Germ-killing plastic wrap

Biodegradable plastic that releases germ killers provides an example of what’s known as active packaging, and scientists report progress toward taking this concept to market.

Paul Dawson and his colleagues at Clemson (S.C.) University are fashioning plastics from proteins found in corn, soy, and wheat. While these biodegradable polymers are being heated or compressed to make a thin film, the food scientists add a sprinkling of a natural antimicrobial agent–usually nisin. This is a bacteriocin, an antibioticlike substance secreted by bacteria such as those harnessed to make yogurt and cheese. Nisin has long been added to food to prevent the growth of pathogens (SN: 2/7/98, p. 89), especially Listeria, a potentially lethal food-poisoning agent among the few that grow well under refrigeration.

Dawson’s team intended for the nisin to slowly diffuse into and protect any food the plastic contacted. At a poultry-science meeting in Montreal, Dawson reported promising data on a soy-based biopolymer.

In one set of experiments, his group inoculated a broth with Listeria, then inserted some of the soy-derived plastic into the liquid. Concentrations of the bacteria increased with time when the broth was incubated with the plain soy plastic. However, bacterial concentrations dropped to 1 percent of the initial amount within 2 hours of adding the plastic impregnated with nisin. And plastic containing a combination of nisin and another antimicrobial–the saturated fat known as lauric acid– “completely eliminated the bacteria,” Dawson says.

For a more realistic test, his team laced some lunch meat with Listeria, then they vacuum packed it inside the soy-based plastic. In the absence of antimicrobial additives, the bacterial count within 3 weeks climbed to about fivefold the initial amount. However, over the same period, the number of Listeria on meat encased in plastic laced with either nisin or the nisin-lauric acid combo dropped to 10 percent of the starting count.

Though the new biopolymers release antimicrobials, they don’t flex and seal as well as the polyethylene films that now dominate the food-wrap market. As an interim measure, Dawson’s team is currently investigating how nisin-impregnated biopolymers might be laminated onto the surface of polyethylene wraps that will make contact with food.