Problem: Bacteria in the pitching yeast. Acid washing can help to reduce the number of some bacteria, but is not a complete solution. We usually do not recommend acid washing because:
*Acid washing only reduces the number of bacteria, but does not completely remove them. After one or two more generations, the bacteria can regenerate up to numbers that impact flavor.
*Acid washing has different impacts on different yeast strains, and usually reduces the quality of their performance and viability. Acid washing is not an effective way to kill wild yeast.
However, if acid washing is done, here is an effective method:
Begin with and keep the yeast between 36-40oF.
Determine how much yeast will be needed to pitch, put in a suitable container such as stainless vessel, and begin the acid washing procedure 120 minutes before the yeast will be pitched.
Add food grade phosphoric acid until the pH of the slurry is between 2.0-2.5. Mix thoroughly to get an accurate reading.
Continue to mix/stir constantly until pitching, 60 to 90 minutes.
Add the entire mixture to the fermentor
This can result in cold shock, but it important that the temperature of the acid wash stay cold or yeast damage will take place. There is already some yeast damage with acid washing, but you want to minimize it.