Process optimization and structural characterization of resistant dextrin prepared by plasma-assisted acid-thermal method

Resistant dextrin (RD) has been widely produced as a low-molecular-weight soluble dietary fiber using pyrodextrinization via thermal-acid starch treatment. However, conventional approaches have been limited by high energy consumption, prolonged reaction times, as well as suboptimal yield and resistance. In this study, cold plasma (CP), a green physical field technology, was introduced as a pretreatment to produce resistant dextrin from waxy maize starch. Thermal-acid treatment was enhanced to characterize the structural, physicochemical, and rheological properties of the resulting product. Waxy maize starch was pretreated under atmospheric air using a dielectric barrier discharge plasma system. Single-factor experiments were conducted to evaluate the effects of plasma generation voltage, plasma treatment time, hydrochloric acid dosage, thermal-acid treatment time, and thermostable α-amylase hydrolysis temperature on resistant dextrin yield. Subsequently, an orthogonal array design was employed to optimize the parameters. The resulting resistant dextrin samples—with (CP-CRD) and without (CRD) plasma pretreatment—were analyzed for solubility, whiteness, freeze-thaw stability, and rheological behavior. Structural characterization was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and proton nuclear magnetic resonance (¹H NMR). The optimal combination of parameters was determined for preparation: plasma generation voltage of 80 V, plasma treatment time of 90 s, hydrochloric acid concentration of 12%, and thermal-acid reaction time of 60 min at α-amylase hydrolysis temperature of 94 °C. The yield of CP-CRD reached 83.12% under optimal conditions, higher than that of the control (72.60%). Physicochemical analysis revealed that CP-CRD exhibited improved solubility (96.66%), along with high freeze-thaw stability, as evidenced by smaller reductions in transmittance in multiple freeze-thaw cycles. Rheological tests showed that both CRD and CP-CRD displayed shear-thinning behavior; whereas CP-CRD exhibited higher elastic and viscous moduli, indicating a strengthened molecular network, due to plasma-induced cross-linking and incorporation of polar functional groups. Structurally, SEM images revealed that surface roughness and granule fragmentation increased in plasma-pretreated resistant dextrin. XRD and FTIR analyses confirmed complete loss of crystalline structure in both dextrins, indicating a more pronounced amorphous content and a higher R_1022/995 ratio. ¹H NMR analysis indicated that CP-CRD contained a lower proportion of α-1,4 glycosidic bond (36.68%) and higher levels of α-1,6 glycosidic bond (20.91%) and β-type linkages (β-1,2 glycosidic bond, β-1,4 glycosidic bond, β-1,6 glycosidic bond), along with an increased number of reducing ends. These structural variations contributed to a higher branching (42.18%) and a lower polymerization (3.32) in CP-CRD. In conclusion, cold plasma pretreatment enhanced the efficiency of thermal-acid treatment conversion of starch into resistant dextrin. The synergistic mechanism involved plasma-mediated depolymerization, short-chain fragments, surface microstructural modification, and introduction of polar groups, which collectively promoted more frequent and random glycosidic bond rearrangements and transglycosylation reactions during thermal-acid treatment. A final product was obtained with higher yield, solubility, and freeze-thaw stability. More branched, complex structure enriched in digestion-resistant linkages. These findings can also provide a theoretical basis for the green and efficient preparation of resistant dextrin using plasma-assisted technology.