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(Nanowerk News) During your journey from open fields to produce displays in supermarkets, fresh vegetables and fruits can sometimes become contaminated with microorganisms. These items can spoil other products, further spreading contamination and increasing the amount of food that can cause illness.
To prevent cross-contamination between fresh produce, researchers at Texas A&M University have created a coating that can be applied to food contact surfaces such as conveyors, rollers, and collection pails. In addition to being germicidal, researchers have designed its coating to be extremely water repellent. The researchers said that without water, bacteria cannot stick to or multiply on surfaces, dramatically reducing contamination from one product to another.
“Consuming contaminated raw food makes hundreds of people sick each year, so food contamination is not only a major health problem but also a major financial burden,” said Mustafa Akbulut, associate professor at the Artie McFerrin Department of Chemical Engineering. “In our study, we showed that our new dual-function coating, one that can repel and kill bacteria, can greatly mitigate bacterial spread, preventing cross-contamination.”
The results of the study appear in the February issue of the magazine. Applied materials and ACS interfaces (“Dual function superhydrophobic coatings with bacterial anti-contact and antimicrobial characteristics”).
Foodborne illness can be caused by a large number of pathogens including multiple strains of viruses and bacteria. To remedy any infection after harvest, fresh produce is generally washed and then disinfected in powerful antimicrobials, such as hydrogen peroxide or acetic acid.
However, bacteria can still escape unscathed if they manage to hide in hard-to-reach places on fruit and vegetable skins. Furthermore, if the amount of bacteria is large enough, they can form protective sheaths, called biofilms, which further protect them from the action of disinfectants.
Contaminated products can spread pathogens directly, by touching other foods, or indirectly, through food contact surfaces. Currently, there are several ways to prevent indirect transmission ranging from antimicrobial surface coatings to antifouling polymer surfaces that act as springs to drive bacteria away. But the researchers said that these approaches, while effective at first, can lose their effects over time for a variety of reasons.
To overcome the obstacles posed by current technologies, Akbulut and his team proceeded to create an antimicrobial surface coating that is also extremely hydrophobic. They noted that the water-repellent property of the coating can help food contact surfaces retain their germicidal action for much longer.
“Most bacteria can only survive in an aqueous environment,” said Akbulut. “If the surfaces are super hydrophobic, then the water, and along with it, most bacteria will repel. With fewer bacteria around, fewer germicides are used, increasing the overall life of the coating.”
To make their dual-function coating, Akbulut and his team started with aluminum foil, a metal commonly used in the food industry for contact surfaces. On the metal surface, they chemically bonded a thin layer of a compound called silica using high heat. Then, with this layer as a substrate, they added a mixture of silica and a natural germicidal protein found in tears and egg white called lysozyme.
Together, the silica-aluminum layer bonded to the silica-lysozyme layer formed a coating that had a rough texture when viewed at microscopic scales. The researchers noted that this submicroscopic roughness, or the small bumps and cracks in the coating, is key to superhydrophobia.
“Generally, if you increase the roughness, you increase the hydrophobicity of a material, but there is a limit,” said Shuhao Liu, a graduate student at the College of Engineering and the lead author of the study. “If the coating is too rough, bacteria can hide behind cracks again and become contaminated. Therefore, we modified the ratio of silica to lysozyme so that the roughness produces the best possible hydrophobicity without compromising the overall function of the coating.”
When its lysozyme-infused superhydrophobic coating was adjusted and ready, the researchers tested whether it was effective in slowing the growth of two strains of disease-causing bacteria, Salmonella typhimurium and Listeria innocua. Upon examination, they found that the amount of bacteria on these surfaces was 99.99% less than that of the bare surfaces.
Despite the high efficacy of its coating to prevent bacterial spread, the researchers said more research is needed to determine if the coating works equally well to mitigate viral cross-contamination. Although they are more durable than other coatings, they noted that their coating would also need to be reapplied after a certain amount of use. As a next step, therefore, Akbulut and his team are working on developing more permanent dual-functional coatings.
“Our goal is to create smart surfaces that prevent any type of pathogen from adhering and multiplying,” said Akbulut. “In this regard, we have developed surface coatings that can prevent bacteria from accumulating on surfaces, which is one of the main reasons for cross contamination. We are now working with agricultural researchers to put our bank invention into practice. ”
