High Gravity Brewing
Brewing beers with high original gravities (above 1.065) requires some modifications to normal brewing procedures. Factors that will determine success or failure in high gravity brewing are pitch rates nutrient addition, and oxygenation.
Increased wort gravity causes increased stress on yeast due to increased osmotic pressure and increased alcohol levels. It is necessary to increase your pitch rates when increasing the wort gravity. A general rule of thumb is to pitch one million cells per milliliter per degree Plato. Thus, 20 degree Plato (1.080 O.G.) wort would require 20 million cells per milliliter.
It is very important to use healthy yeast when brewing a high gravity beer. Choose yeast from a fermentation that had a moderate gravity (1.060 O.G. or less, 15°P or less) and a normal fermentation profile. If a hemacytometer or other cell counting method is not available, use weight or volume to determine a good pitch rate and remember that it is better to pitch too many cells rather than too few. Yeast slurry that is approximately 40% solids has about 1.0 x 109 cells per ml. Pitching 1 liter (1.1Kg or 2.2 lbs.) per barrel of a 40% solids slurry will give you approximately 10 million cells per ml in the fermenter.
Yeast harvested from a high gravity fermentation will be in poor health and should not be used in subsequent fermentations. Because of this, we do not recommend pitching new yeast into a high gravity beer unless absolutely necessary because the cost becomes prohibitive.
Essential nutrient levels are typically reduced during high gravity brewing. This is a function of either increased binding of nitrogen during boiling or dilution of essential nutrients by adjunct addition. Additions of Wyeast Nutrient will restore essential nutrient levels to provide healthy fermentation.
As wort gravity increases, solubility of oxygen decreases. It is very important to compensate for the decrease in solubility by oxygenating more aggressively. When brewing high gravity beers, it is best to oxygenate with pure oxygen through a scintered stone.