Bioactive film could help keep strawberries fresh for longer

Strawberries and other soft fruits are delicate and difficult to keep fresh. In a bid to preserve them for longer, avoiding food waste, Professor Monique Lacroix of the Institut National de la Recherche Scientifique (INRS) led the development of a packaging film to retain freshness. The packaging protects against mould and certain pathogenic bacteria and can keep strawberries fresh for up to 12 days.

The film is made from chitosan, a natural molecule found in shellfish shells which can be considered a food industry by-product. The substance contains key antifungal properties which can be used to curb mould growth.

The film also contains essential oils (concentrates of botanical aroma compounds) and synthesised silver nanoparticles, both of which possess antimicrobial properties. Silver nanoparticles adhere to the cell wall and membrane of a bacterium; silver can enter the bacterium. The chemical reactions caused by the introduction of silver causes its DNA to condense, preventing replication and causing cell death. Among other uses, silver nanoparticles are being used to develop wound dressings which counter infection.

“Essential oil vapours protect strawberries and if the film comes into contact with strawberries, the chitosan and nanoparticles prevent mould and pathogens from reaching the fruit’s surface,” said Lacroix, who led the study.

The composite film has the advantage of being effective against several types of pathogens when used in combination with a standard irradiation process, which kills microbes on the strawberries before packaging.

Lacroix and her INRS colleagues tested it on four microbial cultures: Escherichia coli, Listeria monocytogenes, Salmonella typhimurium, and Aspergillus niger. The composite films showed strong antimicrobial activity against them all. This is particularly good news regarding Aspergillus niger, which is a highly resistant mould responsible for substantial losses during strawberry production. The other three pathogens come from contamination during food handling and are sources of concern for the food industry.

They found that the irradiation and packaging combination resulted in a much longer shelf life, halving loss levels compared to the control. On day 12, they recorded a 55 per cent loss for the control group; 38 for the group with the film alone, and 25 per cent with the film and irradiation. Irradiation does not only extend shelf life; it also helps preserve or increase the quantity of polyphenols in the strawberries (the antioxidant molecules responsible for giving strawberries their colour.)

The researchers suggest that the composite film could be inserted into the blotting paper the industry currently uses for storing strawberries.