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Barrel-Aged Imperial Stout – Part 3: Fermentation

Healthy pitch

One of the best ways to improve the quality of your homebrew is to pitch the right amount of healthy yeast. Moreover, while it may be tempting to merely pour a single pack or vial of liquid yeast into the fermentation vessel, this isn’t usually enough. Under-pitching can cause stuck fermentation and produce off flavours, so it’s best to plan and have enough yeast ready on brew day.

Here’s an easy way to estimate a reasonable pitch rate for your next batch of homebrew;

How Concentrated is your wort?

The denser the wort, the more yeast you need to ensure a healthy fermentation. Wort strength can be expressed in gravity points, which are just the three digits to the right of the decimal point of the original gravity:

  • A 1.050 pale ale wort has 50 gravity points.
  • A 1.080 double IPA wort has 80 gravity points.
  • A 1.120 barleywine wort has 120 gravity points.

Ale or lager?

Much like people, the yeast becomes sluggish when it’s cold. Moreover, since lager ferments at a much lower temperature than ale, you need more yeast to get the job done. A good rule of thumb is to pitch about twice as much yeast for a lager as for an ale:

For an ale, you need about 0.007 fresh liquid yeast vials or packs per gallon per gravity point.

For a lager, you need about 0.015 fresh liquid yeast vials or packs per gallon per gravity point.

How much yeast do I need?

Just multiply the number of gravity points by the multiplier you selected above, and then multiply by your batch size in gallons.

Five gallons (19 litres) of 1.050 ale will need 
(0.007 packs per gallon per gravity point) × (50 gravity points) × (5 gallons) = 1.75 yeast packs.

Whereas 2.5 gallons of 1.080 lager would require 
(0.015 packs per gallon per gravity point) × (80 gravity points) × (2.5 gallons) = three yeast packs.

Note that it is better to have a bit too much yeast than not enough so if dealing with a part pack always round up to the nearest pack.


Oxygen in beer is mostly undesirable except at two points, and only one of these is intentional. That one point is when the post-boil wort has been chilled down to fermentation temperature, but before the yeast has been pitched into it. The unintentional point is in the ageing of particular styles of beer like old ales, barleywines and imperial stouts where the brewer doesn’t intentionally add oxygen but monitors the oxidation of the beer to develop positive oxidation characteristics like sherry notes in the beer.

Oxygen dissolves into the wort as a function of temperature and specific gravity. As such, the colder and less concentrated the wort, the more oxygen will be able to enter into solution. The high temperature and bubbling during the boil drive most of the oxygen out of the wort. Therefore, oxygen must be replenished after the wort is cooled and able to retain the oxygen in solution again.

Oxygen is essential for yeast growth and reproduction. Yeast must grow and reproduce first, before actually fermenting the wort to make beer. Yeast needs oxygen to synthesise the material for expanding cell walls; namely sterols and fatty acids. Miss-managing proper wort aeration can lead to problems such as long lag times before the start of fermentation, stuck or incomplete fermentation, or excessive ester (fruit flavour/aroma) production, any of which would produce less than desired results.

Commercial breweries typically shoot for 8 to 10 ppm dissolved oxygen in the wort before fermentation. Five ppm dissolved oxygen in wort is considered a bare minimum for proper yeast growth. Eight ppm of oxygen in chilled wort can be achieved using plain old air (which is 21% oxygen). Oxygen saturation above eight ppm in wort usually requires the use of pure oxygen.  Again, as the specific gravity of wort increases, its ability to absorb oxygen decreases, thus making wort oxygenation of big beers even more critical to their successful production. Because it is difficult and expensive for homebrewers to measure dissolved oxygen in the wort, experimentation will be needed to determine if the oxygenation method you perform is sufficient. If your fermentation temperature and yeast pitching rate are right, but fermentation is sluggish or incomplete, you should look at stepping up your oxygenation technique for future batches until you achieve success. There are two kinds of approaches homebrewers typically use to oxygenate wort; Whirlpool/ splashing and injection of either air or pure oxygen.

Whirlpool/ Splashing

After the wort has been chilled and transferred to the primary fermenter, some methods can be employed to agitate the wort to introduce oxygen. If your wort is in a glass carboy, you can cover the mouth of the carboy with a loose-fitting cap, get a firm grip and rock the carboy forth and back to slosh the wort around inside. Care must be taken to support the carboy on a cushioned surface and to maintain a secure hold on the carboy at all times while agitating.

If your wort is fermented in a bucket or other vessel with a wide-open top, a stainless steel whisk borrowed from the kitchen can be sanitised and used to whip the wort until it has at least a couple of inches (several centimetres) of foam on top. If you don’t think your arm will last long enough to whisk the wort manually, you can sanitise a whirlpool paddle or a (new) paint stirrer, attach it to an electric drill and agitate the wort accordingly. A word of caution if using an electric drill and whirlpool paddle or paint stirrer. Take care not to damage the fermenting vessel (or yourself), and avoid splashing the wort out
of the fermenter.

Regardless if your wort is in either a conical fermenter, carboy or bucket if you need to add cold water to make up the volume of wort to the desired level, splashing or spraying the cold water into the wort can also increase the amount of oxygen that will go into solution. Agitation is the simplest and least expensive method for aerating wort but to reiterate the maximum amount of oxygen that can be added by this method is limited. 3-4 minutes of aeration by this method will get you in the range.

