Don't try this at home
Before I get to the few useful applications of blending in homebrewing, we need to discuss when not to blend. When faced with an awful batch of homebrew, many new homebrewers wonder if they could blend it with another beer to save it. This never works. A mix of good beer and bad beer always yields bad beer.
By the numbers
If, however, you have a beer that simply has a recipe error — too much or too little bitterness, specialty malt character, color or other character attributable to adding the wrong amount of ingredients — blending is an option if you are willing to brew a second batch of similar beer.
Let's say, for example, you brewed a pale ale for which you wanted an amber color around 14 SRM. However, you somehow undercrushed your crystal malt and the resulting beer had an SRM around 10. What could you do to correct this? If you can put a number to the character you are blending for, there is a simple formula to guide your blending:
N(1) + N(2)X = N(B) (1 + X)
where N is the value of the character in your 1st and 2nd beer (N(1) and N(2)) and in the blend (N(B)). X is the volume of the second beer, relative to your first beer, you will need to blend.
Let's work two examples to show how you can use this equation:
In the first case, let's say you have 5 gallons (19 L) of your 10 SRM beer and want to brew a second 5-gallon (19-L) batch to correct it to 14. What should the target SRM of your second beer be?
In this case, the value of X would be 1 as you're planning for a 1:1 blend of the two beers. (If you were going to only make one gallon of blender beer, X would be 0.2.) By substituting in the other known values, you can solve for N(2), the target SRM for your second beer. So, 10 + N(2) = 14(2). Solving for N(2) yields a value of 18. Thus, you'd have to brew the second beer to have an SRM of 18. (This makes sense as the first beer was four SRM points low and the second beer is planned to be four points high.)
But let's say you brew the second beer and mess up again. This time you overcrush the crystal malt and brew a beer with a color rating of 20 SRM. How much of the 20 SRM beer would you need to blend to the 10 SRM beer? In this case, X — the proportion of beer to blend — becomes the unknown variable. Substituting in for the known variables yields 10 + 20X = 14 + 14 X. Solving for X gives us a value of 0.66 (the fraction being 2/3). So, to get a beer of 14 SRM, you would need to blend the beer in a ratio of 1 part of the 10 SRM beer to 0.66 parts of the 20 SRM beer. Given that you have 5 gallons (19 L) of the first beer, you'd need (5 X 0.66) = 3.3 gallons (12.5 L) of the second beer for blending.
Stouts and blending
As I mentioned at the beginning, there is at least one circumstance in which blending homebrews can be useful. For most beer styles, the target ranges for most characters are fairly wide. If a beer is little more or less bitter, within reason, it isn't wrecked. The same goes for original gravity, color and most flavor attributes. We like to be able to nail our targets, but — in all but the most delicate, balanced styles — we have a little leeway.
In the case of dry stout, however, there is one character for which you have very little "wiggle room" — the amount of roast character. Many homebrewers view stout as one of the easiest styles to brew, but I don't buy it. If you end up with too little dark roast character in a stout, the beer ends up predominated by a coffee-like flavor, which isn't right. (There are hints of coffee in a great stout, but it isn't the primary flavor note.) Too much roasted grains and the beer is undrinkably harsh. Many homebrewers claim that stouts are so big and flavorful that the overall "volume" of flavor masks most errors in the relative amount of dark, roasted grains. I don't believe this.
Hitting the right amount of roast in a stout can be difficult for a few reasons. You need to not only get the right amount of roasted barley (perhaps with other dark malts) in the recipe, you also need to have the grains crushed properly. This can be difficult as the dark roasted grains found in stout are typically smaller than base grains. If you crush your grain bill with the base malts and specialty grains mixed, you may not get an adequate crush of the dark grains. (You also need to realize that roasted barley comes in different roasts. You need to use the highly-roasted version (around 500 °L) for the bulk of the dark grain bill in your stout. This is sometimes called black barley, not to be confused with black patent malt.
