Acid Washing of yeast is an effective method to remove contaminating bacteria from cropped yeast prior to repitching. It is commonly used in breweries that reused their yeast for many generations. Yeast can be washed every generation or whenever contamination is suspected. Food-grade phosphoric acid is recommended to acidify the yeast slurry down to pH 2.2–2.5; the temperature should be maintained at 2°C–4°C for 2 h with gentle stirring to protect the cells against damage. It has been demonstrated that a combination of an ammonium persulfate wash acidified with phosphoric acid was more effective at removing most bacteria. Some lactic acid bacteria may show resistance even to the treatment; however, hop resins can be added as an extra precaution. The washing process will not remove wild yeasts, which are resistant to low pH. Whether pitching yeast is negatively affected by the washing process has been the subject of many studies, and many brewers view acid washing as a person might regard a life-saving drug with powerful side effects—as a last-ditch resort. However, some breweries employ yeast washing as a regular part of their yeast handling and culture maintenance. Under standard brewing conditions, with the procedure carried out correctly, the yeast should remain healthy. Yeast slurries harvested from high-gravity brews or under poor physiological conditions may result in a loss of cell viability resulting from the acid treatment. Acid-washed yeast must be pitched into wort directly after treatment. It is important to note that acid washing should not be necessary if hygiene standards are kept high, yeast is stored for a minimum period of time between brews, and is not reused for more than 5–10 generations.