Off Flavors in Beer Series- Diacetyl

Overview

Two common pronunciations are die-ASS-a-teel and DIE-a-see-till.

Responsible for buttery or butterscotch flavors. Also described as adding a slick mouthfeel.

The flavor threshold is quite low at 0.10 mg/L. In today’s commercial beers the average is near 0.05 mg/L . However, Some stouts can have levels as high as 0.60 mg/L, and a few British pale ales have diacetyl levels near 0.30 mg/L. It is estimated that 20% of beer drinkers do not detect the presence of diacetyl even at rather high concentrations.

It is easy for the novice beer evaluator to confuse the buttery flavor of diacetyl with the caramel flavor of certain malts (especially in young beers) and the toffee-like character of beers that have gone stale. Given time it is easy to distinguish the two; diacetyl tends to be unstable in most beers and can take on raunchy notes. The flavoring imparted by caramel malts, on the other hand, tends to be stable.

Diacetyl Formation

Several pathways lead to the creation of diacetyl. The first is through normal yeast metabolism. Brewer’s yeast forms a precursor called alpha acetolactate (AAL), which is tasteless. This compound is converted to diacetyl as the beer ages. The reaction that changes AAL to diacetyl is accelerated by high temperature. At cool temperatures it will still occur, but more slowly.

Brewer’s yeast contains enzymes for both producing and reducing diacetyl. Modern brewing practice dictates that beer be aged on live yeast until the vast majority of AAL is converted into diacetyl. Brewer’s yeast, while unable to metabolize AAL, will readily absorb and break down diacetyl into relatively flavorless compounds. By giving the beer enough contact time with the active yeast, the brewer can eliminate the diacetyl. It generally takes only about two weeks of aging an ale to assure that it will have no buttery flavors.

Lager beers can take a bit longer to “diacetyl proof,” because they are usually fermented and matured at lower temperatures than ales. The cool environment slows the conversion of AAL to diacetyl. Some brewers will warm their lagers to 55-60 °F to help speed the oxidation of AAL to diacetyl and its subsequent metabolism by the yeast. Still, it can take a minimum of four weeks to produce a stable lager.

Highly flocculent yeast can leave perceptible levels of diacetyl in beer, which is one reason why most commercial yeast strains are powdery and fully flocculate only after chilling. Yeast behavior for a given strain can also vary with reuse.

Diacetyl is also formed by mutant yeast. Brewer’s yeast that has lost its ability to properly utilize oxygen are called respiratory mutants, or petite mutants (because they form abnormally small colonies on laboratory plates). These yeast are also unable to properly metabolize diacetyl, thus leaving it in the beer. Increases in the diacetyl formed with re-pitching parallel increases in the level of respiratory-deficient mutants in the pitching yeast.

A bacteria called pediococcus can also form high levels of diacetyl in beer. While this bug cannot hurt humans, it can make beer sour as well as buttery. Tartness is desirable in Lambic beers, but it is most unwelcome in most other beer styles. The vast majority of brewers do their best to avoid pediococcus!

A widely observed but little discussed phenomenon occurs when diacetyl appears spontaneously in a beer that seemed to have normal flavors. Strong evidence indicates that this can occur when marginally dysfunctional yeast have been used in the main fermentation — they tend not to metabolize all the acetolactic acid in the wort. The acetolactic acid spills over into the finished beer and later is oxidized to diacetyl.

Diacetyl Test

How do you know if your beer contains an excessive amount of alpha acetolactate (AAL), thus necessitating a diacetyl rest to yield shelf-stable beer? The test is not only easy and accurate, but it requires only some glassware, hot water, and the very sophisticated aroma analyzer found in the middle of your face!

This test is based on the fact that heat will rapidly oxidize the relatively flavorless AAL into butter-like diacetyl. You will need two glasses with covers; aluminum foil works well in this role. You will also need a hot water bath big enough to hold one of the glasses. The water should be heated to 140-160 °F.

Place a sample of your young beer in each glass. Cover and put one in the hot water bath while keeping the other at room temperature. Keep the beer in the water bath for 10-20 minutes.

