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Filamentous fungal fermentations.

Suppose a microbiologist finds a compound produced by a fungus that--if developed into a drug--could revolutionize medical treatment of a disease.

But what if the microbiologist then learns that this newly discovered compound can't be economically massproduced using today's fermentation methods?

There are two losers in this hypOthetical scenario: the company that could manufacture the pharmaceutical, and--more importantly--the user who would be denied this new treatment.

Presently, some fungi produce antibiotics such as penicillin, enzymes that convert com syrup to sugar, and chemicals such as citric acid and industrial alcohol.

But bridging the gap between biological breakthrough and practical application may be eased by an invention developed and now being patented by ARS scientists.

Called an attached-growth biological reactor, the invention compensates for the problems certain fungi have trying to grow in standard fermentation tanks and facilitates their successful harvest.

Most fungi currently used in fermentation processes grow as pellets and secrete their products into a surrounding fluid. However, some fungi form as long strands or clumps in a filamentous type of growth. It is these fungi for which the biological reactor was designed.

While our hypothetical microbiologist might grow small batches of filamentous-type fungi in laboratory flasks, this is insufficient for industrial production, says chemical engineer Dennis J. O'Brien, who is at ARS' Eastern Regional Research Center in Philadelphia, Pennsylvania.

O'Brien and co-inventor Wolfgang Heiland, a supervisory mechanical engineer also at the center, have designed a new biological reactor that encourages filamentous-fungal growth.

Currently, industrial fungal fermentations are done by placing the microorganism in a sealed tank filled with liquid containing essential nutrients. The tank is supplied with air, and at the end of fermentation, the mixture is pumped to a machine that separates the cells from the liquid.

But if filamentous-type fungi are grown under these conditions, the mixture thickens as they grow, reaching a point where it becomes as thick as oatmeal, O'Brien says. The results are poor fungal growth due to lack of oxygen and a mixture that is difficult to further process.

O'Brien says the biomass yield of fungi that form in pellets in current industrial fermentations is about 15 grams per liter, while that of filamentous-type fungi is only about one-third of that.

"If you are basing a process on filamentous fungi, this new reactor offers an economical choice for production," O'Brien says.

A key to the invention is eliminating the need to ferment microorganisms in submerged tanks. Rather, a horizontal cylinder inside the reactor is submerged to half its depth in a nutrient solution.

Sterile air is pumped into the reactor through a tube. A shaft continuously rotates the cylinder, so the fungus gets an equal distribution of nutrients and oxygen, he says.

"It's a better way to grow the organism," O'Brien says. "In submerged fermentation, air is pumped into the solution, but oxygen is not very soluble in water."

Twelve to 19 hours after a fungus is added to nutrient fluid in the reactor, the fungus begins attaching to the rotating cylinder. It continues to grow until it eventually covers the entire cylinder.

"The fungus grows outward and looks similar to fibers on a paint roller," O'Brien says. "It has the look and feel of raw chicken skin."

The attached material is periodically shaved off the cylinder and caught in a small pan. Once harvested, the fungal growth can be retrieved for extraction of desired compounds.

"Now it's possible to have continuous fungal growth and harvesting," says O'Brien. "This invention gives industry a choice that could improve the economics of fermentation and encourage the development of new microorganisms and products."--By Bruce Kinzel, ARS.

Dennis J. 0 'Brien and Wolfgang K. Heiland are in the USDA-ARS Engineering Science Research Unit, Eastern Regional Research Center, 600 E. Mermaid Lane, Philadelphia, PA 19118. Phone (215) 233-6601.
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Title Annotation:includes related article on eicosapentaenoic acid
Author:Kinzel, Bruce
Publication:Agricultural Research
Date:Jul 1, 1992
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