- Yeast make amino acids that don't exist in wort
- Yeast will make them if they exist, but can't take them up
- Making one in particular, valine, causes diacetyl to be produced
- Diacetyl is reabsorbed to make needed coenzymes
- Certain conditions will cause yeast to produce valine, but not take up diacetyl
So we covered a few things in Yeast Flavours 1 and 2, now lets talk about diacetyl, or more specifically VDKs. VDKs, or Vicinal DiKetones produce buttery flavours (not to be confused with “toffee” flavours derived from malt), accompanied by a greasy finish. Diacetyl is the same chemical found in fake popcorn butter, and does not generally belong in beer, though is acceptable in certain styles at low thresholds.
So how are VDKs made and what can we do about them? It helps to understand that yeast, like all cells, absorbs sugar from the wort and then diverts it into different pathways depending on its needs at any point in time. We’re going to focus today on the ilv pathway, or iso-leucine-valine pathway. Valine is an amino acid that yeast require to perform various functions that we won’t get distracted with today. It is present in wort, but when yeast first start to grow, it is impossible for them to take it up directly from the wort. It’s also very difficult for them to take it up if they are young cells, or if they are elderly cells. In those situations, they will make it themselves.
So early in fermentation, yeast will divert some carbon away from alcohol toward the ilv pathway instead, whose end result is valine. The starting point is a chemical called α-acetolactate. Inside the cell, α-acetolactate becomes valine. Nothing keeps α-acetolactate from escaping the cell, however, and once outside, it will naturally degrade into diacetyl.
This normally isn’t a problem because yeast actually like diacetyl. Diacetyl will be absorbed by the cell and used to regenerate NADH into NAD+. Yeast need to do this because in other reactions, NAD+ is turned into NADH. They’re useful coenzymes and having the balance thrown off means the yeast can’t continue to turn glucose into energy.
So you have a situation where under certain conditions, yeast make valine. This process creates α-acetolactate which escapes the cell and becomes diacetyl This in turn is reabsorbed to regenerate coenzymes. The answer is simple then, limit the amount of valine the cell has to make and you’ll limit the amount of diacetyl remaining in your beer (the yeast will find other places to maintain the redox balance.)
For all-malt brewers, this isn’t normally an issue at the beginning of fermentation; all-malt wort contains plenty of valine, so once the dormant yeast goes through it’s wake-up, or lag phase, it no longer has to use the ilv pathway, and so it takes up valine directly from the wort while maintaining it’s redox balance through the conversion of the remaining diacetyl. However, remember that young and old yeast cells have difficulty taking up nutrients, and so will continue to make their own while mature yeast cells are taking valine directly from the wort.
This means that a lower pitch rate will lead to a higher rate of diacetyl production because of a higher proportion of young yeast cells. A yeast that has been repitched too many times will also produce more diacetyl because of a higher proportion of old yeast cells. The other place where this is going to become an issue is during bottle conditioning. Adding oxygen and new fermentables, which both happen during bottle conditioning, gives the signal to dormant yeast that it’s time to wake up. However, because of the lack of nutrients and the very small amount of sugars, the good times run out very quickly, leading to a rapid shut down. Pitching fresh yeast, rather than relying on the capabilities of old yeast is essential in this scenario.
Some brewers might think that an alternative solution would be to simply add yeast nutrient to your bottle conditioning. This is a terrible idea because of a few reasons. 1. Yeast won’t be able to take it up during the lag phase, no matter how healthy they are, and 2. Yeast nutrient contains zinc which shunts carbon to yeast growth rather than CO2 formation. You’ll end up with a less carbonated beer with more new yeast cells that can’t break down the diacetyl that was formed during the lag phase which then rapidly goes dormant. In addition. The new yeast cells will likely not have enough metabolic capacity to prepare themselves for starvation and will end up being destroyed, leeching their contents into the beer and possibly creating some really nasty off-flavours.
The last situation where diacetyl is created through this route is a relatively new – hop creep. Hop creep arises because hops that are added post fermentation contain enzymes that break down dextrins. Since they’re added after the yeast have finished fermentation, the enzymes continue to slowly work away and provide fermentables for any yeast left in suspension. This will be consumed, but a lack of nutrients and old yeast cells mean – you guessed it – diacetyl. The result is an extremely dry, buttery IPA. The solution to this is simply pasteurization, which isn’t feasible for everyone, but it will remove both sides of the equation – the enzymes and the yeast. Filtration will also remove yeast from the equation, but again, it’s out of the reach of many. Many brewers have had success hopping only when the yeast are still active, but there’s currently no research outlining whether yeast are able to denature these enzymes, so dry hop at your own risk!
In conclusion, the things you’ve always been told continue to be true – pitch a sufficient amount of healthy yeast for your beer and don’t repitch too many times. Limit oxygen exposure after pitching and pitch fresh yeast when bottle conditioning.