Cool the hot beer to about the same temperature as the cool sample; a cold water bath can be used to good effect for this. Remove the covers and smell each sample. One of the following conditions will exist:

1. Neither beer smells buttery. This is good! It means that all of the AAL has already been converted to diacetyl and your beer is ready for packaging.
2. The heated sample smells buttery, but the cold one does not. This means that there is excessive AAL still floating around your beer, and you should age it at 60 °F or so for a few days to allow diacetyl to form and then be metabolized by the yeast. Repeat the test to determine the proper time for packaging.
3. Both samples smell like butter. This can be a bad thing. It can be indicative of a pediococcus infection, in which case you should dump the batch and start over, or it can mean that your yeast is incapable of metabolizing diacetyl (see respiratory mutants, above).

It could also mean that your beer is still kind of young and you should try the test again after a few more days of warm aging. Hopefully the diacetyl will fade. Kräusening with fresh yeast may also help, unless the problem is bacterial.

Diacetyl Prevention

Start Strong

A brew that experiences a long lag time due to weak yeast or insufficient aeration will produce a lot of diacetyl before the main fermentation begins. In this case there is often more diacetyl than the yeast can consume at the end of fermentation and it can dominate the flavor of the beer.

Diacetyl rest

Diacetyl production and reduction are strongly influenced by temperature, and the rates for both increase as temperature increases. Thus, an ale fermented at 20 degrees C (68 degrees F) typically has a higher diacetyl peak than, say, a lager fermented at 10 degrees C (50 degrees F). The rate of diacetyl reduction, however, is much higher in the ale than in the lager, which is why most lager brewers prefer to get diacetyl levels below 0.10-0.15 mg/L at the end of the main fermentation. Some additional reduction occurs in cold storage, but at a very slow rate. For this reason, some brewers raise the temperature of a cold-fermented beer to 20 degrees C (68 degrees F) for a brief period following the end of the main fermentation, a practice that is usually called diacetyl rest.

Narziss fermentation.

In this procedure the first two-thirds of the fermentation is done at 8-10 degrees C (46-50 degrees F). During the final third of fermentation, the temperature is allowed to increase to 20 degrees C (68 degrees F), after which the beer is transferred to cold storage.

Kraeusening

Another alternative is to add freshly fermenting wort (kraeusen) to diacetyl-laden beer in cold storage.

Sanitization

It is unnecessary for fully developed pitching yeast to be sterile. As long as bacteria relevant to beer are kept below the level of 1-10 cells per 10 million yeast cells, the finished beer will remain unaffected. The situation is dramatically different in the initial stages of yeast propagation. Here, sterile conditions are needed, as is pure culture yeast. This is particularly true when propagating yeast from slants, but it also applies to starting up semidormant liquid yeast.

Sources:

http://morebeer.com/brewingtechniques/library/backissues/issue1.2/fix.html

http://www.professorbeer.com/articles/diacetyl.html

Additional resources:

https://byo.com/mead/item/546-diacetyl-homebrew-science

Pumpkin beers date back to the colonial America when colonists where introduced to the native gourd. Since malts were hard to come by in the New World, early settlers turned to broad range of fruits, vegetables, and sugars to ferment.   There is even mention of pumpkins being used to produce alcohol in what is believed to be America’s first folk Song “New England’s annoyances”

“Stead of pottage and puddings and custards and pies,
Our turnips and parsnips are common supplies.
We have pumpkins at morning and pumpkins at noon,
If it was not for pumpkins, we should be undone.

[If barley be wanting to make into malt,
We must be contented and think it no fault;
For we can make liquor to sweeten our lips
Of pumpkins and parsnips and walnut tree chips.]”

Furthermore, in the Oxford Companion to Beer, there is reference to how early settlers would produce wort from pumpkins “Pompion Ale”

“Let the Pompion (pumpkin) be beaten in a trough and pressed as apples. The expressed juice is to be boiled in a copper a considerable time and carefully skimmed that there may be no remnants of the fibrous part of the pulp. After that intention is answered let the liquor be hopped cooled fermented & c. as Malt beer.”

There is historical records showing that founding father George Washington used pumpkin to brew a pumpkin porter.

