I learned heaps this past weekend. And the good news for you is that I took notes.
As part of a relatively large group of avid home brewers, I enter my beers into competitions. Partly because I think they are good, but more because it’s good to get feedback from people who know what they are talking about. Any punter can taste my home brew and think it’s great when in reality, it has a major fault of diacetyl. This is why it’s easy to believe your beers are great and not understand when you continue to get low scores in competitions.
But what exactly is diacetyl? Or Acetaldehyde; or DMS for that matter?
I contacted one of my home brewing friends, who happens to brew at Lion. He put me in touch with their guy in charge of sensory quality control. Bradford was excited to share his wealth of knowledge and his chemistry kit to teach 25 home brewers what 5 of the most common brewing faults smell and taste like. This was a great opportunity to taste and smell the faults we keep hearing about. DISCLOSURE: Bradford works at Lion but he was there representing himself with his own opinions, not those of Lion.
Smell and taste sensations are genetically linked. So not everyone will be able to pick up all of these faults. I know a guy who can’t smell Acetone but for me once I have gotten a tiny whiff, I can’t get rid of it – it’s like a contamination of my sinuses.
Each of these chemicals were spiked, one at a time, into a common base beer (Lion Red) at around 4x threshold. The threshold is the concentration where 50% of the general population can detect it.
The first was DMS or Dimethyl sulfide. I would describe this as a rotting vegetable smell, old cabbage, or when you forget what’s in the veggie drawer of your fridge for a couple weeks. It’s caused by the malt and it’s more common in pale malts than dark malts. Usually, DMS dissipates during the boil. A 60 minute rolling boil is ideal, 90 minutes works too but there is not much use in going longer as you probably want to avoid caramelising the sugars in the wort. It is important to boil with the lid off your kettle so the DMS can evaporate. It is equally important to cool the wort quickly to prevent DMS from being reabsorbed. This is where a wort chiller makes a big difference.
The next Fault we reviewed was acetaldehyde. This is commonly described as “green apple or fruity, paint, or even cardboard. It can be caused by the yeast at the beginning of the fermentation as a result of the yeast being in poor condition and have a slow start. It can also be caused by bacterial contamination. You can re-pitch with more yeast if you end up with a slow start, but acetaldehyde is hard to get rid of once you have it. We talked about adding oxygen to the fermenter for the first day or two, but for those of us who are too cheap to fuss with tanks or fancy equipment, it’s easier to start growing up your yeast culture on the day of your brew, and keep them happy so you can pitch them at a good concentration and a great start.
Our third fault was Acetone or Acetate. Most women will recognise this as finger nail polish remover. This is a nasty flavour and should never be present in your beer. It’s caused by hot fermentation or poor oxidation for the yeast at the start. This is where having a controlled temperature for your fermenter is so critical.
Forth was Diacetyl. In some beer styles a bit of Diacetyl is acceptable but for others (lagers) this should not be present. It smells like buttered microwave popcorn and a lot of people like this flavour (which is a good thing for my last batch – at least someone in my house will drink it). We learned diacetyl is a normal product from yeast which builds up at the beginning of the fermentation. Towards the end, the yeast start to use it up again. The big problem comes from having a slow start to your yeast where they produce more diacetyl than they can use later on. It is possible to encourage the yeasties to eat more by raising the temperature a bit (to 20C max) for about 3 days. Here, again, is where a temperature controlled fermentation fridge is useful.
Here’s my fridge. It looks horrible and even worse, it has a huge Lion Brown sticker on the front (which covers up most of the rust). It was cheap on trademe and it works with the little heater plugged into the controller. (the larger fermenter looks low because I pulled several litres off to make my chilli-beer entry – that’s it on the right)
Now, the last fault we tried was Acetic acid – also known as vinegar. Again, in some beers this is expected but if you’re not brewing a belgian or sour beer, you have a problem. Acetic acid is a result of either bacterial contamination or sluggish yeast. It wouldn’t hurt to add some zinc to your water. Bradford wrote me later to say the following “At LION we add between 0.3mg/L and 0.5 mg/L of Zinc ions to our fermenter. We add it in the form of Zinc Sulphate. SO for a 30L ferment 10mg-15mg of zinc is what you need.” (Keep in mind that New Zealand malts are lower in zinc due to our deficient soil so this can be scaled back to 0.2mg/L if you’re using malts grown in another country – ed)
“Have a look at your zinc tablets and work out how many you need to add to get those levels. One of our reference books here warns against levels over 0.6mg/L.”
Starting to detect a common themes? Basically, if you invest a bit on your kit set up to control your fermentation temperatures, and have happy yeast, you can avoid the major issues. This assumes you have a tight control over your sanitation!
If you see errors in the write up, please let me know. I was trying to write and listen at the same time – it’s been a long time since I was in school.
I had lots of people express interest in attending (more than our location could accommodate) so if you would like me to arrange another go, contact me and I will arrange it. Thanks again to Bradford for a great class! Huge thanks to the Crown pub for hosting us too.
Edited to add a clarification to zinc levels – Thanks Bradford!