Nonflavonoid Phenols in Grapes

It's pretty cold out there (9 ^oF), so it's the perfect time to hunker down and review the literature that I read while taking the UC Davis Viticulture course. The phenolic compounds that occur in grapes can be found in the skin and in the seeds. The low molecular weight nonflavonoid phenolic acid biosynthesis in grapes begins with the amino acid phenylalanine. The deamination of phenylalanine catalyzed by phenylalanine ammonia lyase results in the formation of trans-cinnamic acid. T-cinnamic acid is the precursor of the nonflavonoid phenolic compounds. The addition of a hydroxyl group to t-cinnamic acid results in the formation of the first phenolic compound in the grape called 4-coumarate. The part of the 4-coumarate structure colored in red is the "phenol". All phenolic compounds contain this chemical moiety.³

Another hydroxylation of 4-coumarate yields caffeic acid and a methylation on the hydroxyl group on caffeic acid results in the formation of ferulic acid shown below:

These cinnamic acids can exist either in the free form or as esters of tartaric acid shown below in blue:

Caftaric acid is the most common nonflavonoid in grapes. In the process of winemaking, these cinnamic acids are the first to be extracted due to their occurrence in the cell vacuoles of skin and pulp.⁴
References:
1. Understanding Grape Berry Development Practical Winery and Vineyard, Issue July-August 2002, Illustration done by Jordan Koutroumanidis of Winetitles.
2. Vintage Direct 2.10 Making Sparkling Wine (Methode Champenoise)
Illustration is from Dunsford, P.A. and Sneyd, T.N. 1989. Pressing for quality. In: Proceedings of the Seventh Australian Wine Industry Technical Conference
3. Douglas Adams, VI257, Lesson 8, pg. 4-11.
4. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008, pg. 285-287.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.