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update to pasteurization
===Pasteurization===
Pasteurization is measured in terms of "pasteurization units" (PU). One PU is equal to exposure of 60°C (140°F) for 1 minute. The total PU is determined by plotting time against temperature in degrees Fahrenheit. A total of 15 PU's has been given as the target for pasteurizing beer. The following equation can be used to calculate PU's using different temperatures and times <ref name="Haas_1960">[https://www.sciencedirect.com/science/article/abs/pii/S0065216408701242 Gerhard J. Haas. "Microbial Control Methods in the Brewery". Editor(s): Wayne W. Umbreit. Advances in Applied Microbiology. Academic Press, Volume 2, 1960. Pages 113-162. ISSN 0065-2164. ISBN 9780120026029.]</ref>: [[File:PU calculation.jpg|none|360px|]] There are two types of pasteurization methods used in brewing: tunnel pasteurization and flash pasteurization. In tunnel pasteurization, which is more widely used in breweries, cans or bottles of packaged beer is moved slowly through a tunnel of fixed temperatures. In flash pasteurization (or plate pasteurization), large quantities of beer are pasteurized at the same time via a heat exchanger and is usually performed before the beer is packaged <ref name="Vaughan_2005" />. Since thermal death rates for beer spoilage organisms has been identified to be 140°F (60°C) for 20 15 minutes <ref name="Haas_1960" /><ref>[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1946.tb01593.x THERMAL DEATH POINTS OF MICRO-ORGANISMS IN BEER. Aage Lund. 1947.]</ref>, this is the baseline temperature and time for pasteurization, although higher temperatures and shorter times are used for some pasteurization methods (see the below links). The complete thermal death of ''Brettanomyces'' in wines has been reported to be 50°C for 5 minutes. <ref>[https://pubmed.ncbi.nlm.nih.gov/15996781/ Thermal inactivation of the wine spoilage yeasts Dekkera/Brettanomyces. José António Couto, Filipe Neves, Francisco Campos, Tim Hogg. 2005. DOI: 10.1016/j.ijfoodmicro.2005.03.014.]</ref><ref name="Nunes de Lima 2020" />. Some strains of ''Lactobacillus'' have been shown to potentially survive pasteurization temperatures for at least some amount of time; see [[Lactobacillus#Tolerance_of_Extreme_Temperature|''Lactobacillus'' Heat Tolerance]] for more information.
Microfiltration is an alternative technology to heat pasteurization that can be used to pasteurize beer. Microfiltration uses a set of membranes, usually in the 0.45–0.65 μm range, for filtering bacteria and yeast. Bacteria have a cell size of about 5-10 μm and yeast species have a cell size of about 5–16 μm, while flavor compounds such as phenols are filtered out when using a smaller diameter filter such as 0.2 μm. One study by Bernardi et al. (2019) found that filtration with polyethermide membranes removed around 1-2 IBU, ~30% of yeast-produced phenolic compounds (most polyphenols from hops were not filtered out), and larger tannins (which were only a small portion of the total polyphenol content). The antioxidant activity was largely not impacted. After filtration, the beers were 26%-33% lighter in color, depending on the style of the beer, and were 100% clearer. The filtration that was used, which was 1.2 μm, also produced fully pasteurized beers <ref>[https://www.sciencedirect.com/science/article/pii/B9780128152584000135 Microfiltration for Filtration and Pasteurization of Beers. Guilherme dos Santos Bernardi, Jacir Dal Magro, Marcio A. Mazutti, J. Vladimir Oliveira, Marco Di Luccio, Giovani Leone Zabot, Marcus V. Tres. 2019. DOI: https://doi.org/10.1016/B978-0-12-815258-4.00013-5.]</ref>.