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Dimethyl sulfides

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DIMETHYL SULFIDE (DMS) DMS ( Dimethyl Sulfide) is an organic sulfur compound present above its flavor threshold in most beers. Because of its low flavor threshold, 10 - 150 ppb, it is a primary flavor and aroma compound that makes a significant contribution to beer character, especially in lager beers. It has a characteristic taste and aroma of cooked or creamed corn.

Contents

DMS in Beer

Some detectable level of DMS is characteristic of many lager styles, and is especially noticeable in light lagers. However, DMS is present in most beers at some level. It is excessive DMS that gives some home brewed ales a "cooked corn" character.
The amount of DMS found in beer is lowest in British ales, 10 - 20 ppb and highest in German lagers and all-malt beers, 50 -175 ppb, while the United States' lagers generally contain 40 - 100 ppb.
Beers with high adjunct ratios or low gravities allow the DMS taste or off-taste to be more detectable, while German beers, all-malt beers, flavorful beers, especially dark beers, make the taste of DMS less discernible even at higher levels.3

Causes of DMS

DMS is created whenever wort is heated, by the breakdown of precursors found in pale malts. Under ordinary circumstances, most of the DMS that is created by heat is then evaporated during the boil. Some DMS is also removed during vigorous ale fermentations, which is why higher levels are often found in lagers.

Covered boil
Covering the brew kettle during the boil prevents DMS from evaporating, and results in high levels of DMS in the finished beer.
Slow cooling
Because DMS is created at temperatures below boiling, cooling the wort too slowly means that excessive levels of DMS can be created which cannot be evaporated once the boil has stopped.

Preventing DMS

The level of SMM in malt is responsible for the DMS level in wort. During mashing the SMM, DMS and very soluble DMSO are brought into solution. No SMM is hydrolized to DMS at this time.

Kettle boiling hydrolizes SMM to DMS which is removed during evaporation. The half life or time needed to remove half of the DMS is 40 minutes so that three-fourths is removed in 90 minutes. Narssis recommends a 100 minute boil to reduce the level of SMM and DMS to acceptable levels in most beers.

The level of DMSO does not change during the kettle boil. A small amount of DMS, 0.4 ppb, may be contributed by hops, especially if added in large amounts late in the boil. As long as the wort is hot SMM will be converted to DMS. It is important to convert SMM to DMS in the kettle so that build up during the hot wort stand is minimized.<br

The following steps should insure low levels of DMS in the finished beer:

  1. Boil the entire wort 90 minutes or longer
  2. Ensure that the boil is vigorous - rolling
  3. Allow at least 8% evaporation
  4. Minimize the hot wort standing time
  5. Rapidly cool the wort

Fermentation and DMS

During fermentation, the evolution of CO2 removes and reduces the level of DMS. At moderate DMS levels of 30-60 ppb a 30-35% reduction can occur, while a 35-60% reduction can occur at higher initial DMS levels, 60-150 ppb.

The yeast's metabolism does not convert SMM to DMS but certain yeast can produce higher DMS levels by reducing DMSO to DMS, especially in lager beer production at cooler temperatures. Certain wild yeasts and Enterobacter agglomerans can produce DMS.5 Table VI shows the reactions that take place in malting and wort boiling.

DMS After fermentation

No change in DMS level occurs unless some form of purging or contamination occurs. Water dilution of high gravity beers may reduce the perceived threshold of DMS due to dilution of other mashing flavors.

Creating DMS

The threshold of DMS can assume to be 40 ppb and n for DMS was determined to be 1.12.5

The major source of DMS in finished beer is derived from its precursor, S-Methylmethionine (SMM), an amino acid, which is formed during the germination and kilning process of malting barley.

Barley does not contain DMS or SMM. However, both are formed by the biosynthesis occurring during germination. SMM, also known as DMS precursor (DMSP), is heat-labile and decomposes on heating to form DMS during kilning, wort boiling and hot wort storage.

No DMS is formed during germination. The amount of SMM produced during germination is dependent on the barley crop year, the growing conditions, the barley variety, the germination temperature, the germination humidity, the germination time, the steep-out moisture content, airflow rate, the use of alkaline steeping liquor (KOH) and the use of potassium bromate.

The most effective way to reduce SMM during germination is by slightly under- modifying malt, specifically by reducing the moisture content of barley at steep-out to 40-42% and reducing the germination temperature to 55-60oF. It has been shown that a reduced airflow during germination resulted in a 50% lower SMM level in the finished malt.4

Alkaline steeping liquor and use of potassium bromate and other factors which reduce the metabolic growth rate during germination have been shown to significantly reduce SMM and insuring DMS levels in finished malt.

Two row barley which has a normally lower nitrogen content than six row barley, has been shown to produce significantly less SMM during the malting process. European malt has less SMM and DMS than North American and Canadian malt.1

Regardless of variety or growing conditions, the most important factor for reducing SMM and DMS occurs during kilning. The SMM formed during germination is converted by the heat of kilning and air flow to DMS. The DMS formed is either removed or volatilized in the kilning draft, oxidized to (Dimethyl sulfoxide) [DMSO], or remains in the finished malt. Since DMS is easily removed during the kettle boil, it is important that the ratio of SMM to DMS be as low as possible in the finished malt.

The conversion of SMM to DMS occurs at about 1401F. Therefore, by increasing the withering temperature, increasing the final kilning or curing temperature and extending the final curing time the level of SMM and DMS will significantly be reduced in the finished malt. The stability of SMM in malt is greater at higher moisture levels.

It has been reported that the half life of SMM in malt at 7.5% moisture is as follows:4 7.5% moisture: half life SMM: @ 185oF 1.9 hrs. @ 175oF 4.8 hrs. @ 150oF 20.0 hrs.

British pale ale malts, Munich malts and other malts cured at higher temperatures show very low levels of SMM and are of little significance to final DMS levels in beer.

Some general indicators of higher SMM in malt are corresponding higher degrees of modification, soluble nitrogen and alpha - amino nitrogens.

It has also been mentioned that long storage of malt may cause an increase in DMS levels in malt.

PROPERTIES of DMS

(CH3)2S
H3C-S-CH3
Boiling Point: 99oF
Density: 0.848
Flavor Threshold (Beer):
Perceived Threshold: 10-150 ppb Depends on amount of flavor in beer
Flavor: Cooked corn, creamed corn
Aroma: Same - highly volatile
Source: Precursor in malt (SMM), hops (minor)

External Links

Dimethylsulfide and Truffles