Overcoming Fusel Poisoning Requires Patience, Diligence

Lessons learned from bringing plants back from the brink of shutdown. This article is published in the June issue of Ethanol Producer Magazine.
By Dennis Bayrock | May 14, 2016

Fusels are a family of 400 organic acids, higher alcohols, aldehydes and ketones that yeast and bacteria produce in small quantities as part of their metabolism. However, fusel compounds are approximately 15 times more toxic to yeast than ethanol and, if they build up at fuel ethanol plants they can cause real problems.  Systemic plant shutdowns have occurred with concentrations of mixed fusels as low as 100 to 500 parts per million (ppm).   

A June 2012 article in Ethanol Producer Magazine, “Fusel Oil Recycle—A Silent, Odorous Killer,” provided a basic understanding of fusels. Here we’ll discuss strategies to detect and combat fusel toxicity emergencies.

The simplest, fastest qualitative way to detect fusels is to use your nose. Fusels have a characteristic varnish-like odor that is different from ammonia, ethanol and denaturant.

If fusel odor can be detected at the fusel draw pump on the rectifier, prompt action is needed. By the time one detects fusel odors in the slurry, it already is at 20 ppm—the human detection threshold—and probably increasing. Severe yeast inhibition can occur at fuel ethanol plants with as few as 500 ppm mixed fusels, although individual fusels can be much higher.

One simple fusel test method for samples drawn from the sidestripper or rectifier column involves adding a saturated salt solution to the sample in a 100 milliliter graduated cylinder. Most fusels are insoluble in saturated salt solutions and will separate.

Unfortunately, this method cannot resolve yeast inhibition issues caused by fusel compounds at concentrations as low as 30 ppm. The saturated salt test is designed for samples containing concentrations greater than 50,000 ppm. Using this method to measure lower concentrations can lead to significant errors because a 0.5 ml error in reading the graduated cylinder potentially can translate to a 2,000 ppm, plus or minus, difference. It also will not detect fusel compounds that are completely soluble in saturated salt solutions.

Plants have other tools that can help. High performance liquid chromatography (HPLC) and gas chromatograph-mass spectrometry (GC-MS) can detect fusels, along with their primary role in benchmarking fermentation.  While specialized columns are made for fusel detection, most fuel ethanol plants utilize one column designed to benchmark fermentation.  Many of these existing columns can resolve and quantitate some fusel compounds by increasing the processing time. The accompanying HPLC chromatogram illustrates fusel peaks appearing after ethanol’s. These tests can detect fusels at concentrations near yeast inhibition levels.

Plants can determine whether their HPLC can resolve fusel compounds, by starting with at least a 1-to-200 dilution of a sample from the fusel draw pump. If fusels are detected, a diethyl ether extraction on the sample will provide better sensitivity at lower concentrations.

Another fusel test that can be outsourced is headspace analysis. Seal a test sample in a vial containing headspace and incubate it overnight at a prescribed temperature, typically 80 decrees Celsius. Inject a portion of the headspace gas into an HPLC or GC-MS. Caution must be exercised in interpreting fusel contamination issues with this method because there are many yeast-inhibiting fusel compounds that will not volatilize at the set temperature of the test and will remain undetected.

Five ethanol plants dealing with fusel toxicity confirmed the validity of this technique to detect fusels. These plants turned on their mash hydroheater for a week to flash off fusels before the mash reached the propagators and fermentors. Although volatile  fusels indeed were being removed, evidenced by smell, the inhibition continued. Only after steps were taken to prevent yeast overproduction of fusels and the plant was purged of fusels did the yeasts return to their normal vigor.

Overproduction of fusels caused by yeast cell stress conditions can take weeks to concentrate to toxic levels and may, at first, appear to be similar to bacterial contamination because fusel compounds behave analogously to other inhibitors such as lactic and acetic acid chemicals from bacteria.

