Is Recycling Yeast an Option?

Sugarcane ethanol producers typically recycle the yeast used in their fermentation process. While yeast recycling can offer a variety of benefits, it can also introduce problems with contamination and shift a plant's fermentation profile.
By Erin Voegele | September 15, 2009
Brazilian consulting company Fermentec recently announced its researchers have developed a new strain of yeast that allows for a relatively high concentration of ethanol in the fermentation process while reducing the volume of vinasse that is produced. Vinasse, the primary coproduct of the sugarcane ethanol production process, is commonly used as a fertilizer.

In Brazil, the yeast used to ferment sugarcane mash during the ethanol production process is often recycled. For this reason, the alcohol content of the fermentation process generally ranges from 8 percent to 10 percent. A higher concentration of alcohol would kill the yeast, making it impossible to recycle the microbes. By comparison, the alcohol concentration in the fermentation stage of corn ethanol production typically reaches approximately 16 percent to 20 percent.

Through the use of improved temperature control and the use of its unique yeast, Fermentec's CEO Henrique Amorim says that his company's researchers were able to increase the concentration of ethanol in the fermentation process up to 16 percent without killing off the yeast. This achievement was possible due to the company's new strain of yeast, which has been designed to tolerate higher concentrations of ethanol and more acidic conditions.

According to Amorim, the next step in Fermentec's research process will be to increase the ethanol concentration in the fermentation process up to 21 percent while maintaining the viability of the yeast. He also says he is interested in studying the possibility of recycling yeast in the corn ethanol production process, and is currently awaiting an opportunity to do so.
While South American sugarcane ethanol plants typically use a continuous production process and recycle yeast, North American corn ethanol plants, which typically employ a batch process, do not. Instead, a new batch of yeast is added during each batch of fermentation. Differences in the two production process, as well as specific feedstock-related challenges, may make it difficult to recycle yeast in corn ethanol plants.

Lallemand Ethanol Technology supplies yeast to facilities that make ethanol from a variety of feedstocks, including corn and sugarcane. According to Craig Pilgrim, Lallemand's global marketing and product development manager, the specific varieties of yeast supplied to each type of facility often differ. This is due to the specific characteristics of each feedstock.

"We select the best strain for sugar-based fermentations based on different criteria, such as the ability to dominate fermentation, mutation resistance, etc., which may be different than corn-based fermentations, which are usually batch fermentations and have different sets of criteria," he says. While yeast is utilized similarly in each type of production process and the main goal is always to ferment the sugars into alcohol, he says the substrates, nutrients, environmental conditions and end product concentrations are not always the same.

Recycling Yeast in Sugarcane Ethanol Production

In the continuous production process employed by most sugarcane ethanol plants, the feedstock is continuously added and pushed through the production line. Once fermentation is complete, which takes approximately 8 to 10 hours, Amorim says the beer is pumped to a continuous centrifuge, where the yeast is separated out of the solution. The resulting yeast cream is then treated with water and acid and pumped back to the fermentor.

"The sugarcane process lends itself very easily to separate the yeast by centrifugation," says Dennis Bayrock, the global director of research and development for PhibroChem's Ethanol Performance Group. "Essentially, [sugarcane/molasses ethanol producers] recycle their yeast almost indefinitely. Some plants actually claim they don't add new yeast whatsoever. Other plants throw in a fresh batch of yeast every so often, just to make sure that the yeast population doesn't change."

The recycling of yeast is possible in the sugarcane ethanol production process because the substrate is a relatively clear liquid, Pilgrim says. "It is basically a sugar solution that has been extracted by squeezing the juice out of the cane," he continues. "Because the liquid is relatively free of insolubles, yeast can either be centrifuged out or separated and/or recycled using membranes. The yeasts can then be washed with acid or treated with an antibiotic to lower bacterial contamination and then re-added to the next fermentation, resulting in large numbers of dead and live yeast. The fermentations go quickly due to large numbers of yeast and the low levels of sugar and ethanol."

Pilgrim says a main driver of recycling yeast is cost reduction. "One only has to use a fraction of the yeast that would be used in a corn ethanol plant," he says. However, he also notes that the cost of yeast is relatively low compared to the other inputs required for ethanol production.

"In reality, yeast is not a major component of the cost for running a fermentation," Bayrock says. Raw materials, enzymes (if used) and utilities account for the majority of this cost. "So, if you are looking [to yeast recycling] as a cost savings for the plant, it wouldn't be all that great," he says.

However, Amorim says that, when done correctly, yeast recycling can increase yields because more yeast is added to the fermentation, which speeds the process.

