Keeping yeast fit and healthy: combating organic acids in fermentation

By Amanda Moser, Senior Scientist, Novozymes
Special to The Digest

It’s harvest time, and that means organic acids—a common stressor for yeast—are a top-of-mind concern for ethanol producers. But while some yeasts are more resilient than others, most still require constant attention; after all, each stressor can potentially reduce performance, and those reductions can quickly add up.

A yeast that is less susceptible to stressors is the best way to create a solid foundation for your fermentation. It also helps, however, to have a strategy for keeping your yeast fit and healthy. In our last article, we discussed common ways to effectively deal with temperature stress and avoid premature yeast death. So today, let’s look at another stressor plants routinely deal with: organic acids.

The effect of organic acids on yeasts

The two major organic acids that are detrimental to yeast metabolism are lactic and acetic acids. Both are products of fermentation by bacteria and/or wild yeast, and both become more of a problem over time due to the use of other wastewater streams in the recycling backset..

Acetic acid is the more potent inhibitor of yeast. However, acetic and lactic acid have different inhibition mechanisms, and together those mechanisms can exponentially worsen conditions.

Thankfully, due to faster growth rates, yeast typically overtake the contaminating bacteria or wild yeast, which suppresses the organic acid production. If contamination levels are high, however, the yeasts will not be able to outcompete for nutrients, resulting in sluggish or stopped fermentations.

Methods of combating organic acids

A key element of controlling organic acids is to maintain a low amount of infection in incoming mash as well as a pH that will inhibit the bacteria more than the yeast—this is typically <pH 6. Higher initial yeast counts can also reduce the effect of organic acids on the yeast population.

The most fundamental way to control infection, though, is to control the fermentation environment. Here are a few key ways to do so:

• Avoid dead legs in the lines in the process.
• Avoid filling multiple fermenters with the same mash train heat exchanger, and ensure they are cleaned between each fermentation.
• Implement and follow proper cleaning programs.
• Clean the fermenters on a regular basis.
• Control fermentation temperatures and maximize yeast growth.

Technological next steps

Again, when it comes to dealing with organic acids, having a yeast with organic acid tolerance is always best. But don’t forget to have a tried and true strategy that will help create the ideal environment for your yeast to thrive. As simple as it sounds, this is a big piece of the fermentation puzzle.

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.