Aside from the few of us who get paid to think about these things, not many realize how commonly fungi intersect our lives. Most people eat some fungus daily: yeast, sure, but lots of others too. I think you should know about this, not to gross you out, but so you can appreciate the contributions of fungi to your ordinary-seeming life.
Today’s lesson in fungal utility covers pullulan, a useful polysaccharide made from fungus slime. That’s right, FUNGUS SLIME.
Pullulan is neat stuff. It’s made of long chains of modified glucose molecules. It tastes inoffensive and dissolves easily. Because of its low digestibility, it has a low glycemic index–eating it doesn’t cause a big insulin spike like sucrose. Early reports claimed pullulan was completely indigestible, but later research showed that it is slowly broken down in our bodies. Alas, a generous dose of it “increases the incidence and frequency of flatulence,” according to the hardy volunteers recruited by Dr. Wolf and colleagues. But those folks drank 50 grams of it in 2 cups of viscous, sugar-free lemonade–more than I’d recommend.
Pfizer introduced Listerine PocketPaks™ in 2001, and they took off like a rocket. Inside the package we find flat squares of what appears to be plastic. It’s not plastic– it’s pullulan. Put one on your tongue and it quickly melts, releasing its minty flavor. Refreshing! Since then pullulan has shown up in a variety other products: children’s medicines, which my son prefers to icky syrups, doggie breath strips and marijuana-laced medi-strips (both sadly discontinued in the US), gelatin-like capsules for medicines that don’t offend vegetarians, and as an edible coating that protects foods from drying.
Pullulan was approved for food use in Japan in 1976, but it took til 2002 for the American FDA to recognize it as a GRAS food additive (Generally Regarded as Safe, E1204). That’s why it’s just been in the last little while that pullulan’s been popping up all over. It can be used as a film or thickener or to encapsulate just about anything, and now that you know about it, you can seek it eagerly in ingredient lists. Not every melt-in-your-mouth strip is pullulan-based, though. There are other food-grade films, including hypromellose, pectin, xanthan gum (a bacterial product), gellan (from algae), and others. But this is a fungus blog.
Aureobasidium is an interesting, sticky, goopy, ubiquitous thing. You can admire its slimy brown mugshot here. It’s really, really good at growing superficially (like many politicians it is impressive, but lacks depth). It is easy to find, for example, on the surfaces of apple leaves, or on apple fruit, where it is can lead to russetting. It likes just about any plant surface, really, where it degrades the cutin layer that protects the plant, opening the door for pathogens. Perversely, it sometimes fights off pathogens– some strains have been deployed as biological controls of plant disease. You might also find it flaking the paint off your house, which it does by growing under and in the paint, and nibbling at the lignin of the wood surface. You can imagine that paint doesn’t stick too well atop a mucusy layer of pullulan. Aureobasidium is growing on the wreckage of the Chernonbyl nuclear plant. If you’re really unlucky, Aureobasidium might grow on your eyeball, but it much prefers plants. Oh, and it might just be the fungus that’s growing on your shower curtain and making your cheap white lawn chairs look like cheap trash.
So you see, Aureobasidium was already common in your life. And now you’re eating it.
Editor’s note: Is fungus snot the same as human snot? No, it’s not (heh). Human snot is primarily mucins– proteins linked to a bit of sugar, with other stuff mixed in: enzymes, antibodies, and such. Pullulan, you recall, is a polysaccharide, not a protein. I know a child who says boogers taste good, whereas pullulan is said to be “almost tasteless.”
- Cooke, W.B. 1959. An ecological life history of Aureobasidium pullulans (de Bary) Arnaud. Mycopathologia 12: 1-45.
- Horvath, R.S., M.M. Brent, D.G. Cropper. 1976. Paint deterioration as a result of the growth of Aureobasidium pullulans on wood. Appl. Environ. Microbiol. 32: 505-507. [PDF]
- McGrath, M.J. and J. H. Andrews. 2007. Role of immigration in the colonization of apple leaves by Aureobasidium pullulans. Appl Environ Microbiol 73: 1277-1286.
- Gildemacher P., B. Heijne, M. Silvestri, J. Houbraken, E. Hoekstra, B. Theelen, T. Boekhout. 2006. Interactions between yeasts, fungicides and apple fruit russeting. FEMS Yeast Research 6: 1149-1156.
- B.W. Wolf, K.A. Garleb, Y.S. Choe, P.M. Humphrey and K.C. Maki. 2003. Pullulan is a slowly digested carbohydrate in humans. J. Nutr. 133:1051-1055. Best quote: “In the first 24-h postprandial period, the frequency and intensity of flatulence was higher (P < 0.05) after subjects consumed pullulan compared with control."
- P. Zalar, C. Gostincar, G.S. de Hoog, V. Ursic, M. Sudhadham, and N. Gunde-Cimerman. 2008. Redefinition of Aureobasidium pullulans and its varieties. Stud Mycol, 61: 21-38.
Pullulan molecular structure, thanks Wikimedia Commons.
Listerine PocketPak™ image by flickr user ErikJaeger under creative commons license. Thank you Erik.