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Hydroxycinnamates

Nonflavonoids important in juice and white wine. Martin Angulo, 2016

Cinnamates are a group of non-flavonoid phenolics containing only one aromatic ring and one carboxylic acid group; they are produced by phenylalanine precursors through the shikimate pathway by non-animal organisms (Taiz and  Zeiger, 2010).  Its most simple form is cinnamic acid and alternatives contain a hydroxyl (-OH) group at carbon 4 (coumaric acid), di-hydroxyls at carbons 3 and 4 (caffeic acid), and (ferulic acid) which has a hydroxyl at carbon 4 and a methoxyl (-OCH3) a carbon 3.  In grapes they are found in the skins and in the pulp and can be esterified with ethanol.  They are mostly found as esters of tartrates.

Caffeic acid is the most abundant of the three forms and is unique from the others because of is ortho di hydroxyl positions.  This makes it prone to enzymatic oxidation by phenol oxidases and oxygen.  During wine fermentation the tartrates can be hydrolyzed freeing the cinnamates, and wines with high levels are susceptible to microbial spoilage by Brettanomyces yeast.  Saccharomyces and other wine organisms contain decarboxylases that can convert for example coumaric acid to 4-vinyl phenol.  Brettanomyces contains an enzyme called vinyl phenol reductase that can further reduce this compound to the volatile 4-ethyl phenol which gives a wine an unwanted sweaty horse, and barnyard smell (Chatonnet et al, 1992).  This is how cinnamates are implicit to microbial spoilage.  Caffeic acid can be metabolized to 4-ethyl catechol and ferulic acid to 4-ethyl guaiacol all of which have similar sensory characteristics and can have an economic impact to wineries that do not have adequate sanitation conditions.  Studies have shown that the tartrate esterified forms are not metabolized by Brettanomyces (Schopp et al, 2013).  The thresholds for these compounds are 605 µg/L and 110 µg/L for 4-ethyl phenol and 4-ethyl guaiacol, respectively (Chatonnet et al, 1992). No data is available for 4-ethylcatechol.

References:

Taiz, L and E Zeiger.  2010.  Plant Physiology 5th ed.  Sinauer Associates.  Sunderland, MA

Chatonnet P, D Dubourdieu, JN Boidron, and M Pons.  1992.  The origin of ethylphenols in wines.  J. Sci. Food Agric. 60: 165-178

Schopp LM, J Lee, JP Osborne, SC Chescheir, and CG Edwards.  Metabolism of Nonesterified and Esterified Hydroxycinnamic Acids in Red Wines by Brettanomyces bruxellensisJournal of Agricultural and Food Chemistry.  61: 11610-11617

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