Curcumin, the compound that gives turmeric spice its characteristic bright yellow hue, has well-known antimicrobial properties. Researchers have now put curcumin to work to create a food-safe antibacterial surface (J. Agric. Food Chem. 2015, DOI: 10.1021/jf505442w). Packaged inside nanosized vesicles attached to glass, the curcumin kills bacteria on contact. The work could lead to cutting boards, cleavers, and countertops that could actively prevent contamination during food preparation, the researchers say.
Commercial products made with antimicrobial materials range from stink-free socks to self-cleaning toilet seats. A countertop that could proactively kill bacteria would address one of the major problems of the food industry: cross-contamination, says Victor Rodov of Israel’s Agricultural Research Organization. When, for example, Escherichia coli from a piece of lettuce get onto a counter, the bacteria can contaminate the next piece of lettuce contacting that surface. Yet the germicides now laced into fabrics or plastics, including silver particles, synthetic polymers, or ammonium salts, are not generally recognized as safe for surfaces where food is prepared.
So Rodov and collaborators at Southern Illinois University (SIU), Carbondale, set out to create an antibacterial surface from food-grade materials.
They first screened 11 natural food compounds with known antimicrobial properties, including resveratrol from grapes, hydroxytyrosol from olives, and curcumin. In their tests, curcumin was the best at curbing E. coli growth.
The SIU researchers then packed curcumin inside custom-built nanovesicles consisting of a membrane bilayer constructed from diacetylene fatty acids and a phospholipid. Some of the diacetylene molecules were tagged with N-hydroxysuccinimide groups, which the researchers used to attach the nanovesicles to the glass. The vesicles also have molecules of glucose hanging on their surface, which stick to bacterial cell walls. The glass-bound vesicles, stocked with curcumin, can then nab passing bacteria.
The researchers compared glass slides coated with nanovesicles prepared with and without curcumin for their activity against bacteria. When the slides with curcumin were immersed for 48 hours in flasks spiked with E. coli, less than 0.5% of the bacteria survived, while no significant decline occurred in the flasks without curcumin.
How it works is still something of a mystery. The researchers know that curcumin tends to hang out within the bilayer more than in the vesicle’s central space. They suspect that when the sugar on the vesicle surface sticks to the cell wall of a passing bacterium, curcumin migrates into the cell and kills it from the inside.
Using a food ingredient as an antimicrobial is a good idea, says Kay Cooksey, who works in packaging science at Clemson University. But for commercial use, she says researchers will need to show that the antimicrobial surface works long term and that it does not convey an odor to the food or shed nanoparticles. She also would like to see if it fights the resistant biofilms that form on food processing equipment. Rodov’s group is now testing whether or not the active surface blocks the formation of biofilms.