"Typical batch fermentations for a dry-grind ethanol facility in North America are about 48 hours," Bayrock says. "Many continuous plants in South America run only about a 9 or 10 hour fermentation. With approximately 5 times to 10 times as much yeast present in fermentation due to yeast recycling as compared to batch plants, fermentation rates are significantly increased." They are able to economically add this much yeast because they recycle it. However, Bayrock notes that it can be difficult to maintain the quality of yeast over a significant period of time both genetically and microbiologically.

While Bayrock agrees that recycling yeast can be beneficial from a production point of view, he also points out that recycled yeast can shift their metabolism over time. This can significantly affect a plant's fermentation profile.

"The disadvantage of using continuously-recycled yeast is that over time, you can introduce genetic changes within the yeast," Bayrock continues. This can shift the fermentation profile, which also affects other areas of the plant.

The batch system typically employed by the corn ethanol industry enables a plant to be more responsive to problems with yeast or contamination. "If you have any process upsets, or difficult fermentors, or bad performing yeast, you sacrifice one batch maybe two but the rest of the fermentors are essentially isolated from each other and production can be recovered," Bayrock says. In a continuous production process, issues resulting from process upsets and contamination could be much more severe.

Feedstock Related Issues
The primary difficulty with recycling yeast in the corn ethanol production process stems from the corn feedstock itself. While the concentration of solids in sugarcane mash are extremely low, the concentration of solids in corn mash are very high.

According to Pilgrim, recycling yeast in the corn ethanol production process would require that the corn mash is centrifuged several times to remove these solids. He says this is not very practical and that yield losses would occur.

He also notes that the ethanol content in the fermentation process would add to the difficulties of recycling yeast. "In dry-grind grain fermentation in North America, the yeasts almost always produce more than 10 percent to 12 percent volume to volume (v/v) ethanol," he says. "Many plants are producing 16 percent to 20 percent v/v. At levels of ethanol over 10 percent to 13 percent v/v, yeasts will not grow and many are dead. To recycle dead yeasts would not be very effective, and particles are present that would increase solids and viscosity dramatically. Moreover, backset that is recycled to conserve water usage would become impossible to handle."

In addition, Pilgrim says recycling yeast reduces efficiency. "Due to the fact that the yeast are not as robust or viable after recycling, it opens up opportunities for other organisms to compete for substrate," he says. "This can lead to increased levels of acetic and lactic acid that reduce yeast growth and vitality, effecting yield and productivity. It also opens the door for wild yeast to take over the fermentation. Wild yeasts with less sensitivity to such end products will survive and continue to thrive and, ultimately, ethanol yields will be reduced. This leads to suboptimal fermentations."

Putting Recycling into Practice
Amorim says that the ability of corn ethanol producers to recycle yeast would only be possible in plants that utilize a wet-mill process. He also says recycling yeast could be beneficial to these facilities because reducing the solids present in the fermentor will make the heat exchange process more efficient, and will reduce the needed volume of fermentors. However, he also acknowledges that several challenges would need to be overcome in order to recycle yeast used in the corn ethanol production process. Specifically, a process would need to be developed to economically remove solids while keeping sugar loss to a minimum.

According to Bayrock, some North American dry-grind ethanol plants have experimented with using a centrifuge to settle out the particulates of corn mash prior to fermentation. "Some plants are actually separating out [the solids] to fermentation. This clearer liquid is then transferred to the fermentor and yeast is inoculated," he says. "This essentially allows these plants to fit more sugar into the fermentor (since corn fiber is not used and also takes up volume in a fermentor), and could also allow them to separate out the yeast in a similar fashion to that of the South American sugarcane ethanol plants. Because you have a clearer suspension, the yeast can actually be centrifuged out fairly easily," Bayrock says. "That would require two centrifuges. One at the beginning of the fermentation to remove solids from the mash and then another, smaller one at the end of fermentation to separate out the yeast from the mash."

While technically it would be possible to recycle yeast in this way, Bayrock says it probably wouldn't offer much of an economic advantage. In addition, any economic advantage that is realized would likely be overshadowed by other issues. "If you are recycling yeast, you have a greater chance of having a contaminant re-circulate through your entire system, causing problems," Bayrock says. Using new yeast directly from a manufacturer essentially guarantees very low levels of contamination. In fact, Bayrock says that recycled yeast often have a contamination level that is 1,000-fold higher than the contamination of new yeast purchased from a yeast manufacturer.

Pilgrim also notes that the batch production process allows for much tighter controls over infections. If there is an infection, it is also much easier to contain. He says that some new sugar beet ethanol plants in Europe use a batch process and do not recycle yeast, even though sugar beet mash is comparable to sugarcane mash. He also says he expects that many U.S.-based sugarcane ethanol plants that are under development will choose a batch production process as well. EP

Erin Voegele is an Ethanol Producer Magazine associate editor. Reach her at evoegele@bbiinternational.com or (701) 373-8040.