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→Tolerance of Extreme Temperature
====Tolerance of Extreme Temperature====
Most ''Lactobacillus'' species have a thermal death rate of ~145°F 151°F (63°C66°C)after 5 minutes <ref>[https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1957.tb02903.x SOME INVESTIGATIONSON LACTOBACILLUS INFECTION. By R.T.Dean,M.Sc.(Carllon and United Breweries,Ltd.,Melbourne,Australia). Received 11th October,1956.]</ref>. Freezing without glycerol will kill most cells, but it is possible for a very small number of cold-resistant mutant cells to survive <ref>[http://fermentationnation.net/2015/11/episode-26-quality-assurance-w-jessica-davis-of-the-bruery/ Fermentation nation Podcast interview with Jessica Davis, QA for The Bruery.]</ref> (~1:19:00 in). In very rare occasions, some strains have been identified as being extremely thermotolerant. For example, [https://patents.google.com/patent/US20180042256A1/en this patent] claims that a strain of ''Lactobacillus delbrueckii'' subsp. ''lactis'' can acidify milk within 2.5 hours when held at temperatures between 45-65°C (113-149°F).
In another example, [https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.1999.00607.x Jordan and Cogan (1999)] described the heat tolerance of three strains of ''Lactobacillus'' isolated from cheese (two strains of ''L. plantarum'' and one strain of ''L. paracasei''). When temperatures were finally able to kill the strains, they observed a non-linear death curve with either very little death for the first 15 minuets, or a tailing of the curve. For the ''L. paracasei'' strain, the culture was able to survive temperatures between 50 and 55°C for two hours. At 60°C, there was no significant cell death for 15 minutes, but after 15 minutes the culture began to slowly die, and then more quickly die after an hour of heat exposure. At 65°C, the ''L. paracasei'' strain had a fairly steep death curve for the first 10 minutes, and then a tailing curve showing a small number of cells surviving up to 25 minutes. Even at 72°C, which is typical for pasteurization in the milk industry, the death rate for this strain was slow enough to survive 15 second pasteurization methods. The two strains of ''L. plantarum'' were less heat tolerant, with death starting to occur at around 50-56°C <ref>[https://pubmed.ncbi.nlm.nih.gov/10499302/ Jordan KN, Cogan TM. Heat resistance of Lactobacillus spp. isolated from Cheddar cheese. Lett Appl Microbiol. 1999 Aug;29(2):136-40. doi: 10.1046/j.1365-2672.1999.00607.x. PMID: 10499302.]</ref>. Assuming the heat penetrates all surfaces, exposure to water heated to 72-80°C for 1 minute should be more than adequate for eliminating heat tolerant strains of ''Lactobacillus'', as well as any other beer contaminate species, from the brewing environment.