Changes

There have been anecdotal reports of other forms of mousy off-flavors. During growth of lactic acid bacteria (LAB), mousy off-flavor detection fluctuated with high levels detected early on, and lower levels detected towards the end of growth. This indicates that there may be a transient, strain-dependent form of THP that can occur during malolactic fermentation. There have also been sensory detection of mousy off-flavors at different levels than the documented levels of ATHP, ETHP, and APY, which were not associated with LAB or ''BrettBrettanomyces'' <ref name="Snowdon"></ref>.

[[File:THP Pathway.JPG|thumb|400|Proposed pathway for THP production by ''BrettBrettanomyces'' <ref name="Elsevier">[https://books.google.com/books?hl=en&lr=&id=KJJwAgAAQBAJ&oi=fnd&pg=PA346&dq=brettanomyces+Tetrahydropyridine&ots=ktbn8PR_fF&sig=r3lkcV-gBa-pK86HSOgFDVIJVDk#v=onepage&q=brettanomyces%20Tetrahydropyridine&f=false Managing Wine Quality: Oenology and Wine Quality. A Reynolds Elsevier, Sep 30, 2010. Pg 359.]</ref>]]

Oxygen has a stimulatory effect in ATHP and ETHP production (particularly ATHP), but its exact role is not understood. It has been speculated that since ATHP production is associated with ''BrettBrettanomyces'' growth, and ''BrettBrettanomyces'' grows better under aerobic conditions, that this is why more ATHP is produced under aerobic conditions <ref>[http://www.brettanomycesproject.com/dissertation/introduction/ Yakobson, Chad. The Brettanomyces Project; Introduction. Retrieved 3/10/2015.]</ref><ref>[http://pubs.acs.org/doi/abs/10.1021/jf071243e The Role of Lysine Amino Nitrogen in the Biosynthesis of Mousy Off-Flavor Compounds by Dekkera anomala. Paul R. Grbin, Markus Herderich, Andrew Markides, Terry H. Lee, and Paul A. Henschke. J. Agric. Food Chem., 2007.]</ref><ref name="Oelofse">[http://scholar.sun.ac.za/handle/10019.1/8437 Significance of Brettanomyces and Dekkera during Winemaking: A Synoptic Review. A. Oelofse, I.S. Pretorius, and M. du Toit. 2008.]</ref>. It has also been hypothesized that oxygen may have a direct effect on the THP molecules themselves <ref name="Snowdon"></ref>. ATHP production was also shown to increase when anaerobically precultured cells were transferred to an aerobic environment, indicating that oxygen has a direct role on the production of ATHP, not just a byproduct of ''BrettBrettanomyces'' growth <ref name="Snowdon"></ref>. Limiting oxygen exposure during kegging/force carbonating is recommended for helping to reduce ATHP production; even very small amounts can have an effect. Pitching fresh ''Saccharomyces'' at bottling/kegging time and naturally carbonating the beer with sugar has reportedly reduced mousy off-flavor detection, perhaps because ''Saccharomyces'' metabolizes both the oxygen and sugar faster than ''BrettBrettanomyces''.

Interestingly, for unknown reasons ''BrettBrettanomyces'' cells grown under aerobic conditions and then transferred to an anaerobic environment still produced significant amounts of ATHP in the anaerobic environment. It has been suggested that the aerobic conditions made the ''BrettBrettanomyces'' cells predisposed to creating ATHP <ref name="Snowdon"></ref>. Oxygen exposure during ''BrettBrettanomyces'' starters could potentially stimulate ATHP production later on down the road, even if the beer is not exposed to oxygen, although anecdotal evidence shows that this may not be a concern for brewers. It is still advised to use an aerobic or semi-aerobic starter for ''BrettBrettanomyces'' unless the brewer believes this might be the direct cause of ATHP problems in their beer. Any other oxygen pick up after the beer has finished fermentation is the more likely cause and the brewer's post-fermentation processes should be examined first.

The level of ATHP production varies widely between species and strains of ''BrettBrettanomyces'', with some strains producing insignificant amounts and others producing very high amounts above taste threshold. Additionally, ATHP production requires glucose or fructose, which explains why ATHP may be seen more often in stuck wine fermentations rather than wine that has finished fermenting. ATHP production by ''BrettBrettanomyces'' was observed in wine with glucose or fructose added, along with synthetic growth media, suggesting that the type of growth substrate does not effect production <ref>[http://www.ncbi.nlm.nih.gov/pubmed/18194246 Growth and volatile compound production by Brettanomyces/Dekkera bruxellensis in red wine. Romano A, Perello MC, de Revel G, Lonvaud-Funel A. J Appl Microbiol. 2008 Jun.]</ref>.

Although ''BrettBrettanomyces'' is capable of producing APY from L-ornithine, the amount produced is much less than that of LAB. In wine, there isn't enough L-ornithine present to production significant amounts of APY from L-ornithine. Therefore, the presence of APY (which is much easier to detect aromatically than ATHP) indicates a bacterial contamination in wine (it is unknown if this applies to beer) <ref name="Snowdon"></ref>.

The presence of the "mousy off-flavor" caused by forms of THP appears to be temporary in beer. Although not much is known about the degradation or metabolic breakdown of ATHP/ETHP, it tends to age out of beer after 2-6 months. Since the odor/taste threshold for ETHP is much higher than ATHP, and ATHP appears to be metabolized into ETHP by ''BrettBrettanomyces'' over time, this may be one of the mechanisms by which the mousy off-flavor ages out of beer. The possibility of ETHP breakdown is not mentioned in any studies that we know of. Another unknown is why does ''BrettBrettanomyces'' produce ATHP shortly after kegging and force carbonating a beer that has reached final gravity. The most likely cause is oxygen pick up during the kegging process. Pitching fresh ''Saccharomyces'' at bottling/kegging time and naturally carbonating the beer with sugar has reportedly reduced mousy off-flavor detection, perhaps because ''Saccharomyces'' metabolizes both the oxygen and sugar faster than ''BrettBrettanomyces''.