Forestall Fermenter Fouling

Readers suggest how to properly clean unit to eliminate scourge of wild yeast .

We're suffering wild yeast problems in our ethanol fermenters. Our clean-in-place system had run flawlessly for 15 years with little attention. The cycle is a rinse with steam/water at 160°F, followed by a weak caustic wash at 210°F, a hot water rinse, a rinse with an organic iodine solution with phosphoric acid, and finally another rinse with hot water. A few months back during the winter we increased the number of times we reused the caustic from three to six before sending the stream to waste treatment. Now, it's spring and we're seeing wild yeast in our cream tanks. We went back to using the caustic three times and then to just once — but that only modestly affected the wild yeast we're seeing in the product. Is the change in the number of times the caustic is used to blame? Or is it something else? How can we solve this problem?

You're not doing an effective biocidal treatment to rid the system of the wild yeast. The steam and hot water treatments should be supplemented with a broader spectrum biocide than the organic iodine. A peracetic acid/hydrogen peroxide biocide has been used successfully in bioethanol plants for several years.
Jon Howarth, senior vice president technology
Enviro Tech Chemical Services, Modesto, Calif.

Not being familiar with wild yeast or tame yeast, I have only a small suggestion based on your description of the cleaning process. Could reducing caustic strength have resulted in an increase in effective acid strength of your organic iodine/phosphoric acid rinse? Could you have corroded some sections of your process that developed microbiological growth tubercules that you're now unable to remove with normal procedures? An internal inspection of your equipment or aggressive cleaning may be required to restore the balance.
 Pete Bisila, system reliability engineer - utilities
 Pine Bend Refinery, Flint Hills Resources, Inver Grove Heights, Minn.

Once wild yeast gets into your plant you're doomed to tearing it apart to clean it. When I worked at the Anheuser-Busch (AB), a new plant manager wanted to impress management by reducing costs. He cut the caustic back and infected the entire plant with wild yeast. He was invited to lunch and never came back, which became a joke — don't go out to lunch with a vice president. The problem was solved by replacing a lot of defective spray balls, flushing the entire system with caustic and, after a rinse, a chaser of saturated steam for good measure. After several weeks, the fermenters were able to make clean product. We learned a hard lesson: don't ignore a system just because it's never a problem. Sadly, this is a lesson I've seen plants learn over and over again.

The first thing to do is to inspect the clean-in-place (CIP) system and then the circuit it takes: 1) make sure all surfaces aren't corroded and tanks are smooth to a minimum finish of 1.32 micron roughness average (Ra), which corresponds roughly to 120 grit, and welds are smooth to 6.32 micron Ra, i.e., about 60 grit —this meets standards for dairies and breweries but pharmaceutical operations require electropolishing to Ra < 0.6 micron, corresponding to a 220 grit; 2) check drainage for all lines and equipment — there's nothing worse than standing water for bacteria, insects and corrosion; 3) confirm flows and heads to all spray balls used to clean tank surfaces; 4) replace corroded equipment in the CIP system — this can act as an appendix, storing bugs for future colonization of your plant; 5) test the surface temperatures during cleaning — a cold spot also can keep bugs safe; initially, during the decontamination phase, you'll want to steam out equipment directly as it's disassembled for inspection; 6) check the gaskets — non-food-grade gaskets can creep into inventories, sometimes with incorrect serial numbers; 7) watch the spray balls to ensure they're working and not leaking; 8) inspect the fermenter chimneys for areas allowing poor heating, dripping back into the fermenter and corrosion; 9) don't forget ancillary equipment — at AB we found wild yeast collecting in the air blower supplying our fermenters: once it was because the high efficiency particulate air (HEPA) filters collapsed on a foggy day; another time the HEPA filter was accidentally removed; and yet another time, air leaked around the filter; and 10) inspect spargers and other equipment that may not have the correct finish or have sharp angles where bugs can hide — spargers were a big red flag at AB.

Lastly, you may want to include saturated steam in the cleaning process. This must be done cautiously because nitrile rubber gaskets (the most common kind) won't stand high temperature steam without cracking and becoming a crevice for bugs. Another reason is thermal expansion, which can cause problems with agitators, thin-walled tanks and even pumps.

The killing capability of steam is a function of time (heat-up, dwell time, cool-down), mixing created by high flow rate, and temperature. Time and temperature are inversely related. The killing power generally is compared to its value at 121.1°C (Fo = 1 at 121.1°C) and ratioed according to the equation: Fo = 10(T – 121.1)/10, where T is a temperature other than 121.1. So, if we assume 265°F (129°C) for a dwell temperature, achieved by sparging steam into a fluid stream, Fo will be more than six times greater and only about one-sixth the time needed at 121.1°C will be required, or about 10. To reduce product damage, such as caramelization, when steaming out at high temperature, provide fast quenching to limit exposure time.

For a worst case, with a colony of wild yeast, I recommend steaming equipment at as high a temperature as possible, at the equivalent of 30 minutes at 121.1°C. At no time should you try sterilization below 180°F (82°C). Use an infrared gun to confirm the temperature using a black spot as a transmitting surface. Air or a hot gas can substitute for steam with a reduction in heat transfer: 1,000 Btu/hr-ft2-°F for saturated steam but only 1 Btu/hr-ft2-°F for air.

If you're interested in more information on metal surfaces and standards, check out this webpage: For information on sterilization practices refer to: and
Dirk Willard, contract senior process engineer
Superior Engineers, Hammond, Ind.

Our polyphenylene ether (PPE) process generates a lot of dust and PPE resin is a severe fire hazard. I'm tasked with figuring out a way to collect and reprocess this dust. I'm considering agglomeration. Do you think this is a good option? If so, what do I need to do? If not, what would you suggest?

Send us your comments, suggestions or solutions for this question by June 14, 2013. We'll include as many of them as possible in the July 2013 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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