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Coating fruit to reduce moisture and retain freshness, though standard practice in citrus and apples, has proved difficult in small fruit like sweet cherries. Among the various materials studied by scientists thus far, a chitosan coating is showing promise and will be evaluated further this season.

Oregon State University researcher Dr. Jinhe Bai, and U.S. Department of Agriculture’s Dr. Anne Plotto, have evaluated the performance of more than 20 different edible coatings on cherry fruit and stems during the last two years.

“For fruit coating research, we found that cherries have thick, natural wax on the fruit surface,” Bai stated in a report on the research. His research showed that the washing process used in normal cherry packing partially destroyed the natural wax layer, thereby increasing water loss and shriveling of the fruit.

Cherries are not especially suited to edible coatings because they do not tolerate the heat needed to dry the coatings, and are packed wet. Some coatings tried on cherries left a white or crystallized residue.

The scientists observed that some coating applications did reduce water loss, but that the efficiency of the coating depended on the formulation and fruit variety. “However, the coatings used in the experiments did not match the fruit’s natural wax very well,” Bai said.

Finding an edible coating that reduces both fruit moisture loss and stem browning is more problematic.

To look at the coating on cherry stems, the researchers used scanning electron microscopy to observe the surface structure of the coated and uncoated cherry stems. They found that the natural wax of cherry stems is destroyed quite rapidly at room temperature. Most of the coating formulations they evaluated did not control stem drying because of poor surface finishing and microcracking of the stems.

One of the next steps in research will be to find a coating that matches natural cherry wax.

As part of the project, the scientists are assessing commercially available waxes used on apples for any benefit on cherries, but they are also looking to develop new types of coatings.

The scanning electron microscopy showed that a shellac coating covered the stomata of the cherry stem, but severe stem cracking was still observed.

“But a chitosan coating showed promise,” Bai said, adding that it provided good finish although the application seemed to be too thick. “We will optimize the formulation of chitosan coating and further evaluate it this coming season.”

Thus far, the coatings evaluated have not reduced the incidence of pitting.