Broader Scope May Enhance UV Screening of Navel Oranges

Sweet, juicy navel oranges owe much of their high quality to the behind-the-scenes work of growers, packers, shippers, retailers and researchers.

U.S. Department of Agriculture (USDA) scientists David M. Obenland and Joseph L. Smilanick develop and conduct research at their Parlier, Calif., laboratories and at packinghouses to determine how to better protect the freshness, flavor, and other qualities of citrus, table grapes, and other highly perishable fruits throughout packing, shipping, and storage. Obenland and Smilanick work for the Agricultural Research Service (ARS), the chief intramural scientific research agency of USDA.

In one study, published in 2010 in the journal HortTechnology, Obenland, Smilanick and their colleagues investigated UV (ultraviolet) screening and sorting of oranges. According to Obenland, California packinghouses have used UV screening for more than 50 years to detect spots—about the size of a quarter, or larger—that glow a bright fluorescent yellow when navel oranges are viewed under UV light.

Packinghouse workers know to promptly cull any oranges that have this distinctive "fluorescence signature" on the peel. That's because the spots, more likely than not, are telltale indicators of the presence of Penicillium microbes that cause blue mold or green mold.

But other, less-studied patterns of fluorescence on navel orange peels may warrant more attention. Fluorescence in the form of specks, smears, smudges or blotches, for instance, may indicate the presence of cuts, punctures or other peel wounds that may pave the way to attack by decay microbes.

To learn more about these less familiar patterns, the researchers sampled about 5,000 navel oranges over a two-year period. Oranges were sorted by fluorescence level-zero, sparse, moderate or high-noted during UV screening. In addition, the oranges were evaluated under normal light—not UV—within 24 hours after UV screening and after the fruit had been stored at 59 degrees Fahrenheit for three weeks.

As expected, fruit with high fluorescence developed further decay and peel-quality problems during storage, but so did many of the oranges that had only moderate fluorescence.

Taken as a whole, the findings suggest that packers who are not already doing so might want to expand UV screening to take several fluorescence levels and patterns into account when sorting navel oranges.

The idea of expanding UV use to include more than detection of the classic decay signature is not new. But the Parlier study, though preliminary, is likely the first to present as detailed a look at this approach.

Obenland, a plant physiologist, and Smilanick, a plant pathologist, work at ARS' San Joaquin Valley Agricultural Sciences Center in Parlier. The California Citrus Research Board and ARS funded the research, which is highlighted in the August 2013 issue of Agricultural Research magazine.