Tuesday, 20 January 2015

Brewing long-aging beers - fermenting high-gravity beers

As promised last week, this is the fourth instalment in my "Brewing Vintage Beers" series. In this post I'm going to cover some of the methods that homebrewers can use to ferment beers that push - or even exceed - the rated alcohol tolerances of the yeasts being used. Experienced brewers will likely see nothing new here, but I've tried to include a bit of the science behind what the different methods do, so that my readers have a better idea of why these things work and are often necessary.

There are a number of things you can do to get full attenuation when brewing high-gravity ales, namely:
  1. Yeast selection
  2. Pitch rates
  3. Late/repeated oxygenation
  4. Late sugar additions
  5. Late yeast additions
  6. Managing the fermentation
  7. What if the batch doesn't ferment completely?
Details below the fold

Yeast Selection

This is the most obvious and easy tweak for high gravity brewing. A number of ale and Belgian yeasts are known to be good fermenters for high gravity ales - often companies will even say "high gravity" or "strong ale" or the like in the name they give to these yeasts. Obviously, picking an appropriate strain with a good history of ethanol tolerance is a key part in getting the degree of attenuation you desire.

What makes a particular strain of yeast ethanol tolerant? The answer may surprise you - we don't know. In fact, the term "ethanol tolerant" is somewhat of a misnomer - pretty much all yeasts have the same ethanol tolerance [1] if grown under conditions which limit other cellular stresses! So ethanol tolerance of a particular strain has far more to do with how the strain responds to stress than any intrinsic biological difference between strains; with less-tolerance strains going into dormancy at lower ethanol (i.e. stress) levels than more tolerant strains. What we do know is that there are several hundred genes involved in determining where this "stress thermostat" is set [2], and there appears to be two main regulatory genes that control this overall process [3].

While exactly how the stress level that leads to dormancy, and thus determines alcohol tolerance, is set remains unknown, we do what traits alcohol-tolerance yeasts have. In particular, they have high levels of certain unstaurated fatty acids and sterols in their membranes, and this appears to enhance tolerance (perhaps by stabilizing the membrane against ethanol's solvent-like properties) [4, 5]. If those sound familiar, it because those same lipids and sterols are what we are trying to generate when we make a starter - indeed, one of the above studies [4] even went on to show that supplementing yeast growth media (i.e. wort) with the various minerals and nitrogen sources found in quality yeast nutrients greatly enhances the ethanol tolerance of yeast.

The take home message here is simple - pick an alcohol tolerant strain and make sure you are running it though a well oxygenated and yeast-nutrient supplemented starter before you pitch it. This will maximize the chance of your yeast fermenting to completion.

Pitching Rates

Even in normal gravity brewing, many brewers under-pitch their worts. While this may only lead to off-flavours or slightly slower ferments in low-gravity worts, it can be lethal in high gravity worts. There is no magic number of yeast needed that will ensure success, but as a rule more = better. Its almost impossible using conventional homebrewing techniques to over-pitch a high-gravity wort.

Conventional ale pitching rates are usually cited as 0.75 million/ml/oP. This rate is typically sufficient for beer expected to finish at 80% or less the maximum alcohol tolerance of the yeast (i.e. an 8% beer for  a yeast with 10% alcohol tolerance). As you move closer to the tolerance level you need to up your pitch rate - in my experience at least 1 million/ml/oP is required, and I usually aim for 1.25-1.5 million/ml/oP for most of my big beers. Believe it or not you can usually exceed, by a few % ABV, a yeasts maximum alcohol rating - but to do that you need to be pitching at larger-like pitching rates (2 million/ml/oP) or higher. For example, my Gnarly Roots Barley Wine exceeded Wyeast 1056's rated ethanol content by nearly 3%, and I achieved that by pitching at a rate of ~3.5 million/ml/oP.

As mentioned in the "Yeast Selection" section, above, its not enough to simply have the right yeast, in the right numbers - it also needs to be healthy. Again, this is achieved by using a proper, well oxygenated starter, dosed liberally with a quality yeast nutrient. Alternatively, you can brew a low-gravity beer (I'd recommend an OG of 1.035 or less), using a well oxygenated starter & well oxygenated wort, and then pitch your big beer onto the yeast cake of this "starter beer". This method works really well for really big beers - you can easily get upto pitch rates of 5-10 million/ml/oP, which will let you push well beyond the gravity limit of the yeast without too many off-flavours, and the low gravity of the starter beer ensures that the yeast your are pitching are not stressed.

Late/repeated oxygenation

As mentioned in my previous article, vintage (i.e. high-gravity) beer brewing requires that you follow proper brewing techniques to the letter. This includes ample oxygenating of your wort prior to pitching the yeast. A couple minutes of pure O2, or 20-30 minutes of filtered air, is required. But if you are pushing the gravity limit you can add oxygen at later time points to aid the yeast in its fermentation. But when you add these late additions is key - too early doesn't matter much, but too late can lead to oxidation of your beer.

