Diacetyl Time Line
Christopher White, Ph.D.
Yeast is an amazing organism. It is responsible for producing bread, wine, distilled beverages, and ....beer! When yeast ferments beer, it produces over 500 different compounds. Many of these compounds give beer it's characteristic flavor and aroma. One of these compounds, although usually considered undesirable, is diacetyl.
Diacetyl gives a buttery, butterscotch-like flavor to beer. The flavor threshold of diacetyl is 0.1 parts per million (ppm) in "light" beer. Homebrewed beer can have levels from .05 to greater then 1.0 ppm. Factors that influence the diacetyl level in beer are fermentation temperature, aeration level, bacterial contamination, and the yeast strain used. This article will explain how diacetyl is formed and how the levels vary during the course of fermentation and maturation. (Fig. 1)
Typical Diacetyl Time Line
Diacetyl is a small organic compound (Fig. 2), belonging to the chemical group called ketones. Another ketone commonly found in beer is 2,3-pentanedione. When the diacetyl level is checked in beer, 2,3- pentanedione is so similar that its level comes out in the test and the combined result is called the VDK (vicinal diketone) level.
Diacetyl Level (ppm)
CH3 C C CH3
Fig 2: Diacetyl
After yeast is pitched into beer, the yeast undergo a lag phase, followed by a phase of very rapid growth called the exponential growth phase. During both the lag and exponential phase, yeast build amino acids, proteins, and other cell components. Most of these components do not make the flavor of the beer, but the various pathways produce individual compounds that leak out of the cell to effect beer flavor. One of the amino acids produced by yeast is valine. An intermediate compound in valine production is acetolactate (Fig. 3). Not all of the acetolactate produced eventually becomes valine, some will leak out of the cell and into the beer. This acetolactate is then chemically (not enzymaticly) converted to diacetyl in the beer. The chemical reaction is an oxidation, and high fermentation temperatures favor this reaction. Other factors that will increase diacetyl production in this phase are insufficient nutrients (i.e. the amino acid valine), which forces yeast to manufacture their own. For example, the more valine yeast produce, the more acetolacate intermediate is required, and hence the more diacetyl made. There is a strain specific phenomena here, because given the same conditions, different strains will produce different levels of diacetyl.
Pyruvate acetolactate valine Inside Yeast Cell
As yeast slow down in fermentation, they enter what is known as the stationary phase. where beer undergoes a maturation process to develop the correct balance of flavors. One of the key elements of maturation is diacetyl reduction. Not only do yeast produce the precursor to diacetyl, they also consume the diacetyl produced, and enzymaticly reduce it. Yeast reabsorb diacetyl and convert it to acetoin and subsequently to 2,3-butanediol (Fig.4).
This phase is
Diacetyl Diacetyl Acetoin 2,3-Butanediol Inside Yeast Cell
Both acetoin and 2,3-butanediol can escape the cell, but neither contribute much in terms of flavor given their high flavor threshold.
It is important to provide sufficient maturation time for diacetyl reduction, commonly known as a "diacetyl rest". Diacetyl reduction is slower at colder temperatures, so it is essential to incorporate the diacetyl rest when making cold fermented lagers. The process is simply to raise the fermentation temperature from lager temperatures (50-55F) to 65-68F for a two day period near the close of the fermentation. Usually the diacetyl rest is begun when the beer is 2 to 5 specific gravity points away from the target terminal gravity. The temperature is then lowered to conditioning temperature following diacetyl reduction. For ale production