Effects of plasma treatment on the structure and functional properties of flaxseed gum

Flaxseed gum is an anionic heteropolysaccharide located in the mucinous cell layer of flaxseed hull, accounting for 8%-10% of flaxseed mass. Since the type of hydrophilic colloid has favorable thickening, emulsifying and weak gelling properties, the flaxseed gum can be used to construct the delivery system of sensitive bioactive components using the electrostatic interaction and/or Maillard reaction with proteins. The potential of flaxseed gum to stabilize the sensitive bioactive components, such as n-3 fatty acids and carotenoids largely depends on the naturally occurring 2S storage protein (conlinin). Moreover, the space network structure formed by the intermolecular cross-linking also partly contributes to the potential of flaxseed gum acted as a delivery vehicle. However, it is still necessary to further improve the delivery stability of the system constructed by the single flaxseed gum through specific physical modification, and thereby broaden the promising application in healthy food. As new non-thermal technology for food processing, plasma is gradually applied to the moderate modification of food macromolecules, including protein, starch, and chitosan. It is unknown whether the plasma can pose a beneficial effect on the flaxseed gum and the coexisting protein or phenolic compounds in the aqueous phase. The current study aims to explore the effects of different plasma treatment time (0, 5, 15, 30, 60, 90 and 120 s) on the selected techno-functionality of flaxseed gum at the mass concentration of 5 mg/mL, including the thermal property, rheological behavior, emulsion-stabilizing and in vitro antioxidant capacities. An atmospheric pressure plasma jet (APPJ) treatment during 0-120s was used to explore the changes of pH value, zeta potential, monosaccharide composition, molecular weight distribution, microstructure, interfacial adsorption property, total phenolic, and flavonoids. The results showed that the pH value and zeta potential of flaxseed gum gradually decreased, due to the acidification of solution, indicating the production of NO2-, NO3-, ONOO- following plasma treatment. The depolymerization of flaxseed gum was induced with the plasma treatment time extending from 30 to 120 s, resulting in a decrease in average molecular weight. Thermogravimetric (TG) analysis showed that the plasma treatment reduced the carbonization temperature of flaxseed gum due to its depolymerization. The total phenolics, flavonoids, and in vitro antioxidant activities of flaxseed gum showed a repetitive trend of first decreasing and then increasing, as demonstrated by the DPPH and FRAP assays. It can be attributed mainly to the dissociation, release, and oxidative depletion of small amounts of free phenolic acids as well as the subsequent depolymerization of lignan macromolecules in flaxseed gum following 0-120 s of plasma treatment. Moreover, the plasma treatment further affected the apparent viscosity and interfacial adsorption properties of flaxseed gum. A short-time plasma treatment improved the potential of flaxseed gum to stabilize the flaxseed oil emulsion, characterized by a Turbiscan Lab stability analyzer with multiple light scattering technique. The cryo-SEM images revealed that the flaxseed gum was involved in the formation of interfacial film for lipid droplets, and simultaneously maintained the spatial distribution of lipid droplets using spatial network structure following 0-15 s of plasma treatment. Consequently, a short-time plasma treatment can be used to effectively improve the emulsion stability potential and in vitro antioxidant capacities of flaxseed gum, and thereby serve as an effective way to tailor the techno-functionality of flaxseed gum for the extensive application in health food.