An eco-friendly, plastic-free casing can keep fruits and vegetables, such as avocados, fresher for longer. To do this, fine threads of pullulan, which consists of sugar molecules and is edible, are applied to the food. The pullulan is enriched with natural substances that are effective against microorganisms, as reported by a group led by Philip Demokritou and Kevin Kit Parker from Harvard University in the US in the journal “Nature Food”.
In trials, avocados wrapped in this way lasted significantly longer than unwrapped fruit. After seven days of storage at 22 degrees Celsius, 90 percent of the untreated fruit showed visibly rotten spots. For the encased specimens, it was only half.
In addition, there is a greatly improved environmental balance compared to many plastics: According to the researchers, pullulan can be washed off without any problems and is broken down in the earth in three days. So far, packaging has mostly been made from petroleum-based plastics. However, because the plastic is hardly degraded in nature, a major environmental problem has now arisen.
It could grow even more in the coming decades. The Organization for Economic Co-operation and Development (OECD) expects global plastic consumption to increase from 460 million tons in 2019 to over 1.2 billion tons in 2060. Plastic waste would almost triple by 2060.
“We knew we had to get rid of the petroleum-based food packaging and replace it with something more sustainable, biodegradable and non-toxic,” says Demokritou.
The researchers added thyme oil, citric acid and nisin, a natural antibiotic produced by lactic acid bacteria, to the sugar polymer pullulan dissolved in water, which is also used for drug capsules, for example. These substances are effective against microorganisms such as the bacteria Escherichia coli and Listeria innocua and the mold Aspergillus fumigatus. These occur naturally on the skins of fruit and vegetables and are largely responsible for fresh food rotting. Because the growth of microorganisms is inhibited, coated foods remain edible for longer.
To apply the protective layer to the fruit, the scientists used a process in which an apparatus similar to a hair dryer heats the pullulan dissolved in water with the antimicrobial substances. Most of the water evaporates and the pullulan is expelled from the apparatus in tiny threads, wrapping itself around the fruit.
This currently takes two to four minutes. The researchers state the costs as a few cents per fruit. But improvements are still possible. “I’m not against plastic, but I’m against petroleum-based plastics, which we keep throwing away because only a tiny fraction of them can be recycled,” says Demokritou.