The key is to add the oxygen at a time when the yeast can 1) still benefit from it (i.e. before they start entering dormancy), and 2) when the yeast are still active enough to consume all of the  O2 before the oxygen starts reacting with other wort components t create off flavours. You can do more than one additional oxygenation, although I've never had the courage to go beyond two late  O2 additions (i.e. 3 total, counting the pre-pitch oxygenation). Sadly, there is no perfect way to do this, but I have a series of "rules" that I follow which appear to work well:
  1. Don't add additional oxygen until a good kraussen has developed. This indicates that they yeast are highly active and likely have likely consumed the O2 added initially to the wort. Assuming you pitched a goodly amount of healthy yeast, this will be around 18 to 24 hours post-pitch.
  2. At this time point aim for an oxygenation of 50% or so that added when you first oxygenated your wort - i.e. if you ran your  O2 for 5 minutes prior to pitching, run it for 2.5 minutes for the first addition.
  3. If you are considering a second late addition, make sure the beer is still actively fermenting and has a strong kraussen; if it does not it is unlikely that there is sufficient yeast activity to consume the O2 you are about to add, meaning oxygenation-related off flavours may develop.
  4. Rouse your yeast by gently stirring the beer - in a manner which lifts the yeast off the bottom of the fermenter - before adding the late addition oxygen. This ensure that you have the maximum number of yeast consuming and using the oxygen you are adding. Even if you're not adding additional oxygen, gently rousing the yeast at 24 and 48 hours can greatly help the completion of fermentation.

Late sugar additions

This is a method of fermentation management that I don't think gets enough credit or use. Many high-gravity beers involve the addition of simple sugars (table sugar/sucrose, dextrose/corn sugar, invert sugar, etc) to build gravity and thin the body of the beer. Most people simply throw these into the kettle - which isn't a good idea for big beers for two reasons:
  1. Keeping the sugar out of the kettle means your yeast start off with a lower-gravity wort; this is less stressful and acts as a "starter" for the later addition of the sugars.
  2. Yeast are not passive sacks of enzymes - they actively adapt to their environment. Yeast must import sugars using transporter proteins - proteins which grab hold of sugars and them pass them through the membrane of the yeast cell [5, 6]. Simple sugars - dextrose and invert sugar - can be imported passively (i.e. the transport proteins act as holes that selectively let the sugars enter the cells), while more complex sugars are imported by a variety of energy-consuming and passive mechanisms. In addition, more complex sugars like matose must be split into their constituent sugars inside the cell before being metabolized. In other words, more complex sugars are harder (and more energy consuming) for yeast to import and use. As such, a yeast in a simple-sugar rich environment may not upregulate the genes needed for dealing with the more complex sugars, leading to underattenuation as they yeast consume only the simple sugars and then enter dormancy without processing the more complex malt-derived maltose.
So adding the sugar late has two obvious advantages - you'll have more yeast to process the sugars and you'll force your yeast to process the complex malt sugars before giving them the simple sugars.

But when do you add the sugar? Typically I add them somewhere between 24 and 48 hours - generally, I am trying add the sugar once kraussen hits its maximum, or even has begun too drop slightly. By then most of the yeast will have entered stationary phase (i.e. will not be dividing any more) and thus will not readily change their expression of those sugar importers. Plus, you'll be upping the gravity while the yeast are still highly active. I've always added all the sugar in one go, but it may be advantageous to do it over two additions if you're adding more than 10% fermentables with the sugar addition. 

In terms of what sugar is best, in theory simple sugars (invert or corn/dextrose/glucose) are best as they require the least pre-processing and no energy consumption prior to fermentation, but I've found no evidence that this has been tested. I prefer to err on the side of caution and always use invert sugar, which is rather simple to make:
  1. Measure out the amount of sugar you require, put into a pot at least 5X the volume of the sugar. Make sure you know the volume of sugar as well as its weight.
  2. Blend in a small amount of acid - a teaspoon/kg of lime juice or lemon juice, or 1/2 tsp of cream of tartar/kg works well.
  3. Dissolve the sugar in 1 cup (250 ml) water per kg of sugar; you will need to heat this on the stove to get it to dissolve, but heat slowly - you do not want it to boil until all the sugar is dissolved.
  4. Using a candi thermometer, your stoves temperature control, and small water additions, hold the sugar at 125-135C (260-275F) for 30 minutes. How to control sugar temps is covered in my article/video on Making Belgian Candi Sugar.
  5. Kill the heat after the 30 minutes has passed - your sugar is now inverted. Next slowly add in an 1X to 1.5X volume of cold water (i.e. 1 to 1.5 cups water per cup of sugar), stirring to prevent the sugar from crystalizing. This should convert the sugar into an easy-to-add invert syrup. Pour into a sanitized mason jar and store at room temperature until needed - the sugar concentration is enough to keep this sterile until used.

Late yeast additions

Adding a new pitch of yeast, fresh out of the starter, is often a good way to ensure you beer ferments out completely. You can either pitch the same yeast, or another more alcohol tolerant strain to ensure completion. Using a yeast like White Labs Super-High Gravity ale is a great way to finish off a strong beer brewed with a yeast that has a more desired flavour profile; by limiting this yeast to the end of the ferment you can minimize its particular flavour profile (which is actually pretty nice, for English-style beers), letting the flavour of your first yeast be the star of the show.

