Green beer aromas
Regarding beer flavors, there are byproducts that are formed during fermentation and can influence the beer by giving it a stronger body, or conversely can negatively affect flavor, aroma, head retention, etc.
Those byproducts can be divided into two groups:
„ Green beer flavors: These byproducts can negatively influence the taste and aroma of the beer. They can be removed or neutralized during the process of fermentation and maturation through the ways of biochemistry.
One of the goals of the maturation process is to contribute to the removal and neutralization of green beer flavors, namely diacetyl and sulfur bonds.
„ Mature beer flavors: Unlike green beer flavors, these byproducts cannot be removed or neutralized from the beer. It is only possible to control its formation during the brewing process.
Mature beer flavors determine the aroma of the finished beer.
In this article, only green beer flavors will be discussed, in details.
• Diacetyl (vicinal diketones): Diacetyl is the most important green beer flavor. When its concentration in the beer is higher than 0.12mg/l, the beer will have a sweet taste, reminiscent of rancid butter.
The structural formula of diacetyl (a ketone):
H3C – C – C – CH3
H3C- C- C- C2H5
Formation and decomposition of vicinal diketones occur in three stages:
g First stage: Only the first stage of vicinal diketones is formed through yeast. They are formed from the synthesis of amino acids, during yeast propagation (budding of the yeast, mitosis).
In this stage, they do not have aroma or taste and are not perceptible in the beer. In case of a high amount of the amino acid valine in the wort, it is possible to reduce the formation of 2‑acetolactate in the first stage.
A starting point for the synthesis occurs in the fermentation or aerobic process, in which it occurs the formation of pyruvic acid between reactions. In this state, the yeast releases 2‑acetolactate, acetohydroxy acid, in the substrate (wort being fermented).
This stage does not impart any taste or aroma to the fermentation wort. It is not possible to identify the 2‑acetolactate in the beer.
The element 2,3-pentanedione is not as perceptible as diacetyl. A concentration of 0.6 to 0.9 mg/l in the beer must be reached in order for this element to be perceived.
The formation of 2,3-pentanedione depends on the following factors:
§ Type of yeast: the amount of acids formed by the yeast cell;
§ Yeast dosage in the wort: a higher amount of yeast leads to an increased production of 2‑acetolactate;
§ Oxygen: the presence of oxygen causes an elevated formation of 2-acetolactates.
g Second stage: vicinal diketones, diacetyl, and pentanediones that occur outside of the yeast cell are formed through the 2-acetolactate, via oxidative decarboxylation.
The formation depends on the following factors:
§ Acidic pH: with a pH value between 4.2 and 4.4, there is a fast transformation in vicinal diketones; with the increase of the pH value, there is a reduction in the transformation speed;
§ Temperature elevation: a fast transformation occurs with heating;
§ Oxygen: absorption of oxygen by the beer causes a fast transformation of the first stage to vicinal diketones.
The first-stage transformation into vicinal diketones limits the speed of the maturing (beer maturation).
Vicinal diketone reduction
Diacetyl and 2,3-pentanediol can only be decomposed with the help of the yeast cell contained in the green beer and, therefore, lighten or reduce the unpleasant taste in the finished beer.
The decomposition takes place through the following reduction:
diacetyl g acetoin g butanediol g 2,3-pentanediol
Butanediol is hardly perceptible in the beer. Its concentration in the beer can be very high (high threshold of butanediol).
The decomposition of diacetyl into butanediol is favored by the following stages:
- During fermentation, the yeast has an enormous capacity to decompose diacetyl into butanediol. That capacity is ten times higher than its formation during fermentation.
- Diacetyl decomposition greatly depends on the temperature.
- Diacetyl decomposition speed depends on the yeast concentration in the beer maturation.
- There must exist an intense contact of the beer with the diacetyl and the yeast (pumping and pressure release inside the
Conclusion for correct fermentation control
- The concentration of diacetyl (vicinal diketones) in the green beer can be a criterion to determine the degree of maturation of the beer (performing fermentation control via diacetyl concentration, instead of the degree of fermentation of the sugars);
- For the maturation stage, active and healthy yeast cells are necessary.
Summary of the process
O g acetolactate g diacetyl g acetoin g butanediol g yeast cell g 2,3-pentanedione
w Acetolactate or acetohydroxy acid – It is a carboxylic acid found in plants and microorganisms. It catalyzes the synthesis of the branched chain of amino acids, such as valine (CH3COCOHCH3COO + CO2).
w Acetoin – It has one hydroxyl group and a short chain (C4). Formed during glucose fermentation, it is commercially prepared from diacetyl, with zinc and acid, and used as flavoring.
w Butanediol – A carbon, hydrogen, and oxygen compound, a diol that has two –OH groups. It is produced by a variety of microorganisms, and can be found in cocoa butter (C4H10O2).
w Decarboxylation – Break-up of CO2 from the carboxyl group of the carboxylic acid.
w Sulfur compounds – Volatile sulfur compounds are formed through yeast metabolism, as well as thermobacteria.
Sulfuric acid (SM2) is a very volatile compound formed during fermentation through amino acids that have sulfur in their composition. With its movement from bottom to top in the green beer during fermentation and maturation, carbon dioxide (CO2) removes volatile elements from the sulfur.
w Mercaptans – They are compounds in which the alcohol’s hydroxyl (–OH) is replaced by –SH. Being the compound that affects the beer flavor the most, they are responsible for the taste that beer gets when it receives excessive light incidence.
Mercaptans are formed in beers with a fermentation degree of up to 65%. They are transformed due to the acidification of the environment, making the aroma less intense. They react with acid and turn into disulfides.
w Dimethyl sulfide (DMS) – The yeast is not capable of transforming DMS. Because of that, the DMS contained in the wort remains almost untouched in the finished beer.
HEYSE, Karl-Ullrich. Handbuch der Brauerei-Praxis. Nuremberg, Germany: Hans Carl.
KUNZE, Wolfgang. Technologie Brauer und Mälzer. Berlin, Germany: VLB.
NARZISS, Ludwig. Abriss der Bierbrauerei. Stuttgart, Germany: Enke.