The use of atmospheric cold plasma technology in reducing the antibiotic residue and microbial load in discarded dairy milk

Background and objective cold plasma of a semi-ionized gas includes ions, electrons, ultraviolet photons and radicals. Cold plasma is a non-thermal technology that uses high-energy and reactive gases to inactivate microorganisms on the surface of food products. In general, cold plasma is used to destroy chemicals or organic bacteria, airborne bacteria, and to disinfect food and equipment packaging. The purpose of this experiment was to investigate the use of atmospheric cold plasma technology in reducing antibiotic residue and microbial load in dairy milk.
Materials and Methods: In this study, in order to investigate the effect of atmospheric cold plasma, atmospheric dielectric barrier discharge device was used. 50 ml of milk containing 6 μg/ml antibiotic enrofloxacin and bacteria Staphylococcus aureus were used for the experiments. In order to investigate the effect of plasma on the reduction of antibiotic activity, sterile milk was exposed to plasma between 0 and 20 minutes. Then, for the antibiogram test, Mueller Hinton agar culture medium was prepared in the amount of 20 ml in a plate with a diameter of 10. After 18 hours of incubation of bacteria in Tryptic Soy Broth culture medium. Its antibacterial activity was determined by measuring the inhibition zone diameter. In order to count bacteria, 10 ml volume of milk after inoculation with desired bacteria at the rate of 107 CFU/mL was subjected to plasma flow for a certain period of time. CECIL-1100 HPLC device was used to evaluate the amount of antibiotic reduction. The column of this device was Eclipse pluse C18 from Zorbox company. To evaluate the quality characteristics of milk, 25 sterile milk samples were subjected to plasma flow at zero, 5, 10, 15 and 20 minutes. LactoScope FT-B device was used. Color values included L* (light), a* (red) and b* (yellow), which were measured using a chlorometer (CI-7600, X-Rite, Michigan, USA).
Results: The results showed that increasing the time of plasma flow in milk caused a significant decrease in the growth rate of bacteria (P<0.05). Also, the rate of bacterial inactivation decreased significantly and became similar to sterile milk prepared from the market (P < 0.05). The antibiotic effect of enrofloxacin decreased by 45% after 10 minutes of plasma flow and by 88% after 20 minutes of plasma flow, therefore, oxidizing agents and high-energy electrons in cold plasma effectively destroy enrofloxacin. With increasing plasma time, milk factors including fat percentage, electrical conductivity (milliseconds), non-protein nitrogen (mg/100 grams), percentage of trans fatty acids and percentage of saturated fatty acids and melamine (mg/kg) increased as well as percentage of non-fat solids, lactose, fatty acid in cis state, sucrose, pH, freezing point, ratio of both volatile fatty acids to milk fat (milliequivalent to 100 grams of fat), and citric acid to citrate (mg/kg) also increased. No significant changes were reported in the density (g/ml) and percentages of each crude protein total solids, mono fatty acid, casein, water and total protein. No changes in ΔE were observed during the cold plasma process, while a color change of 1.15 was recorded during the pasteurization process.
Conclusion: According to the results of this study, it can be concluded that atmospheric cold plasma with a cold plasma flow time of less than 10 minutes and a voltage below 8 kV can be used as a powerful strategy for discarded milk sterilization should be discussed.