13,758
edits
Changes
added to Acid Production section
====Phenol Production====
Phenols such as 4-vinylphenol (4VP, barnyard, medicinal) and 4-vinylguaiacol (4-VG, clove) can be produced in beer by the decarboxylation of hydoxycinnamic acids, which are found in malt. While both ''Saccharomyces'' and ''Brettanomyces'' strains are have varying capabilities based on strain of converting hydroxycinnamic acids to their vinyl derivatives <refname="Lentz">[http://www.mdpi.com/2304-8158/4/4/581/htm Analysis of Growth Inhibition and Metabolism of Hydroxycinnamic Acids by Brewing and Spoilage Strains of Brettanomyces Yeast. Michael Lentz and Chad Harris. 2015.]</ref>, ''Brettanomyces'' is also able to reduce these vinyl derivatives to ethyl derivatives. These vinyl derivatives have similar tastes to the ethyl derivatives but have lower flavor thresholds. Levels of all compounds produced vary depending on species and strain of ''Brettanomyces''.
* [http://phenol-explorer.eu/foods See the Phenol Explorer website for more information on sources of precursors.]
====Acid Production====
In the presence of oxygen, ''Brettanomyces'' strains are capable of producing acetic acid. Depending on the brewer's palate and the degree of acetic production, this can be a desirable or undesirable trait. The degree of acetic acid production varies among different ''Brettanomyces'' strains. Acetic acid produced by ''Brettanomyces'' may also be used in the synthesis of [[Secondary metabolites|acetate esters]] such as ethyl acetate. ''Brettanomyces'' has been shown to produce enough fatty acids in anaerobic fermentation to drop the pH to 4.0, which can also be esterified (see the ester table above) <ref name="yakobson1"></ref>. Many of these acids can have an unpleasant rancid odor and/or taste, which may be noticeable in young ''Brettanomyces'' beers before these acids are esterified.
Michael Lentz and Chad Harris tested whether or not the hydroxycinnamic acids (HCAs) inhibit the growth of ''Brettanomyces''. They found that high levels of hydroxycinnamic acids (HCAs), which includes ferulic acid, p-coumaric acid, and caffeic acid, do inhibit the growth of ''Brettanomyces''. Ferulic acid is the strongest inhibitor of these three HCAs with most strains tested not being able to grow in wort that contained 12 mM (millimolar) of ferulic acid. Caffeic acid was generally shown to be the weakest inhibitor of the three HCAs tested. Levels of 25 mM p-courmaric acid inhibited growth of all strains tested, and levels of 30 mM of caffeic acid inhibited all strains tested. The ability of HCAs to inhibit growth is different from strain to strain of ''Brettanomyces''. Inhibition does not appear to be species dependent. Some strains display a lag time and grow more slowly in the presence of high amounts of HCA's, but still eventually achieve maximum growth compared to if they were grown without exposure to HCAs, while others lag and then stop growing before reaching maximum growth <ref name="Lentz"></ref>.
The amount of HCAs varies widely from plant to plant, and the amount that is found in must or wort also varies on how the raw ingredients are treated. These measurements are generally not a consideration for maltsters or grape growers <ref name="Lentz"></ref>. The one exception to this is the ferulic acid rest that German brewers have used to create more clove-like flavors in certain beer styles.
{| class="wikitable sortable"