Mycology lessons at the SF Food Bank

[Updated below]

When I volunteered at the San Francisco Food Bank with other food bloggers on Saturday (an outing organized by Amy and Sam), one of our tasks was to transfer oranges from huge crates to smaller boxes. As we moved the oranges, we ran across quite a few that had patches of white, green and blue mold. This got me thinking about the mold on oranges. What is it? Is it useful to humans, or just a pest?

After a bit of research, I found a short report from the Indian River Research and Education Center (PDF) about oranges that identified the green mold as Penicillium digitatum and the blue mold as Penicillium italicum. Although it is not so pleasant to grab a mold-covered orange, the color can be quite lovely, and as I’ll explain below, molds in the Penicillium genus are quite important to humans. The light green reminds me of the classic celeste green of a Bianchi road bicycle, which then leads to thoughts of the open road.

I didn’t discover any useful applications of the orange molds, but the Penicillium genus has some important species. Foremost, of course, is the antibiotic drug, penicillin, which was initially derived from Penicillium notatum and has more recently been replaced by Penicillium chrysogenum (according to the McGraw Hill Science and Technology Encyclopedia). Several cheeses are inoculated with a Penicillium species during production, including Roquefort (P. roqueforti), Gorgonzola (P. glaucum), and Camembert (P. camemberti).

George Barron’s website on Fungi has some more information about Penicillium fungi and great picture.

At Green Car Congress I ran into yet another use for spoiled oranges and orange peels: making ethanol to use as a transportation fuel. The post states that “[c]itrus waste is rich in pectin, cellulose and hemicellusic polysaccharides, which can be hydrolyzed into sugars and fermented into ethanol.” The proposed project will use waste material (instead of real food) as the feedstock and the ethanol plants will be near the citrus facilities: “Citrus Energy notes that the 5 million tons of citrus waste produced annually from Florida’s 100 million citrus trees is available on a continuous basis for 8 months a year with no transportation costs to ethanol plants co-located with citrus producers. Existing road and rail transportation systems servicing the citrus plant would take the ethanol to market.” No details about whether the mold has any special properties in the ethanol process…

Photo credit: Orange photo from Clearly Ambiguous’s Flickr Collection, subject to a Creative Commons License.


  1. Yeah, but that doesn’t make it any less gross when you have to grab on orange that is completely encrusted with the stuff and so squishy from decomposition you can’t pick it up intact. 🙂

  2. I agree with Sean. And i have to wonder whether oranges that were touching the moldy ones are going to get moldy in the box before they even reach their destination?

    Another question: did anything you find indicate that organically grown oranges fall prey to mold faster and easier than others? Because mine sure seem too; lemons, too.

  3. We probably could’ve filmed a punk rock video with all the mohawked oranges we found (Citrus Vicious and Oranjohnny Rotten?). And I wonder what was up with those oranges that had a weird indentation around their circumference, as though they had been strangled. OK too much anthropomorphism.

    Next time I sort through moldy oranges, I’ll try to think about their delicious cheesy cousins.

  4. Orange peels, dried, with the mold, are important in traditional Chinese medicine. I can’t remember what for, because I’m a bad bad ex-TCM-student. They’re far from the grossest thing that TCM considers to have medicinal properties.

  5. Sean — some of those ex-oranges were challenging, like picking up a handful of water.

    Bonnichiwa — Unfortunately, I don’t have a definitive answer to your question about mold spreading. Whether it happens probably depends on how long they were in contact, whether the “target” orange has any imperfections to which the spores can attach, the temperature, the humidity, and so on.

    I didn’t run across any comparisons of mold susceptibility between organic and conventional oranges, but found a hint in the document by the Indian River Research and Education Center (PDF link in the post): “Postharvest decay can be reduced by harvesting at optimum maturity; gently handling fruit during harvest and postharvest operations; maintaining sanitary facilities and water handling systems; prompt cooling; optimum temperature and RH; and use of approved fungicides or biological control agents.” Washing the fruit after harvest might remove some of the mold that was collected in the field, and applying fungicides will kill some of the mold. I imagine that waxes also can prevent mold.

    Shelly — a punk rock video would have been a great way to play with our food. Perhaps some of the near-liquid ones that Sean mentioned could have been used for some abstract splatter art by throwing them against a canvas.

    k.a. — that’s interesting that the mold is considered a medicine. I wonder if it needs to be fully aged, whether green is preferred to blue, and so forth. Something to look up on my next trip to the library.

  6. Interesting article and chat. I found it because of this question: What is the best way to wash the rest of the oranges that were in the bag or box with the moldy ones? Plain dish detergent? A light bleach rinse (1:10, 1:15 bleach to water)?

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