Dr. Dennis Bayrock

Global Director Fermentation Research Lactrol (Phibro epg)

Session: Speaker – Acidity/Stuck Fermentation

Topic: The critical role of pKa and pH in the understanding of bacterial contamination and yeast inhibition.


In the fuel ethanol industry, it is common practice and one of the first actions taken by a
plant is to lower pH in the propagators or fermentors when bacterial contamination is detected by HPLC or with diagnostic kits. In many cases a lower pH will aid in controlling bacterial contamination and has been used since the start of the fuel ethanol industry.

Lowering the pH however may have undesired consequences on fermentation and
propagation. Many enzymes added to the plant have a narrow range of pH optimal activity which could upset the overall fermentation performance if the pH is lowered. Enzymes within the yeast are extremely sensitive to pH changes and the yeast cell actively expends energy (at the expense of growth and fermentation performance) to strictly maintain internal pH against any changes in external pH. Although many yeasts can tolerate low pH’s (down to 3.7), their overall growth rate and fermentation performance can also be severely affected as a direct consequence of lowering pH. Furthermore, a lower robust yeast crop is less effective at competing for nutrients with bacteria – allowing the bacteria to potentially persist, grow, and produce toxic metabolic by-products. Lastly, the amount of sulfuric acid required to lower the pH in one fermentor can be considerable. To lower the pH from 5.7 to 5.2 in one fermentor will require an additional ~4000lbs of sulfuric acid.

Chemicals inhibit bacteria and yeast by different mechanisms. Caustic chemicals
saponify the yeast and bacterial cell membranes – causing direct physical damage and leaking. Hypochlorite and ozone oxidizes and damages the same cell membranes. Quaternary compounds solubilize the fatty acids in the cell membranes – weakening its structure.

For all chemicals that have a pKa (organic acids such as lactic and acetic acids and fatty
acids), only the undissociated forms can cross cell membranes and cause a change in the internal pH. The concentration of these undissociated forms are dependent on the pH and it’s associative pKa. Thus the pH of mash modulates the degree of inhibition that yeasts and bacteria experience with these pKa chemicals. Lowering the pH creates a higher concentration of the undissociated forms of these chemicals and thus increases the level of inhibition on yeasts and bacteria while raising the pH does the opposite.
While the direct effects of pH on inhibition of yeast and bacteria are well known and understood to the fuel ethanol industry, the secondary inhibition effects of chemicals that have a pKa is not.

This seminar will initially educate on the concepts of pH and pKa. The seminar will then rapidly accelerate to raise awareness and discussion about the secondary inhibition effects caused by chemicals that have a pKa, what countermeasures the yeast and bacteria have with chemicals that have a pKa at a low pH, and what countermeasures the plant can do to minimize chemical inhibition for stalled fermentors, high organic acid recycle, and fermentors that are brought back online after a prolonged period of dormancy.

Bio: Dr. Dennis Bayrock completed his PhD from the University of Saskatchewan. He specializes in multistage continuous fermentations and in microbial contamination in fuel alcohol fermentations. In total, Dr. Bayrock’s career in practical research and international experience has spanned over 26 years in the fuel ethanol industry.

Dr. Bayrock is an Adjunct Professor at the University of Minnesota where he conducts research and provides teaching in fuel ethanol fermentations. In addition to his faculty position, Dennis continues his responsibilities in research and development for Phibro at PhibroChem’s Ethanol Performance group lab in St. Paul, MN.

During his career he has visited numerous fuel and potable alcohol plants in Canada, US, Jamaica, Puerto Rico, Brazil, and Thailand and carried out plant microbiological audits, optimized addition of antibiotics on a plant-by-plant basis, and optimized plant fermentation conditions. Dr. Bayrock has demonstrated his expertise in identifying, cataloging, and screening various microbial contaminants from the fuel and potable alcohol industries. He has made numerous emergency “house calls” to plants suffering from contamination and has successfully diagnosed, treated, and restored plants to normal operation. As an internationally invited lecturer, Dr. Bayrock has been well received at conferences in the fuel and potable alcohol industries, and has presented numerous seminars on general plant operations and on multiple topics relating to yeast and bacteria.