Well-Fed, Stress-Free Yeast Required

FROM THE JUNE ISSUE: The best way to minimize issues from stress or contamination and maximize yield is to cultivate healthy, actively growing yeast.
By Jim Miers | May 30, 2018

Ethanol plant managers have many factors to watch in the quest for efficiency. Optimizing yields requires maintaining consistency through tight controls on plant parameters—an important step in establishing a solid baseline to use when evaluating tweaks to the process or new products. Proper yeast nutrition, particularly in propagation, can translate into fermentation consistency. In addition to nutrition, keeping stress to a minimum is important, with two environmental factors needing special attention: pH and temperature.

The ideal mash pH is 5.0 to 5.2, although the range at which yeast will ferment is much broader. If acidity is off, which can happen when using anhydrous ammonia as the source of the nutrient nitrogen, the yeast cell will expend energy adjusting its internal pH to the ideal, sacrificing ethanol yield.

Yeast also have optimal temperatures in which they thrive, with bittersweet results. If the temperature is too low, yeast slow the conversion of glucose to ethanol but gain better viability and vitality. If it’s too high, conversion speeds up but yeast viability is sacrificed. Stressed yeast produce more glycerol, decreasing ethanol production. Fermentation itself, of course, produces heat. One strategy is to lower temperature near the end of fermentation when ethanol levels—another stressor on yeast—increase.

Combating Stress
Fermentation broths can be inherently stressful environments, with yeast naturally producing heat, ethanol and organic acids, along with other compounds impacting pH. Combating those potential stressors and keeping yeast optimally producing ethanol starts in the propagator. It’s the critical first step, keeping the mother cells in the yeast culture healthy so they produce healthy daughters from the start, because the daughters, as exact duplicates of their mothers, are going to become mothers producing exact duplicates.

Oxygen is an important nutrient for healthy yeast. Proper aeration is needed to produce the unsaturated fatty acids and sterols required for strong cell membranes. Not only do strong membranes allow the passing of needed nutrients, but they play a role in the cell’s ability to withstand stress.

Corn, and all other grains used for ethanol production, are rich in starches that get converted to the nutrient glucose, but are limited in another key nutrient—nitrogen. The need for useable nitrogen, called free amino nitrogen (FAN), and its management is well-known in the ethanol industry.

Anhydrous ammonia or urea have long been used for nitrogen and, more recently, proteases have been introduced to enzymatically break down the protein in the grain itself into FAN.

Less understood is the impact of other nutrients, including phosphorus, sulfate, copper, potassium and others. Many micronutrients play a big part in the enzyme functions within the yeast cell. Zinc, for example, plays a part in the ethanol dehydrogenase enzyme and deficiencies can cause stuck fermentations. Magnesium helps insulate the cell from the osmotic stress that occurs if the glucoamylase gets ahead of the process and converts more starch to glucose than the yeast can immediately consume. Magnesium also helps the cell withstand stress caused by temperature and ethanol. Calcium helps make the cell wall more permeable, so important nutrients can easily pass into, and byproducts pass out of, the cell. Too much calcium can be detrimental, however, thus maintaining the correct ratio between calcium and magnesium is important.

Nutrient Packages
Corn is a great source for a broad range of nutrients, although not all are 100 percent bioavailable. A nutrient analysis on corn will show both bound nutrients as well as the soluble portion that can be utilized by yeast. Also, alternative feedstocks have slightly different nutrient profiles. For example, the lower yields often experienced by producers using grain sorghum is a result of a slightly different balance of essential bulk and trace minerals than seen in corn.

Micronutrients also likely play a role in the differences observed in corn from year to year and region to region. Lallemand Biofuels & Distilled Spirits field personnel have long noted that plants in some areas will see process changes when new-crop corn comes in, while plants in other areas of the Corn Belt observe little impact. Corn also appears to change in nutrient availability the longer it is stored, exacerbating the difference between old and new crop.

With these factors in mind, Lallemand has formulated yeast nutrient packages for different applications, including one when sorghum is used. One of the newest, Nutri-Plex Pro, is even formulated for plants working through yeast stress. The high degree of recycled process streams in today’s ethanol plants means fusels can inadvertently become elevated. With all the naturally occurring yeast stressors—temperature, pH, osmotic and ethanol—in the broth, the addition of fusel stress can be the straw that breaks the camel’s back. Once identified, however, high fusel levels can take time to correct. Lallemand’s Nutri-Plex Pro nutrient package was designed specifically for high-stress loads from fusels, as well as high temperature and lactic stress. While the package won’t eliminate the stress, it can help minimize losses as the plant tackles the root cause.

The bigger role for yeast nutrition, however, is to ensure consistency and insulate yeast from naturally occurring stressors. But nutrient benefits may not be easily quantifiable in a single ferm tank’s yield. Often, the true benefit is seen in improved kinetics. A well-designed yeast nutrition program can speed up fermentation by as much as four hours, reducing a 52-hour fermentation to 48 hours to completion. It quickly adds up. A 100 MMgy plant dropping three fermenters every day will be able to add a fermenter every four days, given the back end can handle the increased throughput. In a year, the additional 80 fermenters give a nice return on investment for the added nutrition.

Today’s ethanol industry aims for efficiency and optimization in all operations, but it is only in fermentation where ethanol is made. Producers should keep yeast well-fed, stress-free and productive.

Author: Jim Miers
Nutrient Category Manager
Lallemand Biofuels & Distilled Spirits
[email protected]