Creating a Pumpkin Beer-

Base Beer

Although a broad range of styles can be used to create pumpkin beers- ranging from pale lagers to stouts, even sour’s and pumpkin hefeweizen (letting the yeast provide the spice / clove notes), most recipes are in line with amber or Octoberfest styles. A traditional pumpkin beer is lightly hopped, and has moderate caramel / toasted flavors (think pumpkin pie and associated spices). A good traditional base is utilizes neutral base malts with layers of specialty and Munich malts to give flavor, body, color and sweetness.

Yeasts

There seems to be no clear consensus on yeast choice, so if designing your own recipe pick a yeast that blends in well with the character you are looking for in the finished product. English strains can add esters that work well with the toast and caramel notes, while neutral strains let the malts and spices take center stage.

Hops

Hops should be kept subtle. The goal is to use hops to offset the sweetness of the malt, without imparting excessive bitterness. You do not want the beer to be bitter or the hops to overpower the spice character of the beer. Keeping IBUs below 25 is a good rule of thumb.

Pumpkin

Adding the pumpkin can create some real challenges in the brewing process. Pumpkins are high in both sticky glucans and proteins, and while there is no shortage of starches, it’s difficult to get fermentable extract due to the high water content. Because of this you would have to add a relative high concentration of pumpkin pulp to boost your OG. In fact you would need to add ~ 1lb of pumpkin pulp to boost your OG a modes 0.005 in a 5gal batch.

You have options in picking the type of pumpkin to use. Canned pumpkin puree (with or without the spice), fresh or roasted pumpkin. Roasting the pumpkin to caramelize the sugars can add some additional character; such as biscuit or roasted hints- like a pumpkin pie crust. If you are going to roast your pumpkin one hour in a 350⁰oven should do the trick. You will know when it is done when some browning appears and the pulp is soft- not mushy.

Pumpkin can be added at any (or all) point(s) within the brewing process- Mash, boil, fermenter, or secondary. Since there are starches that can be converted to sugars, traditionally it has been added to the mash so that some conversion can take place. However with only a 0.005 lift per pound you have to consider if it is worth adding all of the sticky gums to your mash. Please also be aware that high concentrations of pumpkin can add a slightly sour / astringent note. If the answer is yes you should strongly consider rice hulls (1/2- 1 full lb per 5 gal batch). Additionally, you may consider a betta glucan rest of 15 min at 122⁰ to help break down some of the gums. Lastly, if you are fly sparging you can constantly rake the top of the mash since the gums will rise to the top of the mash and prevent sparge water penetration through the mash bed. If used at all, pumpkin concentrations added per barrel of wort range from ½ lb-10lbs of puree. Most agree that the pumpkin addition doesn’t make or break the end product, it gives nice carotene colors and some flavors (especially if you are roasting).

Spices

While many spice recipes / combinations can be used (think again about pumpkin pie), the standard mix is generally comprised of cinnamon, nutmeg, ginger, cloves, allspice and vanilla. There are also pre-made pumpkin spice blends that can be used, however you do trade convenience for control when using this method (fine tuning the combinations & consistency). Additional adjuncts such as brown sugar, molasses, maple syrup and honey are also sometimes used. Creating balance is the key, as you do not want the spices to overwhelm the beer. Ideally, you want to pick up the spices, the pumpkin, and the base beer in the finished product. As with all herb & spiced beers, make sure that the base beer does not get lost in the final product. If you plan on going for the “pumpkin pie in a glass” beer, you will need to increase the final gravity / sweetness and the ABV so the alcohol can help balance the flavors.

Many brewers tend to treat the timing of the spice addition is very similar to the timing of aroma hops, adding in the final minutes of the boil. Remember, if you can smell the spices after you add them you are boiling of their volatile oils. Another group of brewers add the spices to the fermenter, like you would do typically for dry-hopping. Making a tincture also works well. This allows you to fine tune the amount of spice in the finished beer.

Most of all, have fun (serve your pumpkin beer out of a pumpkin), experiment with different types and levels of pumpkin, spices, and when you use these in your process. This is a perfect style to experiment with, and the perfect style to serve at your Holiday gatherings. Enjoy!