Air or oxygen injection

To accomplish oxygenation using air, an aquarium pump (or another air compressor) can be used to pressurise the air and send it through the tubing and some type of filter to remove dust and microorganisms. A HEPA (High-Efficiency Particulate Air) filter with a .023 micron sized filter or a filter improvised from cotton balls wetted with alcohol is recommended to keep the air as contaminant-free as possible as it enters the wort. Many homebrew suppliers now carry air pumps, filters and diffusers for this method of wort oxygenation. 2-3 minutes of aeration by this method will get you in the range.

When using pure oxygen (ensure you get food grade) then you do not need to filter the oxygen. CAUTION oxygen is highly flammable, do not use around open flame and electronics. 1 minute of oxygen at a flow rate of 1L per min will get you in the range.

Carbonation stones of 0.5-micron size are the way to apply the air/oxygen to the wort, bubbling can be done but is much less efficient.

Dissolved oxygen meters are slowly coming down in price, and if you want to be sure you are in the ppm range, then this is a great investment. Otherwise, just use the guide above and be consistent to make adjustments after a few batches.

Feeding the yeast

The basic idea behind “feeding” is to brew the beer at an OG lower than planned, but add the remaining fermentables to the beer during fermentation. For example, one way to brew a 14% ABV beer would be to make some wort with an original gravity of around SG 1.140, then ferment it down to an FG of around 1.035. Another way would be to produce a wort with an OG 1.120 — which would yield a roughly 12% ABV beer, assuming the same degree of attenuation — but “feed” the fermentation with the additional carbohydrates that would have been required to make the wort 1.140. This process adds extra steps to the fermentation, so why would you do it?

 The rationale behind “feeding” is that the yeast never encounters a sugar concentration over a certain level. For example, in the second example above, the yeast never has to ferment wort over SG 1.120. After the yeast is pitched, the specific gravity would drop. The fermentables added later would boost the specific gravity, but not beyond the original wort strength. For example, perhaps the wort fermented down to 1.080, then the fermentation was “fed” so that the specific gravity was boosted to 1.100. When this beer finished fermenting, it would have the strength of a 14% ABV beer, but the yeast would never have had to deal with anything beyond a projected 12% ABV level of wort sugars. Very high gravity fermentations can be stressful, and feeding is a way to ease some of this stress. (A side benefit is that, because your original boiled wort is lower in gravity, you may experience slightly higher hop utilisation.)

You can feed your fermentation pretty much anything that would work as a kettle adjunct. The most obvious “food” would be dissolved malt extract. You could dissolve the extract to make a very thick wort, heat it to 170 °F (77 °C) to sanitise it, cool, and add it to your fermenting wort. Likewise, thick solutions of simple sugars such as cane sugar (sucrose), corn sugar (glucose), or honey would also work. Feeding the fermentation ‘simple sugars’, which are 100% fermentable, would not raise the expected FG of the beer as much as malt extract (which is not 100% fermentable).

The sugar solution you make needs to be thick enough to raise the overall “ virtual OG” of your beer, but not so thick that it doesn’t dissolve readily in fermenting wort. When you add the sugar, it should be stirred in with a sanitised spoon or racking cane. However, the wort should not be aerated at this point. Adding a small quantity of yeast nutrients at each feeding is a good idea. The quantity of yeast nutrients added for the final feedings should be tiny enough that the yeast can be reasonably expected to use them all up. If the yeast doesn’t consume the nutrients, any contaminants in your beer will have a source of nitrogen and other minerals to utilise.

As fermentation proceeds, the yeast consumes the sugars in the wort and excrete ethanol and CO2. (The metabolic activity of the yeast also gives off heat.) The specific gravity will drop steadily due to the decreasing sugar content. (The increasing alcohol content also contributes to the decrease in specific gravity, as ethanol is less dense than water.) At some point, it will be time to “feed” the fermentation.

The best time to “feed” is after high kräusen has passed, but the fermentation still has some momentum. A steady bubbling from the airlock is a good sign of this. At this point, adding additional sugar will not cause the sugar level to exceed the original OG, and the yeast should be healthy enough to ferment it. You can take a hydrometer reading to see if the specific gravity has dropped sufficiently, but it isn’t necessary. Once the peak of fermentation has passed, a sizable chunk of the sugars will have been consumed.

You may want to break your feedings up into two or more sessions. For example, you may wish to only boost the potential alcohol at each feeding by a projected 1–2% ABV. That way, if the fermentations bogs down, you’re not left with many sugars in your wort.

You may also want to add small amounts of yeast nutrients to the “feeder” wort. If you do, add only as much as would be required for the quantity of wort added — for example, don’t add enough nutrients for a 5.0-gallon (19-L) batch when you’re only adding a quart of wort.

 The feeding itself is simple. Mix up your sugar solution, perhaps with a tiny amount of yeast nutrients. Heat it to 170 °F (77 °C) to sanitise it. Cool it back down to fermentation temperature and add it into your fermenter. Use a sanitised spoon or racking cane to stir the mix gently. Do not aerate the wort when you add the sugar solution.

Expect the fermentation to pick up in the next few hours. However, unless you’ve added a ton of sugar (beyond that required to boost the ABV by 1–2%), it should not approach the vigour of high kräusen.

As with any other very high gravity fermentation, a couple of things might help you finish. After the final feeding, when the fermentation is slowing significantly, you may want to raise the temperature. At a minimum, raise it to the top of the yeast’s stated temperature range. In most cases, with just a little fermentation left, a few degrees beyond that won’t hurt.

Check out Part 4 for advice on selecting the right woods for your Barrel-Aged Imperial Stout.

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