defined as an ionized gas consisting of electrons, positive and negative ions, free radicals, excited and non-refractory gases, as well as electromagnetic rays (UV and UV photons( which these particles are constantly interacting with each other( Conrads and Schmidt 2000). One of the applications of cold atmospheric pressure plasma is a modification of biopolymer structures through surface modification. Exposing the surface of biopolymers to bombard electrons, free ions, free radicals, and the emission of electromagnetic radiation causes some chemical and physical reactions on the surface of the material, which also leads to changes in the functional properties of matter. To date, no data have been published on the effect of plasma on gum arabic.
Material and Methods: A dielectric barrier discharge apparatus was used for plasma formation and sample treatment. The device consisted of two parallel rectangular aluminum electrodes (17 * 8 cm) with a 28 mm gap between them, and each electrode was covered with a mica sheet as a dielectric barrier. A certain amount of gum arabic powder was placed on the lower plate and treated for 10 minutes. The humidity and ambient temperature were adjusted to 85 and 20 °C, respectively, and were monitored during treatment with a thermo hygrometer. Then flow behavior, FT-IR, XRD, water contact angle, color, and FE-SEM measurements were conducted to evaluate the effect of plasma treatment. The SPSS software using t- student test was applied to evaluate the effect of plasma treatment on gum arabic compared with untreated gum.
Result and discussion: The results showed that the oxidation and etching were two effective reactions in plasma treatment of gum arabic. The XRD analysis showed the plasma treatment had no significant effect on the gum arabic amorphous structure, although the increase in the peak of the treated gum compared with control depicted some changes in the structural order of gum arabic happened after plasma treatment. A comparison of the IR spectrum of the treated and control gum confirmed the presence of all informative peaks in relatively similar positions. The most noticeable changes in the IR spectrum of treated gum was recognized between 1750-1200 cm-1. The peak intensity of treated gum increased in band 1722 cm-1 compared with control, which might be due to the formation of carbonyl groups as a result of aldehydes oxidation. Other changes confirmed generation amine groups, carbon-hydrogen bonds, carbon-oxygen bonds as well as destroying some hydroxyl and hydrogen-carbon groups. A comparison of the obtained micrographs revealed that plasma treatment increased the surface roughness of gum arabic powder. Measuring the contact angle of the specimens indicated the ability of the plasma to change the hydrophilic nature of gum arabic to hydrophobic. The main cause of this occurrence was physical and chemical changes due to surface etching and changes in the ratio of hydrophilic and hydrophilic groups. Investigating the variation of viscosity versus shear rate showed that with increasing shear rate, the apparent viscosity of 40% (w/w) of solutions was gradually reduced, indicating that shear-thinning behavior of solutions. The results showed that the application of plasma treatment reinforced the shear-thinning behavior of treated gum arabic solution. The apparent viscosity of the treated gum solution at all applied shear rates was higher than the natural gum solution, which suggesting increasing intermolecular bonding and particle bonding with water as a consequence of plasma treatment. The power-law model properly described the natural and treated gum arabic flow behavior. The color analysis of the samples showed plasma treatment changed the color of gum arabic powder as a consequence of chemical and physical reactions.
Conclusion: Gum arabic is mainly used as a stabilizer/emulsifier in the food industry. The present study was performed to evaluate the influence of cold atmospheric pressure-air plasma on physical, rheological, and structural properties of gum arabic. According to the FT-IR and XRD data, plasma treatment did not affect the skeleton and amorphous nature of gum arabic, however, the structure order and some functional groups varied upon plasma treatment. Results of the current paper revealed that air plasma treatment significantly (p < 0.05) increased gum solution viscosity and shear-thinning behavior. The plasma treatment increased the roughness of gum arabic powders as a result of the etching process. The measurement of the contact angle of the specimens indicated the ability of the plasma process to change the hydrophilic nature of gum arabic to hydrophobic. The color of plasma-treated gum changed compare with untreated gum. These results will help researchers to deep insight into the effects of cold atmospheric pressure plasma on composite protein-carbohydrate structures and their new properties and thus finding the new capabilities of this technology as well as modified gum arabic in the food industry.