Pitching yeasts collected from brewery fermentations are never absolutely free of microbiological infection. In spite of whatever care and sanitary precautions are taken, some bacteria and wild yeast will contaminate the pitching yeast. The pitching yeast can contain healthy yeast cells and trub (dead yeast cells and organic residues) and may contain 5 to 15% dry solids (35). To minimize microbiological infection, brewery yeast can be washed using the following procedures:
Distilled or Sterile Water Wash
In the first method, the yeast slurry and cold, distilled or sterile water are mixed thoroughly in a decantation tank. The yeast is allowed to settle and the supernatant water is decanted, taking with it dead cells, trub, grain, and hop particles.
The second method is to wash the yeast with acids, e.g., tartaric, citric, sodium metabisulfite, sulfuric, or phosphoric acid which is typically the most commonly used acid. Acid washing lowers the pH of the yeast slurry to the point at which bacteria and weak yeast cells are killed off, but it does not harm the healthy yeast cells.
Acid Wash with Ammonium Persulfate
Some brewers use an acid-persulfate combination rather than just acid claiming that it is a more effective treatment than treatment with acid alone. Briggs et al. recommend the addition of a strong oxidizing agent as ammonium persulphate (0.75% w/v) with phosphoric acid (8).
Chlorine dioxide, an alternative to distilled water or acid washing, is relatively new to the brewing industry and is gaining acceptance as a method for washing yeast. It kills via microbes by reacting chemically with sulfur-containing amino acids, the building blocks of protein which are used to form cell membranes.
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