Korean scientists detail the use of a rapid spray coating of Iron (III)-tannic acid-metal-organic coordination complex (Fe(III)-TA-MOC) for fruits.
The five second of spray coating forms a film around 5 nanometres (nm) in thickness, which can be varied in a controlled, linear fashion by the number of coating cycles.
Results of a field test that looked into the post harvest shelf-life of mandarin oranges found that after 28 days of storage at 25°C, 27% of the uncoated mandarin oranges (10 out of 37) were rotten and covered with mould, while the coated mandarin oranges remained edible.
Additional tests on strawberries stored at 25°C and humidity (32–45% relative humidity) found after 58 hours of storage, unsprayed strawberries became rotten with white mould, but the sprayed strawberries remained intact.
The strawberries were divided into edible and inedible ones: while 56.3% of the sprayed strawberries were edible, only 6.3% of the unsprayed looked edible.
Nanocoating the waste issue
“Nanocoating technologies are still in their infancy, but they have untapped potential for exciting applications,” said Professor Insung Choi of the Department of Chemistry at The Korea Advanced Institute of Science and Technology (KAIST).
“As we have shown, nanocoatings can be easily adapted for several different uses, and the creative combination of existing nanomaterials and coating methods can synergize to unlock this potential."
The idea of an edible coating for fruits and vegetables is attractive to the food and agricultural industry, where product spoilage and waste are issues the sector are under pressure to address.
Food waste is difficult to quantify but the Food and Agriculture Organization of the United Nations (FAO) reckon that overall, on a per-capita basis, food waste by consumers in Europe and North-America is 95-115 kg/year, while this figure in sub-Saharan Africa and South/Southeast Asia is only 6-11 kg/year.
These figures are mainly connected to financial, managerial and technical limitations in harvesting techniques, storage and cooling facilities in difficult climatic conditions, infrastructure, packaging and marketing systems.
One such example of nanotechnology usefulness is the edible coating of strawberries and bananas by dip coating with silk fibroin, that involved multi-cycled dip coatings and long annealing (up to 12 hours).
Here, US scientists provide details of using edible silk fibroin as a coating to enhance fruits’ shelf life at room conditions by reducing cell respiration rate and water evaporation.
Led by Professor Choi, the team developed a sprayable nanocoating technique using plant-derived polyphenols.
These polyphenols are a nontoxic food additive and exhibit antibacterial capabilities. They can also be used with iron ions, which are naturally found in the body, to form an adhesive complex.
The research team combined this polyphenol-iron complex with spray techniques. As well as coating select areas more quickly sprays can also prevent cross contamination.
The solutions of TA (10 millimols (mM)) and Fe(III) (10 mM) were freshly prepared with water, and poured into each container of the spray brushes.
Mandarin oranges were sequentially sprayed five times for five seconds each. For bulk-scale coating, simultaneous spraying was employed.
After washing the oranges were then simultaneously sprayed for three seconds. The coated mandarin oranges were washed with tap water, dried and placed in a cardboard box.
Similarly, the strawberries were simultaneously sprayed with solutions of TA (5 mM) and Fe(III) (5 mM) and then washed.
The TA and Fe(III) solutions were sprayed simultaneously onto the strawberries on trees for three seconds per strawberry and then washed.
The coating was performed at around 5–6 pm and the coated strawberries, along with the uncoated ones as a control, were harvested at 9 am on the next day.
Two layers of strawberries were stored at ambient temperature (25 °C) and humidity (32–45% relative humidity).
“The on-site studies in the strawberry field clearly showed the direct applicability of the spraying protocol developed herein to edible coating of fruits and vegetables,” the study observed.
“We also believe that this rapid and simple method for forming stable films on bulk substrates would find its applications in various industrial sectors other than agricultural technology.”
The research team acknowledged that while detailed studies on the toxicity of coated fruits might be needed, the edible coatings used in the experiments were recognised to be safe under Food and Drug Administration (FDA) regulation.
Source: Scientific Reports
Published online ahead of print: DOI: 10.1038/s41598-017-07257-x
“Antimicrobial spray nanocoating of supramolecular Fe(III)-tannic acid metal-organic coordination complex: applications to shoe insoles and fruits.”
Authors: Ji Park, Sohee Choi, Hee Moon, Hyelin Seo, Ji Kim, Seok-Pyo Hong, Bong Lee, Eunhye Kang, Jinho Lee, Dong Ryu, Insung S. Choi.