Phenolics: Location and Attributes in Grape Berries

After getting the Parts of the Grape Berry straight, I thought I would tackle the importance of grape phenolic because:

Grape berry phenolics¹:

• contribute to organoleptic properties
• color
• aging properties
• protection against environmental challenges

(In the table below, clicking on the blue highlighted compounds will lead to a blogpost containing the chemical structures.)

Phenolic Compound	Attributes
Hydroxycinnamates acid esters of tartaric acid	→Hydroxycinnamic acid esters of tartaric acid form the predominant phenolic components of the pulp →Caftaric acid at 77% is the most abundant, followed by coutaric and fertaric acids →Caftaric acid is found in the pulp as well as in the skin in a white grape, so it is the most abundant phenolic in white wines →The level of hydroxycinnamates declines significantly during ripening →The major decline begins just before veraison and is virtually done by the second berry growth phase
Anthocyanins	→Only the epidermis and first hypodermal layer of the skin of red varieties is normally darkly pigmented →The next two hypodermal layers may contain smaller amounts of anthocyanins, and subsequent layers tend to be sporadically and weakly pigmented. Pigmentation seldom occurs deeper than the sixth hypodermal layer →Anothocyanins are responsible for the color in red wines
Flavan-3-ols	→Flavan-3-ol are primarily seed compounds with none found in the pulp and very limited quantities found in the skin →Catechin and epicatechin are the dominant flavan-3-ols found in grapes →The skins contain relatively low levels of catechin and epicatechin →Free catechin and epicatechin lie outside of the true seed coat which is comprised of lignin →Catechin and the epicatechin in the seeds show a very steady decline during ripening →Catechin and epicatechin have been shown to impart bitterness
Tannins also called Proanthocyanidins	→Polymeric flavan-3-ols are condensed tannins and are also referred to as proanthocyanidins →Tannins can be composed of four different subunits: catechin, epicatechin, epigallocatechin, and epicatechin gallate →Tannins are responsible for astringency →Tannins are found both in the skin, as well as in the seeds →Skin tannins are very different in character from seed tannins begin greater in size than seed tannins →Polymerized skin tannins can be as long as 40 or 50 units mDP (mean degree of polymerization) of catechin, epicatechin, gallocatechin, epigallocatechin, and/or catechin-3-O-gallate →Tannins are made very early in berry development; nearly all of the tannin that’s going to be present at ripening at harvest is already present in the skin of the grape at veraison →Tannins in the skin do not change significantly during berry ripening →Tannins in the seed show a very dramatic decline during ripening

References:
1. Douglas O. Adams, "Phenolics and Ripening in Grape Berries", Am. J. Enol. Vitic., 57:3 (2006).
2. Douglas Adams, VI257, Lesson 8, pg. 1-50.
3. Illustration from Vintage Direct 2.10 Making Sparkling Wine (Methode Champenoise), Dunsford, P.A. and Sneyd, T.N. 1989. Pressing for quality. In: Proceedings of the Seventh Australian Wine Industry Technical Conference.
4. António Teixeira, José Eiras-Dias, Simone D. Castellarin, and Hernâni Gerós, Berry Phenolics of Grapevine under Challenging Environments, Int. J. Mol. Sci. 2013, 14(9), 18711-18739.