Irish moss is a fining agent used during the boil. Irish moss is not a type of moss; it’s really a kind of seaweed. At the pH of wort (typically 5.0 to 5.5), Irish moss carries a negative charge. At the same pH, haze forming proteins carry a positive charge. Adding Irish moss to your wort attracts haze-forming proteins to it. The “moss” settles out after the boil, taking the proteins with it. To use Irish moss, follow this procedure:
Step 1. Wet 0.04 to 0.125 g/L Irish moss with just enough water to cover it. For a 5-gallon batch, use approximately 1.0 to 2.5 grams of Irish moss. A gram of Irish moss is approximately one teaspoon.
Step 2. When there is 15 minutes left in your boil, add the Irish moss.
Step 3. Whirlpool your wort before siphoning to a fermenter. The trub should settle into the middle of the brew kettle.
Step 4. Siphon your wort to your fermenter. Minimize the amount of trub carried over from the kettle.