In order to keep vegetables fresh, there should be an appropriate exchange of gases and moisture. Bacterial cellulose with the correct water content helps in the exchange of those gases from the atmosphere. Cellulose is nothing but an organic compound produced by bacteria. It is characterised by elastic strength, plasticity and the ability to hold water. Bacterial cellulose is also nanofibrous, which played to the advantage that we will see later on.
The problem here is that this exchange also leads to the building up of microbial activity which contributes to the spoilage of the vegetables. Which is where the silver nanoparticles come in. These nanoparticles can promote antimicrobial activity.
Keeping vegetables fresh without a refrigerator for more than a week seems impossible, let alone doable with bacteria. However, researchers at Indian Institute of Technology (IIT) Hyderabad have kept tomatoes fresh and without any microbial spoilage for as long as 30 days with a special cellulose wrap!
A two-member team comprising of Dr Shivakalyani Adepu and Dr Mudrika Khandelwal from IIT-Hyderabad's Department of Materials Science and Metallurgical Engineering has managed to create a food packaging material made of bacterial cellulose impregnated with silver nanoparticles, and the results of this experiment was published in the Journal of Material Science.
So, how does this bacterial cellulose impregnated with silver nanoparticles warp protect the food?
Well, to keep vegetables fresh, there should be an appropriate exchange of gases and moisture from the atmosphere to the vegetable itself.
Bacterial cellulose with proper water content helps in the exchange of those gases from the atmosphere.
The problem here is that this exchange also leads to the building up of microbial activity which contributes to the spoilage of the vegetables. Which is where the silver nanoparticles come in.
Silver nanoparticles can promote good antimicrobial activity, but the nanoparticle colloids tend to form aggregates and hinder the process of the antimicrobial activity.
So, the scientists took the bacterial cellulose prepared by using Gluconacetobacter xylinus bacteria to produce semi-crystalline nanofiber cellulose. The bacterial cellulose was then treated with sodium hydroxide to remove all bacteria and then dipped in sodium borohydride solution. This allowed silver nanoparticles to be impregnated inside the pores of the bacterial cellulose.
This tackled the problem of silver forming aggregates as the bacterial cellulose matrix restricts the growth of nanoparticles, thereby controlling their size. The bacteria and silver composite wrap was ready to be tested.
Batches of tomatoes were then wrapped in three different materials-the bacterial wrap, polyethylene (polythene) and polypropylene.
Tomatoes wrapped in the composite remained fresh without any wrinkles or microbial spoilage even at the end of 30 days, at room temperature.
However, tomatoes wrapped with polyethylene started wrinkling within the first week, and microbial spoilage was seen within 15 days. In 30 days, the tomatoes had completely deteriorated.
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.
"This is because, besides antimicrobial activity, the composite also allows the appropriate exchange of gases and moisture. The water holding capacity of the composite helps maintain optimum moisture transmission," said Dr Shivakalyani, who is the first author of the paper, to The Hindu.
"The composite also acts as an ethylene blocker thus preventing excess ripening of fruits. It ensures that fruits age slowly," she added.
With even the Vice President of India appreciating the team on Twitter, this innovation will be groundbreaking as the science behind it can be applied to a range of fields.
"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 covers in hospitals," said Dr Mudrika.