Ester production in yeast

Ester production in yeast
By Moritz Kallmeyer
Master Brewer of Drayman’s Brewery & Distillery, Silverton Pretoria, April 2006

Introduction:

The opposite of what we read is usually true regarding the increase in ester production during the growth phase. Ester production is directly related to biomass production. Everything that increases biomass production (intense aeration, stirring, sufficient lipid etc.) decreases ester production. The more biomass is produced, the more acetyl Co-enzyme A is used for and therefore not available for ester production.

Question: Does high temperature early in fermentation, during the growth phase increases ester production?
Answer: No, during the growth phase ester production is reduced because acetyl- CoA is used (tied up) for yeast growth and is not available for ester production.

Question: Does higher pitching rates result in higher ester formation?
Answer: Yes, because there is less growth before they reach the stationary phase.

Question: Does higher initial gravity result in higher ester production?
Answer: Yes, because there are in general more metabolites produced that can react with each other and higher gravity worts hold less O2 – unless the brewer specifically compensates for that.

Question: Does low DO (dissolved wort oxygen) increase ester formation?
Answer: Yes, it inhibits yeast growth, thus increases ester production. Dropping wort DO from 8ppm to 3ppm has a four fold increase in ester.

Question: Is it better to pitch at warmer temperature than at cooler temp for ester control in lager beer?
Answer: Pitching at warmer temperature will begin the production of more yeast. While the yeast is growing there will be less ester production (see above). You will need adequate cooling to control fermentation temperature. Pitching at cooler temperatures with a higher pitching rate will result in less yeast production and more ester production. It would be difficult to speculate which would end up with the most esters.

Question: Does a low pitching rate produce more ester?
Answer: No. Low pitching rate result in less esters. Pitching rate would vary as a means to increase or decrease total fermentation time and could be influenced by shortage of fermenter space. If lack of refrigeration or temperature control is a problem the fermentation needs to be spread out over a longer period by pitching with less yeast and restrict O2 addition in order to control yeast growth and thus fermentation temperature.

Question: Would agitation or stirring of a normal gravity wort increase ester production?
Answer: No, stirring of normal gravity wort would decrease ester production.

Question: How about high gravity wort?
Answer: Stirring in high gravity fermentation would increase ester production.

Question: Does tall fermenters produce less esters than shorter and wider fermenters?
Answer: Yes. Tall fermenters produce less esters than short fermenters because of CO2 buildup. At 0.5 atmospheres the pressure begins to exert a negative effect on yeast growth.

Question: Does wort which contains higher maltose component have lower ester production?
Answer: Yes. The use of high maltose syrup as an adjunct instead of glucose, sucrose or fructose adjuncts will give lower ester formation. New very high maltose syrups of 70%+ are now becoming available. Higher maltose wort gives higher yeast viability and thus increased growth.