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. So a 20 °Plato (1.080 SG) wort would require 20 million cells per milliliter.
Home brewers will either need to pitch more packages of yeast or make a starter. When using an Activator 125 mL package, 1 package to 5 gallons will give around 6 million cells per milliter. This is clearly too low for high gravity brewing and will cause inconsistent results and usually high terminal gravities.
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.