Effects of cold plasma treatment on the quality andantioxidant properties of mixed fruit juice

Abstract

In recent years, non-thermal processes have been used for food sterilization instead of thermal treatment methods. Cold plasma is an example of these new technologies. The fourth state of matter is called as plasma. Main plasma types are classified as cold and termal plasma. Cold plasma refers to plasma with a gas temperature below 1000 K. In this type of plasma, molecules, ions, and electrons are not thermodynamically in equilibrium due to its low temperature. The electron temperature ranges from 10^4-10^5K, while the ion temperature is approximately 25 °C. Cold plasma is applied at low pressure and low energy levels. On the other hand, thermal plasma refers to plasma with a gas temperature above 1000 K. It is used for the modification of heat-resistant inorganic materials due to its ability to reach high energy levels. Cold plasma can be applied at low energy levels, hence it is suitable for use in food applications. Food sterilization, packaging sterilization, surface disinfection of food, and enhancing seed germination speed obtained by plasma in different studies. The sterilization mechanism is achieved through reactive species released by plasma. It relies on the interaction between reactive components in the gas phase and microorganisms, leading to the disruption of their structure. Plasma is a mixture containing gases such as oxygen and nitrogen. When these gases are excited by plasma, reactive compounds generated such as free radicals and ions. These compounds can attach to the surfaces of microorganisms or react with their surroundings, causing damage to the cellular structure of microorganisms. Another inhibition mechanism is the destruction of microorganisms' genetic material by UV radiation and atomic erosion at the atomic level. This method can effectively play a role for inhibition of both vegetative cells and bacterial spores at ambient temperature and in a short time. Additionally, reactive compounds created by cold plasma can interact with microorganisms on the surface of food, leading to their inactivation. These interactions can disrupt the cellular membranes of microorganisms, denature their proteins, or render their enzymes inactive, as a result of this reactions product staling time can be longer. Various mechanisms are being used to generate cold plasma. One of the most commonly used mechanisms is Dielectric Barrier Discharge (DBD) which is having a dielectric material placed between two electrodes and applying an alternating electric voltage to these electrodes. The dielectric material interacts with the electric field, leading to the formation of plasma. Another system is microwave plasma, which generates plasma using microwave energy. Microwave energy is delivered to the plasma chamber through a resonator or waveguide. Microwave radiation stimulates gas molecules, resulting in plasma formation. Corona discharge is a mechanism in which an electrical discharge occurs between a high-voltage electrode and the ground, ionizing the electrical gap to create plasma. The electric field on the electrode ionizes the gas molecules, leading to plasma formation. High-frequency plasma involves applying high-frequency electric current between electrodes to trigger plasma formation. Radio frequency plasma is another mechanism that utilizes radio frequency energy to create plasma. A radio frequency electric voltage is applied between electrodes, resulting in plasma formation. In this study, DBD mechanism was used and plasma treatment was carried out using a pulsed DC power source at 40 kV (56 Hz, 10 mA) for 0 min (control) , 10 min, or 20 min. Its effects on mixed fruit juice was compared to pasteurized samples to observe differences in the quality properties of juice. The outputs of study submitted that cold plasma treatment has positive effects on the quality and antioxidant peculiarities of fruit juice. The color change in samples operated with plasma was barely noticeable compared to pasteurized samples. However, there was an impressive difference in color between pasteurized juice and the control sample. The Brix° values remained unchanged as 10.5 in both processes. The total acidity significantly decreased in the pasteurized sample but no significant difference was observed between the plasma-handled samples and the control sample (p>0.05).The total viable count was found to be extremely low (Log <2), indicating effective microbial reduction. Despite the inhibition observed, there was no significant decrease in mold and yeast counts during the plasma application (p>0.05). Nonetheless, a decrease in mold and yeast count was observed in the pasteurized sample compared to the control sample (p<0.05). It was concluded that cold plasma did not have an effect on the microbial count, but this can be improved by optimizing process parameters. The activity of antioxidant was evaluated using three different methods. No difference was observed according to the DPPH assay. The ABTS value decreased after the cold plasma treatment. The highest CUPRAC value was recorded after the 20 min treatment. The DPPH and CUPRAC values declined in the pasteurized samples compared to the cold plasma samples, but no significant difference was observed in the DPPH assay (p>0.05). The total phenolic content showed a slight decrease in the pasteurized sample compared to the cold plasma samples, but this distinction was not statistically important (p>0.05). The color properties of the fruit juice were recorded after both processes. In the pasteurized samples, a significant color change was observed compared to the plasma- samples. The total color difference was recorded as 20.86±1, while the ratio in the plasma-treated sample was 2.86±2.3, indicating that this color difference was too small to be noticeable. In conclusion, cold plasma process preserved the appearance, taste, and antioxidant properties of fruit juice compared to pasteurization. By optimizing process parameters for microbial inhibition, this method can be applied to fruit juice production.