Composite has antimicrobial activity, allows optimum exchange of gases, moisture

Researchers at Indian Institute of Technology (IIT) Hyderabad have been successful in keeping tomatoes fresh and without any microbial spoilage for as long as 30 days. This was possible thanks to the food packaging material developed by a two-member team led by Dr. Mudrika Khandelwal from the institute’s Department of Materials Science and Metallurgical Engineering. The food packaging material is made of bacterial cellulose impregnated with silver nanoparticles.

Bacterial cellulose was first prepared by using Gluconacetobacter xylinus bacteria to produce semicrystalline cellulose nanofibre from a standard glucose media. “We can use any fruit juice that is rich in sugar or even beer and wine, which are fermented, to produce bacterial cellulose,” says Dr. Khandelwal.

Nanofibrous

Bacterial cellulose is highly crystalline, has high porosity and water holding capacity and possesses great mechanical properties. Also, bacterial cellulose is nanofibrous unlike plant cellulose, which is microfibrous. The results were published in the Journal of Materials Science.

The bacterial cellulose was first treated with sodium hydroxide to remove all bacteria and then impregnated with silver nanoparticles. This was done by dipping the bacterial cellulose in silver nitrate solution and subsequently in sodium borohydride solution. Reduction of silver nitrate to form silver nanoparticles happens inside the pores of the bacterial cellulose.

The nanosized pores present in the bacterial cellulose matrix restricts the growth of nanoparticles, thereby controlling their size. It prevents the nanoparticles from forming aggregates. “We found that the smaller the size [5-6 nanometres] of the silver nanoparticles the better was the antimicrobial activity. There was also sustained release of nanoparticles,” she says. This was not the case with silver nanoparticle colloid where the nanoparticles tend to form aggregates.

The antimicrobial activity of bacterial cellulose was first tested on bacteria and fungi isolated from rotten tomatoes and later on mixed culture. Compared with controls, the composite (bacterial cellulose impregnated with silver nanoparticles) showed 99% killing efficiency. The antibacterial activity was successfully tested up to 72 hours. The antibacterial activity of colloid was only 90%. “The bacterial cellulose with silver nanoparticles not only had activity against bacteria but also against fungus,” says Dr. Khandelwal.

The researchers then tested the antimicrobial efficacy of the composite by using it to wrap freshly harvested tomatoes. Tomatoes wrapped in polyethylene (polythene) and polypropylene served as controls.

Fresh tomatoes

At room conditions, tomatoes wrapped in the composite remained fresh without any wrinkles or microbial spoilage even at the end of 30 days.

“This is because besides antimicrobial activity, the composite also allows appropriate exchange of gases and moisture. The water holding capacity of the composite helps maintain optimum moisture transmission,” says Shivakalyani Adepu from the Department of Materials Science and Metallurgical Engineering at IIT Hyderabad and first author of the paper. “The composite also acts as ethylene blocker thus preventing excess ripening of fruits. It ensures that fruits age slowly.”

On the contrary, tomatoes wrapped with polyethylene started wrinkling within the first week, and microbial spoilage was seen within 15 days; tomatoes had completely deteriorated within 30 days. But in the case of polypropylene, tomatoes remained fresh for a week; they started wrinkling within 15 days and became soft and wrinkled all around within a month.

“We want to test our composite on exotic fruits,” says Dr. Khandelwal. “We would also like to extend the same principle to healthcare products. The composite can be used as antimicrobial lining in sanitary napkins, and disposable clothing and covering in hospitals.”