New Tool Offers Insight into Dynamic Yeast Performance

Measuring yeast metabolic activity helps optimize plant performance
By Jason Van’t Hul and Phil Bureman | July 11, 2013

In today’s corn-to-ethanol plants, the industry standard of counting viable cells using light microscopy and a viability stain in order to evaluate yeast health suffers from several limitations. The method is labor-intensive, error-prone and often fails to provide useful information for diagnosing or optimizing industrial yeast propagations and ethanol fermentation.

In the case of one established Midwestern dry mill ethanol plant, a 110 MMgy ICM design, traditional cell counting was an accepted part of the fermentation management program. Plant managers, however, were concerned about the variance in cell counting data from one technician to the next, and they believed that operator time could be better spent in activities more valuable than counting cells.

The plant was planning several new optimization trials and looking for methodologies to monitor and measure the impact of these changes on their normal operational program. One methodology the plant decided to investigate was designed to be a completely nonsubjective measurement of yeast metabolic activity rates, which is fundamentally different and more useful than counting how many yeast cells are present. This methodology was first introduced in 2009 and allows the fermentation manager to better understand the dynamic performance of the plant’s process while enabling advanced diagnostics and optimization steps based on real performance information. The test requires only 30 seconds of hands-on time followed by an automated series of measurements requiring less than three minutes. It draws its power from fast, fluorescence-generating reactions between native yeast enzymes and biochemical reporter molecules.

This approach gave ethanol producers unique insight into their fermentation processes that had never before been possible. Unlike traditional methods, the Nalco Yeast Activity Monitor measurements correlate with the rate of ethanol production, as shown in the accompanying graph. With the Nalco YAM technology, ethanol producers had a real-time tool to evaluate the impact of new antibiotics, the performance of a new yeast or nutrient program, or most any other potential process change that affects yeast metabolic activity.

The dry mill ethanol plant described above combined regular use of the NalcoYAM technology along with careful analysis of other fermentation data and ethanol yields to establish new standards that help in better understanding, measuring and troubleshooting the fermentation process, including the following:

• Fermentation samples are collected at 10 and 18 hours. If the Nalco YAM activity is less than 1,000 at 10 hours or less than 6,000 at 18 hours, a cell count is performed to verify the readings.

• Low activity and cell count trigger the addition of additional yeast to produce acceptable fermentation and ethanol yield.

• The plant found that, if additional yeast was not added as indicated by the low Nalco YAM activity measurements, the fermentation would end with high sugars, excess syrup, and poor ethanol yield. Also, the DDGS produced would be discolored, sticky, and chunky, and dryer issues would be experienced. Each of these issues would be potentially costly for the plant.

Testing yeast activity using Nalco YAM technology can help producers detect problems earlier in the process while there’s still time for corrective action. It can help ethanol producers catch poor yeast health in propagation or during the first 15 hours of fermentation. Typically, plants would do one viable yeast cell count using a microscope by hour 15. It has been determined, however, that many ethanol fermentations see the highest ethanol productivity from hour 12 to hour 18. After this time, ethanol productivity falls dramatically and yeast activity slows, making any corrective action at that point almost insignificant.

Troubleshooting Aid
One of the most exciting applications of the Nalco Yeast Activity Monitor technology was the evaluation of the yeast itself in an incident that occurred when the plant started using bulk refrigerated cream yeast, replacing its traditional active dry yeast. 

The plant maintained two separate refrigerated bulk tanks for cream yeast. In December 2011, shortly after the receipt of a fresh batch of cream yeast, the plant noticed very low 10 and 18 hour activities in a fermentation using yeast from the most recent delivery. Within 24 hours, a troubleshooting process began. The Nalco YAM technical consultant was able to assist by confirming the low measurements. More importantly, he introduced the plant to the newly developed technique of using the Nalco YAM technology to test for activity of the cream yeast itself. A major difference was quickly confirmed between the activity of the cream yeast in one bulk tank versus the other. The following differences were noted:

• A previous batch of cream yeast in bulk tank No.1 (about 10 days old) showed the typical high activity of about 27,000.

• The most recent cream yeast batch in bulk tank No. 2 (about three days old) showed an unexpectedly low activity of less than 7,000.

• The low 10 and 18 hour activities measured the day before were confirmed to be associated with the cream yeast from tank No. 2.

The cream yeast supplier was contacted and the company indicated the new batch met specifications. The supplier recommended the plant simply add more yeast to each fermentation when using product from the batch contained in tank No. 2—a costly corrective action.

Despite the addition of more cream yeast, when tank No. 2 cream yeast was used, ethanol yield continued to drop from a normal 13.2 percent to 9.0 percent, and the back end of the plant experienced a significant and costly slowdown. The incident resulted in a total of five bad fermentations and an estimated loss of $500,000. As a result of this incident, the plant instituted new procedures to prevent recurrence.  All cream yeast deliveries are now tested with the Nalco YAM technology upon receipt. Since the test takes only a few minutes, it is even possible to confirm the activity of the bulk cream yeast prior to offloading into plant bulk storage. Any YAM readings for newly delivered cream yeast of less than 15,000 trigger an additional cell count. Overall the activity of new cream yeast routinely measures 23,000 plus.

This plant, like most fuel ethanol plants operating in today’s highly competitive environment, continues to optimize and innovate its process and operation. Part of that process is early adoption of low-risk, low-cost innovations from valued suppliers. The yeast monitoring technology has become an integral supporting tool of nearly every fermentation trial at this facility. Yeast activity results are reviewed during every trial to verify the impact on yeast. YAM technology has been utilized to measure the impact of several operational changes including enzyme trials, hydrolysis utilization, overall yeast optimization and enzyme modifications with urea to reduce glycerol. These optimizations are expected to save the plant more than $350,000 each year.

In today’s liquid fuels industry, a producer needs every cost-effective tool available to stay competitive. Understanding yeast metabolic activity helps plants better understand the dynamic performance of their propagation and fermentation processes and allows producers to perform advanced diagnostics and optimization steps based on real yeast metabolic performance information.      

Authors: Jason Van't Hul
Senior Industry Technical Consultant, Nalco

Phil Bureman
Senior Industry Technical Consultant, Nalco

The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).