Edible coatings enhance shelf life of ready-to-eat blueberries

Scientists from Oregon State University found using a variety of edible coatings was able to slow the decay and prolong the storage of Duke and Elliott types of blueberries. The treatment also extended the fruit’s quality and slow the rate of water loss after they were washed, said Professor Yanyun Zhao from the Department of Food Science and Technology.

Fresh untreated, unwashed blueberries have a typical shelf life of one to eight weeks depending on several factors, including variety, ripeness at harvest, method of harvest, presence of disease, and storage conditions, he added. But washing the fruit prior to packing and storing can increase the rate at which the fruit deteriorates.

Methodology​

Fresh blueberries were washed in chlorinated water prior to application with a range of edible coatings: Semperfresh (SF), acid-soluble chitosan, water-soluble chitosan, calcium caseinate (CC) and sodium alginate (SA). Semperfresh is a sucrose ester; chitosan is a derivative of chitin – a natural substance often found in the exoskeletons of insects and crustaceans, and alginate is a polysaccharide commonly found in the cell walls of brown algae.

The fruit was then packed in vented and non-vented clam-shell containers and stored at 2C for a week, followed by storage at room temperature of 20C for up to 15 days. The trial was carried out in the growing seasons of 2006 and 2008.

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Results​

The scientists said results indicated that use of edible coatings could allow for the development of “ready-to-eat fresh blueberries with no reduction in shelf life”.​

They found results differed depending on the packaging and the coatings used. Non-vented containers provided better barrier protection against water evaporation and gas exchange – delaying ripening and dehydration. But the water accumulation allowed for by this packaging type could also promote mould growth, which means an anti-fungal coating would need to be employed.

Differing coatings also had different effects on preserving the blueberries, said the team. For example, the hydrophobic coating material, SF, demonstrated superior water barrier properties than other hydrophilic coating materials, thus reduced the weight loss of coated ‘Duke’ fruit during the room temperature storage. The protein-based CC coating delayed post-harvest respiration of fruit by providing a strong gas barrier on the surface of fruit. Use of chitosan coatings best controlled the mould growth in non-vented containers.

“The use of edible coatings can increase the market value of the fruit by allowing the establishment of pre-washed, ready-to-eat fresh fruit as a convenience for consumers,”​ concluded Prof Zhao.

The team is further investigating refining application methods – such as electrostatic techniques as opposed to the current dripping system. A separate study to evaluate the extent to which the coatings affect taste, texture and flavour was also needed.

Duan, J., et al., Effect of edible coatings on the quality of fresh blueberries (Duke and Elliott) under commercial storage conditions. Postharvest Biol. Technol. (2010), doi:10.1016/j.postharvbio.2010.08.006​