Screening fresh oranges with UV: Study pinpoints new value of detection tactic

August 16, 2013 by Marcia Wood
During ultraviolet (UV) screening at citrus packinghouses, navel oranges that display the characteristic “fluorescence signature” associated with the presence of blue or green mold are typically culled. Credit: David Obenland

Fresh, deliciously sweet navel oranges, on display at your local supermarket, may have been quickly inspected with ultraviolet (UV) light when they were still at the packinghouse. Usually, the purpose of this special sorting and screening is to see if circular spots—which glow a bright, fluorescent yellow and may be about the size of a quarter or larger—show up on the fruit's peel.

More often than not, these spots, which scientists refer to as "lesions," are telltale indicators of the presence of microbes that cause decay, namely Penicillium italicum, responsible for blue mold, or P. digitatum, the culprit behind green mold.

It isn't the microbes that are fluorescing under the packinghouse UV lamps. Instead, it's tangeritin, a in citrus peel oil. When the peel is damaged, such as by decay, tangeritin moves closer to the peel surface, or perhaps seeps out of it, becoming easier for UV to detect.

The characteristic "fluorescence signature" of the decay lesions is easily recognized by packing-line workers who monitor the fruit as it speeds past them on a . All navel oranges that display this distinctive pattern are promptly culled—an established practice that dates back more than 50 years in California citrus packinghouses.

But studies by Dave Obenland and plant pathologist Joe Smilanick—both with the Agricultural Research Service in Parlier, California—suggest that 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 example, may indicate the presence of cuts, punctures, or other peel wounds that might not be visible to the naked eye, yet may pave the way to attack by decay microbes.

Oranges with less-studied patterns, such as blotches, may also warrant attention, ARS research has shown. Credit: David Obenland

To learn more about these less-familiar patterns, the researchers sampled about 5,000 navel oranges over a 2-year period. For the study, navel oranges sampled at two California citrus packinghouses were sorted by fluorescence level—zero, sparse, moderate, or high—noted during UV screening. Next, the oranges were evaluated twice under normal light—not UV. The first time was within 24 hours after UV screening and sorting; the second was after the oranges had been stored at 59°F for 3 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 might want to expand UV screening to take several fluorescence levels and patterns into account when sorting navel oranges. Many of the patterns that the researchers investigated, such as glowing specks no bigger than the tip of a ballpoint pen, might be quickly and easily detected with modern UV-equipped machine-vision sorters.

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.

ARS and the grower-sponsored California Citrus Research Board, in Visalia, funded the research.

Obenland, Smilanick, ARS plant pathologist Dennis Margosan at Parlier, and ARS statistician Bruce Mackey at Albany, California, documented the UV findings in a 2010 peer-reviewed article in HortTechnology.

"Screening Fresh Oranges With UV: Study Pinpoints New Value of Detection Tactic" was published in the August 2013 issue of Agricultural Research magazine.

Explore further: Fish can recognize a face based on UV pattern alone

More information: www.ars.usda.gov/is/AR/archive/aug13/

Related Stories

Fish can recognize a face based on UV pattern alone

February 25, 2010

Two species of damselfish may look identical -- not to mention drab -- to the human eye. But that's because, in comparison to the fish, all of us are essentially colorblind. A new study published online on February 25th in ...

Discovery could help save citrus from dreaded disease

March 7, 2013

A nutrition expert at UC Davis has discovered important clues to the deadly attack strategy of a puzzling plant pathogen that has destroyed hundreds of thousands acres of citrus across the world.

Bright birds make good mothers

August 13, 2013

Female blue tits with brightly coloured crowns are better mothers than duller birds, according to a new study led by the University of York.

Recommended for you

Silencing cholera's social media

May 24, 2016

Bacteria use a form of "social media" communication called quorum sensing to monitor how many of their fellow species are in the neighborhood, allowing them to detect changes in density and respond with changes in collective ...

Evolution influenced by temporary microbes

May 24, 2016

Life on Earth often depends on symbiotic relationships between microbes and other forms of life. A new theory suggests that researchers should consider how symbiotic microbes can influence the evolution of life on Earth, ...

Great apes communicate cooperatively

May 24, 2016

Human language is a fundamentally cooperative enterprise, embodying fast-paced interactions. It has been suggested that it evolved as part of a larger adaptation of humans' unique forms of cooperation. In a cross-species ...

Rare evolutionary event detected in the lab

May 23, 2016

It took nearly a half trillion tries before researchers at The University of Texas at Austin witnessed a rare event and perhaps solved an evolutionary puzzle about how introns, non-coding sequences of DNA located within genes, ...

In changing oceans, cephalopods are booming

May 23, 2016

Humans have changed the world's oceans in ways that have been devastating to many marine species. But, according to new evidence, it appears that the change has so far been good for cephalopods, the group including octopuses, ...

Shedding light on the 'dark matter' of the genome

May 19, 2016

What used to be dismissed by many as "junk DNA" is back with a vengeance as growing data points to the importance of non-coding RNAs (ncRNAs)—genome's messages that do not code for proteins—in development and disease. ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.