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→Yeast Washing
Yeast washing is the practice of exposing a yeast slurry to extreme acidic conditions in order to destroy bacteria contaminants in the slurry, and it has been used for over a century in the brewing industry to help reduce the potential for lactic acid bacteria spoilage. Although techniques might vary throughout the brewing industry, the most typical technique is to add phosphoric acid to a slurry of yeast until a pH of 2 is reached, and then the slurry is stored for 2 hours at 5°C (41°F). While phosphoric acid is a good choice for acid washing because of being inexpensive compared to other acids, its tendency to not kill yeast, and its lack of affecting beer flavor, it also does not kill some contaminants such as ''Shimwellia pseudoproteus''. It has also been proposed that chlorine dioxide, a disinfectant that is often used in the vegetable, meat, and water treatment industries, can be successfully used to wash a yeast slurry, with the first study on this reporting that a concentration of 78 mg/L (concentration value is for the entire slurry) and stored for 30 minutes at 8°C (46.4°F) was effective <ref>[https://link.springer.com/article/10.1007/s00253-020-10534-x Modeling the inactivation of Lactobacillus brevis DSM 6235 and retaining the viability of brewing pitching yeast submitted to acid and chlorine washing. Munford, A.R.G., Chaves, R.D., Granato, D. et al. Appl Microbiol Biotechnol (2020). https://doi.org/10.1007/s00253-020-10534-x.]</ref>.
Lysozyme, an enzyme that is often extracted from hen egg whites, is known to inhibit Gram-negative bacteria such as ''Lactobacillus'' but not Gram-positive bacteria such as ''Acetobacter'', and has been shown to be an enzyme that can help inhibit spoilage bacteria in wine and cider fermentations <ref>[https://en.wikipedia.org/wiki/Lysozyme "Lysozyme". Wikipedia. Retrieved 0319/2020.]</ref>. Lysozyme is normally added to wine with a stuck fermentation or to limit malolactic fermentation, and several yeast companies offer a lysozyme-based product <ref>[https://scottlab.com/content/files/Documents/Handbooks/ScottlabsHandbook2018.pdf 2018 Fermentation Handbook. Scott Laboratories. Retrieved 03/19/2020.]</ref><ref>[https://www.academia.edu/16244381/Lysozyme_in_Wine_An_Overview_of_Current_and_Future_Applications?email_work_card=title Lysozyme in Wine: An Overview of Current and Future Applications. Marco Esti, Ilaria Benucci. Comprehensive Reviews in Food Science and Food Safety. 2014.]</ref>. It has also been suggested to be useful for limiting lactic acid bacteria in yeast slurries, but one experiment reported that the sensitivity of different species of lactic acid bacteria varies, with ''Pediococcus inopinatus'', ''Lactobacillus brevis'', ''Lactobacillus brevisimilis'' showing similar levels of sensitivity, but ''L. linderi'' showing less sensitivity. Bacteria were inhibited more at 22°C than at 4°C. At 300 mg/L, although lactic acid bacteria was inhibited, it was not killed completely <ref>[https://www.researchgate.net/publication/293048096_Antibacterial_properties_of_hen_egg_white_lysozyme_against_beer_spoilage_bacteria_and_effect_of_lysozyme_on_yeast_fermentation/citation/download Van Landschoot, Anita & Villa, A. (2005). Antibacterial properties of hen egg white lysozyme against beer spoilage bacteria and effect of lysozyme on yeast fermentation.]</ref>. Nisin has also been proposed as a potential preservative that can be added to wort during boiling or cooling as well as to the fermenter in order to limit the growth of lactic acid bacteria by up to 90% <ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.233 Müller-Auffermann, K, Grijalva, F, Jacob, F, and Hutzler, M (2015), Nisin and its usage in breweries: a review and discussion. J. Inst. Brew., 121, 309–319. doi: 10.1002/jib.233.]</ref>.
The homebrew practice of mixing distilled or sanitized water into a yeast slurry, letting the slurry settle into three layers, and then removing the bottom and top layer and re-pitching or saving the middle layer, is different than "yeast washing". This process is known as "yeast rinsing", and is primarily employed by homebrewers who wish to separate trub material from their yeast slurries before reusing the yeast slurry. This might have the benefit of removing unwanted flavors from the slurry (although there is a lack of evidence that we know of for this claim <ref>[http://brulosophy.com/2015/03/02/sloppy-slurry-vs-clean-starter-exbeeriment-results/ "Sloppy Slurry vs. Clean Starter". Brulosophy website. 2015. Retrieved 03/19/2020.]</ref>) or hop material that could inhibit yeast growth, but it does not inhibit lactic acid bacteria or any other contaminants (in fact, this process increases the chances of contaminating the yeast slurry). See [https://www.homebrewersassociation.org/how-to-brew/yeast-washing-yeast-rinsing-whats-difference/ this AHA article] for more details on yeast rinsing.