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Abstract

Plasma consists of ions, electrons, and free radicals. They can interact with surrounding molecules and radicals of bioactive compounds in Centella asiatica juice. Plasma can interact hydrogen radicals from antioxidants, resulting in losses of anti-oxidative capacity of the original bioactive compounds. This study aimed to investigate the effects of plasma gas type, plasma radical generation, plasma inactivation of polyphenol oxidase, and effects of plasma on quality of Centella asiatica juice. This research study consisted of 1) a study to find suitable concentration of chlorine solution for cleaning of fresh Centella asiatica; 2) a study of plasma generation from different plasma gas types in a model system; 3) a study to find suitable juice concentration for plasma treatment; 4) a study on the effects of plasma on quality of Centella asiatica juice; 5) a study on effects of air on plasma generation and quality of plasma treated Centella asiatica juice, as compared to heat treatment; and 6) a study on resistance of polyphenol oxidase two plasma. Plasma power was kept constant at 15 kV for all study. The study to find suitable concentration of chlorine solution for cleaning of Centella asiatica leaves before juice extraction was done at 5 concentrations (100, 200, 300, 400, 500 ppm). Microbial analysis was conducted on the sample after cleaning. It was found that chlorine concentration of 100 ppm suitable for cleaning Centella asiatica leave to reduce initial microbial load of the sample. The study on plasma generation from different plasma gases in a simulated system utilized argon and argon mix with air. Two types of plasma gases were used: argon and argon mixed with air. The plasma species were measured using an optical emission spectroscopy (OES). It was found that plasma from argon gas alone consisted of hydroxyl radicals (OH-), hydrogen atoms (H+), and argon radicals (Ar). When air was mixed with argon gas, the same the same plasma species were generated, plus nitrogen radicals (N2) from the air. Therefore, the type of gas determines the plasma species that would be obtained. Because plasma species from argon gas are most likely to have less reactivity, therefore it was selected as the main plasma gas for further study to fine the effect of plasma on quality of Centella asiatica juice. Centella asiatica leaves cleaned with 100 ppm chlorine solution was used in the study to find suitable juice concentration for plasma processing. 3 concentrations (leave:water ratio of 1:6, 1:8, 1:10) were studied. The juice samples were subjected to plasma treatment using argon at flow rate of 10 L/mim as the plasma gas. Plasma treatment took 120 min and the antioxidant activity (DPPH assay) of the juice was analyzed. The juice concentration of 1:8 was found to be suitable for plasma treatment since higher DPPH activity than that of the juice with other concentrations was found. The study on the effects plasma gas on the quality of Centella asiatica juice used juice concentration of 1:8, and 4 plasma conditions (PL1 (argon gas at 10 L/min), PL2 (argon gas at 10 L/min + 0.4 L/min of oxygen), PL3 (argon gas at 15L/min), and PL4 (argon gas at 15 L/min + 0.4 L/min of oxygen). Analysis of DPPH activity was conducted at different times for 120 min. It was found that PL3 (argon gas at 15 L/min) resulted in an increase of antioxidant activity. Therefore, the argon gas flow rate of 15 L/min was selected for further experiment. The study on the effects of air on plasma and quality of Centella asiatica juice was conducted and compared to heat treatment to find suitable plasma condition for juice treatment. Three different plasma conditions were used: P1 (argon gas at 15 L/min), P2 (argon gas at 15 L/min + 5 L/min of air), and P3 (argon gas at 15 L/min + 10 L/min of air). The plasma treatment was conducted for 120 min. The antioxidant activity (DPPH assay), total phenolic content, asiaticoside content, and asiatic acid content of the juice were analyzed at different times. The results showed that P1 (argon gas at 15 L/min) was a suitable condition. It gave a significant increase of DPPH activity (p≤0.05), while the ABTS slightly increased (p>0.05). FRAP slightly decreased (p>0.05). Total phenolic compounds and asiaticoside, and asiatic acid contents were found at 149± 3.11, 0.94±0.27, and 24.99±0.76, respectively, which were slightly decreased as compared to the initial values (p>0.05). The physical quality of the juice was not significantly affected (p>0.05). Plasma could reduce polyphenol oxidase activity by 2% after 120 min treatment, whereas heat treatment could reduce the enzymatic activity by 77%. Studying the resistance of polyphenol oxidase enzyme by plasma technology. The D-value, Z-value, and Ea of the polyphenol oxidase enzyme were determined. The results showed that the D-value was in the range of 203.92 ± 17.03 to 375.54 ± 65.90 minutes, the Z-value (10-30°C) was 76.96 ± 14.20°C, and the Ea was 16.19 kJ/mol. These results suggest that plasma treatment is not as effective as heat treatment in destroying polyphenol oxidase enzyme. Because the previous study showed that plasma treatment could slightly reduce polyphenol oxidase activity, resistance of the enzyme to plasma was investigated. D and z-values of polyphenol oxidase were determined using enzyme concentrations of 100 and 200 units. Initial temperatures of the juice sample were 10, 20 and 30oC. Activation energy (Ea) was also determined. D values were in the range of 203.92±17.03 – 375.54±65.90 min; z values were in the range of 76.96±14.20°C. Ea of the polyphenol oxidase was 16.19 kJ/mol, which was lower than the Ea in thermal process (64.22 kJ/mol). Therefore, the suitable conditions for plasma treatment of Centella asiatica juice found in this study were: using 100 ppm chlorine solution for cleaning sample before extraction; juice extraction using plant to water ratio of 1:8; and using argon gas at the flow rate of 15 L/min as the plasma gas, not necessary to mix air in the plasma gas. Overall, the results of this study suggest that plasma treatment is a promising technique for improving the quality of Centella asiatica juice. However, further studies are needed to optimize the plasma treatment conditions and to assess the long-term stability of the Centella asiatica juice treated with plasma.