Process of zein self-assembly fibrosis assisted by plasma treatment

Protein fibers are characterized by the self-assembled linear aggregates of proteins or peptides. Furthermore, the protein fibers can be expected to serve as an effective delivery carrier for water-insoluble bioactive compounds in food systems. Specifically, a better carrier can be generated for the delivery of hydrophobic bioactive substances, due mainly to its unique structure and high aspect ratio. Conventionally, protein fibrous aggregates are usually formed by proteins rich in β-sheet structure under the conditions of far below isoelectric point pH value and low ionic strength using long-term heating. However, the low preparation efficiency and high energy consumption have greatly limited to select raw materials for protein fibers. The traditional acid-thermal method has also been used to obtain the zein hydrophobic plant proteins at the low conversion rate, due to its special solubility and secondary structure. The application of such protein fiber is still lacking in the field of protein fibrosis. Among them, zein is one of the most important by-products in the process of industrial extraction of corn starch. The self-assembly properties of zein can have promising potential to be an ideal substrate for the fabrication of nano-biocomposite delivery systems in food and nutritional applications. Alternatively, the plasmas were the quasi-neutral particle systems of semi-gas and semi-fluid mixtures with high active moieties. The green, emerging, and non-thermal plasma technology has also great application potential in the food industry. This study aims to improve the fiber conversion rate of zein using plasma technology. An assessment treatment was innovatively used to modify the zein before acid-heat induction. A systematic investigation was then implemented to explore the influence of different process parameters during acid-heat induction on the degree of fibrosis of zein after plasma treatment. Single factor and response surface experiments were conducted to obtain the optimal condition of the fibrosis process in the plasma treatment power, plasma treatment time, heating time, heating temperature, and zein concentration. As such, the thioflavin T (ThT) fluorescence intensity showed that the plasma-assisted acid-heat induction effectively improved the degree of fibrosis of zein. The optimal parameters of fibrotic zein were as follows, plasma treatment power of 64 W, plasma treatment time of 61 s, heating time of 10 h, heating temperature of 70℃, and the concentration of zein of 30 mg/mL. After that, the ThT fluorescence intensity of fibrotic zein reached 2 272±23, which was significantly higher than that of the fibrotic zein without plasma treatment (1 239±19). The fitting analysis was then performed on the amide I region (1700-1600) cm^-1 in the Fourier transform infrared (FT-IR) spectrum. It was found that the plasma treatment increased the β-sheet structure, but decreased the α-helix structure of zein. Transmission electron microscopy (TEM) images showed that the particle size of zein decreased significantly after fibrosis, where the fiber structure was formed by linear aggregation of spherical protein particles along the horizontal direction. Therefore, the plasma treatment can be expected to induce the formation of more zein fiber structures. The finding can provide a strong reference and theoretical support for the development of high-efficiency fibrosis of zein, thus extending the application field of plasma technology in protein fibrosis.