Once confirmed, field experience teaches us that adding more yeast, nitrogen or antimicrobials fails to salvage the situation.  Any fresh yeast are placed in the same inhibiting conditions as existing yeast and antimicrobials are of little benefit, because fusel compounds are toxic to bacteria. In most cases where fusel toxicity is confirmed, bacterial viable counts in diagnostic plating are extremely low as well.
Tackling Toxicity
Once a plant detects fusel toxicity, there is no magic wand to quickly eliminate it. In the worst fusel situation to date at a fuel ethanol plant, it took a week of patience and diligence to fully regain the plant. The following recommendations are important, but temporary, strategies to cope until fusels are purged.

Increase fusel draw rate: The rectifier is the only place in the plant designed to separate fusels, therefore use every means to remove fusels from this location. Use the previously described salt saturation test to verify that more fusels are being drawn out. If there are none, or the amount is very low, check immediately for a fusel dam beneath the fusel draw point. Fusel dams can enter a system with little warning and cause sudden serious inhibition to the yeast once recycled in the mash.

Turn on the liquefaction hydroheater:  Although the yeast did not recover from the fusel inhibition with attempts to purge as described earlier, volatile fusels were detected coming off the mash stream. This suggests the technique could reduce fusels enough in some situations to benefit the yeast, particularly if the total concentration of fusels is near a threshold. 

Change rectifier operating conditions: By temporarily increasing the overall temperature of the rectifier and/or increasing the temperature delta from the bottom to the top, the fusels move up the column, allowing any fusel accumulation below the draw point, or a possible dam, to leave the column at the current draw point. Conversely, decreasing the overall temperature and/or decreasing the temperature delta will move the fusels down. Try both, starting with the shift up, using conservative increments of about 1 C to help focus the fusels on the draw point.

If multiple fusel draw points are not available, or if focusing the fusels is too difficult to accomplish, the next option is to purge through the top of the rectifier by increasing the temperature and delta. The energy and water balance will be severely affected, but purging through the top is preferable to swallowing the fusels through the bottom to be recycled.

 Raise the pH in fermentors/propagators: Understanding both fusel chemistry and yeast cell membranes provides a unique way to lessen fusel toxicity. Weak acids, such as lactic acid, partially dissociate in water, releasing some of their protons to make the water acidic while others remain undissociated. There is a point of equilibrium that can be shifted by altering the pH. 

This principle can be applied as a temporary solution to fusel emergencies. Raising the pH in the fermentor or propagator by a maximum of 0.2 pH will shift a portion of the fusels to the dissociated form. Because charged chemicals typically cannot cross the yeast cell membrane, the dissociated form of fusels remains outside of the yeast cell and does not inhibit the yeast.  As long as the bacterial contamination at the plant is under control, the risk of promoting bacterial contamination with a 0.2 pH increase is minimal. This recommendation has worked in nearly all the 10 fuel ethanol plants where fusel toxicity was confirmed.   

Raising the pH in a filled propagator or fermentor can be done using aqueous ammonia or caustic, both very strong bases. At facilities where fusel toxicity stalled the plant, the plant manually pulsed the CIP [clean in place] valve to the fermentor for about 10 seconds, followed by a manual measurement of the pH change at a sampling port. Additional pulses of caustic were made to shift the entire fermentor up by 0.2 pH.  It is a case of choosing the lesser evil—fusel toxicity far outweighs any potential inhibition by sodium from the addition of caustic.

Aqueous ammonia would be the preferred choice for this job as it leaves behind no residues to inhibit the yeast and also would provide the yeast with an additional source of nitrogen for nutrition. Urea is not recommended, however, because although it can increase pH, it is a weak base and the amount that is needed to change a 750,000 gallon fermentor would be prohibitive—approximately 10 times the amount of caustic.

Fusel toxicity is indeed a silent, though odorous killer of yeast. Vigilant monitoring of distillation rectifier operation and yeast cell stress conditions that lead to overproduction of fusels is extremely important.

Author: Dennis Bayrock
Global Director Fermentation Research
Phibro Ethanol Performance Group