But when should a (planned) second yeast addition occur. I've not seen any definitive source on this, but I tend to wait until the peak of kraussen has passed and the kraussen is beginning to fall. At this point the original yeast have done most of the work it can, giving the fresh yeast more room to work. Moreover, if pitching a different strain, a lot of the flavour development by the primary yeast would be complete by this point in time ensuring that the profile of the first yeast dominates. If adding a late sugar addition, I usually hold back my late yeast addition until 12-24 hours after the sugar addition. I cannot offer a good reason for this, but it appears to work.

How much should be pitched? Again, I've never seen anything definitive, but I usually aim for something approaching a conventional ale pitch rate (0.75 million/ml/oP). This is about half of what I typically pitch to start the ferment, and seems to be enough to ensure the ferment completes.

Managing the ferment

This is one of the more important parts of the process. The high pitch rate combined with the abundance of sugars means the heat of fermentation can easily raise fermentation temperatures above ideal limits - potentially creating more off-flavours (although this appears questionable), and potentially causing the yeast to stall. Some sort of temperature control - even if its just a fan and wet t-shirt - is not a bad idea.

As with most ferments, you want to be pitching the yeast within its rated temperature range, and ideally nearer the lower end. As it ferments it is fine to let it warm, but try to keep it from exceeding its maximum temperature rating. For Belgians it is not uncommon to warm the ferment further near the end - with higher-gravity versions of these beers its not only a good idea, but its often necessary to get the higher level of attenuation typical of these beers.

Expect the tail end of the primary ferment to drag on - I plan on 3 to 4 weeks. Most of the gravity drop will take place in the first week, but the last few points of gravity seem to take at least twice as long as the first 100 points. Once the primary is complete, determined by either 2 consecutive readings of the same gravity separated by ~48 hours - or alternatively, when your wife asks how many more weeks before you can start her a batch of wine - you need to move the beer quickly into a secondary. Yeast autolysis happens faster with strong beers, so leaving it in the primary for too long can lead to a beer that tastes like blood. If possible, pre-purge the secondary with CO2. You are now at the point in the brewing process where oxygen changes from a friend to a foe, and you need to start taking action to keep it out of your beer.

How long you age in a secondary depends on how strong the beer is and the style; follow the typical guidelines for your style. If adding Brettanomyces to get that English aged character, do so right after transfer to the secondary - there is no benefit to waiting, not to mention the Brett will quickly clean up any oxygen introduced into the beer during transfer. Secondary fermentation should occur at a temperature roughly that of the primary fermentation (ideally a degree or two colder) - this way the residual yeast will continue to clean up off-flavours, without putting the beer into a new temperature range that will create new off-flavours. Typical ageing periods for these stronger beers range from a few weeks to well over a year. Regardless, make sure you are checking the airlocks frequently; both to ensure they stay filled, and to ensure they remain firmly seated. A dry or loose-fitting lock can quickly lead to oxidized beer, wasting all of the time, money and care put into the beer.

Packaging needs to be taken care of carefully as well - if possible, pre-purge bottles with CO2. If bottle conditioning, its a good idea to add champagne yeast to beers that have aged for more than 6 months - this yeast only eats simple sugar, is extremely alcohol tolerant, and is clean fermenting. This means it won't attenuate your beer further; adding a different ale yeast might. If kegging & force-carbonating, pre-purge the keg with CO2 before filling. Oxygen-absorbing caps and wax seals are also excellent ideas - both will limit oxygen entry into the bottle, giving your beer better vintage potential and giving better flavour development as well.

The dreaded stuck ferment

If you brew strong beers eventually - no matter how careful you are - will have a stuck ferment. That beer that should end at 1.025 instead craps out at 1.054. What do you do?

I go through an escalating series of procedures, each intended to be a slightly harder push to get the beer to finish:
  1. Rouse the yeast. Rousing the yeast gets a bit of oxygen in the beer and puts the yeast in suspension. If a beer is truly stuck this is unlikely to fix the problem, but its often enough to reinvigorate a sluggish ferment.
  2. Do #1 and throw in a bit of yeast nutrient and oxygenate at the same time. This is usually enough to get a stuck ferment going.
  3. Do #2 alongside a second pitch of yeast. I had to do this once, and in one day a ferment that was stuck for a week chewed through the remaining 20 points of gravity.
  4. Do #3, but use a dedicated high-gravity yeast. I've never done this, but its always in the back of my mind as a last-ditch effort to save a beer.
  5. Give up, cry, and either enjoy your sweet beer or blend it into subsequent lower-gravity beers. I haven't had to resort to this yet, but at some time you just need to cut the cord and move on.

References:

1: http://www.ncbi.nlm.nih.gov/pubmed/3533426
2: http://www.genetics.org/content/175/3/1479.short
3: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.2009.04657.x/full
4: http://www.ncbi.nlm.nih.gov/pubmed/2178780
5: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC182412/
6: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1144178/


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