Course: BFM 201 Ethanol Production

Unit 6 Readings and Lecture

To Do:
Read Textbook: Ethanol Process Fundamentals – Chapter 5 pages 5.1 – 5.6
The mash is pumped into the fermentation tanks. Glucose (sugar) is converted by yeast to ethanol and carbon dioxide and heat in the fermentation tanks. Fermentation time is dependent on the type of yeast used, the rate of enzymes added, the temperature in the tanks and  on the final targeted ethanol concentration desired. The general target concentration of ethanol is 12 – 18% ethanol by volume. Yeast can withstand the stress of concentrations of 12 – 18% ethanol concentrations in addition to organic acids produced by contaminating bacteria. However, contaminating bacteria can lower the ethanol yield by converting glucose to undesirable fermentation products such as fusel oils, acetic acid and lactic acid. In addition to general cleanliness procedures, the addition of antibiotics may be needed to control unwanted bacterial buildup in the fermentation tanks and other equipment.
Yeast

Yeast is microscopic fungus consisting of single oval cells that reproduce by budding, which can change sugar into alcohol and carbon dioxide by fermentation. (David Blume) They are also essential in making bread. The age of the yeast can be determined by counting the number of bud scars on the surface of the cell. A bud scar occurs each time the cell divides. The yeast cell dies once the maximum division capacity, called the Hayflick Limit.

In ethanol production, the yeast go through two distinct phases. (David Blume) The first is called the aerobic phase, during which yeast breath in oxygen, eat sugar, reproduce like crazy, and breath out carbon dioxide. They don’t really produce any alcohol to speak of.

Once they have used up all the oxygen, they shift to the anaerobic phase, in which they breath in carbon dioxide, eat sugar, excrete alcohol, and breathe out carbon dioxide – without reproducing. The fermentation vessels in an ethanol facility are designed to exclude oxygen in order for the yeast to produce ethanol.

The ideal food for yeast is the simple form of sugar called glucose. Gluco amylase enzymes are added to the Yeast Tank to facilitate the breaking down the dextrin in the mash into glucose.

Different strains of yeast will tolerate different strengths of alcohol before they die. Brewers yeast can stand a mash of 8% alcohol. Distillers yeast can handle 10 – 12 % alcohol. Some special breeds can handle up to 18%.

The strain of yeast most commonly used in ethanol production is Saccharomyces cerevisiae.

The critical factors to control to end up with 10 – 12% alcohol mash are temperature, pH, nutrition, agitation, initial yeast dosages, and contamination.
References:

Chapter 5. (2006). In Ethanol process fundamentals: An introduction to ethanol processing. Granite Falls, MN: Minnesota Energy Enterprise Center.

Blume, D., Fuller, R. B., & Winks, M. (2007). Chapter 7. In David Blume's Alcohol can be a gas!: Fueling an ethanol revolution for the 21st century. Santa Cruz, CA: International Institute for Ecological Agriculture.