High-Gravity Fermentations
Our bodies can tolerate quite a lot of things in small doses that would be harmful, even deadly in larger doses. That’s true of many medications we take and of course, it’s true of alcohol. Yeast, being a living organism, has similar problems and paradoxically can be poisoned by alcohol, the very product it produces and that we want it to produce. In short, yeast has problems dealing with worts of high gravity and fermentation of such worts may become sluggish and even cease as the alcohol content of the liquid increases. So in brewing a “big” beer you can easily get a stuck fermentation, or more commonly the beer ends up being overly sweet. You may think that in big beers like barleywines or imperial stouts there is so much flavor contributed by the malts and hops that a little extra sweetness doesn’t matter. In fact, that sweetness can mask much of these flavors and make the beer bland and uninteresting. That is definitely what you do not want since these big beers are expensive in terms of ingredient cost and brewing labor, and are not the sort of beer you are going to brew every day, so it is important to get it right when you do brew it.
Yeast procedures
First let me define what I mean by high gravity beers; these are beers with an original gravity (OG) of 1.080 (19.3 °P) and up as high as 1.100 (23.8 °P) or more. What you want to achieve is 65–70% apparent attenuation. Apparent attenuation (AA) is defined as:
AA = 100 x (OG* – FG*) / OG*
Where OG* and FG* are the decimal parts of OG and final gravity (FG); that is for OG 1.080 OG* is 80.
That means, for example that for an OG 1.080 (19.3 °P) you need to reach FG in the range of 1.028–1.020 (7.1–5.1 °P). For a wort of OG 1.100 (23.8 °P) you would need to get down to FG 1.035–1.025 (8.8–6.3 °P). In selecting the yeast you should preferably choose a strain that is quoted by the supplier as giving 70–75% apparent attenuation. Some brewers have opted to use a yeast with known high-alcohol tolerance, such as a Champagne yeast, either on its own or in combination with a beer yeast. I think such an approach is not necessary as there are a wide variety of yeast strains and forms available that should be capable of doing the job if properly handled.
In order to achieve this kind of attenuation you need to use a lot of healthy, active yeast. If you want to save a little money by throwing in a pack or two of liquid yeast you’ve had sitting around for months or more you will be making a big mistake. That is just asking for a stuck fermentation that would be next to impossible to restart with high-gravity worts. So first, just how much yeast do you need? It is common brewing practice to aim for about 6×106 cells per mL for a beer brewed at around OG 1.050 (12.4 °P). So for 5 gallons (19 L) you would need:
5 x 3.78 x 1,000 x 106 x 6 = 113 x 109 cells
This is about the number quoted by the manufacturers for their packs of liquid yeasts. I am inclined to think that one such pack is not enough, and if you use the old brewer’s rule of thumb of 1 million cells/mL/°P you would need about 200 x 109 cells for 5 gallons (19 L) of a wort at 1.050 (12.4 °P). Note that White Labs actually suggest two of their packs for wort at 1.050 on their website.
But that’s not the kind of beer we are talking about and you have to reckon that you need at least twice this amount of yeast, that is 4 billion cells for a wort at 1.080 (19.3 °P), and perhaps half as much again, that is 6 billion cells, for a wort of 1.100 (23.8 °P) and above. And that means you would need 4–6 packs of liquid yeast for such high gravity beers, which would add significantly to the cost of the brew. So that suggests you are probably better off by making a yeast starter, beginning with say two packs, adding each into 1 qt. (1 L) of starter wort at about 1.040 OG (10.0 °P) and allowing them to ferment for 2–3 days. At this point take each of these starters, split them, and add more of the starter wort to make each up to 2 qts. (2 L), for a total of 4 qts. (4 L) in all. After another 2–3 days pitch these starters into the wort — not all the liquid, but only the sediment in each container. This procedure is not as demanding as it looks on paper, but clearly requires some careful planning before brew day.
I have only talked about liquid yeasts so far — what about dry yeasts? Well the standard 11 g pack contains about 200 billion cells, not enough from the above to do the job we want with just that one pack. But two packs do provide about 4 billion cells and so should work well at the “lower” end of our high gravity range, although you should use three packs if going for an OG 1.100 (23.8 °P) or higher. Whether in this case you should make a starter is somewhat debatable. The manufacturers suggest it is not necessary to do so, and I know some homebrewers who swear that they have had excellent results using dry yeasts in high gravity worts. Indeed, some state that they have had better results with dry yeasts than with liquid yeasts for these kind of beers. That I suspect is due to the fact that liquid yeasts, unlike dry yeasts, do undergo deterioration on storage for several months. If you do not use a fresh sample of liquid yeast, and do not make a starter you are not giving the yeast a fair chance.
