Phenolics in Grape Berry and Wine

Grape Phenolics
Many phenolic compounds are involved in plant protection as biologically active growth inhibitors of other living systems. They have strong antioxidant activities and play important beneficial roles in the mammalian systems.
Catechins, tannins, and anthocyanins are the most concentrated natural antioxidants present in red grapes and wine.
The berry skin contains tannins and pigments, the pulp contains juice but no pigments, and the seeds contain tannins. The insoluble cutin of the epidermis and the insoluble lignin of the hard seed coat are phenolics, which are as important as the skin tannins and pigments and seed tannins but only the soluble phenolics of the grape berry are important in winemaking.
Phenolic compounds of the grape are divided between the non-flavonoids and the flavonoids group:

Nonflavonoid phenolics (with a simple C6 backbone) are found in grapes and wine, with the exception of hydroxycinnamic acids, they are present at low concentrations

• hydroxycinnamates are present in hypodermal cells along with tannins and anthocyanins and in mesocarp and placental cells of the pulp
• hydroxycinnamates are the third most abundant class of soluble phenolics in grape berries, after tannins and anthocyanins
• caftaric acid is the most abundant hydroxycinnamate in free-run juice of white grapes and consequently in white wines

Flavonoids make up a significant portion of the phenolic material in grapes and include several classes:

• Flavan-3-ol monomers (catechins)
• flavan-3-ol monomers (catechins) are responsible for bitterness in wine and may also have some associated astringency
• the major flavan-3-ol monomers found in grapes and wine include (+)-catechin, (-)-epicatechin, and (-)-epi-catechin-3-O-gallate
• much of the flavan-3-ol monomers originate from seed material, although they are also present in grape skin hypodermal cells
• Proanthocyanidins (tannins)
• tannins or proanthocyanidins are polymers of flavan-3-ols and are the most abundant class of soluble polyphenolics in grape berries
• tannins confer astringency to red wines and are extracted from the hypodermal layers of the skin and the soft parenchyma of the seed between the cuticle and the hard seed coat, as well as, from the peduncle of the grape berry
• tannins are a very diverse set of biomolecules varying in size from dimers and trimers up to oligomers with more than 30 subunits
• Anthocyanins
• anthocyanins are responsible for red wine colour and are co-located with tannins in the thick-walled hypodermal cells of skin
• anthocyanins are anthocyanidins covalently associated with one or more sugar molecules
• anthocyanins can be esterificated by acids, such as acetic and coumaric acid, and substituted with hydroxyl or methyl groups that will confer a specific hue at light
• malvidin-3-O-glucoside and its acylated forms is the major anthocyanin present in grapes

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.
All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.