A blend of lemongrass, clove, and thyme essential oils encapsulated in hydroxypropyl methylcellulose nanofibers by electrospinning: characterization and biological activity

Imagine the world of food preservation as a constant battle between deliciousness and decay. For decades, we've relied on synthetic chemicals to keep our food fresh, but there's a growing desire to swap those lab-made additives for nature's own arsenal: essential oils. Think of these oils as tiny, potent warriors extracted from plants like lemongrass, cloves, and thyme. They are famous for their ability to zap bacteria and mold, but they have a fatal flaw: they are incredibly fragile. Exposed to air, heat, or light, they evaporate or break down before they can do their job. It's like trying to use a superpower that disappears the moment you open your mouth to speak. To fix this, scientists use a technique called "encapsulation," which is essentially building a microscopic fortress around these volatile oils to protect them and release them slowly when needed. One popular way to build these fortresses is "electrospinning," a process that uses electricity to stretch liquid polymers into ultra-fine fibers, creating a web so thin it's measured in nanometers—billionths of a meter.

This study takes that concept and applies it to a specific "dream team" of essential oils: lemongrass, clove, and thyme. The researchers wanted to see if they could trap this blend inside fibers made from hydroxypropyl methylcellulose (HPMC), a safe, plant-based polymer often used as a thickener in food. The goal was to create a material that acts like a slow-release shield, keeping the oils stable until they are needed to fight off food-spoiling bugs like Escherichia coli, Staphylococcus aureus, and various molds. By turning these oils into nanofibers, the team hoped to create a new, natural tool for the food industry that could replace harsh chemicals and keep our food safer and fresher for longer.

The Experiment: Weaving a Natural Shield

The researchers started by mixing their three essential oils together in a specific recipe: 50% lemongrass, 25% clove, and 25% thyme. They then dissolved a plant-based polymer (HPMC) in a mixture of alcohol and water to create a liquid "spinning solution." Into this solution, they added different amounts of the essential oil blend—ranging from 0% (just the polymer) up to 20%.

Using an electrospinning machine, they zapped these solutions with high voltage. This electric charge pulled the liquid into incredibly thin streams that solidified into nanofibers as the solvent evaporated. It's a bit like using a magical hairdryer that shoots out threads so fine you need a powerful microscope to see them. The team created five different batches of these fibers, each with a different concentration of the essential oil "warriors" inside.

What They Found: The Sweet Spot and the Flattening

When they looked at the fibers under a scanning electron microscope, the results were fascinating. The fibers without any oil (the control) and those with low amounts of oil (5% and 10%) looked like perfect, round, cylindrical tubes. They were smooth and uniform, with no beads or defects. However, as they crammed more oil into the fibers (15% and 20%), the shape changed. The fibers began to look flattened or collapsed, almost like a wet noodle that lost its structure. This happened because the oil made the solution less conductive and changed its flow, making it harder for the electric field to stretch the fibers into perfect tubes.

Despite the shape changes, the fibers were incredibly thin. The average diameter ranged from about 63 nanometers for the plain fibers to about 160 nanometers for the ones packed with 20% oil. Even at their "fattest," they were still microscopic marvels.

The team also tested how well the fibers held onto the oil. They found a "sweet spot" at 5% oil concentration. At this level, the fibers held onto a massive 58.5% of the oil (Load Capacity) and managed to trap 94.2% of it inside without it leaking out (Encapsulation Efficiency). When they tried to pack in more oil (10%, 15%, or 20%), the efficiency dropped. The polymer matrix seemed to get overwhelmed, and the fibers couldn't hold the extra oil as tightly, leading to more oil sitting on the surface or being lost during the process.

The Superpowers: Fighting Germs and Mold

The real test, of course, was whether these fibers could actually stop bad bacteria and mold. The researchers put the fibers in a sealed environment with the microbes, letting the volatile oils drift out and do their work.

• Against Bacteria: The fibers were very effective against Staphylococcus aureus (a common skin and food bacteria). The fibers with 10% oil were the champions, stopping about 98.4% of the bacteria. They were also good against E. coli, though the results were a bit more mixed; the 15% oil fibers were the most effective here, stopping about 73.8% of the bacteria. Interestingly, the fibers worked better against the Gram-positive bacteria (S. aureus) than the Gram-negative ones (E. coli), which is a common trend because Gram-negative bacteria have a tougher outer shell.
• Against Mold: The results against mold were even more impressive. The fibers containing 15% and 20% oil completely stopped the growth of Botrytis cinerea (a mold that ruins fruits and vegetables), achieving 100% inhibition. They also did a great job against Penicillium crustosum, stopping 100% of it at the 20% concentration.

Heat, Light, and Stability

The team also checked if the fibers protected the oils from heat. When they heated the pure essential oil blend, it started to evaporate and break down at relatively low temperatures. However, when the oils were inside the HPMC fibers, they held up much better. The fibers seemed to act like a thermal blanket, delaying the evaporation and keeping the oils stable at higher temperatures. This suggests that the encapsulation successfully shielded the delicate compounds from the harsh environment.

They also tested the fibers' "antioxidant" power, which is their ability to neutralize harmful free radicals that cause food to go rancid. The fibers with 20% oil showed antioxidant activity almost as strong as the pure oil itself, proving that the active compounds were still working even after being trapped in the fiber.

The Verdict

This study suggests that turning a blend of lemongrass, clove, and thyme essential oils into HPMC nanofibers is a promising strategy for food preservation. The fibers successfully protected the oils, maintained their ability to fight bacteria and mold, and even improved their heat stability. While the fibers got a bit "flattened" when packed with too much oil (15-20%), they still performed exceptionally well, especially against mold.

The authors conclude that these nanofibers could be a game-changer for the food industry, offering a natural alternative to synthetic preservatives. They could be used in "active packaging" (like a wrapper that actively fights germs) or as small sachets that release protective vapors. However, the paper notes that before these fibers hit the supermarket, more work is needed to test how they behave in real food, how long they last on shelves, and whether they are safe for humans to eat. For now, they remain a highly promising, nature-powered shield for our food.