Sprayable nanocoating extends produce shelf life

Materials World magazine
1 Oct 2017

An antimicrobial nanocoating can be sprayed onto food to increase its shelf life. Kathryn Allen reports. 

With high consumer demand for good-quality, year-round produce, packaging has to keep up. Various coatings exist to control or add properties to goods to extend shelf life, but until now these have been limited by interactions between the coating and the produce and the lengthy timescales needed to coat large amounts of goods. 

However, a research team, led by Professor Insung Choi of the Department of Chemistry at the Korea Advanced Institute of Science and Technology, Republic of Korea, has developed a nanocoating made from polyphenol that can be rapidly sprayed onto produce, forming nanometre-thick films. 

Plant-derived polyphenols – usually extracted from gallnut, according to Choi, and a metabolite of photosynthesis – are commonly used as food additives and are known to have antibacterial and potentially anti-carcinogenic properties. Choi’s team used tannic acid (TA), a plant-derived polyphenol, used as an additive in food, cosmetics and pharmaceuticals.  

Choi told Materials World, ‘We use two coating materials – polyphenols and minerals. Upon contact, they form highly sticky aggregates. The contact – in the air – is made by spraying the two materials simultaneously with a two-nozzle system.’ Iron ions, found naturally in the body, form stable nanofilms when mixed with TA. Nutrients can also be incorporated to improve the health benefits of food, according to Choi.

This spraying technique generally takes less than ten seconds, eliminating the time-consuming process of previous solution-based coatings, and reducing the risk of cross-contamination by allowing more control over the application process. The coating’s thickness can be controlled by the length of spray time – five seconds of spraying forms a 5nm coating. 

While the team’s published work states that this film has promise as an edible coating for produce, it acknowledges that further research may be needed on the toxicity of coated fruits. The nanocoating was tested on mandarin oranges. Uncoated and coated oranges were kept for 28 days at 25°C, after which time ten out of 37 of the uncoated oranges were rotten and covered with mould, while the coated oranges stayed in good condition.  

Strawberries were also tested, but were sprayed before they were harvested, along with a control batch of unsprayed strawberries. While the non-coated strawberries were found to go mouldy after 58 hours of storage at 25°C, the coated strawberries did not. In a blind test, the nanocoating did not alter the taste or texture of the fruit. The spraying also had no impact on the growth of the strawberries. 

As demonstrated by the team, the spray can also coat metals, plastics, glass and fabric. When discussing how long the coating will last, Choi said, ‘The function lasts temporarily, say, a week for shoe insoles.’ 

The technology is expected to be applicable to various industrial sectors in addition to agriculture. Mark Sanders, Managing Director of Clement Clarke International, a manufacturer of medical devices for respiratory conditions, told Materials World, ‘This is a very promising approach and one that we will monitor for its applicability to preventing fungal growth in respiratory devices with a 12 month use – it may complement our existing use of silver ion polymer additives.’ The technology has been patented and Choi expects it to be commercially available at the end of this year.  

To read Antimicrobial spray nanocoating of supramolecular Fe(III)-tannic acid  metal-organic coordination complex: applications to shoe insoles and fruits, visit go.nature.com/2w4dceF