By Greig McGill
I am not easily surprised or shocked these days. I’m getting on a bit you see, and have been firmly in the “beer nerd” camp for over a quarter of a century, and a brewer (both home and pro) for more than half that time. And yet… I’m utterly flummoxed! What brought this on? Well, without meaning to be unkind, you did! Recently, the lovely team at Brewshop asked me and some other industry colleagues to help judge their West Coast IPA competition, leading up to the final six being judged publicly at Beervana. Of course, I said yes. It’s always a fun time with the Brewshop crew, and who’d say no to tasting an array of fine beers and then tasting the best of the best at NZ’s most well-known beer festival? Not this bloke, that’s for sure. I was in!
On the day of preliminary judging, to select the top six to go to Beervana, we had well over sixty beers to judge. As the judging wore on, I was shocked to find myself writing down the same comment over and over again on the judging sheets: “Buttery notes - check yeast health and perform a VDK rest” - or one of what felt like an endless series of variations on that theme. I was really surprised by this. After all, it’s been the most common fault in homebrewing for… well… forever, and every “how to brew” resource talks about how to manage it as an off flavour. I’ve written about it many times here in this cool bloggy thing even! Does nobody listen? Does nobody care? *Greig breaks down sobbing and has to be revived with a frothing pint of fine ale*
In all seriousness though, I really am at a bit of a loss to understand what’s going on. Even two of the six finalist beers that made it to Beervana had some VDK present, leading to heartbreaking elimination. Why is this so common? Well, I think there’s a few reasons. I’m going to talk about some cultural changes in beer that I think may have led to this, after a quick recap of what VDKs are. I’ll cover some technical methods for minimising production and cleaning them up, before finally offering some genuinely excusable reason why VDKs may be above detectable levels in West Coast IPAs - ie. it might not be ALL your fault! OK, let’s get to it…
First, WTF is VDK?
Vicinal diketones (VDKs), namely diacetyl and 2,3-pentanedione, are natural byproducts of yeast fermentation in beer production. While low levels of these compounds can contribute to beer flavour complexity, elevated concentrations can result in undesirable buttery or butterscotch off-flavours. These compounds are typically produced during the early stages of fermentation and are gradually reabsorbed and transformed by yeast as fermentation progresses. Judges and brewers often simply refer to VDKs “diacetyl”, but as 2,3-pentanedione presents similarly from a sensory perspective, it is more technically correct to refer to VDKs, as a beer judge is unlikely to be able to discern one from another unless isolated.
2016 - An Epoch Year
Here we go, Greig blames the hazy beer again! Well, not really, but kinda-sorta-yeah. While hazy beer became “a thing” around the late 2000s, it hadn’t really hit the prime time in the USA until roughly 2015, and I really noticed it between two trips to the USA in 2015, where it escaped my notice, and 2016 where it was suddenly ubiquitous and began to reach New Zealand also. Whether you enjoy this movement or not, there is simply no denying that it has had a huge impact on both how beer is brewed and what the drinker looks for in flavour. Bitterness has become less desirable, sweetness more so. Hops are biotransformed during fermentation into very similar compounds that… well… taste like fruit juice. There are an extreme amount of suspended polyphenols and other compounds that form the haze, but also have the effect of muddling the flavours present in the base beer, and often, of hiding faults. At least for a while. As the beer ages, many of these faults become detectable, even dominant. Because of what I call “the great muddling effect” though, new beer drinkers who have come up with hazy beers as normal have also never developed their ability to recognise critical beer faults - who could blame them when those faults are so hidden? While VDKs are produced by all yeast types (excluding some newer genetically engineered strains which sadly remain illegal in New Zealand), the old-but-new-again phenomenon of Hop Creep has become a real problem due to the sheer load of dry hops now being used in hazy beer styles. I’ll talk more about this technical issue later on. Suffice it to say that things have changed, here in Beer Town, and we may no longer be good at noticing when our beer is faulty.
(Don’t) Pass the Butter
So, outside of GM yeast, you’re simply going to get VDKs in your beer as it ferments. VDKs are produced during the early stages of beer fermentation when yeast metabolises alpha-acetolactate, a precursor compound produced from malt sugars. This compound is enzymatically converted into diacetyl and 2,3-pentanedione. Those are then reabsorbed by healthy yeast as part of normal fermentation. However, some will always still remain. The trick is to keep it below the flavour threshold.
