Some yeast strains are famous for giving away fruity notes of banana, apple, pear, pineapple or even rose and honey. These aromas are due to aromatic molecules called esters.
Why do some strains produce more than the others? Let’s understand where they come from to answer that question.
Esters are products of the reaction between an acid and an alcohol through enzymatic reactions.
More than 100 different esters have been found in beer. They can be made with either ethanol or higher alcohol reacting with a fatty or organic acids.
During fermentation, yeast will grow using nutrients like wort sugars and amino acids to synthesise lipids to produce biomass. The molecule called Acetyl CoA, used to synthesise these lipids is also used by yeast to produce esters, so while it is busy producing new yeast cells, it isn’t available for ester production.
When the wort starts running short of amino acids, yeast stops the synthesis of lipids for biomass, the growth stops and cells start using nutrients to prepare to a new phase of starvation. This means that the acetyl CoA required for ester production is now available.
Ester production isn’t a spontaneous reaction, mixing soap’s fatty acids in alcohol won’t make you esters. A catalyst is needed and in our case, that would be the yeast’s role. Some of them are better at it than others and that is why some strains make a fruity fermentation and some more neutral.
The more ethanol and higher alcohol, the more esters can be produced. Factors favouring growth will increase the production of higher alcohol. But conditions favourable for biomass production should be avoided to have more acetyl CoA free for ester synthesis.
Literature on that topic can be pretty confusing and somewhat contradictory, but essentially, when it comes to esters management, the following should be kept in mind:
- Avoid hot trub into the fermenter: lipids found in trub will encourage biomass production.
- Oxygen has a negative impact on yeast enzymes responsible for esterification and therefore should be kept at minimum necessary to maintain good yeast health and fermentation.
- Higher density seems to encourage esters so brewing at high gravity can be an option. Increasing wort’s density with sugar (or unmalted grain) can also lead to a dilution of FAN and change the general profile of amino acids consumed and therefore the alcohol produced, leading to different esters/aromas.
- Higher temperatures increase ester enzymes activities and higher alcohol formation.
- Pressure tends to be unfavourable to ester production. Fermentation in high tanks would therefore have less esters than the same recipe in a small fermenter.
Pitching rate seems to show contradictory results but it seems that either over and under pitching can lead to more ester formation. This would make sense since in either cases, new cells production is kept low, either due to saturation of yeast population (over pitch) or low biomass produced due to low initial population, resulting in both scenarios in more amino acids available for higher alcohol production and free acetyl CoA for ester synthesis!
So the conclusion is that beer fermentation is very complex and many factors are at play so there is never a clear cut solution. Tests and trials need be done to optimise the desired profile, and that’s what makes brewing so great!
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