High Gravity Brewing
High Gravity Brewing (or Blending) is a technique that allows you to brew a larger end-batch volume without buying larger fermentation equipment. A higher specific gravity beer (strong beer)is first fermented and then blended with water to create a lower alcohol beer with less IBU, SRM and final gravity than the originally fermented beverage. High-gravity brewing (or blending) is used by some of the big US brewers. These "Big Guys" brew this way because they can produce greater end-volumes of beer with high-gravity/blending brewing techniques than if they brewed it all at working strength.
- "A fermenter that holds 500 barrels of beer will end up producing 700 barrels of beer. For home brewers, a 5-gallon (19-L) batch of strong beer can easily be diluted to yield 6–6.5 gallons (23–25 L) of finished beer, more if you’re careful." [Colby,2004]
Stereotypically this technique is used for beers like American Pilsners. These beers are brewed with an original gravity (OG) around 14–16 °Plato (SG 1.056–1.064). After fermentation, the stronger beer is blended/diluted to a "virtual" gravity of around 10–11 °Plato (1.040–1.044). Of course, virtual gravity means the beer measured... had the water been added pre-fermentation. BYO reported this could be done to about a 40% water/to/beer blending.
 Recipe Formulation
The easiest way to formulate a high-gravity/blending brewing recipe is to take a recipe for your normal end-batch brewing volume and increase the recipe to a "pretend" larger volume. Then, you brew the beer at the smaller volume with all the extra gravity from the increased ingredients. Calculating how much to expand the recipe is easy. Simply divide the larger volume of target beer you wish to end up with by the smaller volume of strong beer you are actually boiling and fermenting.
- For an example, let’s say you have a 5-gallon (19 L) pale ale recipe with:
- If you want to end up with 6 gallons (23 L) of beer, multiply all the ingredients by 6 divided by 5, or 1.2. Doing this shows the following amounts, which you can use to brew and ferment 5 Gallons (19L) and then dilute to 6 Gallons (23L)after your fermentation is completed.
- 12 lbs. (5.4 kg) pale malt
- 1.2 lbs (0.54 kg)
- 2.4 oz (68 g) of hops [Examples from Colby,2004]
There is a drawback to this simple formulation, in that hop utilization slightly decreases at higher wort gravities. This means that just because you are using proportionally more hops in your "higher gravity" base beer, you could still end up with an "under-hopped" blended/diluted beer. The differnce is very small in most cases, but grows in correlation with the blending/dilution percentage being added.
To formulate high-gravity/blending calculations, all you need to know is this simple formula: (C1V1 = C2V2), C = Concentration and V = Volume.
 Brewing the Strong Beer
At higher water dilution/blending rates, you may notice that your finished beer is a bit more estery than it would have been had you brewed it normally. Esters and higher alcohols, byproducts of fermentation, are produced at non-linearly higher rates in more dense worts than in less dense ones. Taking into effect the higher specific gravity and lower hop utilization, you are going to want to brew your base beer as you would brew any beer and ferment it to completion. You have to have a good base beer to produce a good final blended beer.
If your final beer tastes too estery, has fusel notes or other unwanted fermentation byproducts, you’ve pushed the technique too far. At higher dilution rates, you may also need to tinker with your recipe a bit to get the beer to taste as you want it. The big breweries typically do not exceed 16 °Plato (SG 1.064) when brewing their strong beer for dilution. Above this gravity and the resulting blended lager is too estery. For most lagers to be blended, 16 °Plato (SG 1.064) is a reasonable maximum starting gravity for the strong base beer.[Colby,2004]
 Blending/Dilution Water
Any dilution/blending water used should be of the same quality as the water you brewed with. It should taste/smell good and be free of contaminants such as heavy minerals or chlorine. Carbon filtering your tap water, buying spring water, or using RO/distilled water should all get you more-than-acceptable dilution/blending water.
Under most normal circumstances, water has some oxygen dissolved in it. At 68 °F (20 °C), pure water that is exposed to air will have 9.0 parts per million (ppm) oxygen at equilibrium. This amount decreases at higher temperatures. When you dilute your beer, you don’t want to introduce oxygen into it. Oxygen will cause the beer to go stale faster than it normally would, resulting in cardboard-like aromas or sherry-like flavors. So, you need to remove the oxygen from your dilution water.[Colby,2004]
Boil your dilution/blending water for 15-20 minutes, and then cool it very fast (without splashing) to reduce the oxygen content to around 1 ppm oxygen content. You should measure out the amount of water you will need for dilution/blending, plus 5-10% to take in account evaporation. You should use your prepared dilution/blending water immediately unless you can store it under CO2. Blending amounts in the BYO article went as high as 40% water to beer rate, but recommend starting the technique off the first couple of times at 10% before going on to 40%.
My technique is very simple. I use a Sanke to ferment Closed-system pressurized fermentation so I have room to do a 15 +/- little batch of beer. I will use my latest Mild batch as a example to explain the rest.
 Mostly Munich Mild:
12 gallon "end of boil" 14# Munich 2# Pale Ale 2# Wheat 2# Par-boiled rice .25# Chocolate 400*L 1# Crystal 40*L
Mashed over a 2 hour time frame (waiting on the sparge water to heat in my E-HLT) with 8.5 gallons of 166*F strike water, mashing at 154*F. Temperature dropped to 150*F at the end of the rest. I slowly opened my bottom-center drain valve and sparged into my lauter grant. The valve is opened more and more as the vorlauf is started. The wort is then recirculated until running clear out of the mash tun. I then drained the tun to my kettle completely and turned on the valve to fill the mash tun again with my remaining sparge water. I started my boil as always and everything else went to the usual routine. I ended up with 12 gallons once I was finished boiling. I didn't take gravity at this point because it wouldn't have mattered anyway during this experimental batch. I had pre-boiled some filtered water on the stove, inside the house earlier. I left it covered to drop to room temperature before adding it to the boil kettle right before cooling. When I add the water I get an instant drop to under 190*F, before the plate chiller even has a chance. I try to get down to pitching temperature as fast as I can, and do so in about 20 minutes running very litle cooling water through the Therminator plate chiller. Aeration came from a Venturi Device placed in-line during the chilling recirculation back to the kettle. I do this for energy reasons, but the pint is it gets chilled.
Now, comes the part that differs from most people. I transfer to a Sanke keg on top of an older yeast cake, seal up the keg, and attach my spunding valve setup to ferment under pressure. This wort was measured via refractometer to 11 brix (10.5*P or 1.042 SG) and went for 2 weeks in the primary being roused every now or so. I then un-tapped my spunding valve from the keg and rolled and shook the keg for 3-4 days. At the start of the fourth week, it was ready to chill, filter, and serve. Crystal clear beer ran via counter-pressure to another keg for serving, straight out of the primary fermentor. Sounds all fancy-smanchy huh??? Maybe... but to summarize I just added 3 gallons of cold water to 12 gallons of wort in my kettle. Then, I fermented. I too would not recommend volumes of dilution over 40%.
[Colby,2004] Chris Colby, editor of BYO magazine,"Blending for volume: Techniques", Jan,2004
--By WortMonger, member of HOMEBREWTALK.COM 11:42, 4 September 2008 (CDT)