Atmospheric Cold Plasma Treatment of Skim Milk for Enrofloxacin Reduction Using a Multiple Plasma Jet–Bubble Column Reactor

Milk that has been treated with antibiotics for a sick cow cannot be sold for human consumption. This "waste milk" poses a serious problem: if it is fed to calves, it can breed bacteria that resist future medicines, and if it is dumped, it can pollute the environment. The challenge is that standard heating methods, which kill germs, do not reliably destroy these antibiotic residues. Scientists have been searching for a way to clean this milk without cooking it, looking for a method that can break down the drug molecules while minimizing damage to the milk's quality. One promising avenue is atmospheric cold plasma, a state of matter often called the fourth state of matter, which is created by electrifying a gas. Unlike the plasma in a neon sign or a lightning bolt, this version stays cool enough to touch. It is filled with highly reactive particles that can attack and break apart chemical bonds, making it a potential tool for scrubbing contaminants from liquids without raising their temperature.

In a recent study, researchers at Shiraz University in Iran tested whether this cool, electrically charged gas could remove enrofloxacin, a common antibiotic, from skim milk. They built a specialized device that looks like a cluster of tiny glass tubes surrounding a central metal rod. When they pumped air through these tubes and applied a high-voltage electrical current, the air turned into a cold plasma. Instead of just blowing this plasma over the surface of the milk, they bubbled it directly through the liquid inside a column. This bubbling action ensured that the reactive particles generated by the electricity mixed thoroughly with the milk, attacking the antibiotic molecules from the inside out. The team wanted to see if they could tune the machine—adjusting the speed of the air, the strength of the electricity, and the duration of the treatment—to strip the milk of its antibiotic properties while causing as little damage to the milk itself as possible.

The results showed that the method worked remarkably well at removing the antibiotic's power. Under the best conditions, which involved a specific voltage and an air flow rate of 700 milliliters per minute, the treatment reduced the antibiotic's ability to kill bacteria by 95 percent after just 20 minutes. To confirm this, the researchers used two different ways of looking at the milk. First, they used a biological test where they placed drops of the treated milk on a plate with bacteria; the bacteria grew freely where the antibiotic had been destroyed. Second, they used a laser-based technique called Raman spectroscopy, which acts like a molecular fingerprint scanner, to see if the actual shape of the enrofloxacin molecules had changed. This scanner showed that the drug molecules themselves had been broken down by about 84 percent. The close match between the biological test and the molecular scan suggested that the antibiotic wasn't just being hidden or masked; it was being chemically dismantled.

However, the study also revealed that this powerful cleaning process comes with a cost to the milk's quality. As the plasma bubbled through the liquid, it did not just target the antibiotic; it interacted with the milk's natural ingredients as well. The milk became noticeably more acidic, with its pH dropping from a neutral 6.38 down to 5.53. The sugar in the milk, known as lactose, decreased significantly, falling from 56.9 grams per liter to 47.3 grams per liter within the first five minutes of treatment. The milk also became thicker, with its viscosity rising from 1.61 to about 2.05 millipascal-seconds, and the total amount of dissolved solids dropped. Visually, the milk changed color, taking on a yellowish-brown tint. This browning happened because the reactive particles broke down the sugars, which then reacted with the milk proteins to form new compounds, a process similar to how toast turns brown when heated, though here it was driven by the plasma's chemical activity rather than heat.

The researchers found that the settings of the machine mattered greatly. If the air flowed too slowly, the plasma didn't generate enough reactive particles to clean the milk effectively. If the air flowed too fast, the gas spent too little time in the electrical field to become potent, and the bubbles became too large to mix well with the milk. Similarly, increasing the electrical voltage beyond a certain point made the process less efficient. While higher voltage creates more energy, it also heats the gas and causes the reactive particles to destroy each other before they can reach the antibiotic. The sweet spot was a moderate voltage of 8.6 kilovolts, which provided the right balance of energy to break down the drug without wasting the reactive particles or overheating the system.

Ultimately, the study demonstrates that atmospheric cold plasma is a viable, non-thermal way to neutralize antibiotic residues in milk, offering a potential solution for managing waste milk that is currently difficult to handle. The technique successfully destroyed the vast majority of the enrofloxacin, proving that the drug's structure was altered and its biological activity eliminated. Yet, the significant changes to the milk's chemistry, texture, and color mean that this technology cannot simply be switched on without careful adjustment. The researchers concluded that while the method is effective, future work must focus on fine-tuning the process to remove the contaminant while preserving the milk's natural qualities, ensuring that the solution to one problem does not create another.