Speaking of flavor, I'd like to put in a kind word on behalf of very dark roasted grains and malts. Often, you hear homebrewers describe these as harsh, acrid or burnt. I've heard many homebrewers say, "I've started using de-husked malt because roasted barley (or black patent malt) is too harsh for me." Well I for one actually like some of the "harsher" flavors from dark roasted grains. Just as a dose of "coarse" bitterness from a high-cohumulone hop can give a double IPA a pleasing aggressiveness, a little acrid "bite" from black patent malt can keep a robust porter or stout from being insipid. Just because you can take the rough edges from something doesn't mean you should. But I digress.
A relatively easy way to make your stout turn out right every time is to blend for the right level of roast flavor. To do this, brew a pale base beer without any of the dark malts. At the same time, make a homemade extract from all of the dark grains in your grain bill (perhaps with a bit of base malt). The dark roast extract will add the color and roast flavor to your beer. A mini-mash of the dark grains will give you some very dark wort. This wort is boiled to make an extract, and at this point you have two options. You can either blend the dark wort with your pale wort before fermentation. Or you can ferment the two separately and blend when you keg or bottle the beer. The first option is easier, but the second yields better results — especially if you do a few small-scale blends to really hone in on the flavor you like.
Dark grain mini-mash
To perform your dark grain steep or mini-mash, multiply the amount of the dark grains in your recipe by 1.2. Using a little extra grain will ensure that you have enough roast character to blend into your stout. Crush the dark grains and take a look at them. Dark grains can be very brittle, but you should aim for a crush that does not reduce them to powder. Likewise, you should not see any (or very few) intact kernels. Adjust your mill and crush again if the grains are undercrushed. Place the dark grains in a nylon steeping bag and heat an appropriate amount of water in a large pot. Using between 1.5 to 2 quarts of water per pound of grain is fine. Submerge the grain bag in the pot and hold the temperature at 155–162 °F (68–72 °C). The temperature here isn't critical, especially if there are no base grains in the mix. Hold the mix in this temperature range for 45 minutes to an hour.
If you have a pH meter, take the pH of the dark roast wort. It may be as low as 4. If you have any calcium carbonate, stir some in until the pH rises to at least 4.8 or so. (If you've made stout before the normal way, don't add more calcium carbonate than you normally would to a full batch.) A pH value of 4.8 is a little lower than you normally want wort to be, but the extract will be blended with pale wort later so it's OK.
Also realize that you won't want to be making your pale wort with highly carbonate water; use a water treatment program suitable for a light-colored beer.
Once the dark grains are done steeping (or mashing), remove the grain bag and give it a light rinse. To avoid pH problems, I like to dilute the "grain tea" (or mini-mash wort) with water then use this diluted mixture as rinse (or sparge) water. Your efficiency may not be as high as it possibly could be, but the extra grains you added will compensate for this.
The dark wort is then boiled. Boiling the wort not only sanitizes the dark extract, it concentrates it — making the flavors and colors more intense. As the commercial color extracts show us, color can be concentrated a great deal, to over 3,000 °L. Your homemade extract will likely not approach this value unless you boil for a long time. If you didn't have any base malts in the mix and plan to ferment the wort, you can add a bit of dried malt extract to the boil give the wort a bit of fermentable sugars.
After the dark wort is cooled, you can add it to your pale wort (or refrigerate it in a sanitized container until you brew the pale wort). Or, you can ferment it and blend it in the bottling bucket or keg. Remember that you made the extract from more grains than your recipe called for, so don't dump all of the extract in. Add extract and watch the color. When it gets close to the right target, take a small sample of the wort or beer and taste it. Keep adding the extract until you taste the right amount of roast character. This is easier to judge in beer than wort, but fermenting the dark roast extract separately takes some extra time and energy. Either way yields good results. When blending your extract with the pale wort, stir it in and let the blend sit for 30 seconds or so before sampling.
You can bottle any leftover roast extract beer just as you would a regular beer. If you're stuck going to a party where you know they will only be serving light American beers, bring it along and whip up your own personalized stout on the spot.
Perhaps you can make great stout simply by mashing (or steeping) your dark grains normally. If so, congratulations. If — on the other hand — you are like me and have been disappointed with your homebrewed stouts, try this method to reach the level of roast
Just like dark roasted grains, Chris Colby has been described as harsh, acrid and bitter.