The keys to this are, happily, also the keys to making great beer in general! Because we don’t make beer, yeast does, so keeping your yeast healthy will go a long way to minimising the problem.
Firstly, ensure you pitch the correct amount of fresh, healthy yeast. Without a lab, ascertaining the health of your yeast can be difficult, but so long as you are buying fresh yeast, and pitching the correct quantity, with enough oxygen, you should have no issues. If you are repitching yeast, ideally don’t repitch after a challenging fermentation such as from a high alcohol beer, or after a long period of storage. Google a yeast pitch rate calculator and target at least 0.75 million cells per millilitre of wort, per degree Plato (or 4 points of specific gravity). So, for example, if you’re pitching into 20L of 1.050 (12.5 degrees Plato) wort, the formula is: 12.5 * 20000 * 0.75 = 187,500,000,000 cells, or 187.5 billion cells. Try also to get as much oxygen into the wort as possible before you pitch the yeast. The old “shake it around for 10 minutes” method will have to do if you have nothing else, but to ensure enough oxygen for a truly healthy fermentation, invest in a small aeration stone, and a cylinder of oxygen. It’ll help your back too!
Secondly, yeast requires plenty of Free Amino Nitrogen (FAN) in order that the yeast can synthesise plenty of diacetyl reductase enzymes which assist in the breakdown and resorption of VDKs in the beer during the final stages of fermentation. FAN is commonly available via various yeast nutrients, but be careful not to exceed recommended dose rates. Most “normal” beer grists containing normally modified malts will have plenty of FAN for the yeast, and adding extra may result in the production of higher alcohols such as isobutanol. This could give your beer a solvent taste, possibly worse than the buttery VDKs you are trying to avoid.
Thirdly, allow your fermentation to complete under optimum conditions, to ensure the yeast remains active as long as possible. I often hear homebrewers asking “how many days before you crash (crash cool) it?” - there’s no good answer to this question! Fermentation is done when it’s done, and there’s some evidence to suggest crash cooling can actually cause yeast to rapidly excrete some of the VDKs they have absorbed in a metabolic panic reaction before hibernation. Let the last 20% of your fermentation warm up. I usually advise ending your fermentation 1-2 degrees Celsius higher than the warmest point of the fermentation. Holding there for at least 48 hours (more for higher gravity beers) should ensure as much VDK as possible is cleaned up. You can then gradually cool the beer.
Finally, there’s a technological approach! It’s not cheap if you’re using it all the time, but for those “insurance” batches, when you really can’t afford any faults, you can get yourself some alpha acetolactate decarboxylase (ALDC) enzyme! You can dose this enzyme at the beginning of fermentation, being sure to mix it well with the wort, and it will break down the precursor alpha acetolactate directly into acetoin, bypassing VDK formation.
Testing? Testing? Is This Thing On?
If the above seems a bit vague (“48 hours is oddly specific! What does HE know?”) Well, you’re right. And good on you for noticing. Some beers may not require any rest at all, some perhaps more depending on the factors mentioned above. For this reason, don’t take my word for it, learn to perform a VDK test and judge for yourself. Unlike many scientific tests, this one is super easy, and anyone can do it at home.1. Get two small containers (small because you don’t want to waste precious beer!) and pour some de-gassed beer into each, sealing them afterwards.
2. Heat some water to between 60℃ and 70℃.
3. Bathe one of the samples in the water until the contents are around 60℃.
4. Cool the heated sample to room temperature - ensuring the first sample is also at room temperature.
Now. Smell both samples. They should smell the same. Any buttery notes from the heated sample indicate the presence of VDK precursors in your beer which will eventually develop into VDK. In other words, “it ain’t done yet”!
A common modern source of VDK in dry hopped beer styles such as IPA is the old-but-rediscovered phenomenon of hop creep. I’ve written a whole thing on this, so I won’t go over it all again, but it IS something to be aware of. While it’s very hard, if not impossible to avoid completely, it’s getting to be more and more manageable thanks to knowledge of hop composition and the chemistry involved. If you’ve got VDK in your IPA because of this… well, it’s not really your fault, but judges gonna judge, so don’t be upset if your beer gets pinged in competition.
So there you have it. Hop creep aside, there’s simply not a lot of excuse for VDKs in homebrewed beer. If you’re getting some, and doing everything mentioned here, you might have some nasty pediococcus bacteria somewhere in your system, so nuke your home brewery from orbit. It’s the only way to be sure.