Techniques to prevent formation of dimethyl sulfide (DMS), which alters the taste of the beer.
By Michael W. Trommer
Dimethyl sulfide or DMS is a volatile element that contains sulfur, and lends the finished beer a vegetable- or rotten egg-like taste. The brewer and the maltster are interested in not allowing the formation of DMS or in eliminating as much DMS as possible during the process of malting and crafting of the beer.
During germination, inactive DMS precursor is formed. This precursor, instable at high temperatures, is converted to active precursor, represented by DMS-P or DMS-Precursor, and subsequently to free DMS.
In the maceration and germination of grain,
according to certain conditions, it occurs the
formation of a greater or lesser amount of DMS-P or free DMS. The more dissolved the grain during maceration and the higher the germination temperature, the more DMS-P and free DMS is formed. Ideally, maceration at low temperature and germination with low moisture (air not very saturated with water) are desirable.
Part of the DMS-Precursor is transferred to the roots and leaves that sprout during germination. This transferred part is removed from the malt in the cleaning process. The amount that remains in the grain during kilning (Darren) is converted to DMS-P and free DMS.
Formation during kilning
While heating DMS-P, part of it is converted to DMS and becomes free and volatile; the rest of the precursor remains in the malted beer. The higher the grain drying temperature (Darren), the greater amount of precursor is converted to DMS and removed from the grain through release of steam from the cereal.
We must keep in mind that each higher degree of temperature in kilning results in a darker grade of malt.
Therefore, for light malts, we have a limit for the kilning temperature. When dried at 100°C, the grain retains significantly less DMS-P than at a lower temperature.
In order to being imperceptible in the beer, the limit must be between 50 to 60 mg/l (Kunze, 1994). According to German legislation (13.8.1969), the level of sulfur dioxide must not exceed 10mg/l in the finished beer.
Influence on wort
We know that DMS is a volatile element rich in sulfur that imparts an unpleasant odor and taste to beer. The goal is to reduce to zero its level in the finished beer. It becomes perceptible at a level of 50 ppm/ml.
Much like in grain kilning, during wort boiling, inactive precursor is converted to active DMS-P and subsequently in volatile DMS. The more intensive and prolonged the boiling is, the less DMS-P is left in the wort.
Therefore, it is always required from the maltster a low level of DMS-P in the malt grain. When quality malt is used in a 60- to 70-minute boil, the DMS-P is converted to DMS and evaporates almost completely in a conventional boiling system.
The DMS-P that reaches the whirlpool can be converted to DMS, but not much is released from the wort through evaporation.
The DMS-P that reaches fermentation can be absorbed by the yeast and metabolized.
Sulfur metabolism in the fermentation
The wort contains inorganic and organic sulfides, such as amino acids, peptides, proteins, dimethyl sulfide precursor, volatile dimethyl sulfide, and vitamins.
The yeast needs sulfur for the synthesis of proteins and for the construction of cell walls.
During fermentation, volatile sulfur bonds arise, which affects beer aroma and taste. They are produced in part in the synthesis of cysteine and methionine amino acids (Narziss, 1986).
To form volatile sulfurous substances, the wort must contain the amino acid methionine, biotin, and pantothenic acid. The absence of those, as well as the presence of a high level of threonine, results in a strong formation of hydrogen sulfides.
DMS also is formed during fermentation, by the DMS-P precursor. This transformation occurs through heat.
Where to find
Cysteine (C6H12N2O4S2) – found in most proteins.
Methionine (C5H11O2NS) – found as constituent of many proteins such as albumin, present in eggs. It is mostly used as protein-rich dietary supplement.
Biotin – a growth vitamin of the B complex, found in baking powder, liver, and egg yolk.
Pantothenic acid – also called vitamin B5, helps the metabolism of proteins, fats, and carbohydrates.
Mercaptans – bonds that replace alcohol hydroxyls with –SH.
Sulfur bonds are hydrogen sulfides, SH2, sulfur dioxide, SO2, dimethyl sulfide, mercaptans etc. They impart an unpleasant odor and taste to beer. In low amounts, they are sensorially perceptible.
The total sulfur level (mostly from peptides with sulfur-rich amino acids) in the wort decreases via hot/cold trub separation.
Sulfur-rich amino acids are absorbed by the yeast during yeast propagation, and release substances rich in sulfur.
Finally, many sulfurous elements are expelled from the green beer along with the fermentation gases. In case there is an excess of elements containing sulfur in the wort, some measures can be taken in order to decrease the sulfur levels:
· Aerating the wort before cooling;
· Reaction of metals with the wort, such as copper.
When unmalted adjuncts are not used, the level of sulfurous elements is very low, not exceeding 0.5 mg/l in the finished beer.
When unmalted adjuncts are used in sizeable proportions, there is a high chance of obtaining high levels of sulfurous elements in the beer.
Sulfur dioxide levels can reach 20 mg/l in fermentation, and decrease in maturation. The DMS level is related to the malt quality, mashing, and boiling.
Slow or high-temperature kilning of malted barley, use of decoction mashing, and intensive wort boils lead to low DMS levels.
Mercaptan levels reach a maximum when beer fermentation is at 60%, after which they drop. Even in small amounts, they give beer a distinct flavor and aroma.
Michael W. Trommer
Ingredion Brasil Ing. Ind. Ltda.