Diacetyl Rests
TroubleShooting
Seth Adams - Annapolis, Maryland asks,
I’ve seen the importance of diacetyl rests mentioned dozens of times. the description always states to increase fermentation temperatures to the 65–68 °F (18–20 °C) range for 24–48 hours or so. However, there is never a discussion of how quickly to raise the temperature. If I have a lager at 50 °F(10 °C), do I raise the temperature only 2–3 degrees per day until I hit the range (adding ~7 days) to avoid shocking the yeast or do I raise it much faster to no ill-effect?
I have really never understood the idea of shocking yeast. Warming or chilling yeast within the temperature range that brewers ferment beer can and does occur quickly in commercial breweries without negatively affecting the yeast or the resultant beer. Fermenting too warm can of course lead to excessive production of esters, higher alcohols, sulfur, phenols, and other aromatic compounds, so controlling fermentation temperature is an important facet of brewing process control. The short answer to your question about the warming rate is not to worry about it. The easiest way to warm a batch of lager is to move your carboy from your lager fridge to a cool closet or corner in your basement and just let things happen.
So what happens during the diacetyl rest and why is it important to warm the beer in the first place? There is a precursor to diacetyl, alpha acetolactate or simply “DP” (diacetyl precursor) that is excreted from yeast during fermentation. Outside of the cell, DP is oxidized and converted to diacetyl by oxidizing compounds, such as metal ions or sometimes oxygen, in beer. Once DP is converted to diacetyl, yeast cells that are metabolically active and in need of a hydrogen acceptor convert diacetyl into the flavorless compound 2,3 butanediol.
Diacetyl is referred to as a vicinal diketone (VDK) because the molecule contains two ketone groups, R-C(=O)-R’ in biochemical shorthand, located next to each other (vicinal). Diacetyl contains four carbon molecules, hence the root name butane. 2,3 pentanedione is another VDK in beer and contains five carbon atoms. While diacetyl smells similar to butter, 2,3 pentanedione smells similar to honey and is less pronounced than its big brother “D.” This is why 2,3 pentanedione doesn’t have a common name and a rest named after it! But a similar reaction follows, where 2,3 pentanedione is reduced to 2,3 pentanediol. When these reduction reactions occur, nicotinamide adenine dinucleotide (NAD+) is regenerated. Suffice it to say, this is important for yeast metabolism and relates to the general topic of biochemical pressure that helps to understand how we as brewers can influence yeast biochemistry. It is worth noting that acetaldehyde reduction to ethanol also regenerates NAD+ and excess levels of acetaldehyde that build in beer during fermentation drop off with aging during the diacetyl rest.
Crawling out of the rabbit hole of biochemistry and back to the practical world of brewing . . . As a commercial brewer, albeit on the smaller end of the scale, I scratch my head about the diacetyl rest because most fermentation vessels are equipped with cooling jackets, not heating jackets. Most commercial brewers simply have no way of using the sort of diacetyl rest employed by so many homebrewers. This essentially means more time for lager beers. There are some breweries that have invested in heat exchangers used to warm beer after primary fermentation so that aging is shortened. Since time is money for commercial breweries, one would think that this method would be more common, but brewers are a cautious lot and process changes are not undertaken lightly.
One reason that lager brewers may not be able to reduce aging time by simply investing in methods to warm beer is tank pressure rating. Lager beers can be fully car-bonated during lagering using tanks rated for 15 psi/1 bar when the temperature is gradually cooled to about 43 °F (6 °C). Although yeast metabolism is slow in this temperature range, lager yeast do remain active. If the beer temperature were warmed to 68 °F (20 °C) the tank pressure would need to increase to 31 psi/2 bar. This is a significant difference and requires beefier tanks stamped for use at pressures in excess of 15 psi/1 bar. In the US, this means that the vessels are ASME (American Society of Mechanical Engineers) stamped. Yet another reason why a relatively simple homebrewing method is a very real challenge in a commercial setting.
Homebrewers often are envious of the equipment used by commercial brewers. But sometimes the opposite is true, and the ease at which homebrewers can accelerate diacetyl and acetaldehyde reduction in lager fermentations is one example. Go for it, I say!
