The Effect of Non-thermal Plasma on Improvement of Functional Properties of Sorghum Starch

Introduction Sorghum is a valuable source of starch for human use, being a drought-tolerant cereal grain that contains a large amount of starch (approximately 70%). However, native sorghum starch has limited application in the food industry due to its poor functional properties. Modification of sorghum starch would overcome its shortcomings and tailor it to the targeted application. Among physical methods, non-thermal plasma is a novel method for starch modifications. Plasma is an ionized gas including electrons, atoms, ions, radicals, and quanta of electromagnetic radiation that affects the functional properties of starch. The effect of plasma on starch is influenced by apparatus type, treatment conditions (feed gas, time, and power), and the source of starch. Two main mechanisms of starch modification are known as  cross-linking and oxidation together with depolymerization. Although the effect of plasma on many types of starch has been investigated, no research has yet been found on sorghum starch modification by non-thermal plasma. So, this investigation determines the effects of non-thermal plasma on sorghum starch to overcome the deficiency of the native form and to explore wider applications for sorghum starch.   Materials and Methods  Sorghum starch was extracted by alkaline steeping and purified using toluene-salt-water treatment. Dielectric Barrier Discharge (DBD) plasma was performed to modify sorghum starch. The applied DBD plasma setup consisted of two flat rectangular aluminum electrodes with the dimension of 6.5×45 cm and 7×18 cm and an electrode distance of 3 and 6 mm for air and argon plasma, respectively. Each of the electrodes was covered with a mica sheet as a dielectric barrier. DBD reactor was supplied with alternating current (AC). The frequency was adjusted to 375 Hz. Starch samples were treated for 1, 10, and 20 min at 23 kV at atmospheric pressure. The amylose content of sorghum starch was determined by iodine binding colorimetry. Evaluation of other chemical parameters including protein, lipid, ash, and moisture was carried out according to AAC methods. The clarity was determined using a spectrophotometer at 650 nm. The swelling and solubility of 1.5% sorghum starch suspension (at 55, 65, 75, and 85 °C) were measured using the centrifuge method. The centrifuge-filtration method was performed to evaluate freeze-thaw stability of sorghum starches up to 4 cycles.   Results and Discussion  Chemical parameters showed that the extracted sorghum starch was purified. The amount of protein, lipid, ash, and amylose was 0.39, 0.15, 0.59, and 29.23%, respectively. Plasma caused significant altering in sorghum starch properties. Compared to the argon plasma, the air plasma was more effective at increasing the clarity, solubility, and freeze-thaw stability. Increasing the time of treatment also improved the above-mentioned functional properties. The clarity of native starch (14.02%) was increased to 56.10% for the sample treated with air plasma for 20 min, probably due to intense oxidation and depolymerization of starch molecules. While the lowest clarity (13.07%) belonged to the 1-min argon plasma treated sample, this value was improved with increasing time of treatment. Probably cross-linked bonds were predominantly formed during the first minute of argon plasma treatment, resulting in a reduction of paste clarity, while a competitive depolymerization and oxidation reaction could be a reason for the increase of paste clarity. Solubility was increased for all treatments (except for argon-1 min). The highest solubility in each of the temperatures was found for 20 min air plasma treated sample. Depolymerization of starch molecules under plasma treatment produces low molecular weight fragments which leach out easily and increase solubility. The swelling power of 20 min air plasma treated starch was lower than that of native starch, probably due to the structural disintegration. Other samples had higher swelling power. The lower freeze-thaw stability of 1 and 10 min argon plasma treated samples may be due to cross-linking which increase retrogradation. The 20 min air-plasma treated sample had higher stability than other samples in 3 and 4th cycles of freeze-thawing. The freeze-thawing stability of other samples was similar to that of the native starch.   Conclusion  Non-thermal plasma treatment improved the functional properties of sorghum starch. The best results were detected for the sample treated with air plasma for 20 min. Cross-linking may be the main reaction in the first minute of argon-plasma treatment. However, this mechanism was suppressed in a longer treatment time. It may also be stated that the effect of oxidation along with depolymerization was predominant in air-plasma treatment.