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There are, however, a few exceptions to this in the scientific literature. For example, ''L. plantarum'', which is a facultatively heterofermentative species, is homolactic without the presence of oxygen and produces only lactic acid. After food sources have been exhausted and in the presence of oxygen, however, ''L. plantarum'' switches to heterolactic fermentation, and further converts lactic acid into acetic acid. In a lab setting the conversion of lactic acid to acetic acid only happened when glucose was no longer available (this may not be the case in wort where other limiting factors such as low pH can prevent ''L. plantarum'' from continuing their metabolic processes before all glucose is consumed, which is the case in kettle souring), and only during the stationary phase (after growth stopped). During the conversion of lactic acid to acetic acid, ''L. plantarum'' also produces hydrogen peroxide (H²0²), which is toxic to microorganisms and is thought to be a protection mechanism for ''L. plantarum'' <ref>[http://www.ncbi.nlm.nih.gov/pubmed/6480562 Physiological role of pyruvate oxidase in the aerobic metabolism of Lactobacillus plantarum. Sedewitz B, Schleifer KH, Götz F.1984.]</ref><ref name="Quatravaux_plantarum"></ref><ref name="microbewiki_plantarum">[https://microbewiki.kenyon.edu/index.php/Lactobacillus_plantarum_and_its_biological_implications Lactobacillus plantarum and its biological implications. Microbe Wiki. Retrieved 6/7/2015.]</ref><ref name="shaner_plantarum"></ref>. This same process has also been observed in one strain of ''L. brevis'' and under similar conditions of depleted nutrients and oxygen <ref>[http://aem.asm.org/content/early/2017/08/21/AEM.01659-17.abstract The oxygen-inducible conversion of lactate to acetate in heterofermentative Lactobacillus brevis ATCC367. Tingting Guo, Li Zhang, Yongping Xin, ZhenShang Xu, Huiying He, and Jian Kong. 2017.]</ref>. Omega Yeast Labs reports that no noticeable acetic acid is produced if the oxygen is not purged with their OYL-605 ''Lactobacillus'' blend which contains ''L. plantarum'', and brewers should not aerate wort during sour but purging O2 is not required <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1222363364458542/?comment_id=1222371257791086&reply_comment_id=1225212894173589&comment_tracking=%7B%22tn%22%3A%22R%22%7D MTF conversation with Adi Hastings from Omega Yeast Labs on the effects of oxygen presence in OYL-605. 02/02/2016.]</ref>. This indicates that acetic acid and hydrogen peroxide production in typical brewing scenarios where little or no oxygen is present is probably not a concern. ''L. brevis'' has been shown to increase growth rates in the presence of oxygen <ref name="brevis_aeration"></ref>. Thomas Hübbe's master thesis showed that under lab growth media "VLB-S7-S", "NBB®-A", and MYPG + cyclohexmide, both strains of ''Lactobacillus'' tested (''L. brevis'' and ''L. parabrevis'') did not show growth in an aerobic chamber, but did grow in an anaerobic chamber, although this might be an effect of the growth media and not representative of brewing conditions <ref name="Hubbe">[https://www.facebook.com/groups/MilkTheFunk/1407620509266159/ Effect of mixed cultures on microbiological development in Berliner Weisse (master thesis). Thomas Hübbe. 2016.]</ref>.

====Hop Tolerance====