Influence of Drying Techniques and Encapsulation Materials on the Antibacterial Activity of Lactiplantibacillus plantarum Cell-Free Supernatants

Food safety is a constant battle, one that grows more complex as the global population rises and the environment changes. For decades, the food industry has relied on chemical preservatives to stop spoilage and kill harmful germs, but consumers are increasingly seeking natural alternatives. Nature already provides a powerful defense in the form of lactic acid bacteria, a group of friendly microbes found in fermented foods like yogurt and sauerkraut. These bacteria do not just make food taste tangy; they also produce a cocktail of natural chemicals that can stop dangerous pathogens from growing. However, these beneficial chemicals are fragile. They can break down when exposed to heat, light, or oxygen, making them difficult to use in a practical, shelf-stable way. To solve this, scientists have turned to a technique called encapsulation, which involves wrapping these delicate substances in a protective shell, much like a seed is protected by a hard coat. The challenge lies in finding the right shell material and the right method to dry the mixture without destroying the active ingredients inside.

In a recent study, researchers set out to find the best way to preserve the antibacterial power of a specific strain of bacteria called Lactiplantibacillus plantarum. This microbe is famous for producing a liquid, known as a cell-free supernatant, which is rich in compounds that fight off spoilage and disease-causing bacteria. The team wanted to know how two different drying methods—spray drying, which uses hot air to quickly evaporate water, and freeze-drying, which removes water by turning ice directly into vapor—would affect the liquid's ability to kill germs. They also tested two different protective coatings: maltodextrin, a powder made from starch, and sodium caseinate, a protein derived from milk. The goal was to see which combination kept the antibacterial properties strongest, specifically against bacteria that spoil fish and those that cause foodborne illness in humans.

The researchers began by analyzing the chemical makeup of the raw liquid from the bacteria. They found it contained sixteen different components, including organic acids, phenols, and hydrocarbons, which are the weapons the bacteria use to fight off invaders. The most abundant of these were a type of phenol and acetic acid, the same compound that gives vinegar its sharp smell. To test the liquid's strength, the team created seven different versions. Some were left as a raw liquid, while others were dried using either spray drying or freeze drying. The dried samples were then coated with either maltodextrin or sodium caseinate. They tested these samples against a lineup of bacteria, including Staphylococcus aureus, a common cause of food poisoning, and several types of bacteria that cause fish to rot, such as Vibrio vulnificus and Photobacterium damselae.

The results revealed a clear winner in the battle to preserve the bacteria's power. The samples that were freeze-dried and coated with sodium caseinate proved to be the most effective at stopping bacterial growth. When placed on a plate with Staphylococcus aureus, this specific mixture created a zone of inhibition, an area where bacteria could not grow, that measured 23.67 millimeters across. This was nearly as effective as the raw, undried liquid, which created a zone of 24.67 millimeters. In contrast, the samples dried with spray drying generally performed worse, especially when coated with maltodextrin. For instance, the spray-dried version without any coating struggled to stop the fish spoilage bacteria, creating zones of inhibition as small as 11.50 millimeters. The study suggests that the gentle nature of freeze-drying, combined with the protective qualities of the milk protein, helps keep the delicate antibacterial compounds intact.

The researchers also measured exactly how much of the substance was needed to stop the bacteria from growing or to kill them completely. They found that the freeze-dried, protein-coated samples required the smallest amount to be effective. Against the fish spoilage bacteria Photobacterium damselae, this specific mixture could stop growth at a concentration as low as 12.5 milligrams per milliliter. For the human pathogen Staphylococcus aureus, the same mixture could kill the bacteria at a concentration of 100 milligrams per milliliter. In comparison, the spray-dried samples often required much higher concentrations to achieve the same result, and in some cases, the bacteria were not killed at all even at the highest concentrations tested. The study indicates that while spray drying is cheaper and faster for industrial use, it may damage the active compounds more than freeze-drying does, unless the right coating is used.

Looking at the physical structure of the dried powders provided further clues. When viewed under a powerful microscope, the spray-dried particles appeared as small spheres, some of which were wrinkled or stuck together in clumps. The freeze-dried samples, however, looked different; the ones coated with maltodextrin resembled smooth, connected layers, while the protein-coated ones were flat and fragile. These physical differences likely explain why the freeze-dried, protein-coated samples worked so well. The structure of the protein coating may have shielded the active chemicals from the harsh conditions of the drying process better than the starch-based coating did. The study concludes that while both drying methods have their place, the choice of coating material is critical. Sodium caseinate appears to be the superior shield for freeze-drying, preserving the biological activity of the bacterial liquid, whereas maltodextrin might be a better choice if spray drying is the only option available. This work offers a clear path forward for developing natural preservatives that can keep food safe without relying on synthetic chemicals.