Tuesday, March 5, 2013

Groovy! Acid wash yeast



Managing Bacteria in a Brewery
Even with using best practices to ensure sanitary equipment, the yeast may harbor an infection which grows with each re-pitching. Acid washing the yeast lowers the amount of bacteria passed from batch to batch. Brewers most often use acid washing as an emergency procedure to combat infection until fresh yeast can be obtained or as periodic maintenance. Some brewers wash every pitch to ensure absolutely every measure is taken to reduce bacteria in the brewing process. Unfortunately information regarding this practice is often conflicting or confusing.
How Acid Washing Works
Acid washing will lower the level of bacteria cohabitation with yeast. It will not rid an infection. We acidify the yeast slurry while keeping a low temperature to kill a large proportion of bacteria and weak yeast cells. Healthy, actively dividing and fermenting yeast produce an acidic environment naturally. We can utilize the ability of healthy yeast to withstand pH fluctuations to reduce the bacteria. Killing off weak and dying yeast cells is also beneficial because they excrete peroxides and autolysis factors which raises the pH of the total slurry and may contribute off flavors to beer.

Acid washing after several pitches can prevent a buildup of bacteria and weak yeast, allowing the healthy yeast to replicate and ferment uninhibited by bacteria and autolysis factors. Pediococcus affects yeast health by depressing yeast in suspension by up to 30% and increasing fermentation length while also producing high levels of diacetyl.

Acid washing has the potential to negatively impact yeast health if performed inappropriately or on certain strains of yeast. Time, pH, and temperature must be balanced to achieve the goals of washing while maintaining highly viable yeast for pitching.

Recommended Procedures
Overall goal:
Reduce yeast slurry pH to 2.0-2.4 range with food grade acid by constantly mixing at temperature range of 32-40ºF for 90 minutes.

Equipment:
Sterile recirculating reservoir with pump or sterile mixing reservoir and paddle.
75% food grade phosphoric acid.
Ability to cool or at least hold cool temperature.
Instructions:
Take initial yeast pH and temperature readings.
Dilute 75% phosphoric acid to 7.5% by a 1 to 10 dilution. The amount of 7.5% phosphoric acid required will depend on volume of yeast. We used approximately 40L (10 gal) per 100 gallons of yeast.
Start mixing yeast by paddle or recirculation pump.
Add 7.5% phosphoric acid using sterile technique (sanitize all connections)
Mix for 2-3 minutes and check pH and temperature.
Adjust pH by adding more 7.5% phosphoric acid or more water or yeast slurry
once pH range of 2.0-2.4 has been achieved. Start timer. Wash for 90 minutes. Maintain temperature between 32-40ºF.
After 90 minutes pitch yeast as soon as possible. Maximum storage for washed yeast is two hours