Preparation of Okara Cellulose Nanofibrils via TEMPO Oxidation Regulated by Ultrasound Assisted Swelling Pretreatment and Their Application in Pickering Emulsions

Okara insoluble dietary fiber (ISF) was used as the raw material to prepare 2, 2, 6, 6-tetramethylpiperidinooxy (TEMPO) oxidized cellulose nanofibrils (TCNF) through ultrasound assisted swelling pretreatment combined with TEMPO-mediated oxidation and high-pressure homogenization. The effects of different pretreatment methods on the structure and properties of TCNF, as well as the stability of the resulting Pickering emulsions, were investigated. The results showed that both ultrasonic and swelling pretreatments enhanced the efficiency of TEMPO oxidation, among which the combination of NaOH/thiourea swelling and ultrasonic treatment exhibited the most pronounced synergistic effect. Under this treatment, the carboxyl content of TEMPO-oxidized ISF reached (1.28±0.03)mmol/g, which was 91.0% higher than that of the control group. Meanwhile, this treatment promoted fiber bundle disintegration, reduced the size of cellulose nanofibril aggregates, and improved the surface accessibility of cellulose, thereby facilitating carboxylation and increasing the absolute value of the ζ-potential of nanocellulose to 44.00 mV, resulting in relatively good storage stability and light transmittance. Microscopic observations revealed that the synergistic pretreatment led to a more uniform fibril dispersion with reduced aggregation, and the fibril diameters were mainly distributed in the range of 2-8 nm. Fourier transform infrared spectroscopy and X-ray diffraction analyses indicated that neither the pretreatment nor the oxidation process altered the type Ⅰ cellulose crystalline structure. Thermogravimetric analysis showed that the combined NaOH/thiourea and ultrasonic pretreatment slightly reduced the thermal stability of TCNF. Compared with the other pretreatment groups, the TCNF obtained using NaOH/thiourea and ultrasonic pretreatment exhibited relatively high water-holding and oil-holding capacities, reaching 70.10 g ·g^-1 and 113.32 g ·g^-1, respectively. When the nanocellulose prepared by NaOH/thiourea treatment combined with ultrasonication was used to formulate Pickering emulsions, the emulsions exhibited a smaller droplet size (20.99 μm) than those in the other pretreatment groups, together with good centrifugal and storage stability. These results demonstrated that the synergistic effect of ultrasonic and swelling pretreatments effectively enhanced TEMPO oxidation and subsequent nanofibrillation, thereby improving the dispersion stability, functional properties, and emulsifying performance of TCNF. This study was expected to provide an experimental basis for the high-value utilization of okara-derived nanocellulose.