The recommended method for dry yeast pitching is to re-hydrate the pack contents with warm (about 90 °F/32°C) water or wort, stir, and let stand for about 20 minutes before pitching into the wort. This is certainly a convenient approach and obviates the need for planning ahead and taking a starter through several stages. However, my preference is to make a starter, partly because that tells me right away whether the yeast is viable, and partly because I want to be absolutely certain I have a sufficiency of healthy, active cells in the wort. In this case I would use only a one-step starter, re-hydrating the contents of the yeast packs as above and then pitching into 2 qts. (2 L) of wort, allowing fermentation to proceed for 2–3 days before pitching the sediment into the main wort.
More on starters
I suggested earlier that the wort for a starter should be about OG 1.040 (10 °P). Do not be tempted to use a wort at the same OG as your high-gravity beer, because once the yeast has fermented out from 1.080 (19.3 °P) or higher it will be too stressed out to do another one! One way to do this is to use something like a hopped liquid malt extract made for pale ale. You need about 3 oz. of this per 2 qts. (2 L); weigh it if you can, but that’s not easy, so a good tablespoonful will be just about right. An easier way is to use dried malt extract, in which case you will need just about 2 oz. per 2 qts. (2 L); of course, this will not give a hopped wort as does the syrup, which some do regard as a disadvantage.
Once you have prepared the wort, thoroughly dissolving the malt extract, boil it for about 20 minutes, cover securely and cool in your refrigerator. Do not let it cool down too much — just let it come down to room temperature. Then decant the liquid off the sediment into a fresh, sanitized vessel and pitch the yeast culture. If you are using liquid yeast you would of course have to split this 2 qts. (2 L) into two lots of 1 qt (1 L) and pitch each with 1 pack of yeast. In this latter case you would have to repeat the whole procedure, but this time pitch the new starter wort with the sediment from the first two starters.
Oxygen or not?
A problem with starters as prepared the way I just described is that you have boiled out the oxygen from the starter wort, just as you would do with the main wort. But the yeast needs oxygen in the initial stages of its growth in a wort. With the actual brewing wort a good splashing around before pitching the yeast is a reasonable solution to this difficulty (although read on for more on this). But that may be difficult to do on a small scale, and to my mind the best approach is to oxygenate the starter wort at each stage of the propagation. This will give you more than you can achieve by splashing and will result in greater growth of the yeast mass, which is exactly what you want in a starter. If you don’t already have them get yourself a carbonation stone and an oxygen tank and slowly bubble the gas into the wort for 1–2 minutes. Do not forget to sterilize the stone and keep the flow rate low – making it faster will not dissolve more oxygen since the gas bubbles will have less contact time with liquid.
An alternative is to use a stir plate in making the starter.All you need is a conical flask, which most homebrew retailers offer, a suitable stir plate, and stir bar. Pour the pitched starter wort (1 qt./1L) into the sanitized flask, add the sanitized stir bar and plug the mouth of the flask with a sterile cotton ball. Turn on the stir plate at such a speed as to give a deep vortex in the yeast slurry, and leave for a couple of days. If you are using a liquid yeast I would still recommend splitting this so as to give 2 x 1 qt (2 x 1 L) as described earlier. The advantage of the stir plate is that air is continually drawn into the slurry, so continually providing it with oxygen. The oxygen is used to produce unsaturated fatty acids and sterols, which results in growth of a greater quantity of yeast cells.
Anything else?
There is the question as to whether to add a yeast nutrient in fermenting a high-gravity wort. An argument against this is that an all-malt derived wort, especially at high gravity, contains all the nutrients that the yeast needs, so extra nutrient should not be necessary. However, Chris White of White Labs asserts that high gravity worts do need extra nutrients, especially zinc. Since you are probably making this kind of beer once in a while, it is wise to follow this advice and add a nutrient. Both Wyeast and White labs offer a nutrient package (although prepared by slightly different procedures) that will work well in this context. Use as directed on the package.
Another question is what sort of fermentation temperature you should use. If you go to higher levels than you would normally use for an ale fermentation, say up to 70–75 °F (21–24 °C) or even higher you will certainly speed up the fermentation somewhat, as you would with any chemical reaction. You might also increase ester formation, which may or may not be suitable to the style you are brewing. But a bigger problem is that you would also increase the growth rate in the early stages of fermentation. That may sound good but you are likely to find that it also increases the rate of blow-off of the head formed at this stage. With a high gravity wort this can be a problem at lower temperatures, so increasing the temperature can result in the loss of a great deal more of the fermenting wort than you would want. So, I would stick to pitching at around 75 °F (24 °C), then cooling the fermentation down to around 65 °F (18 °C).
To wrap up, the core of my argument is “Make sure you have enough active yeast” and your big beer will be fine!
