How EtOH production affects yeast health: 4 tips for dealing with ethanol stress

By Dale Earls, Scientist, Novozymes

Special to The Digest

It turns out that too much of a good thing can actually be a bad thing—even when it comes to ethanol. At least in terms of the fermentation process.

In commercial yeasts available today, as fermentation progresses and the concentration of ethanol rises, yeast cells are increasingly exposed to levels of ethanol that quickly become toxic. This greatly affects yeast’s ability to produce ethanol; in fact, healthy, growing cells produce ethanol up to 33 times faster than non-growing cells.

Of course, we want ethanol levels to increase; that’s the point of fermentation. And we expect cell productivity to slow down somewhat—after all, the ethanol production rate is maintained in late-stage fermentation not because the yeast is highly productive, but because there’s so much more yeast. Yeast counts later in fermentation can be more than 25 times those in early fermentation.

Some yeasts, of course, are more resilient than others, but most currently on the market still require constant attention to maintain performance levels. So, is there a way to boost productivity in those later stages? Are there steps that producers can take to minimize yeast stress? The short answer to both those questions: absolutely.

Four tips for keeping yeast happy

Here are our four recommendations for dealing with ethanol stress in your plant:

1. Maintain fermentation temperature. Heat and ethanol stress have a synergistic effect, as cells are particularly susceptible to ethanol toxicity at higher temperatures. Reducing the temp toward the end of fermentation can decrease the ethanol toxicity to the yeast cells (cell membrane permeability).
2. Monitor supplemental nitrogen. Especially during warmer months, pay close attention to how nitrogen is being dosed, as too much can accelerate yeast metabolism to the point that excess—and detrimental—heat is produced.
3. Maintain yeast health. Yeasts require several micronutrients and growth factors to optimize their growth and fermentation performance, and essential vitamins are required for maximum fermentation rates and strain development.
4. Monitor ethanol at propagation drop: Higher ethanol concentrations will decrease cell membrane thickness, and make them more likely to experience ethanol toxicity.

Though yeast are most prone to stress late in fermentation, ethanol concentrations of just 10-13% can result in slowed growth, which can impact overall performance. Following these tips as general best practices is a good first step toward improving ethanol yields.

Read our previous articles in this series—addressing warm weather ethanol fermentation and combating organic acids in fermentation, respectively—for more ways to keep your yeast fit and healthy amid various environmental stressors.

To learn more about yeast and many other aspects of ethanol production, such as enzyme functionality, laboratory practices and process troubleshooting, Novozymes customers can access Bioenergy University, an online platform offering webinars, course modules and videos.