Solid-state modification of tapioca starch using non-thermal plasma

Abstract

The objective of this research was to investigate the physical, chemical, and functional properties of tapioca starch subjected to solid-state modification by using nonthermal dielectric barrier discharge (DBD) plasma treatment using different gases (helium and argon), voltage levels (10 kV and 15 kV), and treatment times (5, 10, and 15 minutes). After treatment, an increase in L* value and a marginal decrease in pH value of all treated samples were noted. Scanning electron micrographs showed dented starch granules after the plasma treatment but showed birefringence. From XRD measurement, all samples showed a characteristic C-type diffraction pattern but a reduction in the relative crystallinity after plasma treatment. Native tapioca starch contained 99.06% carbohydrate, 0.15% crude fiber, 0.18% fat, 0.14% ash, and 0.46% protein on a dry weight basis. There was no significant difference in the moisture content of argon plasma-treated samples (P≥0.05), but a reduction in moisture content was found after using helium plasma treatment (P<0.05). The modified starch samples tended to have higher amylose content and reducing sugars with increasing voltage levels and times. However, increasing the treatment time from 5 to 15 minutes at 15 kV for helium plasma caused the amylose content to reduce (P<0.05). Fourier Transform Infrared (FTIR) spectra did not show the new formation of functional groups after treatments. No significant change in the intensity ratio of 1047/1022 cm-1 compared to native samples (P≥0.05) except for the treatment at 10 kV for 5 min of argon plasma (P<0.05). After treatment, the weight-average molecular weight of amylopectin was smaller than that of native starch. A significant reduction in peak viscosity, breakdown, and setback was observed (P<0.05). However, no significant change in the pasting temperature of treated samples was found compared to the native sample. The samples with lower breakdown and setback percentages were found for 5 minutes treatment using argon plasma (P<0.05). After increasing voltage levels and times of modification, an increase in solubility and water binding capacity was observed, while swelling power was reduced. The frequency sweep rheological test revealed that 6% (w/w) starch paste samples had higher G' than G", indicating dominant elastic behavior. The higher G' value of starch pastes was observed when using 5 minutes of argon and helium plasma treatment with lower tan δ. However, G' decreased with increasing treatment time due to depolymerization. Flow tests on 6% gelatinized modified starch pastes showed a lower thixotropy and consistency index (K) value after increasing both gas types' voltage levels and treatment time. The results confirmed the effectiveness of DBD plasma treatment in modifying tapioca starch properties.