It's All About the Fermentation
By Greig McGill
In my recent post on brewing for competition, it might have surprised people that I didn’t specifically mention the F-word. Fermentation (that’s what you were all thinking, right?) wasn’t mentioned specifically because the article was primarily aimed at people who don’t really need to be told just how important the process is for making world-class beer. The F-word is there hiding in the shadows of every tip though.
It is common to read homebrewing forums and see brewers agonising over getting their recipe just so. A few grams more of specialty malt X, adjusting hop addition Y by five minutes here or there. There is so little attention paid to managing their fermentation, the common comments are usually limited to a strain of yeast and, maybe some temperature steps. If I had to guess, I’d say brewers seem to spend around eighty to ninety percent of their design process on recipe and ten to twenty percent thinking about fermentation. I think the quality of beer produced would rise significantly if these percentages were reversed!
When I visited White Labs in San Diego on a recent trip, it was hammered home to me just how little impact the recipe often has in comparison to the effect the yeast can have on a beer. In the White Labs tasting room, they present several basic beers, all fermented with different yeast strains, and different fermentation profiles. In some cases the differences are quite subtle, but even in those cases, they are obvious enough that you’d believe you had different base recipes in your glass. The yeast strain, the yeast health, and the fermentation profile all have such a huge impact on the beer that it starts to seem almost pointless to spend so much time agonising over tiny recipe tweaks to a recipe!
So, what should brewers be thinking about in terms of yeast and fermentation? It breaks down into two main areas; yeast selection, and yeast management. Your inner artist informs the former with some assistance from your inner scientist, and the latter allows the scientist to take over while the artist peeks over her shoulder and provides the odd interjection.
Yeast Selection
Yeast selection requires you as the brewer to think about many aspects of yeast and fermentation besides just the manufacturer’s descriptive and qualitative notes. For flavour compounds, it’s wise to apply the same rules you might use in food matching. Apply the three Cs of Complement, Contrast, and Cut. For example, say you are brewing something pale and hoppy, featuring citrus and tropical notes up front. A complementary yeast strain might either stay right out of the way, or provide similar fruity esters to your hop choices, enhancing the overall impact to the drinker. A contrasting yeast strain might enhance the malt characteristics, upping the perceived sweetness of the beer, or add some sulphur compounds in pleasant amounts. An example of a cut would perhaps a phenolic Belgian strain, or a bacterial culture, providing sharpness and acidity to cut right through the lush hops.
You must also think about the desired attributes of your finished beer when choosing a strain. Do you want it dry as a bone? Ensure the attenuation percentage is high. Do you need to turn around a clean bright beer in a hurry? You’ll want a highly flocculent strain. Need attributes from several different strains? Perhaps you should consider blending – but that could be the subject of another very long article in itself!
Yeast Management
Once you’ve selected your strain, it’s time to let the scientist in you take over. Yeast needs careful management in order to do the job properly. For a starter, yeast viability (the number of living active cells able to ferment your beer) falls off drastically with age, so you’ll want the freshest yeast possible. You’ll also want to make a starter unless you’re brewing the smallest batches. Be aware that if your yeast is old, you may have a lot of pretty sick cells in there, just clinging to life. These may reproduce, passing on their sick and mutated genetics to the future generations you create in the starter. It is usual for the healthy cells to out compete the unhealthy, and it’s likely your fermentation will be fine, but yeast health should be, to misquote the Pet Shop Boys, Willie Nelson, and Elvis Presley, always on your mind.
Yeast does amazing and terrible things during fermentation. I won’t go too deep here; you can Google the science if you want to learn all about exciting things such as the Krebs cycle. For our purposes, we’re going to keep it simple. As you know (I assume, since you’re reading this) yeast produces ethanol, which is a waste product to the yeast cell, and therefore toxic. It also produces many other compounds at various stages of fermentation. Perfectly healthy yeast in a perfect environment produces minimal amounts of “bad” compounds that result in off flavours or just balance issues in a finished beer. You can manage the production of desirable versus undesirable compounds by several methods, but the primary two which brewers use are temperature control and pitching rate. It should be noted that current research indicates pitching rate may be far less of a factor than previously thought, but studies are on going. In light of this, we will mainly address temperature control, and assume you are using a tool such as Jamil Zainasheff’s pitch rate calculator to ballpark your pitch rate.
Oxygenation of your beer allows the best possible environment for the yeast to begin fermentation. Oxygen provides the energy for those little cells to do their tasty, tasty work. Not enough and… well… imagine giving someone half a piece of toast with a stain of Marmite on it for breakfast, then asking them to work a 12 hour day of heavy physical labour. How well do you think they’ll do their job? Oxygen is the toast. Yeast nutrient is (almost literally – seriously) the Marmite. Don’t skimp!
Most yeast strains follow a simple rule of thumb – the warmer it gets, the more active the yeast will be, and the more additional compounds – such as esters, vicinal diketones (of which diacetyl is the most commonly discussed), and ethanol precursors such as acetaldehyde – are produced. If you stay within the temperature range the manufacturer specifies for the yeast strain, most of these will be within acceptable levels in the finished beer, and you can use the range to dial in how much “fermentation character” you want to taste. A drop in temperature tells the yeast that it should prepare for dormancy, and it is fairly common advice to “never go backwards”. I often hear of people crash cooling their beer to stop fermentation – never do this! It is best to slowly increase the temperature towards the end of fermentation to keep the yeast active as long as possible until fermentation registers as complete. By that I mean the green beer should taste free of off flavours, and terminal gravity should have been attained as signified by no further decrease in specific gravity readings over 48 hours. The reason for this warm rest near the end is to encourage the yeast to “clean up after itself” – scavenging its own waste products for energy, leaving a much cleaner beer lacking in off flavours.
If you intend to re-pitch your yeast, you should do so as soon as possible - a week at most. If you can’t brew again so quickly and therefore can’t re-pitch, fresh yeast is relatively cheap, compared to the cost of making a batch of terrible beer. Yeast gets even cheaper if you have great sanitation practices when making a starter and/or you can share costs with a brewing friend or local brewing club.
Speaking of costs, “But Greig”, I hear you cry (yes, you Tim in Christchurch, I have excellent hearing), “an oxygen setup, yeast nutrient, temperature control, a stir plate and flasks for starters, pitching more yeast… all this sounds like you’re just trying to make us spend more at Brewshop!” No, I’m not. What I’m saying is that you can brew great beer, or you can brew faulty beer. If you want to do the latter, feel free to skimp on everything here. If you want to make great beer, I’m sorry, but there are no cheap shortcuts.
To summarise, think about your fermentation as much more than just an afterthought. It defines your beer. Spend some money. Look after your yeast, and the yeast will make you great beer, even if your recipe is mediocre!
Cheers and compliments of the season!